Controlling Suspension Travel in Dirt Cars

The ride height changes we see on a typical dirt car could be considered extreme. But, the movement of the chassis is necessary in order to position the car the way we want it to be. We can visually see how the car is reacting on the race track, but if we could view it up close at those attitudes, we could learn a lot more.

Similar to asphalt cars, “normal” ride heights are becoming a thing of the past. On dirt, the attitude of the car for the late models creates a lot of Right Front compression, Left Rear extension and a lot of associated rear steer at times.

But does all of that movement mean a team has to disregard ride heights when they setup the car? No. Ride heights will always be important, maybe more so now than ever, and we’ll tell you why.

There are many different ride heights besides the ones we measure at the shop. Sometimes we need to study the on-track ride height to better understand what is going on with the cars attitude. Then we might be able to better plan out changes to match the track conditions.

We stress the importance of establishing a baseline for your setup. The term “setup” encompasses many different settings on the car. One of the areas of concern are the angles of the control arms, suspension links and positions of the moment centers, front and rear.

Every team needs to establish ride heights for their dirt car no matter which class you will run. This baseline height will dictate many of the other settings and going back to a known height will help you with changes you might want to make to the car from time to time.

 Why Ride Heights For Dirt Cars? – When we set the static control arm angles, we are depending on a certain chassis movement and that causes the dynamic angles to be what we expected. If we don’t keep track of our static ride heights and they end up changing, then the dynamic angles will also change.

Let’s say we make a spring change at the track. We either change the spring rate at one corner of the car, or we move the upper mount in or out on the chassis. These are common changes. If we don’t adjust the ride height back to where it was before we made the spring change, then our link angles will have changed too.

If we had good link angles before the change that produced just the right rear steer, etc., then after the change we will have something other than the ideal link angles. This can become very unpredictable.

A lot of things are dependent on the link angles for dirt late models and dirt Modifieds. Not only is the rear steer dependent on those angles, but the tire loadings for the four tires depend on those angles too. Here is why.

If the angles were such that the LR of the car lifted say 4.5 inches, and we changed the ride height so that the angles were less providing less lift, then we might only be up 4.0 or 3.5 inches. The rear steer of the car would change and the whole attitude of the car would be different. And the force of the LR tire driving forward will change and be less with less link angles.

The aero for a dirt car uses the top view angle created by the rear steer to produce a side force that counters the lateral force trying to push us off the track. This aero effect is called Flat Plate aero. The angle of the “flat plate” side of the car is critical in determining how much aero side force the car will produce.

With this flat plate aero, the angle can be too much, or too little. Once we find that perfect angle, the car works much better and it’s like we gained tire grip when it is really just aero side force holding the car onto the track.

Not only is side force present, but downforce from the front to rear angle of the car is pushing the car down. If the LR lift is diminished then we will have less top side angle to produce flat plate downforce and we then have less loading on the tires.

Less side force and less downforce could be a result of making changes to our ride heights. Sounds crazy, but all of this does matter. There is so little grip in a dry slick track that any help we can get could potentially drop our lap times by a tenth or two.

High Left Rear link angles in a four-bar dirt Late Model cause lifting in the LR corner, which pulls the LR wheel forward to create a lot of rear steer. This steer moves the rear of the car out and creates a side force aero effect that pushes the car towards the inside of the turns.
Low angles of rear steer reduce drag and are best for tight track conditions where there is less need for side force aero. We can directly measure rear steer from different link angles related to left rear shock rebound travel that we can simulate in the shop.

Static verses Dynamic Ride Height – So we have decided that it is good to establish static ride heights and to make adjustments to our ride heights when making spring changes so as not to change the link angles.

You can do practice spring changes in the shop and note how much you need to adjust the ride height back to the baseline. That way, a certain rate change in spring rate might need a 3 turn change in spring height at one corner of the car. Or, moving the RR upper spring mount in 3 inches might necessitate a 4 turn change in that springs height.

What about the dynamic ride height? We can measure the shock travels at all four corners, especially the LR and RF corners. Those two move the most of the four corners on the car. We can position the chassis at those ride heights when the car is in the shop and on a level surface.

With the car at the dynamic ride heights, we can physically measure the rear steer and the angle of the flat plate side from straight ahead. There is a reason why we need to note these angles.

A shock travel indicator, or better yet, a data acquisition record of the shock travels can tell us a lot about the chassis position and attitude on the track. Data can average out the movement whereas a travel indicator shows only the maximum travel that could be influenced by bumps, etc.
Once we know the shock travels at the corners of the car, we can then install links set at those same travels and study the rear steer and other aspects of our setup. You might be surprised at the results and how much the car is moving sideways with the rear steer.

With aero side force, there is also a lot of drag, but if we can drive through the corner faster and keep from sliding, then the drag is worth it in the long run. But if the track is tight with a lot of grip, we don’t necessarily need that side force as much, or not at all.

Then we can reduce the lifting of the LR, which reduces the rear steer and top view angle of the car in order to reduce the drag. Now we can accelerate much quicker down the straights and drive more straight ahead.

The tell tale for all of this would be the shock travel in the LR corner. If we could measure the travel in the shop verses rear steer, then once we hit the track, we could compare the shock travels and adjust the link angles until we read the shock travels we think we need for the track conditions.

Less rear steer and associated LR rebound shock travel for tacky tracks, and more rear steer and more LR rebound shock travel or slicker tracks. This is one more tuning tool we can use for dirt cars to help us adapt to changing conditions.

We know we need to make setup changes to adjust the balance for different conditions. Now we know we also need to make adjustments to the link angles so our dynamic ride heights will produce the degree of rear steer that is needed to gain or reduce aero side force and drag.

Conclusion – We have come to learn that dirt late models and modifieds are complicated engineering marvels. But that doesn’t mean we cannot use simple common sense to set them up. The two areas we need to concentrate on are levels of balance that match the track conditions of grip and the use of aero side force when the track begins to go slick to help hold the car in the turns.

When we study ride heights, both static and dynamic, we can then begin to understand more about what the actual attitude of the car is on the track. We can directly measure this angle and then know how to setup the car.


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