Analyzing a dirt late model chassis

We spend long hours in the shop, sweating over details, but what is the car really telling us it needs when we go to the track. There are some simple techniques to reading or analyzing the car and what it needs. The shape of the track, the way the car rotates in the corner, and watching for the driver inputs are all factors that affect how the car gets tuned.

Track shape

The shape of the track the car is running is probably one of the biggest factors I look for when trying to understand what the car needs. All tracks are somewhat unique in shape, but a car can usually get pretty close, by categorizing them into two basic configurations. The first is what many racers call a paper clip style track. These have long straights and tight corners. These usually require a car that rotates well and has plenty of traction off the corner. Many times a higher wedge car will run better in these conditions, because there will be less side bite into the corner allowing the car to turn quicker, and a little more static wedge to quickly tighten the car off the corner. Adding banking to a paper clip style track can usually lead to diamonding the track. This is where the car runs straight into the middle of the corner, rotates hard and slides up the banking until it is pointing straight to come off the corner hard. With this kind of technique there is no slow progression back into the gas to exit the corner. The car has already fully rotated, so acceleration is hard and fast off the corner. This usually doesn’t work on longer radius corners.

So, you guessed it long radius corners are the other type of track configuration. Long radius corners require more of a balance to the race car. The car needs to drive the radius into the corner, turn free, and finish turning the corner while still accelerating out. Too much wedge, either dynamic or static, will cause the car to be loose into the corner and to tight off the corner, causing the car to lose the nose on exit. Large radius slick conditions have to be one of the most difficult conditions to tune the car. Adding banking to a large radius track will add more down force to the entire car, not just promote more body roll, like a large flat radius corner will.

Chassis Yaw

How the car rotates in the corner is another area to watch for when tuning a dirt late model chassis. Matching the shape of the track with how the car rotates will tell quite a bit of what the car needs. With the amount of roll steer designed into today’s cars it is hard sometimes to determine of the car is loose or if it is following a natural arc into the corner. Driver feedback does help in this area, but the car should travel into the corner at a slight yaw, and yaw should increase gradually as the car gets closer to the middle of the corner. This should be timed so the time off the gas in the middle of the corner should be minimal. The amount of roll steer in that particular type of car will be the determining factor.

Driver inputs; steering angle

Watching the driver inputs will also be an indicator of what is going on with the race car and watching the front tires is the best tell tale sign. This will go hand in hand with how much the car will yaw. A tire is only created with so much slip angle. If the yaw of the car exceeds the slip angle the car will be loose or at least not have as much side bite as it could have entering the corner. This is where the steer angle of the front tires comes into play. So, it could be said that the radius of the corner, the amount of roll steer, and the slip angle of the tires are all interrelated; the front tires steer angle is the indicator of what is going on. Taking all these into account is kind of tough to describe every situation, but practice watching the cars is the best way to figure it out. It seems to me, for most tracks with a large radius corner, and a car with a birdcage four link rear suspension, any more than ten degrees of counter steer in the front tires indicates a loose condition. But, the numbers aren’t written in stone, it is just a rule of thumb.

It will take a lot of practice learning to analyze a car, but using these signs will hopefully help you on the road to success.

Till next time, be fast.

Kevin