Wrapped Up In Traction Control

Traction-limited acceleration is a condition almost every race car driver has experienced. It is the situation where the car’s ability to accelerate is limited by the available traction between the tires and racing surface. In other words, it does not matter how much horsepower you are putting to the tires if they cannot transfer that force to the ground and accelerate the car.

Sanctioning bodies have understood this for quite a while, and many enforce tire rules in an attempt to reduce tire and engine costs. The thinking is if the tires can only make use of a limited amount of horsepower, building a monster engine becomes counterproductive. However, these series may also allow some type of mechanical traction control device.

The two most common mechanical traction control devices are the pullbar and liftbar. Both are essentially spring-loaded replacements for solid control arms in the suspension that react to the drive torque applied to the axle.

How It Works

To understand how these systems work, it is best to first understand what we are trying to accomplish. Let’s start by examining what happens at the tire contact patch. As the car accelerates, there is a limit to the traction that it can put to the ground before spinning. If we can slow the application of torque to the tire, we can decrease the chances of overpowering it and losing traction.

Working our way to the engine, the torque applied to the axle by the drive force on the tire must be opposed by a torque in the opposite direction from the chassis to the axle housing. Said more simply, you’ve got to lock the axle down so it won’t flop over backwards when torque is applied to the tire. At any instant, the amount of torque at the tire must equal the amount of reaction torque at the axle housing. Remember Sir Isaac Newton’s “Every action has an equal and opposite reaction” (also known as the Third Law of Motion)? If you have X pound-feet applied to the tire, you also have X pound-feet trying to spin the axle housing in the opposite direction. Knowing this, we can slow the application of torque to the tire by slowing the application of torque from the chassis to the axle housing. We accomplish this by transferring the force through a spring-loaded bar. In order to increase its force applied to the axle housing, the spring must deflect. This deflection takes time, which slows the application of torque to the tire.

The deflection of the pullbar or liftbar spring causes the axle housing to rotate around the axle. This is called wrapup. The spring is typically adjustable for preload. Preload on the spring creates a torque on the axle housing while the car is at rest. During acceleration, wrapup will not begin until the applied torque is equal to the preload torque. The system’s wrapup stiffness is the amount of torque per degree of wrapup. It is the wrapup stiffness and preload that determines how your mechanical traction control system will perform.

Adjustments

Wrapup stiffness and preload should be adjusted to suit your engine, gearing, chassis setup and, of course, track conditions. The nature of this very dynamic system makes it impossible to reliably recommend ideal settings. The best way to find a starting point is to ask a few trusted racers or your chassis builder.

The spring bar acts like a solid bar until the load is greater than the preload on the spring. As the torque on the tire becomes greater than the preload torque, the axle housing begins to wrap up. The rate and total amount of wrapup (degree of twist) depends on the wrapup stiffness.

If the preload or wrapup stiffness is too low, the axle housing will be allowed to move too soon or too much, and the drive force at the tire will increase at a rate slower than the frictional capabilities between the tire and track. As a result, the tires won’t spin, but acceleration also isn’t maximized. For example, if there is zero preload, the drive force starts at zero and increases with wrapup. Obviously, the drive force will develop much more slowly than the ability of the tire to carry the load to the racetrack, limiting the car’s acceleration.

If the preload or wrapup stiffness is too high, the drive force at the tire is produced at a rate faster than the frictional capabilities between the tire and track. This creates a potential for wheelspin and a loss of traction. However, in this case, a good driver can use the throttle to control the application of torque and maintain traction.

If you want to try something a bit more advanced with your traction-control system, you can experiment with a progressive spring. Progressive springs increase their rate as they are compressed. As a result, the wrapup stiffness increases as the axle housing experiences wrapup. In some situations this setup may be advantageous. Just remember that increasing preload on a progressive spring also increases the stiffness.

Damping

Like any spring-mass system, the wrapup action must be dampened to reduce oscillation of the system and slow the recoiling action when power is dropped or brakes are applied. Typically, more damping is needed during recoil while the spring is releasing the energy it stored during wrapup. Also, if the brake caliper is mounted solid to the axle housing, brake torque will accelerate the recoil action of the housing, which must be controlled. For these reasons, dampers with wide split ratios are commonly used.

The term “traction control” implies there is feedback into a system that knows how much wheel spin exists at any given time and the control system reacts to produce the desired result: zero tire spin. By this definition, mechanical traction control devices are not “traction control” devices at all. Truthfully, they are “traction enhancing devices.” The system merely slows the application of torque to the tires, which helps to match the demand for traction forces with the tires’ ability to supply it. The difference with a mechanical system is the driver can still overpower the tires and lose traction.

Experience is your greatest weapon in tuning a traction-control system. Getting a feel for the torque curve of your engine, and the traction limitations between your tire and racing surface is irreplaceable.