Instant Center Adjustment

Why do we care about the Instant Center in the front suspension? We know our car builder spent plenty of time engineering the proper roll center. Instant Center of the Left and Right side front suspension are a piece in the puzzle that creates Roll Center.

Since I am opposed to over engineering at the track, I prefer to focus on Instant Centers and adjust them with the same freedom that is applied to adjusting the rear roll center. “Just try it” is my philosophy when it comes to adjusting the Instant Center at the track.

Before you start moving parts around it does pay to understand the geometry that creates instant centers. I like to explain mechanical design by using words like “stuff” or “things” instead of complicating simple topics with big phrases such as “dynamic roll propagated via G-Force induced dynamic wheel loading”. Really, we can explain stuff by simply understanding that there is more front end travel at a high speed high banked track than you would find if you set up some orange cones in a parking lot.

Timing your suspension travel by balancing spring rate, roll bars and shocks with the leverage created by Instant Centers can maximize your Big Bar set up. For that matter - any set up is benefited by experimenting with Instant Center locations.

Instant Centers are easy to visualize. You find the little spot the same way on both sides of the front of the car. For this review we will take a snap shot of the RF suspension. Your A-Arm is bolted to the frame via an ear that is welded to the frame horn. Your lower control arm bolts onto the cross member. You have a spindle pin and the tire size sets the height of the wheel center off the ground. Your A-Arm is about 7” to 12” long and your lower control arm is probably around 16” to 18” long for your typical late model that has a 63.0” track width. Basically, if you have a stock car, the parts that create the Instant Center are similar regardless of your brand of car.

My goal is here is to eliminate the fear that can be associated with the big pile of details that create the magical Roll Center. Really, roll center is often found at about 1.5” off the ground to 2.5” off the ground for most cars – give or take an inch. The left to right location moves all over the place depending A-Arm length. Of course, as soon as you run the suspension through travel the roll center moves about. With a huge roll bar, the rules have changed and once the car has been pulled down to the ground with your insanely stiff rebound shocks, the roll center and instant centers move around much less as compared to when we had a pair of 350’s on the coilovers and an 1-1/8” bar.

Big Bars require new thinking. Since planting the nose piece to the ground is the new norm, then it would seem that the suspension layout is less important – or is it? With the nose piece held to the ground by huge rebound numbers and a sway bar that nearly eliminates body roll, then why do we care about roll centers and instant centers at all?

While the movements are less, they are still there. We still have dive, roll and plenty of bumps. But, all of those movements happen faster and the distance traveled, once the nose pieces is sucked down, is less. With this “new” information how can we make an effective adjustment utilizing Instant Centers along with shocks and springs?

It pays to think about the mechanical leverage of the Instant Center. Adjusting the Instant Center can be the subtle adjustment that compensates for the reduced actual center of the corner travel induced by giant sway bars. Since the Instant Center is 2 simple lines per side we can visualize it easily. The first line is drawn through the center of the upper ball joint and extending through the inner pivot. Be sure to find the true center of the ball joint provided by your ball joint manufacturer. The second line extends from the lower ball joint through the inner pivot on the lower control arm. Extend both lines until they intersect. Boom – the Instant Center is created. Through suspension travel the intersect point moves based on the length, angle and connection point of the upper A-Arm and the lower control arm. See the accompanying photo for the visual and you will see that Instant Centers are pretty simple to understand.

The Instant Center is easy to visualize right at the track. Simply follow the lines of the upper and lower arms until they meet. Be sure to utilize the actual ball joint center provided by your ball joint manufacturer.

Now examine the RF Instant Center and how we can use mechanical leverage to our advantage. Let’s assume our track width is 63.0”.  If the hypothetical RF Instant Center is 4 inches off the ground and 3 feet left of the vehicle centerline we end up with about 49.5” (close enough) of leverage. If we make the RF A-Arm Longer and keep the same connection point on the frame ear pivot we move the RF Instant Center more to the left. The longer A-Arm gets flatter and it takes the imaginary line longer to run into the line from the lower control arm. The change lowers the RF Instant Center as well. So, hypothetically, let say we moved the RF Instant Center to left about a foot and down 2” (Since we are starting from a known baseline all we care about is the direction of the adjustment – we can repeat the change by tracking the slugs we use).

 A-Arm length and mounting height have a dramatic effect on Instant Center Location. Carrying an inventory of A-Arm lengths gives you more Instant Center Adjustment choices right at the track.

The longer lever arm created by the adjustment scenario in the prior paragraph compresses the RF spring more than it would have in our baseline set up. The car speed and banking provide the same amount of force, but the longer lever creates more travel at the RF. Really – it is like running a softer RF spring when the chassis rolls. Lowering the RF Instant Center promotes more roll. The longer lever from the Center of Gravity gives an additional boost to roll.

Bolt on tube sections make quick work out of changing A-arm length. Moving Instant Centers at the track is an adjustment you should try more often. The bolt on tube section is a rigid advantage on snouts where the A-Arm wraps around the frame.

Here is where the fun starts – let’s keep it simple. You can draw your suspension and do the actual math and record it for future reference. For now – let’s just think about the concept. Moving the RF Instant Center to the Left effectively softens the RF spring through chassis roll. But, if you want a softer RF spring, why not just put one in? Well, this is where you need to think about the corner entry, when the car is relatively traveling in a straight line, and the corner middle where the car is in full roll. If you balance the Instant Centers, and consider all of the compromises that come with race car set ups, you can make subtle adjustments by manipulating the timing of suspension compression (corner entry) and suspension roll (corner middle).

When your car is going perfectly straight, the giant sway bar is doing about zero. If the car is going straight and you smash the brakes then the ultra soft springs you have up there may not hold the car for a stable entry. You can fix the problem by adding stiffer springs for straight line (entry) braking, but then the middle may suffer due to the stiffer springs you thought you needed? From your baseline, adding front spring to get some help with corner entry stability coupled with moving Instant Center to the left creates a lever to help the car roll – now you get help under braking without suffering more spring rate during roll. The game is in balancing the Instant Center with the entry “dive” and the mid-corner “roll”.

Your car builder has the baseline figured out when it comes to Instant Center and Roll Center. But, track conditions change and driver styles vary. Maybe you can utilize Instant Center changes just like you use the Panhard bar? Try it and see if “Mikey likes it”.

The adjustment idea I like best for Instant Center manipulation is to use the same length A-Arm but simply move the upper frame pivot point up and down to get your desired result. I prefer moving the pivot point of the upper A-Arm for subtle adjustments. The benefit of moving the upper pivot point is that hardware is available to make it easy, you can make subtle changes, you avoid messing up the bump steer and the camber curves stay in line. Your car builder spent a mountain of time and testing on your baseline front end design so it pays to make adjustments that are subtle verses stretching the design parameters to extremes.

Using a slotted ear and slugs makes adjustments easy. You can move the pivot point in small increments by carrying a slug kit. You can raise and lower the Instant Center right at the track. If you want more roll, but don’t feel like you can run softer springs, you can simply change a slug and raise the RF A-Arm Pivot point. Raising the pivot point will move the RF Instant Center farther left and lower. The subtle adjustment gives you some turning help without decreasing braking stability. The RF gives you easy adjustment and you can “feel” the affect of the change just like when you move the panhard bar. You do have to readjust camber – easy deal.

Slugs that are marked give you an easy way to record Instant Center changes. A 1/4" slug makes a profound difference. Draw it out when you have time. At the track, just bolt in a pair!

A slotted A-Arm frame tab works perfectly with the slug system allowing for quick and precise adjustments.

The LF Instant Center is important too. You can use the LF to raise or lower the roll center. You can also use the LF to move the roll center left or right. You can accomplish the same thing with the RF, but this article is trying to provide simple examples to help your team see the concept and give you the confidence to try what may be a new adjustment for your team. You can certainly draw it all out, but for today just think about what happens when you move the Instant Center with simple A-Arm slugs.

When you run a Shorter RF A-Arm you generally move the roll center to the right. The shorter RF A-Arm has more angle and intersects with the lower control arm line faster – that is easy to understand, right? If the Roll Center is closer to the right it speeds the rate of travel and the car reacts quicker. Go too far and you will blast through the travel before the full force of the center of the corner arrives. When this situation occurs, the “soft push” is usually the result. So, this is a magazine article and you have a race car going around a real track. Reading is fun and accepting the limitations within this article will help you to just try adjustments. There are a ton of variables and the goal of this lesson is to simply discuss one element as if Instant Centers were not connected to anything else – of course they are! But, if by forgetting about all of the other “stuff” we can learn how to manipulate Instant Center adjustments to overcome a problem then we have learned something new.

Changing A-Arm length requires the camber to be reset. A billet nut plate speeds changes when time is short. An accurate billet caster camber gauge is a must for any race team.

Since your car builder spent the time to build your car with a proven Roll Center location we want to be careful to not adjust so much that we erase the years of testing and knowledge that our car gives you when you buy a frame. So, from a prior article Roll Center is explained here:

Roll Center Explained:

To simplify the Front Roll Center thought process it helps to understand the creation of the so called magical point.  Front Roll Center is a calculated point verses a physical place. To find it you must first locate the Instant Center both left and right.

The RF Instant Center is found by drawing a line through the center of the RF upper A-Arm ball joint extended out though the center of the A-Arm inner pivot point on the frame. Another line is drawn from the RF lower outer ball joint center though the lower control arm frame pivot. The RF lower control arm line is extended out until it meets the RF upper control arm line. Where these lines intersect is called the Instant Center. The LF Instant Center is found in the same way.

After both Instant Centers are located you can now find the Roll Center. From the RF Instant Center you draw a line back to the RF contact patch center. From the LF Instant Center you draw a line back to the LF contact patch center. Where these two imaginary lines, running from the contact patches to the corresponding Instant Center intersect, is the Roll Center. Remember – the Roll Center moves as the suspension goes through travel.

Spend time to learn about Roll Center when you are in the shop. At the track - trial and error is still common, even for Cup teams with full time engineers. 

Note:

Since the “Roll Center” location is a moving point is space it gets complicated – carrying graph paper at the track is not feasible, I prefer to spend time thinking about roll center during the design stage of building a chassis – and much thought is placed into Roll Center when designing any suspension.

 At the track, it is easy to visualize Instant Centers and difficult to think out roll center. By simplifying, I can adjust Instant Center locations right at the track as I can easily see how the upper A-Arm line passes through the lower control arm line. With simple visual estimation, I can have another adjustment method at the track and I carry slugs to make repeatable changes just like I would move the Panhard bar or change a spring.

If you use a RF A-Arm frame mounting plate that is slotted for height adjustment you can use slugs to ensure you have repeatable and documentable changes. For the Front Instant Center adjustment you can simply record that you moved the RF inner A-arm mounting point up a1/8th inch with a slug. Changing a slug is pretty easy.  If the driver doesn’t like the adjustment you can simply bolt the original slug back in.

Instant Center adjustments at the track can be used to create the feel of stiffer front springs under braking yet have the front springs feel softer in the center of the turn due to the longer lever that is created by the Instant Center length change. Many variables come into play and the teams that get the variables closest wins.

At the track – I usually focus on Instant Center adjustments by moving slugs on the upper A-Arms. You can move the lower points too, but you bring in rack location issues and bump steer corrections. The upper adjustment is easy to understand especially when track time is limited.

Lowering the RF A-Arm inner pivot raises the Front Roll Center and moves it to the right. Negative Camber is added and may need to be reset.

Raising the RF A-Arm inner pivot lowers the Front Roll Center and moves it to the left. Negative camber is reduced and may need to be reset.

Lowering the LF A-Arm inner pivot raises the Front Roll Center and moves it to the left. Positive camber is reduced and may need to be reset.

Raising the LF A-Arm inner pivot lowers the Front Roll Center and moves it to the right. Positive camber is added and may need to be reset.

The reality is that Front Roll Center is simply a derivative of the Instant Center locations.  Instant Centers are simple even through dynamic roll. Why complicate your trackside thought process with imaginary lines? Keep it simple at the track and use slugs to maintain records and repeatability. You can engineer at will after the race and study the Roll Center changes you accomplished and measure the affects of bolting in a few simple slugs.

Engineering becomes more important in racing every day. When time is short, educated experimenting is equally as valuable.

Go Forward – Move Ahead

Jeff Butcher

06/08/12