The way we approach the development of bite is related to how our car is setup, how the suspension systems are designed, and how the race track is shaped. In Part One of this two-part series, we learned something about how tires produce and keep traction. Now, we'll learn how to put that knowledge to practical use to develop more forward bite off the corners.
The way we develop bite for...
The way we develop bite for better acceleration off the corners has evolved over the years. What most teams used to do, even 5 years ago, is not what's being done today. Let's explore the many different ways we can help get more bite out of our race cars.
The Setup Helps
The way in which we set up the car can help us get more traction off the corners on flatter racetracks. One thing we did some years ago is to split the rates of the rear springs so that the left rear spring is a higher rate than the right rear spring. When we accelerate, we transfer weight to the rear of the car. As that weight is applied, the rear springs must compress to absorb the added weight. If the LR spring is stiffer, then it will compress less than the RR spring and this will increase the amount of the total sprung weight supported by the RF and LR tires. This produces an increase in the crossweight percent, usually making the car tighter off the turns while under acceleration.
Now with the newer setups in both dirt and asphalt, we see more and more that teams are stiffening the RR spring rate. Bite suffers on the flatter tracks. It's useful to run the softest spring at the RR that will maintain the dynamic balance along with the stiffer front sway bars on asphalt cars. On dirt, we may reduce the amount of rear steer to compensate for the stiffer RR spring to get more bite.
When making changes to the spring rates, be sure to maintain a balanced setup. If you soften or stiffen the RR spring rate, the rear of the car will want to roll differently, creating an unbalanced setup. We must adjust the height of the rear Moment Center to compensate so the car will be balanced in the middle of the turns.
A torque arm is a device that...
A torque arm is a device that absorbs some of the engine torque when we open the throttle on exit off the corners. Various rates of springs and shocks can be used to adjust the resistance to rotation of the rearend.
The setup package in the car can have an affect 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 to prevent this condition.
As we have explained in the past, if a car is tight in the middle of the turns, we must add steering input to help increase the front traction to compensate. 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 and as power is applied, the rear tires suddenly lose all traction.
Many drivers will swear that the car is loose. 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.
The pullbar third link acts...
The pullbar 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 rearend can be utilized to produce several effects such as introducing rear steer and increasing the crossweight percentage while under acceleration.
Spring and Shock Split Rates
For flatter tracks, the method of using rear spring split does not need to be substantial to accomplish the goal. On Asphalt Late Model cars, a 10- or 15-pound split (RR less than the LR) does what is needed. A 25-pound or greater split may be too much for a coilover car and 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.
For the BBSS setups that are popular, but not necessarily needed, using less spring split (with the RR stiffer than the LR) helps provide bite and running even springs across the back is possible if you can raise the Panhard bar high enough to maintain a balance in the setup.
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. As the driveshaft rotates, the pinion gear (attached to the driveshaft through the Universal joints) engages the ring gear (attached to the axles through the differential) and tries to climb it. This applies a force that will try to rotate the rearend in a counter-clockwise direction when viewed from the right side. If the upper link, on a three link rear suspension, is angled with the front end lower than the rearend, the force that tries to rotate the rearend will try to make the third link more horizontal. This applies an upward vertical force to the front of the link when accelerating that tries to lift the rear of the car. When decelerating, the opposite occurs and the braking forces try to lift the rearend causing the car to be loose. Antisquat should be used in limited amounts.
To a lesser extent, splitting the rear shocks' compression rates will accomplish a similar effect while the shocks are in motion and adjusting to the transfer of weight upon initial acceleration. This effect is very short lived, but can help to compensate for the shock to the tires that comes from the initial application of power. We would increase the compression rate in the LR shock and lessen the compression rate of the RR shock to accomplish this effect.
We have learned that traction can be better maintained if we can decrease the amount of torque that reaches the rear tire contact patches that comes from the initial application of power. Doing this helps the tires adjust to the transition of forces from lateral to longitudinal.
When we're at mid-turn, the lateral forces will be resisted by the tires at the contact patch and all four tire contact patches will be at the limit of lateral adhesion if we're going as fast as we can go 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 that is transferred to the rear tires through the driveline at that same time, we can help the rear tires maintain their attachment to the racing surface.
We do this using devices such as a pullbar or a lift arm. These items momentarily transfer the torque to themselves instantly and effectively at the very moment the throttle is applied.
The Traction Circle theory of tire technology tells us that there is only so much traction available from a particular tire and its contact patch, no matter what direction the forces are directed. The actual number in pounds of resistance is based on the size of the tire contact patch, the adhesion properties of the compound itself as well as that of the track surface, the amount of load on the tire, and the tire slip angle, or angle of attack relative to the direction of travel of the car.
The tire needs to be able to transition from the one direction of resistance (lateral, which is the resisting of the centrifugal forces which 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 surface of the track.
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. If the right trailing arm is mounted higher than the left-side arm, then as the rearend rotates, the LR tire will move to the rear more so than the RR tire resulting in rear steer to the left.
If this transition happens too quickly, the tire is "shocked" and will most likely break loose. This is very 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 pullbar or lift arm can absorb some of the torque going to the tires during initial application of power. By being able to move, these devices will absorb of 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 in the track surface mean more spring rate is needed in the devices. Slicker track conditions require less spring rate and more travel for increased torque absorption.
With the left side mounted...
With the left side mounted lower, the distance the axle moves rearward is greater by a small amount than the right side. In tests and through calculations, we arrived at a movement of 0.050-0.060 inch of movement of the left over the right. This provides just enough bite to help generate more traction without causing the car to be too tight and push.
Antisquat is a geometric suspension design that utilizes the torque that is transferred to the rearend and tries to rotate the differential. On a three-link car, the third link (upper link mounted above the center of the rearend housing) can be mounted at an angle with the front mount lower than the rear mount so that 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 rearend can't move vertically, the front mount can exert an upward vertical force that resists the squat that comes from weight being transferred to the rear under acceleration.
Antisquat enhances rear traction in two ways, first by helping keep the rear of the car higher and with that the center of gravity. This promotes more load transfer. And second, by keeping the rear spoiler higher due to less squatting.
Load transfer is directly related to the height of the CG, and the higher it is, the more load transferred we have to the rear under acceleration. So, a higher CG promotes traction as more load is transferred while under acceleration. Along with that, a higher rear spoiler catches more air and produces more aero downforce and drag for added grip at the rear tires.
There is no truth to the theory that the third link produces mechanical downforce on the rear tires through the rearend. Any force put on the rearend by virtue of the link wanting to straighten out is offset by the reduced compression in the springs and the tradeoff is even. For every action, there is an equal and opposite reaction. We can't pull weight from out of the sky, so as we stated, all added load comes from increased load transfer from the higher CG and/or more aero downforce from a more efficient rear spoiler.
Contrary to popular belief,...
Contrary to popular belief, pinion angle shouldn't be measured relative to the ground. The important measure is the difference in angle between the pinion shaft and the driveshaft. Excess angle here and at the transmission shaft end of the driveshaft causes a mechanical reduction in the amount of power that reaches the rear wheels. The two angles should be nearly equal and opposite in direction in order for the system to work efficiently.
Asphalt cars merely need to maintain a straight ahead attitude when cornering, but dirt cars must steer the rearend to the right on dry slick tracks to develop a more sideways attitude which 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 would need 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 came time to get back into the throttle, the tires had already lost traction and all that happened was that the tires spun producing little forward bite. With the advent of radical rear steer geometry, the cars will now roll over, the rearend will steer to the right to point the car, and the rear tires will maintain grip with the track surface and be ready to provide forward bite when the driver gets back into the throttle.
The improved technology related to the front end geometry and moment center has helped the dirt cars develop better grip in the front and we now see less rear steer, especially with the tighter conditions. This improves mid-turn speed and helps promote bite off the corners.
A fairly new concept for added bite on asphalt involves a geometric design that will produce 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 pullbar, lift arm, or other similar device that allows the rearend to rotate under acceleration, we can steer the rearend to the left only while the car is accelerating.
As the rearend rotates under power with the pinion moving upwards, the whole rearend will move rearward. If the lower control arms are mounted different distances from the axle, then one side of the rearend 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 and this results in rear steer to the left which will tighten a car off the corners.
Be sure to maintain the correct trailing arm angle so that the rear steer from body roll doesn't negatively affect the car. We would move the front end of the arm the same amount as we move the rear in this process.
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. The driver is Kevin Weaver and we can see that as the throttle is applied (purple line), the pullbar (red line) begins to extend. Kevin initially applies about 30 percent throttle and then gradually increases throttle all of the way down the straightaway to 100 percent near the end. The engine rpms (black line) never run up, meaning that the rear tires never break loose. The pullbar has controlled the engine torque and the driver has used proper throttle control. This was a very fast lap.
It's not widely known, but some top dirt racers have in the past adjusted their car's 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 that teams have of reducing engine output. We've 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.
The Educated Foot
When all available and useable methods of promoting traction control have been applied, the car may still be difficult to apply power to without losing rear traction. In that case, it comes down to drivers using their skills to help prevent loss of rear traction 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/she can't apply full throttle without the rear tires spinning, then they 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's a story told that the late and great Dale Earnhardt was at a test at the Richmond racetrack years ago along with many other teams, one in particular that was having unknown problems and going slow. The struggling team's owner knew Dale and asked him if he would take the car out and see if he could determine what might be the problem. The car had a data recording device installed and one of the functions that the system showed was throttle travel.
Some rear suspension systems...
Some rear suspension systems are designed so that any lateral movement of the rearend causes considerable rear steer. If you need to tighten the car, you can cause lateral movement of the rearend during body squat and roll by reversing the angle of the Panhard bar. This means that the right side, for a right-chassis-mounted Panhard bar, would be mounted lower than the left side. This moves the rearend to the right in the corners and off the corners when the right side of the car moves lower.
Dale promptly went out and ran a full second quicker than the usual driver. 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 the New York skyline, straight up and down, or quickly on and off the throttle. He was off the throttle much too quickly going into the corner and had to wait too long to get back into the throttle off the corners until the car was more straight to keep from spinning the tires as he mashed the gas. This example best defines driver induced Traction Control.
Improving traction off the corners is mostly about three things:
Balancing the setup and geometry so that the rear tires are always griping the racetrack so the car is not tight leading to the "tight/loose" syndrome.
Applying one or more traction methods to enhance weight distribution and overall mechanical grip off the corners as needed. This includes reducing the shock to the tires when applying the throttle.
Learn to recognize the amount of traction that is available and help the driver to know when you have done all that is mechanically possible to enhance forward bite. At that point, it's up to the driver to operate the throttle correctly to help 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 will maximize performance through enhanced traction off the corners. Not only will your lap times get better, the car will be more competitive when trying to get by lapped traffic or when passing for position.