by Ross Bentley
RIDE HEIGHT
The ride height is the distance between the road surface and the lowest point on the car. Often, this is different at the front than the rear. This difference is called “rake”—usually with the front lower than the rear. Adjustment of the ride height, particularly the rake, is used to tune the handling.
The ride height is usually determined by running the car as low as possible without the chassis bottoming out (or, at least, just barely touching) on the road surface, or the suspension running out of travel. Usually, the lower the car is run, the better the aerodynamics. Additionally, the lower center of gravity is advantageous.
SPRING RATE
Choosing the optimum spring rate is one of the most important setup factors you’ll have to deal with. The spring rate is the amount of force needed to deflect a spring at a given amount and is usually measured in pounds per inch of deflection. The diameter of the spring wire, the overall diameter of the spring, and the length or number of coils determines this rating.
It’s your goal in developing the car to find the optimum spring rate for the front and rear suspension. Generally, it’s a compromise between having a soft enough spring to allow the suspension to handle the undulations in the track surface, while being stiff enough to keep the car from bottoming out when hitting a bump. There are many more factors involved, such as your driving style or preference, the amount of aerodynamic downforce you are running, the weight of the car, the shape and condition of the track surface, and so on. Perhaps most important though is the balance front to rear. Generally, it’s best to use the softest spring possible on the rear, to help the rear tires achieve maximum traction under acceleration, and then balance the handling with the optimum front springs.
WHEEL RATE
The wheel rate is the amount of force needed to move the wheel a given distance. It is also measured in pounds per inch of deflection. It is determined by the geometry of the suspension and spring-mounting location and the spring rate. Understand that even though you have the same spring rate on the front and rear suspension (or two different cars), the wheel rate may differ due to the amount of leverage a suspension system applies to the spring.
ANTI-ROLL BAR
An anti-roll bar (sometimes, wrongly referred to as a sway bar) is used to resist the vehicle’s tendency to lean during cornering. The anti-roll bar, usually a steel tube or solid bar, is used to alter the front or rear roll resistance. This affects the car’s handling characteristics. Many cars have adjustment controls in the cockpit, so you can make changes as the track conditions, fuel load, and tire wear change throughout a race.
Adjusting the anti-roll bars is probably the easiest and quickest change you can make to the suspension setup. Therefore, it’s important to try the car at full stiff and full soft settings to see what effect it has. When beginning to dial in the setup of the car, I like to do a “bar sweep.” This is where I will adjust the front bar from full soft to full hard, then do the same with the rear bar while noting the change in handling. That gives my engineer and I a good indication as to which direction we will have to go to develop a good balance in the car.
As a general rule, to improve the grip on the front of the car (to lessen understeer), you should soften the front bar or stiffen the rear bar. To improve grip on the rear (lessen oversteer), you should soften the rear bar or stiffen the front bar. However, that’s not always the case (as I’ve discovered a few times), so be prepared to try the opposite.
ROLL STIFFNESS
Roll stiffness is the total amount of resistance to the car leaning or rolling provided by the springs and anti-roll bars. This is measured in pounds per inch of spring travel at the wheel. This is a function of the spring rate and the anti-roll bar stiffness.
The distribution of the vehicle’s roll stiffness between the front and rear suspension is called the roll stiffness distribution, and is expressed as a percentage front to rear. Generally, it’s the roll stiffness distribution that we use to fine-tune the handling balance of the car, using the springs and anti-roll bars. Adjusting the front roll stiffness (with springs or anti-roll bars) in relation to the rear, and vice versa, is the most common method of altering the handling balance of the car.
SHOCK RATE
The purpose of a shock absorber is to slow down and control the oscillations of the spring as the suspension absorbs undulations in the roadway. Actually, a shock absorber is a damper; it damps the movement of the springs.
Shocks work in both directions: Compression is called bump; extension is called rebound. A shock absorber, therefore, is rated by the rate of deflection at a given shaft speed, both in the bump and rebound direction. If the car’s springs are force sensitive, the shocks are velocity sensitive.
You can also use the shock absorbers to alter the transient handling characteristics (how responsive the car is to your inputs). If the springs and anti-roll bars determine the amount of body roll and the distribution front to rear, then how quickly that body roll occurs is determined by the shock absorber rates.
So the shock absorbers are another important suspension-tuning component. And, as with the spring rate, finding the optimum shock setting is a delicate compromise. It takes some experience before you have the sensitivity as a driver to be able to find that perfect setting.
ILLUSTRATION 4-4 A shock-absorber dyno produces a graph that relates the force it takes to stroke the shock, in both bump and rebound, versus the velocity at which it moves. Learn to read and understand shock dyno graphs and especially how their data relates to what you feel when driving.
CORNER WEIGHT
If you place the four tires of a vehicle on four separate scales, they will give you the corner weights of the vehicle. From there, you can determine the front-to-rear and left-to-right weight distribution, as well as total vehicle weight.
Ideally, for a road course, the left-to-right corner weights should be identical. In practically any midengine car the rear corner weights will be higher than the front. For oval tracks, often the setup will be biased to one side or corner.
Adjusting corner weights is one of the most important suspension-tuning tools, one that is often overlooked by many inexperienced racers.
TIRES
One of the most effective ways of checking and optimizing chassis adjustments is by “reading” the tires. Evaluating tire temperatures will indicate if the tire pressures are correct, if the alignment settings are correct, how the overall handling balance of the car is, and to some extent how close to the limit you’re driving.
All tires are designed to operate within an optimum tread temperature range. In this optimum range, the tire generates its maximum traction (as shown in Illustration 4-5). Above or below that optimum range, the tires will not grip the track surface well. Also, if they are operated above the optimum range for too long the tread may begin to blister, chunk, or wear quickly. An average temperature range for a high-performance street radial is in the 180 to 200 degrees Fahrenheit area; for a racing tire, it’s 200 to 230 degrees Fahrenheit.
ILLUSTRATION 4-5 Tire temperature versus traction graph. In this graph you can see that the tire gains traction as its heat builds, until it reaches a point where it then begins to lose traction.
To determine tire temperatures use a tire pyrometer, an instrument with a needle that is inserted just under the surface of the tire’s tread, generally at three points across the tire—the inside, the middle, and the outside of the tread.
Tire temperatures taken after the car has come into the pits are an average of the corners and straightaways. If it’s after a long straightaway or a slow cool-off lap, the temperatures may be misleading, as part of the tread may have cooled more than others. So, it’s important to take temperatures as close to a corner as possible. They must also be taken as soon as the car has come to a stop as they will begin to cool after about a minute.
The optimum camber angle is indicated when the temperature near the outside of the tread is even with the temperatu
re near the inside of the tread. If the temperature near the inside of the tread surface is significantly higher than the outside, there is too much negative camber. The inside is heating up too much. If the outside temperature is hotter than the inside, there is too much positive camber.
If the temperature in the middle of the tread is equal to the average of the inside and outside of the tread, then the tire pressure is correct. If it’s too hot in the middle of the tread, then the tire pressure is probably too high. If it’s too cool in the middle, then the pressures are too low. Ideally, the tire temperatures should be even all across the tread.
If the temperatures on the front tires are even with the rear tires, then the overall balance of the car is good. If they are hotter than the rears, then the fronts are sliding more than the rear, and a spring, shock, or anti-roll-bar adjustment may be necessary. The reverse is true, as well.
If all four tires are not running in the optimum temperature range, it means one of two things: Either the tire compound is not correct for the application or it has something to do with your driving. If the temperature is too low, you’re not driving the car hard enough. You’re not working the tires. If the temperature is too hot, you’re driving too hard. You’re sliding the car too much. There is more about this in the next chapter.
Get used to reading a tire. If you can look at the tread surface in relation to how the car felt and the tire temperatures, and then determine what to do to make improvements, it may make the difference between you and your competitors.
Generally, the surface should be a dull black all across the tread. There should not be any shiny areas. If there is, it probably means that part of the tire is being overloaded. Also, if you are driving the car hard enough (using the tires), the tread surface will show a slightly wavy grained texture. It should be this same texture all across the tread.
A couple of notes on how to treat new tires: When starting with a new set of tires, it is best to break them in. First, “scrub” them in by weaving back and forth (if safely possible) to clean the mold release agent off the surface. Second, don’t destroy them on the first lap by putting the car in huge slides through the corners and getting massive wheelspin under acceleration. Instead, gradually build up the heat in them by progressively increasing your speed. Their overall grip will last longer this way.
The more you understand about the car, the more successful you will be. All the driving talent in the world will not guarantee a win. Take the time to learn and fully understand everything you can about how the car works, how it is set up, and what each change should and does do. Even if you don’t work on the car yourself, being able to tell your mechanic what the car is doing is the only way of getting the maximum performance from the car. As with many other aspects of racing, read, listen, and learn as much as you can. At the end of this book, I’ve listed some additional books I strongly suggest you read.
Before making vast changes to the car’s setup, be sure that you first know the track well, are comfortable with it, and are driving well. I’ve seen drivers (myself included) get so caught up in the idea of making the car work better, they forget about their own driving. Also, when making changes to the setup, only make one adjustment at a time. If you make more than one, how do you know which one made the difference?
I bought my first Formula Ford from a driver who had been racing for a number of years, and who I knew was knowledgeable about the setup and mechanics of the car. I knew the car was pretty good. So I decided I wouldn’t try to out-trick myself. I promised myself I wouldn’t make any drastic changes in the car for at least the first season. I was just going to concentrate on learning to get 100 percent out of the car as a driver and only fine-tune the suspension. The second year I raced it, I made some serious modifications to the car. By that time, I felt like I knew enough to do that.
TIRE TRACTION
You’ve looked at the tires from the perspective of how they relate to chassis adjustments in the last chapter. Now, let’s get back to how to drive them. In fact, to get the most from your tires, you really do have to understand them. You can be somewhat successful in racing without knowing many of the suspension basics I talked about previously, but you must understand how tires work.
Every force that affects your car, and your performance, is transmitted through the four tires. Absolutely everything. So, you better know how they work and be sensitive to them.
SPEED SECRET
You will never win a race without understanding how tires work.
There are only three factors that determine the amount of traction you have available from the tires. The first is the coefficient of friction between the tire and the track surface, which is determined by the road surface itself and the rubber compound of the tire. The second is the size of the surface of the tire that contacts the track surface. Obviously, the more rubber in contact with the road surface, the more traction available. And the third is the vertical load on the tires. This load comes from the weight of the vehicle and the aerodynamic downforce on the tires.
Tires do not reach their limit of traction and then all of a sudden break away into the land of skidding and sliding. Sometimes it may feel like that, but they always give you some warning signs. As they reach their limit of adhesion or traction limit, they gradually relax their grip on the road.
In fact, primarily due to the elasticity of the rubber, tires have to slip a certain amount to achieve maximum traction. The term used to describe this tire slippage in cornering (lateral acceleration) is called “slip angle” and is measured in degrees. As your cornering forces and speed increase, the tire ends up pointing in a slightly different direction than the wheel is actually pointing. The angle between the direction the tire is pointing and the path the wheel is following is the slip angle.
When accelerating or braking, the amount of tire slippage is measured in percentages.
The tire’s traction limit, and therefore its cornering limit, is achieved within an optimum slip angle range, as shown by the “Slip Angle vs. Traction” graph on ILLUSTRATION 5-3. That range may vary slightly for different tires (radial tires slip less than bias-ply tires), but the basic characteristics remain the same. Up until that optimum slip angle range is reached, the tire is not generating its maximum traction capabilities. If the cornering speed or steering angle is increased, slip angle will increase along with tire traction until it reaches a point where tire traction then begins to decrease again.
ILLUSTRATION 5-1 Tire slip angle.
How quickly the tire reaches its optimum range and then tapers off determines the “progressivity” of the tire. A tire that is too progressive (one that takes too long to reach its limit, and then tapers off very slowly) is not responsive enough. It feels sloppy. A tire that is not progressive enough will not give the driver enough warning when it has reached its traction limit and is going beyond it. It doesn’t have enough feel. This tire is difficult to drive at the limit since you never know precisely when you’re going beyond it. Typically, a street tire is more progressive than a racing tire. A racing tire is less forgiving than a street tire.
On a dry track, maximum traction—and therefore maximum acceleration, braking, and cornering (maximum slip angle)—occurs when there is approximately 3 to 10 percent slippage (as shown in the “Percent Slip vs. Traction” graph in Illustration 5-3), depending on the type of tire. This means a tire develops the most grip when there is actually a certain amount of slippage.
Fortunately, as I said earlier, when tires reach their traction limit and then go beyond, they don’t lose grip completely and immediately. They actually lose grip progressively. And even when they are beyond the limit, completely sliding, they still have some traction. Think about it. Even when you have locked up the brakes and you are skidding, you still slow down, not as fast as when the tires are still rotating, slipping 3 to 10 percent, but you do slow down. The same thing applies during cornering. When the car starts to slide, the tires are still trying to grip the road.
And, as they grip the road, they are scrubbing off speed down to the point where the tires can achieve maximum traction once again.
This is a reassuring fact to remember. It’s possible to go slightly beyond the limit without losing complete control and crashing. We’ll talk more about driving at and beyond the limit later.
ACCELERATION
When accelerating, think of squeezing the gas pedal. Don’t pounce on it. Again, the throttle is not an on-off switch. It should be used progressively, squeezing it down and easing off it. This must be done quickly, but smoothly.
As I said before, there is a limit to your tire’s traction, which should be approximately 3 to 10 percent slippage on dry pavement and somewhat less on wet pavement. Should the tires exceed this percent slippage, leading to wheelspin, it will result in less than maximum acceleration. At that point simply ease off the throttle slightly, “feathering” it until you have controlled traction and maximum acceleration again.
BRAKING
The braking system on most race cars is more powerful than any other system in the car. In other words, the car is capable of stopping much quicker than it can accelerate. Take full advantage of this.
As with acceleration, maximum braking occurs with approximately 3 to 10 percent slippage. This means the wheels are actually turning slightly slower—3 to 10 percent slower—than they should be for any given car speed. Exceeding this limit leads to lock-up, 100 percent slippage, and loss of steering control. Braking at the limit, or threshold of traction, is called “threshold braking.” It’s the fastest, most controlled way to slow, or stop, a vehicle. This is what I mean by maximum braking.