Setting up a circle track...
Setting up a circle track car to be a road racing car is an art that requires you to understand the peculiarities. There are similarities between the two and some distinct differences. Many of these cars end up being sold to collectors and are then prepared for racing in vintage classes. Photo by Harold Hinson
At least 10 degrees of that difference is due to the extra work the inside half of the tire does while it is the inside tire of the axle pair. Tire wear will reinforce this method as well, but we still may see a slight bit of extra wear on the inside half of the tires. That is to be expected, and there is nothing we can do about it if we want optimum performance out of the tires.
The geometry of the front and rear needs to be tailored to the needs of turning both ways as well as other settings. For circle track racing, we have the liberty of using a moment center that is offset from centerline to enhance the dynamics. On the road courses, the moment center will have to be centered in the car.
The centering of the moment center must be combined with a geometry that allows the front dynamics to be the same for right and left turns. We also need to adjust our camber change characteristics so that there will be minimum camber change with the dive and roll we will see. If we are used to the dive and roll numbers from banked tracks, we might not have the best control arm angle settings for flat turns such as those experienced on road courses.
When the car dives and rolls in the turns, the moment center will move. If we start with the moment center at centerline at static ride height, then as it moves in the turns, it needs to move the same amount for a right or left turn. This keeps the dynamics consistent and the handling predictable for the different turns.
In this example, the moment center is moving in the opposite direction we normally see, but depending on the arm angles, this is a possibility. The design goal is to keep it near the centerline of the car in the turns and make sure it moves an equal distance for right and left turns.
At the rear, our trailing arm angles and the Panhard bar angle will need to be reset from what we are used to for circle track racing. We can manipulate rear steer in our cars for oval racing, but as said before about the moment center, our rear steer must be uniform for left and right turns.
The trailing arms should be set to the same angle on both sides of the car and level to the ground. The car will tend to rise less on the inside suspension and squat more on the outside suspension. If we have any angle in our trailing links, then this difference in travel of the forward ends of the trailing arms will produce some rear steer. If the links are angled upward and toward the front of the car, then the rear steer will be to the outside of the turn and produce a loose, or oversteering condition.
If the links are angled downward and toward the front, then the rear steer would be toward the inside of the turns and cause a tight, understeer condition. A level setting reduces rear steer in both directions and will produce a slight amount of rear steer to tighten the rear of the car. This gives the car better bite off the corner while not being so extreme as to make the car tight in the middle of the turns.
The Panhard bar should be level to the ground. This is not necessarily the optimum setting for all turns, but it is less detrimental to rear-end lateral movement, which can produce rear steer.
The height of the rear moment center is critical, too. As with oval racing, the car should be balanced front to rear, and the height of the rear moment center works in conjunction with the spring rates to accomplish the overall balance. With the stiff spring rates we will probably install, a lower rear moment center will probably work better.
Some cars have Watts' linkages for a rear location device. This system works well for road racing because it produces no lateral movement of the rear end as the car dives and rolls in the turns. The moment center height of this system remains consistent, making the dynamics more predictable and consistent.
Weight distribution is an often overlooked area of setup for the road racing car, no matter what classification-from Formula One to Mean Miatas. In circle track racing, we tune our setups with varying weight distribution numbers and read and record these as crossweight percentages or pounds of wedge/bite in the left rear (LR). Adding the RF and LR weights and dividing that number by the total vehicle weight (with driver and all weights) gives us the crossweight percentage. The amount of LR weight over the right-rear (RR) weight is referred to as "wedge" or "bite." For road racing, the crossweight and wedge must be zero or very close to zero, because the crossweight or wedge must be the same for turning in both directions. The car would handle differently each way, otherwise.