In order to have the perfect...
In order to have the perfect exit, the entry and middle need to be correct. We want the car to be neutral, hooked up, and ready for application of all the power the track will allow.
In this age of improved engine systems that help us gain more horsepower and torque, what remains as one of the greatest challenges in circle track racing is the issue of handling. It is a generally accepted truth that speed gained in the turns is much more significant than any small legal gain in engine power.
Speed gained in the turns will be carried all the way around the racetrack. If you can gain 1 mph in each of the turns, then you will have gained 1 mph on the straights, too. That 1 mph equals a reduction in lap times of 111/42 tenths for a 16-second-per-lap track with an average speed of 100 mph. For most tracks and series, that represents the difference between the pole and Tenth Place in qualifying.
With the car already loose,...
With the car already loose, the application of power increases the problem. In testing, we ran different setups that were both loose and tight, and the tighter setups were faster through the middle and off the corners.
In Part One of this two-part series, we examined how to recognize basic handling problems and ways to correct them for corner entry and the middle of the turn. We believe that midturn balance is most important to overall handling performance. Entry and exit performance is also very important for both performance and the ability to race other cars and move forward through the field, even if that means passing lapped traffic. We have worked on midturn and entry problems. Now let's tackle the exit problems.
Loose on Exit Being loose off the corner is possibly the most detrimental handling problem we can have in an actual race. Many passes occur as the cars are exiting a turn. Other racers can recognize when you are having problems with bite off the corner and set you up by laying back a bit and making a run off the corners. You are forced high coming out and must steer more to avoid the wall. With a car that is already loose, that means backing off and letting the passing car go. This will happen with many more cars until one comes along that is as loose off as yours.
A car that is loose on exit could have several problems. When we race on asphalt, this condition usually occurs on flatter racetracks and on tracks with worn surfaces. Some of these problems/fixes relate to both dirt and asphalt, but some will not cover both.
There is a design for a three-link...
There is a design for a three-link rear end that utilizes a lift arm or pullbar where the LR spring is mounted in front of the rear end and the RR spring is mounted behind the rear end. When the car accelerates and the rear end rotates as the lift arm or pullbar moves, the LR spring compresses and the RR spring decompresses. This adds load to the LR wheel and takes load off the RR wheel. If this makes the rear tires more equally loaded, then the car will have more traction to help provide better bite off the corners.
1 Shock Compression and Rebound Rates
If the LR shock is too soft on compression or the RR shock is too stiff on compression, the car will lose some amount of crossweight percentage momentarily as we get back into the throttle and load transfers to the back of the car. With that, the rear of the car and the rear shocks compress and the front shocks rebound. To gain bite off the corners, the LR shock should be at a higher rating for compression than the RR shock. This serves to increase the crossweight momentarily as the car accelerates and the rear of the car squats.
Front rebound rates affect the distribution of weight also. Many teams will decrease the rebound rate of the LF shock to loosen a tight car off the corners. This could be making your car too loose off. Match the front rebound rates if a split is making the car loose.
2Rear Spring Rates
The combination of rear spring rates can affect the amount of traction we have on exit at the rear of the car. This could be especially significant with the asphalt big bar and soft spring setups. With the BBSS springs, the RR spring is usually much stiffer than the LR spring. As load transfers to the rear upon acceleration, the LR spring compresses more than the RR spring, and that de-wedges the car. To compensate for this, the LR shock should have a much greater compression rate than the RR shock.
Basic shock tech tells us we need more compression control for a soft spring, because the spring resists compression, too, and there is less spring to resist. For stiffer springs, we need much less compression because that stiff spring will resist compression so well.
The angle of the rear trailing...
The angle of the rear trailing arms is critical in determining the degree of rear steer your car will have. If the car squats excessively as power is applied, it is possible that the amount of rear steer will increase to make the car too loose or too tight off the corners.
For conventional setups, keep the rear springs even in rates or use a slightly softer RR spring if traction is needed. Whichever setup combination you choose, make sure the setup is balanced and that changes to the spring rates are in conjunction with corresponding changes to the Panhard bar height to maintain a balanced setup.
The amount of rear spring split (RR softer than the LR) needed varies with the type of car. A coilover asphalt Late Model car only needs about 10 or 15 pounds of split to get the job done. A dirt LM could use a 50- or 75-pound split to cause the car to roll over more dramatically with dry-slick conditions in order to create a sufficient amount of rear suspension movement to promote the rear steer that is needed for those conditions.
A stock car with the metric four-link rear suspension and big springs mounted in the stock location needs more split of up to 50 pounds of difference in order to overcome the high rear moment center that is associated with that type of suspension.
3 Rear Steer
Your car could develop a loose condition if the rear suspension is designed to steer to the right as the car rolls and squats on exit under full acceleration. The suspension links may produce zero steer under normal cornering attitudes associated with rolling and diving. That may change as the car squats after weight has transferred when the car accelerates so that the new arm angles now produce rear steer to the right.
The LR trailing arm has a...
The LR trailing arm has a significant effect on rear steer associated with excess squatting on acceleration. If the car squats on exit, that motion can be utilized to promote rear steer and increased forward bite. It can also cause the car to be loose. With the arm uphill (front higher), the left rear will move back, providing rear steer to the left and making the car tighter. If it starts out level, the squatting motion brings the left-rear wheel forward, causing rear steer to the right and making the car loose.
Adjust the angles of the rear control arms so that you will have near zero rear steer when the car is at the exit attitude. An increase in the amount of antisquat also serves to limit the amount of rear suspension movement while the car is accelerating, which reduces the effect of rear steer. This effect can be mistaken for added bite. In reality, it reduces unwanted rear steer to the right, fixing a loose-off condition.
4 Mechanical Traction Control
If the car is good through the middle and initial exit but loose off the corner as we get into the gas, then the problem might be one of power-induced wheelspin and we need to further develop our traction-enhancing program. The primary goal is to eliminate the "shock" that reaches the rear tires due to engine torque that comes from initial application of power. We can utilize systems that increase the crossweight and those that produce rear steer to the left to help tighten a loose-off car.
The cars with the metric four-link rear suspensions can benefit from mechanical traction control, too. I have heard of teams utilizing more solid bushings in the left-side connections at the chassis and the rear end, but leaving the stock rubber bushings in the right side. This provides less rear steer in the middle of the turns but introduces rear steer to the left upon acceleration as the right-side bushings compress.
5Weight Distribution and Placement
If we feel comfortable with the balance of the car related to the geometry of the front and rear ends, then we might be set up a little too loose with the crossweight percentage. The car might be able to handle more crossweight and still turn fine through the middle of the corner as long as the front geometry is correct. A car that is more "free" feels really nice through the middle, turning with almost no effort, but we need a slight amount of "tight" to assist our exit performance.
One driver I used to work with (who had won several championships) always drove faster, especially during the race, if the car was a little tight in the middle. He would say after a practice run, "She's pretty good, just a touch tight in the middle." When I would take that "touch" out of the car, he would slow down by 2-3 tenths.
The pullbar is just one device...
The pullbar is just one device that racers use to reduce the "shocking" effect at the rear tires as the driver gets back into the throttle. This Coleman-designed link can be preloaded. It will extend against the force of acceleration to absorb some of the torque. It also works in reverse to dampen deceleration forces on entry. Other types of devices for this use are lift arms and even leaf springs. The leaf type of suspension has a limited amount of wrapup from the torque of the engine and acts like a pullbar to some extent.
A driver must feel that the rear of the car is under him or her to feel more confident pushing the speeds in the turns and taking the car to the limits when passing other cars or holding off a challenge. That is exactly why the front geometry needs to be correct so that when we need to turn the car against a tighter setup, it goes where we point it.
6 Tight/Loose Syndrome
The limit to what was discussed above is when we set up the car to be too tight. We have learned that a tire will actually gain traction as we increase the angle of attack, or in more simple terms, when we increase steering wheel input. That means if we have a tight car in the middle of the turns, we can compensate by increasing the steering input and the front will gain traction to match the rear.
The problem with this is what we call the tight/loose syndrome. When the car is set up too tight, we can still possibly overcome that with even more steering input, but at some point, and this happens very quickly, the balance is reversed and the front ends up with more traction than the rear.
This almost always occurs just about the time when we are getting back into the throttle. So the car is already going loose, and then bam-we gas it up and then the car really goes loose. The driver comes in and swears that the car is loose, but if we can recognize the tightness in the middle, evidenced by the amount of steering input, then we will know the difference and be able to apply the correct fix, which is to loosen the car for a more balanced midturn.