A-Arm Science

Rolling through the center of the turn at maximum speed requires a suspension that is free of flex yet moves smoothly with out binding. A-arms have improved dramatically and choosing the correct A-arm will help you to create suspension systems that are faster and more consistent.

There are many things to consider when selecting the A-arm for your car. For dirt cars many racers have utilized arms that split at the bolt in ball joint allowing the a-arm to be opened for installations where the A-arm wraps around the frame mount. The split causes additional flex and high quality versions should be considered. The split A-arm utilizes a bolt in ball joint. Many top teams are switching to low friction screw in ball joints for better performance.

We built this fixture for this test which can apply from 0 to 1500 pounds of force right at the ball joint emulating the conditions seen on your car. We can also set up the fixture to cycle A-arms repeatedly for destructive testing.


Buying an A-arm shaft with simple drilled holes is a less expensive option yet using an A-arm with slots gives you repeatable adjustability. Successful teams keep an inventory of A-arm slugs on hand and pre-measure the caster at each increment in the shop. Pre-measuring lets teams experiment at the track with the confidence that the caster readings will be spot on. Utilizing the slugs creates a precision locating method helping to dial in the set up with quick caster changes right at the track.






Billet A-arm spacers help to maintain your settings when making changes at the track. These tapered shims keep your A-arm shaft straight preventing binds.

Machined A-arm spacers have replaced the days of caster camber washers falling all over the ground. A-arm spacers can be purchased in straight or tapered styles helping to keep A-arm shafts straight eliminating binding.

To further speed the job many teams use a A-arm nut plate that holds the backing nuts allowing use of one ratchet on the front side. Crew members appreciate avoiding the hot headers while holding an end wrench at an awkward angle.


Beyond choosing a slotted version or the basic single hole version, special attention should be paid to the tolerances between the steel A-arm and the shaft. The tolerance is critical as not enough clearance leads to binding and galling. Too much clearance creates unwanted movement and caster change especially under braking. At times excessive clearance can lead to chattering and an unstable car. In dirt applications too much clearance can allow dirt to crawl between the housing and the shaft and the grinding action causes premature wear, sticking and increased friction. Quality A-arm manufactures hold the tolerance for optimal performance.



4 Bearings spread out the load and this version has a machined bearing housing to keep the shaft in perfect alignment for lower friction.

As the suspension rolls and rides over bumps the A-arm is worked up and down continuously. For more consistent performance many teams are going to bearing style A-arms to eliminate friction. Teams are finding that with roller bearing A-arms that they can sometimes run slightly less spring rate due to the reduced friction. A-arms last longer as the bearings prevent the wear created by the steel A-arm tube contacting the shaft. Heat expansion from the headers and brakes can negatively affect standard A-arms if manufacturers do not use proper clearance. Bearing A-arms effectively eliminate the heat issue.



Bearing Style A-arms reduce friction for smooth roll and repeatable performance on the track. This version has a machined bearing housing allowing racers to replace just the steel tube section for quick adjustments or crash repair.

The smooth action of the bearings builds consistent suspension roll and increased corner speed is obtained over a long green flag run. The tight tolerance and low friction of bearing style A-arms eliminates variables allowing crew chiefs to make chassis adjustments with the assurance that the suspension is rolling up and down in a repeatable free flowing fashion. Removing the binding found in less expensive A-arms creates speed as the adjustment decisions are not chasing inconsistencies. Each crew chief decision becomes more effective and every racer should strive to eliminate variables for consistent speed week in and week out. Quality A-arms are powder coated for a premium finish where as less expensive models are spray painted which quickly wears off.

Adjusting the car with A-arm length can be very successful. Shorter RF A-arms create more camber gain and might help you to improve your tire sheet readings. Shortening the RF A-arm moves the front roll center up and to the right. At times the quicker reaction of the RF suspension, due to a shorter A-arm, can improve lap times. Experimenting at your track with A-arm length is just as effective as making a spring change. New A-arms on the market allow you to change the steel A-arm tube section while leaving the shaft bolted to the frame. These new designs speed trackside changes and reduce costs as racers can carry different A-arm tube sections without the added expense of extra shafts.

Big Bar Set Ups generally use longer A-arms than conventional set ups. Camber gain adjustments that line up with your Big Bar Set Up is a critical piece to the puzzle. At times teams jump between a conventional set up and a big bar set up. The A-arms with a bolt on section speeds the A-arm changes that are needed.

Utilizing low friction ball joints with your bearing style A-arm further enhances free movement of the front suspension. Some low friction upper ball joints are adjustable giving racers another adjustment. Fine tuning A-arm angle with the upper ball joint allows for quick roll center adjustments and easy experimentation. Adjustable upper ball joints can be used in conjunction with slotted frame A-Plates. Slotted A-plates allow the inner pivot of the A-arm to be moved up and down and slugs secure the inner pivot in place. The A-Plate slugs are manufactured for precision adjustment in the up/down axis offering another roll center tuning tool.


The weld on this A-arm is suspect. With out a proper weld bead flex and cracking can occur. High quality A-arms use thick wall .095 tubing and are TIG Welded.

Some A-arms flex more than most race teams think. Prominent teams test the flex of new A-arm brands before they end up on their car. Manufactures that use thick wall .095 tubing know the slightly more expensive tubing reduces flex as compared to .083 which is commonly used. TIG welding is another feature that will reduce flex. Poor quality wire feed welds make for limp noodles verses a rigid A-arm system. Be sure to inspect the weld quality before purchasing A-arms for your rocket.



Shaft type has an effect on flex. Choosing lightweight aluminum for the cross shaft comes at a price – aluminum flexes more than steel and racers should consider if the few ounces of saved weight in an aluminum cross shaft is a good trade. Our test results show that the steel shaft attached to a 9.5” A-arm flexes .110” at 600 pounds of force where as the aluminum shaft flexes .150” at the same force.



For this test we used our custom flex testing fixture applying force ranging from 200 lbs to 750 lbs. The dial indicator illustrates the flex difference between top brand A-arms and less expensive imitations.

A-ARM Flex Chart
.

9- ½” Bearing Steel Shaft No Name Brand

Flex at 200lbs = .060, at 400 lbs = .100”, at 600 lbs = .135”, at 750 lbs = .155”
.
9- ½” Bearing Steel Shaft Quality BrandFlex at 200lbs = .040, at 400 lbs = .075”, at 600 lbs = .110”, at 750 lbs = .130”

.

9- ½” Bearing Aluminum Shaft Quality Brand

Flex at 200lbs = .056, at 400 lbs = .105”, at 600 lbs = .160”, at 750 lbs = .200”
.

9- ½” Alum. Shaft (no slot) Quality Brand

Flex at 200lbs = .056, at 400 lbs = .105”, at 600 lbs = .160”, at 750 lbs = .200”

.

9- ½” Steel Shaft (no slot) Quality BrandFlex at 200lbs = .050, at 400 lbs = .075”, at 600 lbs = .110”, at 750 lbs = .130”

.

9- ½” Steel Shaft (no slot) No Name Brand
Flex at 200lbs = .090, at 400 lbs = .125”, at 600 lbs = .185”, at 750 lbs = .225”

Our fixture for this test was designed to apply varying pounds of force. The flex chart shows the movement associated with applying lateral pressure at the ball joint center line. The fixture is rigid and the frame mount is the same as you would see on your car. The goal in building the fixture was to emulate what your A-arms see in your racer.

Teams that reduce friction and flex will take another step forward. Racing continues to evolve and teams demand better pieces to find speed. Luckily manufactures are stepping up to the challenge.

Go Forward – Move Ahead

Jeff Butcher
Courtesy of JOES Racing Products
5/1/09
http://www.joesracing.com/

Rolling Resistance

Finding speed a little at a time is the way to the front. A few horsepower here, less drag there coupled with a good shock change might add up to half a tenth on a good day. Reducing rolling resistance is one of those speed secrets that works continuously but it is hard to measure in the real world. Still – if your car has less rolling resistance common sense dictates that your race car will be faster.

Cup teams go to great lengths to measure improvements in rolling resistance. Several Cup crews have travelled to a hidden hill to test their ideas relating to reducing rolling resistance. The teams drag the car up to the top of a steep hill and set up to test roll the car down the steep grade to determine how far it will roll with out the help of the motor. They let gravity propel the car until it rolls to a complete stop. Teams test different roll resistance ideas celebrating the smallest improvement in free roll distance.

While your team may not be able to test rolling resistance on a secret hidden hill you can try some of the latest rolling resistance tricks. Running your hubs “wet” with low friction bearings and seals will reduce rolling resistance and increase car speed. You can really feel the difference based on the effort of pushing your car through the pit area. Oil filled hubs are the new rage and coupled with low friction seals you get many benefits. One spin of the hub with your hand is enough to illustrate the reduction in friction.

John Zaretske of JZ Motorsports likes to run oil filled hubs for improved speed. Low friction seals and bearings combined with 85/90 weight gear oil result in hubs that spin freely. John also likes the maintenance aspects of running oil filled hubs. Zaretske says: “Rebuilding and re-greasing standard bearings is an all night job – with oil filled hubs I can quickly drain the oil and inspect the fluid just like I inspect motor oil in a screened filter. Each week, if the hub oil is clean, clear and free from metal particles I just refill with 4 ounces of new fluid and I am ready to race”. John goes on to say: “in the rare event I need to replace my oil filled bearings, the job is much faster because I can just pour in the oil instead of making a time consuming mess packing bearings with old school grease”.

When running the hubs wet it is important to add the right amount of gear oil. 4 to 7 ounces is recommended by the car builers we surveyed and your team should monitor the fill level to ensure proper coating of your bearings. High quality synthetic lube is required.

You can experiment with the right amount of oil if you run your hubs wet. Zaretske recommends 4 ounces but other builders and hub manufactures may have their own recommendation. Our survey of car builders resulted in a recommended hub oil fill range from 4 ounces to 7 ounces utilizing synthetic fluid. As running hubs wet is a new speed secret you should keep an eye out as the fill level recommendations will change based on testing, fluid type and the evolution of manufactures specifications.

Weekly draining and refilling of "wet" hubs is simple. Fresh oil helps your parts and their longevity. Using a locking hub nut will ensure that your hub maintains torque even at lower torque settings.

Chuck Carruthers of Chuck Carruthers Industries says that only 5% of his customers are running hubs wet. Carruthers states: “Using Teflon coated bearings with 4.25 ounces of 50 weight synthetic gear oil gains 6 to 8 coast down horsepower”. Chuck prefers traditional wheel bearings and pays the extra cost for the Teflon coated version. Chuck feels the low friction roller ball style bearings are risky in long races. Carruthers says; “we get plenty of friction reduction from Teflon coated roller bearings and would rather avoid the added wear seen in roller ball bearings”. At Carruthers Industries they are careful and run the minimum amount of hub oil to ensure parts are coated but avoid running too much fluid as excess fluid in the hub can actually add heat. Chuck says “you want enough fluid to coat the bearings but any surplus fluid can foam or even impact how heat transfers and dissipates through the hub”.

Low friction roller bearings or Teflon coated tapered bearings will lower your rolling resistance where you need it most. Roller ball bearings should be inspected often and need continuous maintanance.

With the “wet” system and Teflon coated bearings Chuck recommends running 15 to 20 ft pounds of torque on the front hubs. He advises that you can run less on the rear as the brake heat is less intense. Chuck says 10 to 12 ft pounds on the rear hubs works great. Reduced preload on the bearings reduces rolling resistance creating more speed. Chuck reminds us “be sure to use a locking hub nut to ensure your torque setting stays put from cold to hot”.

We temporarily installed a clear cover to show the fluid level with 4.25 ounces of oil at 2 degrees of negative camber. A quick spin coats the bearings and races evenly.

Carruthers pays attention to detail and has made a special hub adapter for measuring hub torque. Chuck advises: “When our customers are doing their own work we have them torque the hub nut to our recommendations with a good foot pound torque wrench – we give them the setting and they simply torque down the hub nut and lock it down. For the cars that we maintain in our shop we go the extra mile and use a hub adapter that connects to a high quality inch pound torque wrench – the adapter connects to the torque wrench at the hub center. We add oil and give the hub a spin ensuring the hub and seals are lubricated. We then use our inch pound torque wrench with our custom made centering adaptor. We adjust the hub nut until we have 28 inch pounds of drag torque when rotating the hub with the torque adapter at cold temperature. When the hub is hot we know that there is minimal torque on the hub providing for optimal reduction in rolling resistance. At race temp we see about 3 to 4 inch pounds of drag torque with our custom measuring device”. Care should be used with this process as if the specs are not followed exactly you could end up with too little torque resulting in failures. Weekly inspections and pre-race checks are needed when pushing the edge in this fashion.

Rubber seals are the source of friction. You can easily feel the dramatic difference in spin effort from low friction seals as compared to traditional rubber seals. The added expense can be offset if your hubs have a seal retainer system.

While at the 2008 PRI show a confidential source explained the secret testing their team performed relating to floating rotors. Since my confidential source did not want to get fired by his team he wants his name kept out of this article. The secret tests performed by his high profile team revealed that running floating rotors added 6 horsepower on the chassis dyno. Making 6 horsepower due to brake efficiently – seems crazy! The HP gain is due to the floating rotors finding center through out the heat expansion range. The T-nut set up and rotor flange allows for a bit of movement that absorbs rotor warping and isolates it from the hub resulting in less brake pad drag. You get the horsepower gain and the brakes run cooler due to less pad and rotor contact. Drivers report smoother braking with reduced pulsing adding to the efficiency of the braking system. Here we go – a little more speed created by removing unwanted friction.

Using a floating rotor with a T-Nut package allows the rotor to expand through the heat range. The movement allowed by the rotor flange is isolated from the hub resulting in less drag. This hub has a seal retainer system that lets you remove and replace expensive low friction seals with out damage.

Another way to reduce rolling resistance is to square your rear end. There are many thoughts on rear end set ups but I like to keep this simple. I make sure my rear tires run nearly parallel and make sure the rear end is set square in the car. When building a rear end I make sure both tires point straight ahead and then I toe in the RR tire 1/32 of an inch. I figure at speed and under load that the RR will pull back that much and when it counts the rear end is perfectly square. A square rear end will not have any drag as compared to running toe out with the tires dragging all the way around the track. We know Cup teams play around with rear toe settings for aero advantages but for short track racing a square rear end will be more consistent and it will reduce your rolling resistance.

Equally important is the front toe setting. It is common for short track racers to run 1/8th inch or so of toe out and this spec has been around as the standard for a long time. I think the thought process is different now as compared to a few years ago. Today I recommend rethinking the toe out setting in the front end. Our components are simply manufactured better today and there is less free play in the front suspension pieces. A-arms are stronger, racks are better, rod ends, ball joints and tie rods are all built with tighter tolerances. I would recommend running 1/32nd of toe out or even zero toe out in the front. With the tires pointing straight ahead you can find more speed and eliminate tire drag from excessive toe out.

Ackerman should also be considered. Ackerman is sometimes used as a chassis adjustment but the Ackerman effect and additional degrees of steering at the left front tire creates drag. Keep in mind the amount of tire drag you are adding to the car if you have Ackerman toeing out the LF tire as you turn. Excessive Ackerman can sometimes cause a hitch when the driver picks up the throttle. As the power is applied to the rear tires they have to push against and overcome the Ackerman drag on the left front. Depending on the situation the car may break loose or simply not leap off the corner due to the Ackerman drag. Ackerman can be a great adjustment but thinking out the rolling resistance considerations may find you additional speed.

Aerodynamics is another area where rolling resistance gains can be found. Air likes to follow body surfaces and air needs smooth transitions to prevent unwanted turbulence. When ever possible create smooth and rounded body transitions verses letting air fall off of sharp corners or cliffs. When massaging air try to mold the body to allow air to follow surfaces avoiding surprise edges or cliffs.

As race cars have evolved reducing rolling resistance may help make up some of the power limitations created by crate motors, 9:1 compression motors, and carburetor rules. Embracing rolling resistance and massaging your car with a friction free approach may “find” you 15 to 20 new horsepower. If you are running a 400 horsepower crate motor you could end up with a net 5% improvement in power! Even with unlimited horsepower the teams that work hardest on reducing rolling resistance will create horsepower that just might be the nudge you need to get you ahead at the photo finish.

Go Forward – Move Ahead

Jeff Butcher
JOES Racing Products, Inc.
3/31/09
http://www.joesracing.com/

Shock Specialists

Shocks are an important tuning tool that create feel in your car. With the variety of shock brands, components and types partnering up with a shock specialist can help you to navigate through the vast array of component offerings.

To increase our shock knowledge, we have interviewed two successful specialists in the shock field. Mike Naake of Naake Suspension Specialists and Mike Leary of Leary’s Shock Shop offer their suggestions and experience. Both “Mike’s” provide shock hardware, setups and technology to prominent racers across the country and are authorized service centers for many of the major shock brands.

With so many shock choices on the market how can you help racers choose the correct type?
Leary:
It is important to buy the most tunable shock within your track or organization’s rules. If your rules allow for compression and rebound adjustment the additional initial investment will translate into speed. If your rules dictate twin tube designs then the more economical hardware can be maximized through tuning. When rules allow we want our teams to utilize the options that are made available in pressurized mono tube shocks.

Naake:
Rules and budget are part of the equation. We tailor our shock packages to meet the needs of each racer. After a complete interview of each team we determine if pressurized mono tube shocks will work best or if the economics of twin tube shocks meet their needs. Once the shock type is selected we continue the interview to learn more about the goals of each team and build packages based on their input. Understanding driver tendencies and track characteristics allow us to tune shocks for more speed.

What is the racer benefit for partnering up with a shock specialist?

Naake:
Drivers and Crew Chiefs can communicate what they want the car to do and we can create an option based on the feedback. We can customize and offer a linear/digressive piston, digressive linear piston, double digressive piston, and another piston designed specifically for rough race tracks. Our knowledge is based on feedback from many teams and the information base gained over the entire group would be impossible to gain within a single team.

Leary:
With shock manufactures offering ever expanding part options for their shocks i.e.: pistons, shim stacks, shafts with different bleed options, etc, it is almost impossible for the racer to stay on top of his shock program. As shock specialists we are continuously being educated on latest product offerings from the shock companies we support. Racers also benefit from our experience – we are testing all the time and the knowledge gained by working with several teams helps us to understand the changes needed for each individual team. Our shock dyno runs full time creating unique shock packages even for teams competing at the same track.

What is a Mono tube shock?

Naake:
There are two types of late model/sprint car mono-tube shocks, emulsion and De Carbon style. An emulsion shock is typically a mono tube shock with gas and oil in the same chamber. The oil and gas mix creating foam. This is not a desirable situation in any hydrologic system, especially a racing shock absorber. Christian Bourcier de Carbon invented the mono-tube pressurized gas shock absorber. A De Carbon style shock absorber has a dividing piston that separates the nitrogen and oil. De Carbon mono tube shocks with gas pressure and a dividing piston perform better than twin tube shocks. Mono tube shocks cost more to manufacture yet the added expense results in better and more consistent performance.

Naake uses this photo to illustrate the base valve action on the mono tube compression stroke. As the shaft displaces the shock oil the base valve smoothly opens to allow for the shaft volume. Shim stacks can be seen flexing to meter the precise amount of shock oil.

The Naake mono tube rebound stroke photo illustrates shock oil flow and you can see the internal forces and valving action. The mono tube gas separating piston is clearly shown at the top of the shock.
What is a Twin tube shock?

Naake:Many tracks and weekly racing series’ rules require a twin tube shock. They do this to keep the racers’ costs down. A twin tube shock has an inner tube that the piston runs in. At the bottom of the inner tube is a base valve. The function of the base-valve is to make 30%-40% of the compression force and replenish the oil in the inner tube on the rebound stroke. If a base valve is not performing with enough force in a twin tube shock that could result in cavitation or what we call in a twin tube design “dumping”. Dumping is when too much oil moves out of the inner tube too quickly and on the rebound stroke it is not replenished fast enough resulting in a momentary dead spot on the rebound stroke. The dumping can be verified on the rebound opening stroke of a constant velocity test on your shock dyno.

Naake uses this photo to show the low pressure gas filled bags utilized in twin tube shocks. The low pressure bags prevent foaming and deform to account for shaft displacement. The Base Valve adds 30 to 40% of the compression force.

Naake points out twin tube shocks have an inner tube that the piston runs in. The twin tube rebound view shows the base valve metering oil from the outer oil reservoir.
Why run a base valve?

Leary:Base valves are of most benefit on heavy cars and when low gas pressure is used. For most late models we run without base valves in mono tube shocks to save cost. As you run high compression it can make sense to add a base valve to help prevent cavitation on the back side of the piston. When running a base valve the shock valving needs to be tuned to line up with the compression forces that are metered through the base valve.

Naake:
A base valve is an optional component on a mono tube shock and works great with low pressure. Some series that allow mono tube shocks do not allow base valve to be installed. A base valve shock will have a much smoother feel to the driver than a non-base valve shock. Of course, a base valve in a shock adds to the cost. We are big fans of base valves when rules allow. The added control of the oil displaced by the shaft gives us more choices with our rebound adjustments as cavitation is eliminated through transition from compression to rebound.

Is shock oil important?

Leary & Naake:
Shock oil is a part of the shock that most racers overlook. Many manufactures use inexpensive hydraulic oil in their shocks. Inexpensive oil can vary through out the temperature range. You may start the race with a shock that has a 5 valving on compression and rebound. Inexpensive oil may react dramatically with heat and effectively make the 5 shock you built in the shop perform like a 3 shock on the track. Using thin synthetic oil reduces the viscosity change allowing for more consistent shock performance from ambient temperature to race temp.


Using a drip cup for shock rebuilding keeps the oil contained and your workplace clean. An organized shock building station is a must if you are servicing shocks on your own.



Leary tested several 5 shocks from different companies. This graph shows that a 5 shock valving varies significantly from brand to brand. Understanding the brand valving differences will help you to make the proper adjustments when using competing brands.
What Shock tips can you give to our readers?

Naake:We hear a common myth that bag shocks blow out due to rough track conditions or sudden high velocity compression. In my experience, I just do not see this at all. The only bag failures I have seen are due to errors in assembly or a bag had a small puncture prior to assembly.

You can perform a simple hand test of your twin tube style shock. Fully extend the shock. Position the shock so that the shaft end is up. Compress the shock about a half an inch. If you feel any slack or a dead spot, it is an indication of air in the system. This can be caused by the shock being low on oil, either from the shock having an oil leak or from not enough oil placed in the shock during assembly. It could also indicate a leaking gas bag. We perform this test on all twin tube shock prior to running a dyno test. If they don’t pass the hand test, they are guaranteed to fail the dyno test.

Leary:
You should have your shocks dyno’d when they are new to get a baseline, and then should be re-dyno’d after a crash and halfway thru the season. A shaft bent 3 or 4 thousandths or a tiny dent in a mono tube shock body, will change the shock dramatically. In this day in age, it is almost impossible to build your own shocks without testing them on a quality dyno. Using the proper tools such as, shock wrench, shock vice and drip cup will make the building process easier and protect your investment. Keeping your shock work area clean and organized is pivotal to building successful shocks.




If you are going to work on your own shocks then using the proper tools such as a shock vice will help you to build winning shocks.
What shock adjustment tips can you suggest?

Leary:
Most of the handling of a pavement car is controlled in the first 2 inches of shock movement. I like to use the rear shocks to control the entry of the corner, left side shocks and RF for the middle of the corner and the fronts for exit.

I avoid using the compression side of the shock for handling adjustment. I would rather use springs to control compression adjustments. The one exception is possibly the compression on the left rear – by increasing the compression at certain speeds, you can gain bite off the corner. It can make the car think is has more LR spring on exit without changing the corner entry like a spring can.

On a conventional spring setup, the shock settings will work differently than with a soft spring set-up. The more front spring rate, the less compression you need because the spring is doing the work. With a soft front spring set up, you need to use compression to control the speed of the front end movement because you have a 200 lb spring trying to hold up an 800 lb corner.

If you tie down the left rear shock on a conventional set-up, I’ve found you will tighten up the entry of the corner. I believe it keeps the left rear weight from transferring to the RR as fast, which gives you that loose entry feeling and keeps the weight on the LR tire.

With a soft spring set-up, if you tied down the LR, it usually will loosen the corner entry. Because of the rapid weight transfer to the RF of the car, we can overload the RF tire, which can cause a soft push on entry. By delaying that transfer the RF tire has a chance to “grip” the track, so the driver feels the car is looser on entry.

With either conventional or the soft spring set up, the rebound on the RF can help that “tight in the middle” feeling. By stiffening the rebound on the RF it holds weight on that tire and helps it maintain grip – it also keeps the RF from transferring weight to the LR as fast, which will cause the car to pick up a push. The downside of holding down the RF is you can lose some grip off the corner, because you are delaying that weight transfer to the LR.

When we have soft front springs, we need the rebound on the LF shock to help the sway bar to keep all that weight transfer to the RF. That is why we see the extreme tie-down LF shocks. It is important to balance your front springs/sway bar and tie-down in the LR shock. We build LF shocks at 1 inch of travel anywhere from 600 lbs of force to 1000 lbs, depending on the set up.

Changing the sweeps on externally adjustable shocks makes a big change. Leary uses this dyno graph to illustrate the dramatic effect of using sweep adjustments.
Naake:To help free up the car from the center off a quick adjustment is to add gas pressure to the RR shock. By adding up to 200lbs (if you have the right hardware) the car can take on a better attitude and become more stable on exit. This fix is quick and can be removed quickly if the driver is still looking for a better exit.

With today’s set ups we like to maintain rebound in the front shocks. If the car is tight, and if the team has the right hardware, we try to go with high frequency pistons to relieve the tight feeling. In short, there is an o-ring behind the shaft band that delays the metering of oil through the bleed holes for a brief moment and then shuts the bleeds off just as quickly. Our teams can work with us to discuss these options to find more speed – our goal here is to let your readers know that there is high tech hardware out there that goes beyond generic answers. We have proven results in gaining front grip off the corner with the high frequency hardware available. Remote canisters provide another layer of adjustability when allowed.

While high tech is cool sometimes a basic answer works well. If your car is tight then more compression in the RR will help the car turn in the center. You can try a bit more rebound in the LR to free it up. You can take a little compression out of the LF or tie down the RF to help the car turn. Extreme rebound in the front shocks is good in many cases but too much low speed control can reduce front grip. With extreme front rebound you need some bleed to allow the tire to follow the track surface.
When rules allow a remote shock canister gives you more compression adjustability options. Base valves can be built into remote reservoirs and the housing needs to be protected from damage. Using a canister mount allows for quick adjustments when practice time runs short.
Summary:

Butcher:
Shock science and hardware is constantly changing and the application of the set up tips is very dependant on the track, driver and location in the turn. Many times a standard shock tip will change 180 degrees based on moving the car a few feet in the corner and the tips above need to be applied with full understanding of shock mechanics – both Mike’s will change their adjustments based on real world data. When it comes to shocks it is all about timing and transitions. Shocks might hold or delay movements but in the end the springs carry the load. Bumps, braking, throttle and rolling through the middle all provide information that will be analyzed independently by your shock guru. Be aware that you may have soft front springs but you also must consider the overall front spring rate based on the giant sway bar that could be in your car.

Go Forward – Move Ahead

Jeff Butcher
Courtesy of JOES Racing Products
www.joesracing.com

Resources:

425.267.9199

916.771.0109

Tires & Temperature

You spend piles of time figuring out how to make more horsepower, optimizing shocks and building lighter cars which all come together where the rubber meets the road. Every speed secret on your car is applied at the contact patch so creating the optimal footprint is the meeting place for all of your hard work. Tires are the single most important aspect of speed as every adjustment from motors to springs relies on the grip you manufacture at the contact patch.

Measuring tire temperatures accurately will provide the information you need to produce even grip and maximum friction where it matters most. Using pyrometers correctly provides valuable information. Often, a tire sheet is handed to the crew chief and the numbers on the paper sets in motion a flurry of adjustments. The thrash begins and springs fly. Orders are barked and the crew moves quickly to get the car back on the track with hopeful anticipation of new found speed. Temps are taken to see how the new adjustments worked out and too often the intended chassis improvements are 180 from what is needed. The driver reports the car is even worse than before the recent round of adjustments. Did the missed set up attempt occur due to a poor choice on the chassis adjustment? Or - were the wrong changes made due to poorly measured numbers on the original tire temp sheet?

It is worth noting that tire temps and pyrometers are one of the few ways that you can analyze real time data right at the track. If the temperature measurements are taken correctly, chassis specialists can calculate the best changes that will perform for the length of the run. Using up important practice time and valuable practice tires with wrong way adjustments is expensive. Choosing the correct pyrometer to produce scientific information and assigning the crew member that is dedicated to precision is paramount.





To dissipate heat racing tires are very thin. Thick tire rubber holds in heat and the potential for blistering increases. Tire engineers balance the rubber thickness with tire compounds to produce a package that considers car weight, corner speed, track abrasiveness, outside temperature, intended lap use and several other variables. Since the thickness of tire rubber can vary you need a pyrometer with an adjustable tip length probe. We want consistency and measuring tire temperature down at the cord is the best way to ensure accurate and repeatable numbers. If your team is measuring tire temps at varying depths then the information on the tire sheet is going to set in motion changes that could slow your car down.

Pyrometers must be used constantly - scientifically. Rubber is a poor conductor of heat yet it is a great insulator. If you are trying to assess camber temperature curves then you want to know the inside, middle and outside temps based on your camber setting and corner performance. If you measure the inside location at the rubber surface, the middle location and mid tread depth and the outside at the cord your temp sheet is going to have more inconsistency than Michael Jackson has had cosmetic procedures. The cord heat is insulated away from the outside elements and the most heat will be found beneath the rubber and down at the cord. Scientifically – it makes sense to measure all 12 locations at cord depth where the purest temperature is located.

Adjust your pyrometer probe so that the adjuster stops the probe penetration just before the probe reaches the tire cord. Using the stop on the adjustable tip will allow your crew to quickly and consistently get down to a consistent depth near the cord each and every time. Your temp sheet will provide scientific quality information due to the repeatable and consistent probe depth.


Reading rubber temperature down at the cord is best as the friction of your tire pulls and stretches the tire rubber. The stretching effect creates heat just like when you bend a coat hanger back and forth. More friction creates more contact rubber stretch. Measuring down near the cord displays the data resulting in efficient chassis adjustments.

If you have a pyrometer with a fixed tip you can make it work but you introduce depth variables. How many times have you seen a temp sheet that showed you needed to take out RF camber and sent the car back out without an adjustment. A second temp reading shows you have too much camber – I will bet probe depth variation is the issue. With a fixed tip your crew needs to “feel” the cord to ensure the probe is at a repeatable and consistent depth.

Your probe tip is made of thin steel which heats up quickly sucking the heat out of the pin hole made in the tire rubber. Be sure to move quickly. If you leave the probe in one spot in the tire rubber and watch the display you will see the temp rise and then begin to fall as the heat is sucked out of the test location. You need to record the maximum temp in each probe location. High quality pyrometers lock in the maximum temperature automatically increasing accuracy. Utilizing a temp lock feature gets you around the car in nearly half the time.

I am often asked if Infra Red pyrometers are good for tires. You certainly can use Infra Red pyrometers for tires but it is a quick check and the information is simply less precise than using an adjustable tip probe. IR pyrometers measure the tire surface. The surface temperature is impacted by engine heat, brake heat, puddles etc. Camber in the front tires places only a few inches of the tire on the ground at the low speeds encountered when travelling back to the pit area. The track surface is cooler than tire operating temps so the tire surface area in contact with the ground pull heat from the strip in contact with the track at a different rate than the rest of the tire – this difference skews your camber curve readings. The rubber down at the cord is insulated for a longer period giving you more time to measure temps relative to on track performance and probes can reach down past the surface for a better look.

When using IR pyrometers for tire temps bear in mind that the surface recordings will be much cooler in comparison to probes and you will lose the fine detail that can be found with probe type pyrometers. IR pyrometers are great tools for measuring track, header, brake and cockpit temps. Using the right tool for the job is usually sound advice.

To ensure the best relative tire temp readings follow these steps:

1. Use a properly adjusted pyrometer probe tip to measure down at the cord.
2. Get to the car quickly – speed matters!
3. Record the highest temp at each location with an automatic max temp feature or by manually witnessing the highest temperature. Move around the car quickly.
4. Start at the same tire each and every time and record the individual Inside, Middle and Outside for all 4 tires. Consistency is the goal.
5. Record track temp and outside air temperature on your tire sheet to monitor the difference that these variables have on your tire temps – over time you will be able to forecast better compensating adjustments.
6. Keep in mind that tire temps are of more value on a car that is handling well and with tires that are in good shape. Tire temps on cars that are in left field are about as valuable as politicians’ promises.

Using your scientific tire temps you can evolve my rule of thumb tips shown below. My tips are based on a car that is set up properly and just needs fine tuning. Your team should adjust the suggestions below based on your real world testing and document your own pre-determined adjustment to form your own game plan. Creating a game plan in advance will allow you to quickly asses your adjustment options improving your decision making when things get hectic at the track.

Camber Adjustments.
Your tire sheet temperatures suggest a camber adjustment is needed but knowing the adjustment amount is an educated guess. My rule of thumb for adjusting camber is a 1/8” shim for 12 degrees of temp difference between the inside and the outside. A 1/16th shim is a good start for 6 degrees difference. Trial and error starts somewhere and your team can modify my rule of thumb based on your actual conditions. Strive to find and document a pre-determined shim thickness associated with the degree difference across the tires on your car.

Starting Cold Air Pressure.Air pressure and tire temperature work hand in hand. If you take precision tire temp measurements you can gain an advantage over the competition by adjusting your cold air pressures before the race based on data you have collected over time. You can adjust your pre-race air pressures to a finer degree if you record outside and track surface temps in conjunction with your tire temperatures. If it is really hot out and you have tire temps that are 20 degrees higher than your last trip to a given track you can adjust the cold temps for better race performance. The rule of thumb that I used on a 2900 pound touring late model with bias ply tires was 1 degree of pressure gain for every 10 degrees of additional tire heat. You can visualize that there would be more pressure gain in the heat of summer verses a cool spring. Testing dictates your actual heat induced air pressure compensations. Adjusting your pressures based on recorded results will help you to optimize pressures for more speed on a long green flag run. Understanding the correlation between pressure and temperature will help you to optimize pre-race pressures during those times when your race set has residual temperature from practice and the race is going to start before the tires cool completely. Strive to know the actual temperature induced pressure gain based on your driver, track and conditions.

Air pressure adjustments.Inflating each individual tire properly means better grip, more wear and more speed. With accurate tire temps my rule of thumb is to adjust individual pressures 1 pound for every 5 degrees of over or under inflation shown as hot or cold center temps on my temp sheet. Your team can tailor the starting point of this rule of thumb to your actual situation. Your team may decide on 1 pound per 4 degrees or something different but the goal is to find the pressure needs for your car and tires. Establishing a baseline for inflation pressure adjustments will help you to dial in the winning set up and add consistency to your set up process.

Measure and record your tire temps with a quality pyrometer. Use the scientific data to form a pre-race game plan. Use your game plan to make the right call when it matters most. By using science you can take the black magic mystery out of your tires by building consistency in your adjustment process.

Go Forward – Move Ahead.

Jeff Butcher


Tools Courtesy of JOES Racing Products, Inc www.joesracing.com

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Camber and the Tire Contact Patch

Drivers like cars that cut into the turn. Optimizing your camber and understanding the tire contact patch will make for a faster car that stays fast through out the race. With today’s highly engineered tires, bump stop set ups and high level competition adjusting camber for maximum grip will help you win.

Camber is simply the tilt of the tire. Standing in front of the car if the top of either of the tires tilts towards the engine it is negative camber – if the top of the tire tilts away from the engine it is positive camber. To measure camber a quality all billet camber gauge will have the perfect surfaces to provide accurate and repeatable readings.



A billet caster camber gauge has the machined surfaces for accurate readings.
Camber is essential to match the contact patch to the bank of the turn balanced against your Upper A-Arm and Lower Control Arm lengths. Even if the corner were flat we would want camber. We need camber to work with the Upper and Lower Control Arms to achieve proper camber gain through suspension travel. Camber coupled with gain will optimize the tire contact patch taking advantage of the tire construction parameters.


Think about the changing camber angles in your race car through travel. We adjust the suspension and camber to achieve reasonably even tire wear producing tire temperatures that are balanced. That said, what we really want is the entire foot print of the tire on the ground under full corner load by using correct camber to match the spring load of the inner and outer sidewall. Too much camber loads the inside side wall too much and not enough camber loads the outside sidewall too much. If you want your car to cut through the center of the turn then it would make sense to have all of the available rubber firmly in contact with the ground.

Tire Contact Patch Explained
For this tire contact patch exercise think of a trampoline – we are going to compare a trampoline to how a race tire works. Imagine a trampoline to create a mental illustration of how the tire contact patch is stretched. Think of the trampoline outer frame as the tire bead. Now think of the tire side wall as the trampoline springs. Compare the trampoline surface to the tire rubber contact patch and visualize how tightly and evenly the trampoline surface is stretched by springs towards the frame. Our goal is to have the tire contact patch stretch evenly and tightly just like the trampoline surface. The tire bead is very rigid and creates a sturdy frame. The side wall of the tire is a spring that absorbs loads. By using camber to maximize the power of the sidewall springs the contact patch stretches flat and stays in full contact with the ground producing more grip.

The ideal amount of camber is achieved when the inside sidewall sets into the track to provide maximum pull at the tire contact surface. The stretch pulls evenly from the inside sidewall to the outside sidewall. The correct camber setting will utilize the entire contact patch. If you have too much camber the inside of the tire will not have enough initial surface area to pull the contact patch across and the spring of the outer sidewall will not be engaged. Not enough camber and the contact patch will deform and ball up at the inside edge – the contact patch rolls up and off the ground as the inner side wall spring needs more loading.

We use pyrometers to measure the tires on the inside, middle and outside. When the pyrometer shows a hot temp on the outside of the RF we add camber. Too hot on the inside RF and we take camber out. There is an exception that goes against the common thought of taking out camber when the pyrometer shows a hot reading on the inside. When would you add camber even though it opposes the pyrometer readings? Understanding the tire contact patch will help you set the camber through the exception.

With your trampoline comparison at the front of mind visualize the RF tire surface as it rolls through the turn. Think about the part of the turn where cars choose to cut or push. At this full tire load point, if you do not have enough camber, the contact foot print will not be stretched tightly between the two sidewalls (like in our trampoline comparison) and the pyrometer will show excessive heating on the inside of the tire and the exception occurs. In this condition we add camber even with higher inside RF temps. The excessive heating on the inside generally indicates to take camber out – thinking outside of the numbers may be in opposition to typical camber adjustments.


This exagerated view of a contact patch buldge illustrates how too little camber can show a hot reading on the inside. Understanding the contact patch will help you to make the right adjustment.

You may indeed need to take out camber of the RF due to the excessive inside readings but before you do inspect the tire and look for the exception. Look for rolling at the outside edge. If you see scuff marks extending to the outside sidewall this is clue one. If the outside edge looks rounded this is clue two. Now the important clue – closely inspect the inside of the RF tire about 1” inch from the inside edge. Look for a strange wear area that is about ½” wide that looks different than the surrounding rubber. It will look grainy, be cupped or perhaps mimic wind blown sand on the desert. Remember – it is more effective to dial in camber with new tires. Worn tires may have been misused and can provide false readings. Added attention should be paid to tire temps when you bolt on a new set and high quality pyrometers should always be used. Tire Temps make the most sense when the car is close. Feedback from temps can be erratic if the car is way off.

Exception Explained

If you do not have enough RF camber inside sidewall will be under loaded. The outside sidewall gives way and folds in deforming the contact patch. The deformed tire footprint pulls up off the track surface – as the deformed contact patch reaches the inner sidewall it is forced back down creating a protruding bulge as it curves back to the inner sidewall. Remember the trampoline comparison, we need the contact patch stretched evenly from the sturdy outside bead, through the outside sidewall spring, tightly across the contact patch, through the inner sidewall to the firm inner bead. In this exception adding camber at the RF will load the inside tire wall with the ability to firmly hold the inner edge of the tire foot print. The rolling or protruding of rubber at the inside edge is due to inadequate static RF camber. Don’t be fooled by this short term and artificial temperature. The extra temperature is created from the deformed contact patch bulge as it curves back to the inner sidewall. The bulge rubber will quickly grind off permanently damaging the tire. and the true pyrometer reading will show up! Proper camber will give the inside of the tire the maximum grip allowing the contact patch to stretch trampoline tight all the way across. Proper camber will allow the outside side wall to be pulled in by the contact patch rubber connecting the inner and outer sidewalls in unison and with equal load.

Camber - Old School

Camber gain through travel is related to your static camber, your Upper A-Arm and Lower Control Arm lengths. Consider the amount of travel your front end experiences. If you have an old school set up verses a Bump Stop Set Up there is less overall travel and the Upper A-Arm will be short. With Bump Stop Set Ups there is more travel from your static ride height and much longer Upper A-Arms slow camber gain.

For a pavement touring late model old school thinking was about 1 degree of camber gain per inch of travel. This guideline was a rough starting point with traditional set up and would be adjusted as needed to actual conditions. With a 17 ¾” RF Lower Control Arm a typical Upper A-Arm would range from 7 to 8.5” +/-. The increased angle of the Upper A-Arm provided for camber gain from static to maximum load. With standard suspension travel and a static camber of 3.5 degrees negative you would achieve about 5 to 6 degrees of camber in the center of the turn. Gain would be 1.5 to 2.5 degrees of camber through travel. Every car and track is different and these ball parks give are a simple view of old school camber

Camber - Bump Stop Set Ups
Bump Stop Set Ups require Upper A-Arms to be considered in an entirely different way. Longer Upper A-Arms slow down camber gain so it is wise to measure your camber with the car on the bump stops emulating the center of the turn. Static ride height is of zero value on a Bump Stop Set Up – as soon as you reach race speed the car is down on the stops and never sees static height again until you load it back in the trailer. Since shocks with mammoth amounts of rebound hold the car down on the bump stops the ride height static camber is not even worth checking. Bump Stop Set Ups use much longer Upper A-Arms, such as 8” to 12”, yet the Lower RF Control Arm is still around 17 ¾ on a touring late model.
When using bump stops be sure to consider your A -arm lenghts and angles.
The Goal

Your goal in identifying the proper camber is to find the optimal camber amount that creates maximum tension across the tire surface by equally loading the inner and outer sidewall. Dialing in the camber for the conditions will help the car turn. Too much RF camber and the inside edge will not hold – not enough and you will get balling up at the inside edge.

To set camber with your Bump Stop Set Up I would pick a repeatable ride height down on the stops that represents your best estimation of ride height in the center of the turn. A repeatable middle of the corner ride height number will make a better week to week reference point then trying to chase a ride height that varies based on how you adjust the shock body etc. For a Bump Stop Set Up I would start with 4.5 degrees of negative camber at the RF at my mid corner reference point and would not even care about static ride height camber. I would dial in the optimal camber with my pyrometer and tire inspections. I would look for consistent tire temps on short runs with new tires and longer runs with the same new tires. Adding or subtracting from my initial set up would be based on the feedback the car provides. The left front starting setting would be 3 degrees positive with the car on the stops and I would experiment there.

Tire Temp Tip

From experience my fastest cars had RF inside temps that were 10 to 14 degrees hotter than outside temps. The small amount of extra inside heat ensured that I was just reaching over the edge giving me the best shot at a fully stretched contact patch. I made sure to verify the temps on both short and long runs with new tires. The LF has less load so 12 to 16 degrees hot on the outside temp showed me LF outside tire wall was digging in with everything it had.

Go Forward – Move Ahead.

Jeff Butcher
Courtesy of JOES Racing Products

Bump Steer & Bump Stops

Under maximum corner load, where races are won, excessive Bump Steer can slow your car down and make it more difficult to find the optimal set up. Understanding Bump Steer will increase corner speed and give you more options in finding the winning set up.

What is Bump Steer? Bump steer is the toe in and toe out of your front wheels created by the up and down movement of your suspension. Really – bumps aren’t even needed! When the nose lifts under acceleration do you want the wheels to turn in or out on their own? What about when you are under heavy braking? Do you need the Right Front wheel to go one way and the left the other? Think about when the car transitions between compression and extension – we want the driver to steer and not have to correct for the inconsistencies caused by improper front end settings. When the suspension oscillates over bumps the last thing we want is to have the tires turn themselves due to excessive Bump Steer.

Bump Steer is caused when the swing arc of the suspension is not matched to the swing arc of the tie rod. Different swing arcs of the tie rod and suspension are what causes Bump Steer. To match the arcs you must follow a few simple design principles that were considered by your car builder.

Stock car suspensions are comprised of an Upper A-arm and a Lower Control Arm. Your frame sets the inner pivots and your spindle and ball joints set the outer pivots. Your car builder thought long and hard about all of layout dynamics to engineer the hardware that allows you to properly position components to attain Zero Bump Steer (Fig 1).

(Fig 1). Your car builder carefully engineered the pivot points, angles and lengths. Setting the Bump Steer is like Blue Printing the suspension to exactly match the design specifications.
The layout, lengths, and angles of the upper A-Arm work together with Lower Control Arm to encompass what engineers refer to as an Instant Center. To help understand the Instant Center you can visualize a triangle (Fig. 2). Draw a line from the center pivot of the top ball joint down to the center pivot of the lower ball joint. Now draw a line from the center of the top ball joint through the inner A-Arm pivot and extend it towards the middle of the car. Complete the triangle by drawing a line from the center of the lower ball joint through the inner pivot of the Lower Control Arm and extend it to the spot where it meets the Upper A-Arm line. The intersect point of the two lines is the Instant Center of your suspension. The RF and LF have independent Instant Centers.
(Fig.2) Your pivot points work together and intersect at the Instant Center. The illustration shows the RF suspension from the front view.
With your triangle drawing (Fig 2.) you can imagine a very long tie rod – one so long it would not fit on a late model as we know it. Connect your imaginary long tie rod with a mental bolt at the Instant Center. Extend your imaginary tie rod out towards the spindle and connect it to the center of the line between the upper and lower ball joint. With this layout you can see that the imaginary tie rod would follow the same arc through travel as the suspension (upper and lower control arms) and the car would achieve zero Bump Steer.

Matching the arc of your actual suspension to the arc of the tie rod completes a design scenario that points your tires straight ahead through suspension movement. To apply the matching arc concept to the design of your late model you will need to consider three design principles for ZERO BUMP.

Your outer tie rod pivot must fall on a line drawn through the upper and lower ball joints.
Your inner tie rod pivot must fall on a line that is drawn through the Upper A-Arm pivot and Lower Control Arm pivot.
The angle of the tie rod must create a line that when extended intersects with the Instant Center.

Race cars are made from welded steel that bows and twists from the heat of welding. Rack plates, steering box mounts and spindles all can have variations that we need to account for by utilizing shims to locate the pivots considering our 1-2-3 design elements. Setting the Bump Steer is like blue printing an engine – you are simply going the extra mile to match your car exactly to the car builder design specifications (Fig 3.).


(Fig 3.) Typically Stock Car tie rods follow the Lower Control Arm line. In our drawing we are illustrating that you can mount the tie rod elsewhere as long as the outer tie rod end falls on the Ball Joint Axis line, the inner tie rod end falls on the Upper A-arm and Lower Control Arm pivot line – and the angle of the tie rod ends intersect with the Instant Center.

Often rack plates are mounted too low for direct mounting of the rack. To achieve the proper pivot points detailed in our 1-2-3 instructions we may need to space the rack up (Fig 4). Using CNC machined billet rack spacers adds to the precision or simple washers can be used if you have the correct thickness on hand. Shims may also be needed on the spindle side to account for caster changes or spindle variations.


(Fig. 4) Mounting the rack at the proper height allows for zero Bump Steer.

To measure the Bump Steer you need a precision Bump Steer gauge and you will find a digital version speeds up the project. Suspension settings need to be racing ready and the proper components need to be fully installed and tightened. All front end settings need to be set – exactly. Tackling the Bump Steer measuring process should only begin when the car is truly race ready. Prepare your car in the following order and consult your car builder for their recommended front end specs. Make sure you have the right parts on your car!


(Fig. 5) A Precision Bump Steer Gauge that is billet rigid and utilizes one dial indicator will do the math for you for a faster and more accurate Bump Steer process.

Prepare the car to measure your Bump Steer per the following Check list:

1. Set the tires – air pressures and stagger.
2. Set the ride height.
3. Adjust the camber.
4. Adjust the caster.
5. Match your tie rod lengths per the1-2-3 instructions.
6. Center the steering by centering the inner tie rod ends with the Lower Control Arm inner pivot per the 1-2-3 instructions. Lock the steering in place to ensure solid measurements.
7. Set the toe.
8. Record a reference point while your car is on the ground and at your design ride height. Measure from the floor to the lower grease fitting or other repeatable spot such as the sway bar mount on the lower control arm – remember to write the number down.
9. Place the car on jack stands matching your ride heights and adjust for the jack stand height. The goal is to maintain your suspension angles while on jack stands matching the ride height on the ground.
10. Bolt on the Bump Steer plate to the hub and set it to level. Jack the suspension to ride height and note where the dial indicator touches the Bump Steer plate. Setting Bump Steer is a trial and error process and noting where the dial indicator touches the Bump Steer plate indicator marks will allow you to return to your ride height quickly after attempted adjustments.
11. Jack the suspension fully through compression with bump stop set ups (or at least 2”) and through at least 2” of rebound travel. Write down your results and refer to the Quick Shim Guide (Fig. 6).
12. Shim as needed.

To help you shim your way to proper Bump Steer here is a Quick Shim Guide that you can use after taking an initial Bump Steer measurement (Fig 6):

Quick Shim Guide

Toe out in compression & toe in on extension.
Reduce shim thickness at spindle or lower the inner tie rod end by lowering the rack or drag link.
Toe in on compression & toe out on extension.
Add shim thickness at spindle or raise the inner tie rod end by raising the rack or drag link.
Toe in on both compression & extension.
Lengthen the tie rod.
Toe out on both compression & extension.Shorten the tie rod.
Toe out on compression. Toe in on extension and then toes out with additional extension.
Reduce shim at spindle and shorten tie rod.
Toe in on compression. Toe out on extension and then toes in with additional extension.
Add shim at spindle and lengthen tie rod.

Bump Steer is stated as X amount of Bump Steer (in or out) in 1” of travel. The starting point for measuring Bump Steer is your static ride height. In today’s world of bump stop set ups the reward for zero Bump Steer is even greater. Bump stop set ups allow for more travel – in fact bump stop set ups use all of the travel! More travel multiplies Bump Steer geometry errors and spending the time to get it right does mean more speed and more importantly it produces a fast car all the way to the end. Why fade when you can win? Is improper Bump Steer one of the reasons why some cars slow down at the end of races?

If you have excessive Bump Steer you are un-necessarily heating your tires and wearing them out. The tires go over the bumps in a very fast manner and those millions of in and out toe oscillations generated by too much Bump Steer produces un-wanted tire heat and instability. You can think of it nearly as a toe vibration – in and out – back and forth in rapid motion. Remember, the movements occur through travel not just from bumps. Braking, acceleration, roll, transition all create movements that will magnify Bump Steer. Get rid of Bump Steer and let the driver turn the wheels verses letting the tires turn unpredictably on their own!

So – now that we see the Bump Steer light it is time for the golden question. How much Bump Steer should we run? It’s a matter of opinion and every set up guy has their magic formula. My answer is as close to zero as possible. What ever Bump Steer amount you use should be a recorded and repeatable number that is adjusted verses being an accident. Repeatability in race set up is the way to go.

A small amount of Bump Out is stable. Bump In can cause an unstable car – I always stay away from Bump In. A small amount of Bump Out ensures that my cars avoid Bumping In. A small amount of Bump Out ensures that you avoid Bump In through component flex and it covers unforeseen variations. With Bump Stop Set Ups and Big Bar Soft Spring Set Ups my recommended number is .004 of Bump Out per 1” of travel both left and right.

Before Bump Stop set ups - utilizing a common set up in a 2900 pound touring late model my base Bump Steer set up was .002 to .005 of Bump Out on the RF and .005 to .008 of Bump Out on the LF. Consult your car builder and use his experience. Remember, every car builder has their own idea of Bump Steer settings. Consistency and repeatability are the goal.

“A qualifying trick is to bolt in an extra .187 shim on the LF for your qualifying run which adds about .010 of additional Bump Steer at the LF for a qualifying total of .018 verses my standard .008. Sticker tires and their extra short term grip cover the negative effects of the added Bump for a lap or two. The benefit of the extra Bump Steer is that it manufactures some quick heat in the LF. Under the stress of a one or two lap banzai run the added LF quick heat helps set the car into the middle of the turn - sticker tires make it work”.

Go forward – move ahead.

Jeff Butcher

Tools Courtesy of JOES Racing Products, Inc. www.joesracing.com