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.
We can use the engine torque...
We can use the engine torque to our advantage by mounting the rear control arms in a certain way. If the upper link on a three-link rear suspension is angled, with the front end lower than the rear end, then the force that tries to rotate the rear end will try to make the third link more horizontal. This applies an upward vertical force to the front of the link that tries to lift the rear of the car. When decelerating, the opposite occurs and the braking forces try to lift the rear end.
We have learned that traction can be better maintained if we decrease the amount of torque from the initial application of power that reaches the rear tire contact patches. Doing this helps the tires adjust to the transition of forces from lateral to longitudinal.
At mid-turn, the lateral forces are resisted by the tires at the contact patch, and all four tire contact patches are at the limit of lateral adhesion if we are going as fast as we can without sliding. In more simple terms, the tires at that point are about to give up and slide. If we can reduce the initial shock transferred to the rear tires through the driveline when applying power, we can help the rear tires maintain their attachment to the racing surface.
We do that by using various mechanical devices that move as the rear end wants to rotate with the application of power. The rear end, when viewed from the left side, desires to spin or rotate clockwise when we gas up the car. A lift arm, pull bar, or similar device will absorb some of the torque caused by the acceleration isolating that force from the tires.
The traction circle theory of tire technology tells us there is only so much traction available from a particular tire and its contact patch, no matter what direction the forces are coming from. The actual net force of resistance is based on the size of the tire contact patch, the adhesion properties of the tire itself versus the track surface properties, the amount of load resting on the tire, and the tire slip angle, or angle of attack relative to the direction of travel of the car.
The angles of the four bars...
The angles of the four bars on a dirt Late Model car determine the attitude of the rear end as the car goes through dive and roll in the turns and off the corners. There is a science to setting these angles for different conditions. Most top teams are getting less and less radical with the trailing arm angles.
The tire needs to transition from one direction of resistance (lateral, which is resisting the centrifugal forces that are at right angles to the direction of travel) to the other (longitudinal or inline acceleration associated with application of power) over a longer period of time in order to maintain grip with the track.
If this transition happens too quickly, the tire is shocked and will most likely break loose. This is obviously detrimental to performance because in order to recover the grip in the rear tires, we must back off the throttle and allow the tires to reattach themselves to the track surface. This takes a lot of time, and we lose a lot of ground in the process.
The pull bar can also absorb some of the torque going to the tires during initial application of power. By being able to move, these devices absorb some of the torque of the motor for a short period of time, usually long enough to allow the tire to adjust to the new direction of force.
We can experiment with different rates of springs and shocks in these systems to adjust to and perfect the traction enhancement for different conditions. Higher amounts of grip on the track surface necessitates more spring rate in the devices. Slicker track conditions require less spring rate and more travel for increased torque absorption.
Antisquat is a geometric suspension design that utilizes the torque transferred to the rear end and tries to rotate the differential. On a three-link car, the third link (upper link mounted above the center of the rear end housing) can be mounted at an angle, with the front mount lower than the rear mount.
We can cause the car to produce...
We can cause the car to produce rear steer to the left to tighten the car while accelerating off the corners for more forward bite by staggering the heights of the trailing links.
When the car is accelerating, the force caused by the pinion gear trying to climb the ring gear causes the link to try to straighten out. Since the rear of the link that is mounted to the rear end cannot move vertically, the front mount will exert an upward vertical force that resists the squatting that comes from added load being transferred to the rear under acceleration.
Antisquat enhances rear traction in two ways: by keeping the rear of the car and the center of gravity (CG) higher, and by keeping the rear spoiler higher and more in the air stream.
Load transfer is directly related to the height of the CG, and the higher it is, the more weight transferred to the rear under acceleration. So a higher CG promotes traction as more weight is transferred. Along with that, a higher rear spoiler catches more air and produces more aero downforce for added grip at the rear tires.
There is no truth to the theory that the third link produces added mechanical downforce on the rear tires through the rear end. Any pressure put on the rear end by virtue of the link wanting to straighten out is offset by the reduced compression in the springs, and the trade-off in total load is even.
For every action, there is an equal and opposite reaction. We cannot pull weight from out of the sky, so as we stated, all added load comes from weight transfer and/or more aero downforce from a more efficient rear spoiler.
If the right trailing arm...
If the right trailing arm is mounted higher than the left-side arm, then as the rear end rotates, the LR tire will move to the rear more so than the RR tire, resulting in rear steer to the left.
Asphalt cars merely need to maintain a straight-ahead attitude when cornering. Dirt cars must steer the rear end to the right on dry-slick tracks to develop a more sideways attitude that will point the car in the right direction to be better able to get off the corners. In past years, the drivers on dry-slick tracks needed to throw the car sideways in the turns and, in the process, break the rear tires loose in order to point the car.
When it was time to get back into the throttle, the tires had already lost traction and just spun, producing little forward bite. With the advent of radical rear steer geometry, the cars now roll over, the rear end steers to the right to point the car, and the rear tires maintain grip with the track surface and are ready to provide forward bite when the driver gets back into the throttle.
A fairly new concept for added bite on asphalt involves a geometric design that produces rear steer upon application of power. We can utilize a certain type of rear suspension to create rear steer only under acceleration. In the three-link rear suspension system, if we use a pull bar, lift bar, or other similar device that allows the rear end to rotate under acceleration, we can steer the rear end to the left while the car is accelerating.
As the rear end rotates under power with the pinion moving upward, the whole rear end will move rearward. If the lower control arms are mounted at different distances from the axle, then one side of the rear end will move farther than the other. If the left-side trailing arm is mounted lower than the right-side trailing arm, then the LR wheel will move rearward more so than the RR wheel, resulting in rear steer to the left, which will tighten a car off the corners.
Data acquisition not only...
Data acquisition not only tells us exactly how our drivers use the throttle, but also how the traction control devices are working. In this example, we see a graph from a dirt Late Model car at a test at Eldora. We can see that as the throttle is applied (purple top line), the pull bar (red line) begins to extend.
Initially, the driver applies about 30 percent throttle and then gradually increases throttle all the way down the straightaway to 100 percent near the end. The engine rpm (black line) never spikes, indicating that the rear tires never break loose. The pull bar controls the engine torque, and the driver uses good throttle control.
It is not widely known, but some top dirt racers have adjusted their cars' engines to produce less horsepower when slick track conditions would not allow great amounts of torque and horsepower to be put to full use. Using smaller carburetors, adding restrictor plates, unhooking the secondary butterflies, or using electronic traction control that changes the timing or breaks down the ignition on one or more cylinders are all ways teams reduce engine output.
We have seen teams change to a smaller engine when they knew they were going to a traditionally dry-slick track. Again, anything we can do to help the rear tires maintain grip at all times will give us a better chance to apply the power available to accelerate the car
When all available and useable methods of promoting traction control have been utilized, it may still be difficult to apply power without losing rear traction. In that case, it comes down to the drivers using their skills to help prevent loss of rear traction when coming off the corners.
Many top drivers have perfected the art of throttle control to help maintain traction. This means that if the driver knows he or she cannot apply full throttle without the rear tires spinning, the driver will work to apply just enough power to accelerate without breaking the tires loose. This method applies to both dirt and asphalt racing and is much harder to master than most might think. Truth be known, many of our most successful drivers overcame less than perfect setups using this technique.
There is a story about the late and great Dale Earnhardt Sr. at a test at the Richmond racetrack years ago. Among the many teams there, one team in particular was having unknown problems and going slow. The struggling team's owner knew Dale and asked him to take the car out and see if he could determine the problem.The car had a data recording system, and one of the functions that the system showed was throttle travel.
Dale promptly went out and ran a full second quicker than the usual driver with the same setup. Later on, a close review of the throttle graph showed that Dale was rolling on and off the throttle, and the graph looked much like a roller coaster. The other driver's throttle graph looked like a group of large square buildings-straight up and down, or quickly on and off the throttle. He was off the throttle much too quickly going into the corner, and was waiting too long to get back into the throttle. He delayed using the throttle off the corners to avoid spinning the tires as he accelerated. This example best defines driver-induced traction control. It can be a major factor in reducing lap times. Improving traction off the corners is mostly about three things:
1. Balancing the setup and geometry so that the rear tires are always gripping the racetrack, ensuring that the car is not tight (leading to the "tight/loose" syndrome).
2. Applying traction methods to enhance weight distribution and overall mechanical grip off the corners as needed.
3. Learning to recognize the amount of traction that is available and helping the driver know when he or she has done all that is mechanically possible to enhance forward bite. At that point, it is up to the driver to modulate the throttle correctly to further maintain grip between the tires and the track surface.
Your driver may never be able to mash the gas and go, but as Scott Bloomquist once said, "My goal is to go wide open all of the way around the racetrack. I know that's not possible, but the closer I can get to doing that, the better I like it." Like Scott, learn to develop a legal traction control package that maximizes performance through enhanced traction off the corners. Your lap times will get better, and the car will be more competitive while you try to get by lappedtraffic or pass for position.