We need sufficient forward...
We need sufficient forward bite right from the get-go. On this restart, the inside car spins his tires while the yellow No. 1 car pulls away to eventually win. Photo by Todd Ridgeway
Having superior forward bite is oftentimes the difference between winning and, well, not winning. On asphalt and especially dirt, getting in and through the middle is very important, but being able to apply a lot of power off the corners helps us pass to gain position and also to stay ahead of the competition when we are leading.
Mid-turn speed is all about the setup. Having the ability to power off the corners without spinning your tires is many times another story altogether. Our search for more bite off the corners starts with a good, balanced setup like the one we discussed in Part 1 of this series. Once we have achieved that, we work on developing additional bite off the corners. We need to accomplish that without ruining the hard work we went through to improve the mid-turn handling.
The path we need to take to develop more traction while under power is related to how our car is set up, how the suspension systems are designed, and the shape of the racetrack. In Part 1, we learned something about how tires produce and keep traction. Now we will learn how to put that knowledge to practical use to develop more forward bite off the corners.
The Setup Is Very Important The way we set up the car can help us get more traction off the corners on flatter racetracks. We can split the rates of the rear springs so that the left-rear (LR) spring has a higher rate than the right-rear (RR) spring. When we accelerate, we transfer load to the rear of the car. We can cause that load to be redistributed so that the amount of bite is increased.
A torque arm, or lift bar,...
A torque arm, or lift bar, is a device that absorbs some of the engine torque when we open the throttle while exiting the corners. Various rates of springs and shocks can be used to adjust the resistance to rotation of the rear end.
As that load is applied, the rear springs will compress to absorb the added load. If the LR spring is stiffer, it will compress less than the RR spring, increasing the amount of the total sprung weight supported by the RF and LR tires. This produces an increase in the crossweight percentage, or what we might otherwise refer to as bite or LR weight, usually making the car tighter off the turns only while under acceleration.
When doing this, we want to be sure to maintain our balanced setup. If you soften the RR spring rate, the rear of the car will want to roll more, creating an unbalanced setup. We must raise the rear moment center (MC) to compensate so the car will not be overly tight in the middle of the turns. We do this by raising the Panhard bar (or J-bar). With leaf-spring rear suspensions or the stock four-links, changing the rear MC is more difficult.
For those cars, the rear MC is already quite high, so splitting the rear springs with a softer RR is necessary just to balance the mid-turn desires of the front and rear suspension. This is very convenient because we then have more forward bite without having to work too hard at it.
The setup package can have an effect on how the tires adapt to the application of power. Most of the time, if we can keep the car from being overly tight on entry and through the middle of the turns, we can avoid the all too common tight/loose condition that causes a car to be loose off the corners. A balanced setup helps prevent this condition.
The pull bar third link acts...
The pull bar third link acts much the same as the torque arm by extending under acceleration, which serves to soften the application of torque to the rear wheels. The rotation of the rear end can be utilized to produce several effects, such as introducing rear steer and increasing the crossweight percentage while under acceleration.
As we have explained in the past, if a car is tight in the middle of the turns, we must compensate by adding steering input to help increase the front traction. Then, as we pass mid-turn, the added steering generates more than enough front traction to overcome the tight condition and the car begins to get loose. All of this usually happens right about the time we start to get into the throttle. As power is applied, the rear tires suddenly lose all traction.
Many drivers swear that the car is loose when, in fact, it is tight. We need to learn to recognize this tight/loose condition so that proper adjustments can be made to the setup of the car for a more balanced mid-turn handling package. This condition is responsible for a major number of loose-off problems. Professional teams that use data acquisition systems monitor the steering input and can see that excess steering is being used. This helps them diagnose the tight/loose condition.
For most applications, the rear spring split (softer RR spring) does not need to be substantial to accomplish the goal. On asphalt and even the dirt Late Model cars, a 10- or 15-pound split does what is needed. A split of 25 pounds or more may be too much for a coilover car and will cause an unbalanced setup that would be far too tight into and through the middle of the turns. For cars with big springs in the rear and a metric four-link suspension, a larger split is sometimes needed to overcome the high rear moment center we discussed.
To a lesser extent, splitting the rear shock compression rates accomplishes a similar effect. This happens only while the shocks are in motion and adjusting to the transfer of load upon initial acceleration. This effect is very short-lived, but can help reduce the shock to the tires that comes from the initial application of power. Increasing the compression rate in the LR shock and/or reducing the compression in the RR shock accomplishes this effect.