Getting the car to stick is...
Getting the car to stick is a never-ending quest in every form of oval track racing.
Editor's Note: Second of a two-part series
The path to developing more traction while under power is related to our car's setup, suspension system design, and racetrack shape. Last month, we learned 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 way in which we set up the car can help us get more traction off the corners on flatter racetracks. One step we can take 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 left rear spring is stiffer, it will compress less than the right rear spring and this will increase the amount of the total sprung weight supported by the right front and left rear tires. This produces an increase in the crossweight percentage, usually making the car tighter off the turns while under acceleration.
When doing this, be sure to maintain a balanced setup. If you soften the right rear spring rate, the rear of the car will want to roll more, creating an unbalanced setup. We must raise the rear moment center to compensate so the car will not be overly tight in the middle of the turns.
The setup package in the car 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 to prevent this condition.
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 rear end.
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. As power is applied, the rear tires suddenly lose all traction.
Many drivers will swear 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.
For most applications, rear split does not need to be substantial to accomplish our goal. On asphalt Late Model cars, a 10- or 15-pound split does what is needed. A split of 25 pounds or greater may be too much for a coil-over 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 4-link suspension, a larger split is sometimes needed.
To a lesser extent, splitting the compression rates of the rear shocks 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 reduce the shock to the tires that comes from the initial application of power. We would increase the compression rate in the left rear shock over the right rear 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 at the initial application of power. Doing this helps the tires adjust to the transition of forces from lateral to longitudinal.
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.
When we are 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 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 at that same time, we can help the rear tires maintain their attachment to the racing surface.
The traction circle theory of tire technology tells us there is only so much traction available from a particular tire and its contact patch, the direction of the forces doesn't matter. The actual number in pounds of resistance is based on the size of the tire contact patch, the adhesion properties of the compound itself, the amount of weight 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 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 surface of the track.
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.
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 rear end in a counterclockwise direction when viewed from the right side.
The pull-bar 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 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.
Anti-squat is a geometric suspension design that utilizes the torque that is 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 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 rear end cannot move vertically, the front mount can exert an upward vertical force that resists the squat created by weight being transferred to the rear under acceleration.
Anti-squat enhances rear traction in two ways. First, it helps keep the rear of the car higher (as well as the center of gravity). It also keeps the rear spoiler higher because the rear of the car is higher.
Weight transfer is directly related to the height of the center of gravity (CG). The higher it is, the more weight transfer we have to the rear under acceleration. So, a higher CG promotes traction as more weight is transferred while under acceleration. Along with that, a higher rear spoiler catches more air and produces more aero downforce for added grip at the rear tires.
If the upper link on a three-link...
If the upper link on a three-link rear suspension is angled with the front end lower than the rear end, 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 when accelerating that tries to lift the rear of the car. When decelerating, the opposite occurs and the braking forces try to lift the rear end, causing the car to be loose. Anti-squat should be used in limited amounts.
There is no truth to the theory that the third link produces 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 tradeoff 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 weight comes from weight transfer and/or more aero downforce from a more efficient rear spoiler.
Asphalt cars merely need to maintain a straight ahead attitude when cornering. Dirt cars, on the other hand, 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 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 was time to get back into the throttle, the tires had already lost traction and spun, producing little forward bite. With the advent of radical rear steer geometry, the cars will now roll over, the rear end 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.
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 pull-bar, lift arm, or other similar device that will allow 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 upwards, the whole rear end will move rearwards. If the lower control arms are mounted 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 left rear wheel will move rearward more so than the right rear wheel and this results in rear steer to the left which will tighten a car off the corners.
It might be appropriate to mention another element related to geometry that affects traction off the corners and that is pinion angle. We know that excess pinion angle absorbs some of the engine forces, reducing power to the rear wheels. In the past, we have heard racers say they experienced better bite off the corners when they put more pinion angle in the car.
While it is probably believable that excess pinion angle reduces rear wheel tire spin, it is not because of any mechanical enhancement effect, but rather a reduction in the amount of power that reaches the rear wheels. If we reduce the amount of power that reaches the rear wheels, we also reduce the tendency for the rear wheels to spin. This is not a recommended procedure for adjusting power to the rear wheels.
It is not widely known, but some top dirt racers have 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 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.
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 rear end rotates, the left rear tire will move to the rear more so than the right rear tire, resulting in rear steer to the left.
When all available and useable methods of promoting traction control have been applied, you may still have difficulty applying power without losing rear traction. In that case, it comes down to the 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 if the driver knows he/she cannot 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 is 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 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 the system showed was throttle travel.
Dale promptly went out and ran a full second quicker than the usual driver. Later on, a close review of the throttle graph showed 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 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:1. Balancing the setup and geometry so that the rear tires are always gripping the race track, so the car is not tight leading to the "tight/loose" syndrome.
2. Applying one or more traction methods to enhance weight distribution and overall mechanical grip off the corners as needed
3. Learning to recognize the amount of traction available and helping the driver to know when you have done all that is mechanically possible to enhance forward bite. At that point, it is now 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 dirt Late Model star 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 Bloomquist, 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 past lapped traffic or when passing for position.
Contrary to popular belief,...
Contrary to popular belief, pinion angle should not be measured relative to the ground. It is the difference in angle between the pinion shaft and the driveshaft that is important. 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.
Tires are the ultimate connection between the car and the racing surface. That basic principle is not a new one, but a concept that has always been at the forefront when trying to understand ways to increase handling performance in a race car. It is again at the very top of the list when we discuss traction under power.
There are five elements that influence the amount of traction that a set of dirt or asphalt tires, the rears in this case, will develop
1. Vertical Loading-Increasing the amount of vertical loading (weight) on a tire increases the available traction, but in a nonlinear way. As we increase loading on a tire, it will gain traction, but not in exact multiples. If a tire has "X" amount of traction with 400 pounds on it, the traction will be less than double as we apply 800 pounds of loading to it. The amount of traction will be less than 2 times X.
2. Contact Patch - The size and cross-sectional loading of the contact patch helps determine how much traction we will have for a particular tire. An added area related to the contact patch and traction involves grooving and siping dirt tires and will be discussed later on.
Reducing the air pressure will usually increase the size of the tire contact patch. That would seem to enhance traction, but excessively low or high pressures may reduce the loading on portions of the tire so that the total loading of the tire is reduced and we end up with less available traction for that tire. There is an optimum operating air pressure for each tire that will offer maximum contact patch area and equal loading across the width of the patch.
Camber also affects the size and cross-sectional loading of the contact patch. The correct camber angle compensates for the deflection of the tire sidewalls as the lateral force is applied when we turn the car. More or less camber than ideal means one side of the tire will support more weight than the other, which also reduces traction.
3. Chemical Makeup - The chemical makeup of the compound of the rubber will help determine how much traction is available from a tire. A softer tire will provide more traction, but the maximum amount of traction that can be utilized over a long period of time depends on how the tire holds up to heat and wear. A tire that is a little harder may sometimes hold up better and be faster towards the end of the race when the tires have built up a lot of heat and are well worn after a number of laps.
4. Angle of Attack - The amount of traction available from a tire can actually be enhanced simply by increasing its angle of attack relative to the direction of the car, but only up to a point. From straight ahead, we can turn the wheel and, with each degree of angle of deviation from the direction of travel, the traction in the tire increases. There is a point we reach where the gain is reduced and we approach the limit of attack angle that the tire can handle. Once that point is reached, going beyond causes a sudden loss of grip and traction falls off drastically. This principle is true of all four tires whether front or rear. We will provide more on this subject later.
5. Equal Loading - An opposing pair of tires (tires on the same axle at the same end of the car) will develop maximum traction when they are equally loaded. That is a generally true statement, but upon more careful examination of how we do things in circle track racing, there is a unique situation where that is not exactly true.
The situation is when we have a tire on one side of the car (usually the left side) that is built with a softer compound than the opposing tire whereby it may be able to develop more grip under the same loading as the opposing tire. So, increasing the vertical load on the inside tire with the goal of attaining equal loading for both tires, by whatever means, may not actually generate more traction because of the difference in grip per pound of vertical loading created by differences in compounds.
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 with driver Kevin Weaver. We can see that as the throttle is applied (purple line), the pull-bar (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 rpm (black line) never runs up, meaning the rear tires never break loose. The pull-bar has controlled the engine torque and the driver used throttle control. This was a very fast lap.
The shape of the track for both dirt and asphalt can influence the available traction in several different ways. As we apply power, we need to know a little about how the track is banked, how the banking angle is changing coming off the corners, and how the radius of the turn might be changing. A highly-banked racetrack is very forgiving when it comes to needing bite off the corners. There is so much downforce due to the banking and associated lateral forces, that many times the tires are loaded to the extent that the available amount of horsepower cannot break the tires loose under normal conditions with a balanced setup. The tracks we often worry about getting off the corners are the ones that are flatter and with less surface grip.
The severity of change in banking angle of the racing surface in the portion of the track where we are initially accelerating can cause changes to the pitch angle of the chassis that works to unload one or more tires, reducing traction. A track that goes from high banking to low banking fairly quickly can cause the left rear tire to unload quickly, making the car loose.
There are two ways this can happen. One is when the outside edge of the track drops in elevation and the right front tire follows the drop-off. This lifts weight off the left-rear tire, causing loss of traction in that tire.
The other problem occurs when the inside edge of the track rises up to match the elevation of the outside edge of the track. As the left-front tire rises up, the left-front and right-rear pair of tires become more loaded, momentarily causing loss of loading in the opposing pair of tires. The loss of crossweight percentage (right front to left rear) makes the car lose traction in the rear.
A track that has a decreasing radius in the latter portion of one of the turns can cause a car to develop a loose condition at that point. Usually, older tracks that were originally dirt and then paved retain a straight front stretch and a rounded-out back "straightaway". This "D" shape causes Turns 1 and 4 to be a smaller radius than Turns 2 and 3 for that reason. So, it is difficult to accelerate from Turn 4 because of the decreasing radius.
Remember, we said traction increases for a set of opposing tires when we increase the angle of attack (simply put, this is when we turn the steering wheel more). If the car is neutral in and through the middle of the turns, then as we approach the tightest portion of the turn past midway, where the radius is less, we need to turn the steering wheel more and that produces more front traction than rear traction. The balance we enjoyed through the middle of the turn is now upset and the car becomes loose just when we are getting back in the throttle. This causes loss of rear traction. We will study ways to compensate for this later.
The surface largely determines the amount of traction available under power and we will look at dirt and asphalt tracks separately. On dirt tracks, the amount of moisture dictates the amount of grip the track gives us. Bumps, grooves, banking angles, and the overall radius all help determine how much grip is available for traction off the corners. The setup related to shocks, springs, and rear geometry help determine how much traction will be available for a certain set of conditions.
On asphalt tracks, and even some "dirt" tracks that have been oiled to the point of almost being asphalt, the surface is more consistent. Other than holes or bumps and rises in the surface, we can expect the grip to be the same over the course of the event. Flatter banking and older asphalt dictates the need for more traction control efforts.
Now that we have some kind of understanding of just what affects traction in the rear tires, we need to examine how we can use that information to enhance the tractive properties of the rear set of tires. Next month, we offer some suspension tuning suggestions for over-coming the problems some teams have getting enough bite off the corners.