The primary goal of all setups...
The primary goal of all setups is to develop a balance between the two ends of the car so that they will tend to do the same thing in the turns. We need to maintain this setup balance as we try to fix entry and exit handling problems.
When the car turns well but the rear is not gripping as well as the front, the car is said to be loose. The feel of the car as well as the tire temperatures can reveal a lot about what is causing the loose condition.
A loose car usually has a RR tire temperature that is higher than that of the other three tires. That is because a loose car does not have enough traction off the corners and spins the rear tires, especially the RR tire. Here are several probable causes and solutions for a loose car.
1. Rear Alignment The very first consideration is rear alignment. If the rear end is out of alignment, with the RR wheel farther back than the LR wheel, the car will be loose at all three segments of the turns. This should have been checked in the shop long before the car rolled off the trailer.
A simple stringing of the wheels in relation to the chassis side rail will tell you if the rear-end alignment is off. When testing or in practice, do a quick adjustment-bring the RR wheel forward and go back out on the track. The condition should be better. Do a thorough alignment check when you return to the shop.
If the rear end is positioned...
If the rear end is positioned so that it is aimed to the right of the centerline of the car, the rear tires will continually want to roll outside the front tires. This is obviously going to make the car very loose in all three turn phases. It is very important to align the rear end to point straight ahead and also to line up the right-side tire contact patches.
2. High Rear Roll Center The Panhard bar, or whatever represents the rear roll center height, may be too high for the rest of the setup. If you have a J-bar setup or a Panhard bar, lower it to tighten the car.
Small changes will make a noticeable difference. If the car is way off, make changes in 1/2-inch increments. As the car approaches neutral, make 1/4-inch changes on both sides and then on only one side.
3. Stiff Rear Springs The rear springs may be too stiff, resulting in the front tending to roll more than the rear. This was not a common occurrence in years past, but it is very possible with the advent of the BBSS setups. Soften the rate of the rear springs to help eliminate a loose condition.
Soften only the RR spring in a BBSS setup. Rear spring changes have a significant effect on the handling balance, so be careful. The changes you make, if in excess, can cause the car to revert to a tight condition in a hurry.
With these changes, if you are able to set up the car so that it is neutral and balanced through the middle of the corner, then the next step in the process is to work on entry performance.
The rear trailing arms are...
The rear trailing arms are positioned so that as the right side of the chassis moves down in the turns, the right-rear wheel is moved back, causing a significant amount of rear steer to the right and making the car loose.
A car that is loose into the corner may have one or a combination of the following problems. Let's look at what makes a stock car loose on entry.
1. Rear Alignment A rear end that is out of alignment can cause a car to be very loose, especially on entry to the corner. If the rear end is aligned so that it points to the right of the centerline of the car, then the car will probably be loose into the corner as well as through the middle and off the corner. Nine times out of ten, a car that is loose into the corner has a rear alignment problem that needs to be addressed right away.
2. Improper Shock Rates The LR shock may be too stiff in rebound or the RF shock may be too soft in compression, which transfers load from the LR tire to the RR tire, and from the RF tire to the LF tire on initial entry under hard braking. To fix this, reduce the rebound in the LR shock and/or stiffen the compression in the RF shock.
3. Brake Bias Imbalance Incorrect brake bias adjustment can cause a loose-in condition. Make sure your brake bias is tuned correctly. If too much of the bias is on the rear brakes, the car will be looser under heavy braking than if you lightly brake into the corner. Install brake bias gauges and know the amount of pressure at each set of brakes. Usually, a 60 percent front and 40 percent rear bias works for most tracks.
For dirt cars, we sometimes see a bias adjustment between sets of brakes on the same "axle." Usually, this is done at the front, and the RF is made to brake less, or not at all, to prevent it from losing grip on entry. This could overwhelm the LF tire, so a change in front-to-rear brake bias is recommended along with a left-to-right bias adjustment at the front.