Balancing Act: Finding The Perfect Chassis Setup for Speed

If you’re looking for balance in your car’s handling balance, it helps to look in the right places. Suspension setup is one of the keys to fast lap times, but there’s a lot to the subject.

Turn 4, a medium speed left hander requires considerable braking at the end of a long straight. As soon as you turn in, the car picks up a push, causing speed to be scrubbed off and tire wear to accelerate.

Turn 7 is a tight, fairly slow right-hand turn. As soon as you pick up the throttle as you unwind the steering wheel, the car gets loose to the point where you must back off the throttle.

Let’s analyze the possible causes of the problems and more with some fundamentals of how suspension works. It does not matter what type of car you drive, these questions will help you pinpoint the problem in order to find the quickest path to a solution:

• Is it a tire traction problem?
• Is the issue consistent on all corners?
• Is the problem only in corners in one direction?
• Does the handling issue persist through the entire corner?
• Is the problem only at corner entry?
• Is the problem only at corner exit?
• Is the issue only in low speed or high speed corners?
• Does the issue occur all the time or just occasionally?
• Does the problem appear after a few laps?

Is it a tire traction problem?

Worn tires, too many heat cycles or a flat spotted tire could be issues. Overheated tires also can cause handling problems. Most likely, tire traction problems are caused by a poor setup. Camber, toe, tire pressures, spring rates and antiroll bar rates are all factors, which we will cover in greater detail a little later.

Is the issue consistent on all corners?

If the answer is yes, it’s a tire traction problem. Stay tuned!

Is the problem only in corners in one direction?

This could be caused by a bind in the suspension control arms, the antiroll bar or a bad shock absorber. This could also be caused by diagonal weights (also called crossweight) not at 50 percent. Crossweights other than 50 percent are used on ovals where the car only turns left.

Does the handling issue persist through the entire corner?

If the problem exists in all corners, then the cause is most likely tire traction. If it occurs in only one corner, it’s more complicated. This also could be an issue caused by the driver.

Is the problem only at corner entry?

Transitional handling problems are usually caused by shock absorbers or by the driver.

Is the problem only at corner exit?

Transitional handling problems are usually caused by shock absorbers or by the driver.

Is the issue only in low-speed or high-speed corners?

In low-speed corners, it’s a handling balance (tire traction) issue. In high-speed corners it’s an aerodynamic balance problem.

Does the issue occur all the time or just occasionally?

If occasionally, this is almost certainly a diver error problem.

Does the problem appear after a few laps?

Overheated or worn tires, or changing track conditions will cause this problem.

REVIEWING THE BASICS

Tire Traction

Factors affecting tire traction include the softness of the rubber compound — softer rubber compounds make more traction, but wear more quickly — the vertical load on the tire, the temperature of the tire surface and the cord structure of the tire carcass. In most cases, tire compounds are limited and often, especially with spec tires, only one choice is available. Carcass construction is not something that can be changed, though tire pressures can alter how the carcass reacts under loads.

Traction increases as vertical load increases. However the relationship is not linear. Increase the vertical load on a tire by 10 percent and traction may only increase by 8 percent. This fact is why weight transfer has a major effect on handling. It is also why a heavier car on the same tires will accelerate, brake and corner more slowly. Weight transfer can easily be manipulated to alter how a car handles. On the other hand, it is difficult to reduce vehicle weight especially in a spec class.

Understeer/Oversteer

Understeer exists when the front tires lose traction before the rear. Or as my oval track friends say, the front of the car hits the wall first! Oversteer is the opposite. Rear tires lose traction first — or the rear of the car hits the wall first. The technical definition of understeer says that the slip angle of the front tires is great than the slip angle of the rear tires. Oversteer exists when the slip angle of the rear tires exceeds that of the front tires.

What works for one car doesn’t always work for another. For example, American Iron has three allowable power to weight ratios, and lots of different eligible models to race, which makes for many different suspension setups.

Items Affecting Understeer and Oversteer

Roll Couple Distribution – the relative amount of roll resistance at the front vs. the rear, controlled by spring and antiroll bar rates. These affect steady state (constant speed) cornering.

Shock Absorber Rates – Shock rates affect transient handling balance during corner entry and exit as the load on each tire changes. Compression and rebound both affect transitional handling balance, though differently.

Aero downforce – especially in faster corners.

Driver – When and how abruptly a driver uses the controls – brakes, throttle, steering – can affect handling balance.

Weight Transfer

Weight transfer’s effect on handling balance is profound. How much weight is transferred, where is goes and how quickly it gets there are all major factors. Weight transfers forward under braking, to the rear during acceleration and laterally to the outside during cornering. How much weight is transferred depends on the weight of the vehicle, the height of the center of gravity above ground, the track width of the car and the force being applied to the platform by the tires. Maximum weight transfer occurs when the car is being driven at the limits of tire traction, especially while braking and cornering. When weight transfer occurs is dependent on the driver and when the driver uses the controls. How quickly weight is transferred depends on shock absorber valving. Check out previous Speed News article on Weight Transfer.

What Springs, Antiroll Bars and Shocks Do

Springs

The job of the springs is to keep the tire contact patches on the racing surface over bumps. Springs also keep the chassis from bottoming out on the track surface or the bump stops. The bumpier the track the softer the springs should be. Cars generating high amounts of downforce need stiffer springs to keep the chassis from bottoming. Springs also contribute to roll resistance but controlling chassis roll is not the primary job of the springs.

Antiroll Bars

As the name implies, antiroll bars are designed to control chassis roll when cornering. Stiffer antiroll bars reduce roll, but do not affect the amount of weight transfer. They do affect where weight is transferred. Stiffen the front antiroll bar compared to the rear and more weight is transferred to the front tires. A stiffer rear bar transfers more weight to the rear. Stiffer front bars increase understeer — or reduce oversteer— while stiffer rear bars increase oversteer — or reduce understeer.

Shock Absorbers

Shocks have two important jobs. First, as the correct name for a shock absorber implies — vibration damper – a shock will dampen oscillations or bouncing of the springs. Ideally the shock will limit the bounce to one cycle, which minimizes extra unwanted weight transfer. Aerodynamics are affected by bouncing as well. Controlling spring oscillations especially over bumps and ruts is handled by the high-speed valving within the shock body or remote reservoir.

Shocks also control how quickly weight is transferred from front to rear during acceleration, rear to front under braking and from inside to outside while cornering. This can have a major affect on transient handling balance as we will explore a little later.

Center of Gravity

The center of gravity is the point on a vehicle where, if suspended in the air, the car would be in perfect balance. If the car has equal 50-50 weight distribution front to rear and left to right, the center of gravity would be in the exact center of the vehicle at some distance above the ground. And it is that distance above the ground that affects traction and handling. The higher the center of gravity above the ground the more weight will transfer during cornering, braking and acceleration. The center of gravity height is mostly a function of car design, but the location can be lowered by lowering the vehicle and by mounting components like fuel cells, batteries and other items lower in the car. The driver’s seat can also be lowered to help lower the center of gravity.

Roll Couple Distribution

When a vehicle is cornering, weight transfers from the inside to the outside of the vehicle. Vertical load is reduced on the inside tires while the same amount of load is added to the outside tires. This occurs independent of body roll, which has a negligible effect on weight transfer. The question becomes how much weight is transferred on the front compared to the rear? The roll couple distribution determines the answer to that question. Roll couple distribution determines how much load (weight) is transferred at the front vs the rear of a car. Roll resistance is provided by the springs and antiroll bars. Stiffen the front springs and/or antiroll bars and more weight is transferred to the front outside tire. Increase rear spring rates and/or antiroll bar rates and more weight is transferred to the rear tires. If a car is perfectly neutral during steady state cornering, increasing rear roll resistance will cause the car to oversteer. Increasing front roll resistance will cause the car to understeer. The opposite happens when roll resistance is reduced at one end of the car. Soften front roll resistance and understeer is reduced or oversteer increased. Soften the rear roll resistance and oversteer is reduced or understeer is increased.

Roll couple distribution determines how much load (weight) is transferred at the front vs the rear of a car. Roll resistance is provided by the springs and antiroll bars.

Roll resistance is increased when springs or antiroll bar rates are increased. Since springs and antiroll bars both affect roll resistance, the big question is which do you change in the quest for handling balance? Spring rates need to be stiff enough to keep the car from bottoming out on the race track or on the shock absorber bump stops, but soft enough to keep the tire contact patches on the road surface over bumps. Bumpy tracks require softer springs to accomplish this. High-downforce cars require stiffer springs so that aero balance is not effected too much. Antiroll bars are designed to limit body roll. But they make the perfect tool for adjusting the oversteer/understeer handling balance in steady state cornering. Springs rates should be determined by the smoothness, or bumpiness, of the racing surface. Antiroll bar rates should be stiff enough to limit body roll to control camber change. Then the antiroll rates can be fine-tuned for optimum handling balance.

Spring Rates, Wheel Rates, Motion Ratios and Suspension Frequencies

While knowing the rate of a spring is important, knowing what the spring is doing at the tire contact patch is critical. All suspension systems apply leverage to the spring. The amount of leverage affects the spring rate acting at the wheel. And the control arms applying the leverage also cause the spring to travel a different distance than the tire contact patch anytime the suspension moves. The difference between the amount of tire travel versus the spring compression or extension is called the motion ratio. If the tire travels one inch, but the spring is only compressing by 0.50 inches, then the motion ratio is .50. Since we have both a travel ratio and a leverage ratio, the actual motion ratio must be squared. Our example of one inch of wheel travel versus a half inch of spring travel means that the motion ratio must be squared to determine the wheel rate. The spring rate times the motion squared equals the wheel rate of the spring. In our example the motion ratio squared is 0.25. If we start with a 400-pound spring, the wheel rate of the spring is 100 pounds. Motion ratios and wheel rates are important when tuning a suspension system for optimum performance.

Suspension frequency refers to the rate of oscillation of the undamped springs on a vehicle. The rate of “bounce” is measured in cycles per minute or sometimes in cycles per second. Suspension frequencies provide a measure of overall spring stiffness on a vehicle. Stiffer springs overall will have a higher frequency, softer springs will have a lower frequency. The frequency range for track vehicles is in the range of 80 to 150 cycles per minute. For high downforce vehicles, the amount of downforce in pounds should be added to the spring weight at each corner — use the average lap speed for weight — which can be complex to calculate, or increase the suspension frequency to between 200 and 300 cycles per minute. The suspension frequency applies to any vehicle. The factors used to determine suspension frequency include the sprung weight of the vehicle and the wheel rates of the springs. The frequency of the front springs should be offset from the rear by about 10 percent. Which end is stiffer depends on the characteristics of the vehicle – rear drive, front drive all-wheel drive.

Calculating Suspension Frequencies

Here is the formula for calculating suspension frequencies.

Tire Temperatures

Unless you have sophisticated data acquisition and a crew of engineers to analyze and interpret the data, taking tire temperatures is the best tool for making setup decisions for improving handling performance. To understand what the car is doing, monitoring tire temperatures is the best way to gain insights into the forces affecting the performance of your vehicle.
Tire temperatures can provide information on:
• Front-to-rear handling balance
• Optimum tire pressures
• Optimum camber settings
• Overall traction
• If you have the best tread compound for your application

See the tire temperature article from a previous Speed News.

Tire pressures and temperatures go a long way toward indicating what’s really happening with the handling balance of your car.

The Driver

The driver is potentially the most sensitive instrument in the car. Conversely, the driver is most often the primary cause of handling issues.
Here are several areas the driver can cause problems:
• Overdriving, especially under braking.
• inducing wheelspin at corner exit.
• Scrubbing the tires, too much steering in corners.
• Abrupt use of controls.
• Ineffective lines into, through and exiting corners.

Feedback from the driver is crucial to get a good handling car. But driver feedback must be accurate and honest.

Feedback from the driver is crucial to get a good handling car. But driver feedback must be accurate and honest. One issue I have encountered in working with drivers on car setup is the lack of heat in the tires. If tire temperatures are just above ambient temperatures, then the driver may be going to slowly to really tell if the car has a handling problem. Most cars feel pretty good when the driver is driving a 5/10ths. The driver needs to be at or near the limits of traction to really determine the handling balance of the car. The driver can often offer input about the balance. A really good driver can distinguish between corner entry and exit, braking, and even individual corners. Excellent drivers can tell if the problem comes in under certain conditions such as when applying more throttle or while trail braking on slow corners.

Image courtesy of Brett Becker

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