The No. 84 car driven by Shane Huffman in the USAR Hooters ProCup Southern Division handle
In the opinion of most circle track racers, good handling provides more performance than horsepower in most cases. Last month, we examined how to recognize basic handling problems and ways to correct them for corner entry and the middle of the turn. We believe mid-turn balance is the most important to overall handling performance. And entry and exit performance is very important for both overall performance and being able to race other cars and move forward through the field, even if that means passing lapped traffic. We have worked on mid-turn and entry problems, now let's tackle the exit problems.
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 of the turn 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 happens to be as loose off as you are.
A car that is loose on exit could have several problems. When we race on asphalt, this condition usually occurs on flatter race tracks and ones where the surface is worn. Some of these problems/fixes relate to both dirt and asphalt, but some will not cover both.
1.Shock Compression and Rebound Rates - If the left-rear shock is too soft on compression or the right-rear shock is too stiff on compression, the car will lose some amount of crossweight percentage momentarily as we get back into the throttle. The rear of the car and the shocks compress and the front shocks rebound. Usually, the left-rear shock should be at a higher rating for compression than the right-rear shock. This serves to increase the crossweight percent momentar-ily as the car accelerates and the rear of the car squats. Front rebound rates affect the amount of crossweight percent also. Many teams will increase the rebound rate of the left-front shock to loosen a tight car off the corners. This could be making your car too loose. Match the front rebound rates if a split is making the car loose.
There is a design for a three-link rear end that utilized a lift-arm or pull-bar where the
2. Rear Spring Rates - The combination of rear spring rates can affect the amount of traction we have on exit at the rear of the car.
If the left-rear spring is rated much less than the right-rear spring, the amount of weight supported by the right-rear and left-front wheels will increase as the weight transfers to the rear under acceleration with a corresponding decrease in the weight supported by the right-front and left-rear wheels. Since the right-front and left-rear wheel weights represent crossweight, this momentary movement reduces the crossweight and causes the car to be loose while under initial acceleration.
If possible, keep the rear springs even in rate or use a softer right-rear spring if traction is needed. In any event, make sure the setup is balanced and that changes to the spring rates are in conjunction with corres-ponding changes to the Panhard bar height to maintain a balanced setup.
The amount of rear spring split (right rear softer than the left rear) needed will vary with the type of car. A coilover late model asphalt car only needs about 10 or 15 pounds of split to get the job done. A dirt Late Model could use a 50- or 75-pound split to cause the car to roll over more dramatically with dry, slick conditions. This would cause a sufficient amount of rear suspension movement to enact the rear steer that is needed for those conditions.
A stock car with the metric 4-link rear suspension and big springs mounted in the stock location needs more split, up to 50 pounds of difference, in order to overcome the high rear moment center that is associated with that type of suspension.
The angle of the rear trailing arms is critical in determining the degree of rear steer yo
3. Rear Steer - The car could develop a loose condition if the rear suspension is designed to steer to the right as the car 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 under acceleration so that the new arm angles now produce rear steer to the right.
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 anti-squat also serves to limit the amount of rear suspension movement while the car is accelerating, which reduces the effect of rear steer.
4. 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 that of power-induced wheel spin. In this case, 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 percentage and that also produce rear steer to the left to help tighten a loose-off car.
5. Weight Distribution and Placement - If we feel comfortable with the balance of the car as well as the geometry of the front and rear ends, we might just be setup 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. 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 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 lost 2-3 tenths.
The pull-bar is just one device that racers use to reduce "shocking" the rear tires as the
A driver must feel that the rear of the car is under him/her so that they feel 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 we need the front geometry 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 simpler terms, when we increase steering wheel input. 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 right around the time of getting back into the throttle. The car is already going loose and then, bam, we gas it and then the car really goes loose. The driver comes in and swears the car is loose, but if we can recognize the tightness in the middle evidenced by the amount of steering input, then we will be able to apply the correct fix-loosen the car for a more balanced mid-turn.
7. Sway Bar Preload - One effect that serves to tighten a car that is loose off the corner is sway bar preload. It has been demonstrated many times that we can help provide more traction off the corners by adding preload to the sway bar. A caution here is in order. If we know the correct crossweight the car needs for mid-turn performance, then we need to arrange weight on the car after we preload the sway bar. Adding preload, especially when using a larger diameter bar, really adds to the crossweight percentage.
There is a correct amount of cross-weight percentage for every combination of setup and we
8. Racetrack Transition - The shape of the racetrack can affect how the car is balanced when exiting the turns. If the transition is abrupt and the top of the track drops to match the inside edge elevation, the right-front will follow the drop-off and unload the left-rear wheel. Shock rebound rates need to be adjusted to allow the left-rear tire to stay in contact with the racing surface.
If the left-rear shock has too much rebound, that tire will lose a lot of weight and not be able to provide traction. The right-rear tire will be the only one trying to accelerate the car and it will spin. Decrease the amount of rebound in the left-rear shock and/or soften the rebound in the right-front shock to help this situation.
9. Excess Rear Stagger - Every race car needs a certain amount of tire stagger (tire size difference in the rear tires to compensate for the turn radius so the rear wheel rpm will be equal at mid-turn) and no more or less. Excess stagger should never be used as a crutch to help make the car turn in the middle or while under power if it is tight. Doing this will probably make the car loose off the turns while under power.
Stagger only affects mid-turn handling if the car has a locked rear differential or a Posi-traction type of rear differential. With a Detroit locker, one of the axles (usually the left side) will be unlocked going in and through the middle and lock back up on exit while under power. So, with this style of rear end, stagger should match the radius of the turn where the car is starting to accelerate.
Many racers could say they wished they had this problem, but nonetheless, it does exist. This condition is many times tied to a geometry problem or possibly a shock package mismatch. It hurts our performance because as we accelerate off the corner, we might need to eventually lift to avoid the outside wall if the push is severe enough.
The front mounting bracket on a three-link rear suspension is adjustable to allow the team
If we are racing someone and get under them coming out of the turns, we would most likely be forced to lift to keep from pushing up into the outside car. Many cars have been taken out because the inside car did not lift when they should have. Here are some common reasons why the car might be tight on exit off the corners.
1.Front Shock Compression Rates - If the right-front shock is too stiff on rebound or the left-rear shock is too soft on compression, the right-front corner of the car will lose weight as the car accelerates, causing the left rear to also lose weight. The result is more weight distributed to the left front and right rear, which reduces the crossweight percent. This will obviously make the car loose initially as the car starts to accelerate.
2.Rear Spring Rates - The rear spring rate combination could affect the car to make it tight off the corners. Excess rear spring rate split with the right-rear spring rate less than the left-rear spring rate would make the car very tight off the corner while under power. In most cases, reduce the spring split until the desired affect is less than needed and go back up to the next level. If you have a 25-pound split in the rear springs, try a 15-pound split and if that produces enough bite, leave it there. The car must be neutral off the corners so that we can maneuver around other cars in the race.
3. Rear Steer - A small adjustment to the geometry of the rear suspension can reduce the degree of rear steer to make the car more neutral while under power. If the car squats as power is applied, it is possible that the added movement is making too much rear steer to the left, which tightens the car considerably.
With three-link rear suspensions, the arms are fairly short and a 1/4-inch height difference at the front of the arms can be felt by the driver. A 1/2-inch change can make the car undriveable. Make height changes to your trailing arms in small increments.
A "push-rod" trailing arm is designed to compress when the car is accelerating and this sh
4. Front Wheel Camber Change - If the front of the car raises up as we get back into the throttle, it is possible this movement will cause the front wheels to change camber quickly and lose traction on initial application of power.
In a test session we were running at Concord Speedway in North Carolina a few years ago, the car was developing a slight push just as the driver was accelerating off the corner. We could see the front end raise up evidenced by the increase in the gap between the top of the left-front tire and the top of the wheelwell. We pulled the front shocks and increased the rebound rates in both front shocks. That solved the problem. Since we maintained the rebound split between the shocks, we determined that it was the sudden change in the camber off the front wheels that caused the front of the car to lose grip.
5.Rear Stagger - If we do not run a sufficient amount of rear stagger for the radius of the turns, especially where the car is initially accelerating, the rear end will drive the car to the right and towards the wall. I have known teams who had otherwise nicely balanced race cars that could not get off the turns without heading for the outside wall. The problem was insufficient rear stagger. In one case, the driver/owner was a former road racer who did not believe in "large" amounts of stagger. He used 1/2-inch of stagger when the track really wanted 1 3/4 inches. It took him over a year to wise up and then he started winning races.
We have talked about rear stagger and what the car wants at various types of racetracks. Use those guides and do not tune the cars handling with stagger. Give it what it wants and some of your exit problems will go away.
When attempting to tune your car's handling balance at the racetrack, always start with the middle phase of the turns. Run the car at a moderate speed through the middle well below race speed and note how far the steering wheel is turned. Speed up and do a few hot laps and again note the position of the steering wheel. If the wheel is turned farther at speed, the car is tight and if it is turned less, it is loose. If you have to steer to the right at mid-turn, bring the car back in! That simple test has helped many teams quickly determine the status of their setups so they will quickly know which direction to go in tuning the mid-turn handling.
Next, tune the entry balance and then last, tune the exit balance. When all three phases are balanced, work on driver finesse and practice passing maneuvers running high and low off the corners. With the car set up correctly, it is just a matter of experience and a little racing luck that brings that first win.
Radius to inside tire = 200 feet or 2400 inches (12" x 200')
Radius to outside tire = 2465 inches (2400 + 65" track width)
travels 2465 x 2 x Pi (3.1416) 2 = 7744 inches
Inside tire travels 2400 x 2 x Pi 2 = 7540 inches
Outside tire = 85 inches in circumference
Inside tire = 85 x (7540 7744) = 82.75
Correct Stagger = 85 minus 82.75 or 2 1/4 inches.
Multiply by 2 to achieve circle's diameter. Multiply that number by Pi (3.1416) to determine circumference. Divide total by 2 to reach half circle. You can get the same result by multiplying inches x Pi.
The rear stagger should be matched to the racetrack and not used to correct handling problems. A particular car at each racetrack will require a certain amount of rear stagger. The track radius used to determine stagger matters the most where the car will be accelerating. On some tracks, with cars using Detroit locker rear differentials, the accelerating portion radius will be larger than the mid-turn radius. Therefore, less stagger should be used so that both rear wheels are turning the same rpm off the corner to avoid wheel spin.