This pavement car is well...
This pavement car is well balanced and able to drive right on the white line at the bottom of the racetrack. The consistency that a balanced setup provides will create the opportunity for winning. This team won this race.
Chassis setup, especially the tuning part at the racetrack, is largely a matter of experience. Over the years, the most sought after crew chiefs in stock car racing were the ones who have demonstrated the ability to make quick and accurate decisions needed to solve handling problems.
Today, as a direct result of a lot of research and development taking place over the last few years, we might look at these common fixes in a little different light. Basic chassis setup techniques are common to both dirt and asphalt racing.
The difference in the track surfaces creates special problems for each, but the raw basics remain the same. The following is a list of "dos and don'ts" related to creating a good, balanced setup and solving basic handling problems. We will note where differences may occur related to dirt or asphalt.
Unstable Entry-Many times a driver will feel very uncomfortable with entering a turn. The car just does not take a good set as we begin to brake and turn left. This feeling causes him/her to back off early and not go as deep into the corner as other cars.
At higher banked tracks, say 12 degrees or more, a setup utilizing a stiffer right front (RF) spring will tend to load the crossweight percent on entry as the car is braking and starting to turn left. That is because the LF will dive more than the RF, which will load the RF and LR tires momentarily. This feels good to the driver and gives him confidence knowing the rear end will stay under the car.
A stiffer compression setting on the RF shock can produce a similar weight and aid deeper entry into the corner by momentarily loading the cross, just like the stiffer RF spring does.
At flatter racetracks, especially on dirt, we have a very different cause and solution to the problem. When we brake going in the corner on a flat surface, we want the car to begin to roll in the same direction as it ultimately will in the middle of the turn. If we set up the car with a stiffer RF spring, the car will sort of flip-flop, first starting to roll left when braking and then right as we roll through mid-turn. This is sometimes visible to the observer watching from outside the car.
The solution is to run a stiffer LF spring. As we brake into the corner, the RF spring will compress more than the LF spring and the car will begin to roll to the right. As the car enters the mid-turn, it will continue to roll more to the right and the transition will be much smoother, enabling deeper corner entry.
Loose Entry-A car that is loose on entry to the corner can have several problems. If the car is severely loose and nothing seems to help, it is almost always rear end alignment that is the problem. Typically, the rear end is pointed to the right of the centerline of the car and wants to swing right as the car is steered left, much the same as a hook-and-ladder fire truck does when the guy in the back steers right going around a corner. The car will feel much the same as this. Regardless of how straight you think the rear end is, start moving the RR forward 11/48 inch at a time until the car is no longer loose-in.
At racetracks with a tight entry, and/or requiring heavy braking on entry, the loose condition is most likely brake bias or shock related. First, try moving the brake bias to the front. If that does not help, then decrease the rebound in the LR shock and/or increase the compression in the RF shock. A common fix to a tight car on entry used to be increasing the LR shock rebound to pull weight off the LR corner. That made both ends loose and corner speeds suffered.
Push on Entry-A car that develops a push on entry may have too much brake bias toward the front end. Another cause that would develop at the same point of heavy braking, and is hard to distinguish from bias problems, is when the shock/spring combination is too soft.
The crew chief can tell a...
The crew chief can tell a lot about the car's handling balance by observation. From this vantage point, he can observe how the car looks as it enters and negotiates the turns.
Notice that this upper control...
Notice that this upper control arm mount has anti-dive built in. Seen just above the upper control arm are the two mounting bolts. The front bolt (to the right in the picture) is higher than the rear bolt.
Special slotted upper control...
Special slotted upper control arm brackets are fitted with offset slugs that allow you to lower or raise the mounts. This car lowered the rear mount by 11/44 inch and raised the front mount by 11/44 inch to make a 5-degree angle in the control arm shaft for anti-dive.
A good quality spring rate...
A good quality spring rate tester is essential to finding your correct spring rates. Rate the spring at the correct installed height and not the first few inches. The spring will never be working at the first one or two inches. A 200-pound-per-inch rated spring will be compressed three inches at ride height with 600 pounds on it.
The tendency towards using softer springs on asphalt, and to a lesser degree on dirt, has increased recently and so this is a more common problem today. The primary culprit is the greater amount of dive on entry caused by soft springs, which creates excessive camber change. When the RF camber changes too quickly, that tire loses grip and the car begins to push. The best fix for this problem is to add anti-dive to both upper control arms. The best part of using anti-dive is that it has no negative affect on the rest of the setup. Touring dirt Late Models and touring-type asphalt Late Models have traditionally gone without anti-dive. That trend is changing and many teams are discovering the benefits of anti-dive for their short-track cars.
Anti-dive is created when the upper control arms are angled in relation to the ground with the front end of the pivot shaft higher than the rear end. Most upper control arms are mounted with two bolts that are six inches apart. On these types of arms, 11/42-inch difference in height of the two mounting bolts will produce a five-degree angle. This is sufficient anti-dive for the RF of the car. Use half that amount, or 11/44 inch of difference in height, for the LF upper control arm mount.
The lower control arms also contribute to anti-dive, although less so, and the opposite is true-meaning that the rear chassis mount should be higher than the front chassis mount.
Creating anti-dive in your race car should reduce the amount of dive on entry and the excess camber change that goes with it. Remember to keep the average heights of the mounting points the same to keep from altering the roll center location.
Loss of "Feel" for the Car-For a car to be fast, the driver must have a good feel for what the car is doing. A soft ride that feels sloppy does nothing to give a driver confidence in knowing what the car will do next.
Many racers think that increased compression in the shocks should help this problem, but too often that has an opposite effect. The car may indeed need more compression in the shocks, but the rebound should always be higher than the compression to control excess body sway. In most cases, just increasing the rebound resistance will solve the problem.
The angle of the trailing...
The angle of the trailing arms is adjustable by moving the front mount up or down. The greatest effect on the left rear corner is caused by increasing the angle to create rear steer as the car squats on exit off the corners.
For dirt cars where a lot of vertical movement is desirable to get more weight transfer on dry slick racetracks, reduced compression and rebound is helpful, and a lot of movement should be expected.
Tight in the Middle-The Car Won't Turn-This is the "A-number one" complaint for most ill-handling stock cars on dirt or asphalt. The problem usually lies with a combination of poor front-end geometry and an unbalanced setup.
Generally speaking, we want to first reduce or eliminate Ackermann effect in the steering system. We need to position our roll center so that the front suspension dynamically will work to make the LF tire work harder. As we move the roll center left, the front end becomes more efficient and turns better.
Finally, we need to balance the setup. What we have discovered is that a tight setup is usually related to how the front and rear suspensions are arranged. A front end that is stiffer than the rear end will produce an unbalanced car that is tight and won't turn the corner. A stiff RF spring and/or a soft RR spring will cause this imbalance.
Reversing the front springs and putting the stiffer spring on the LF is an old trick that a few modern day racers know about. [Although many dirt racers with 4-bar suspensions are doing this-Ed.] This is only good for lower banked racetracks, but really does help the car to turn.
A softer RR spring helps traction up and off the corners, but a 10 or 15 pound difference for asphalt cars is usually all that is needed to get the job done. More split (50-75 pounds) is required for dirt racing on dry slick tracks in order to produce the proper amount of rear steer, and to help load the LR tire. Excess rear spring split makes the car too tight.
This car has the panhard bar/track...
This car has the panhard bar/track bar mounted on a serrated mounting plate making small adjustments possible. Always mark a reference height.
Poor Forward Bite-At the higher banked racetracks, we usually do not have a problem with traction off the corners because of the downforce created by the banking. On many flatter tracks, we can never get enough bite.
Bite can be enhanced by several methods: a) soft RR spring (only 10-15 pounds split on asphalt and more on dirt, say 25-100 pounds split), b) use of a pull bar or lift arm to reduce the shock to the rear tires when we get back into the throttle, c) introduce rear steer as the car squats on exit. This is done using the rear trailing arm angles. On a three-link rear suspension, a higher angle (front pivot mounted higher) on the left trailing arm will push the LR back as the car squats, providing rear steer to the left that promotes forward bite.
On a four-bar rear suspension, the arms can be positioned to provide rear steer similar to the three-link as the car squats, usually in the higher holes.
Slow Off the Corner-Fast at End of Straightaway-A car that is sluggish off the corner and tends to be a rocket at the end of the straightaway is geared all wrong. The place to accelerate quickly is not at the end, but at the beginning of the straightaway for several reasons.
Using a lower gear to get off the corners will produce more increase in mph in the first half of the straight than a higher gear will produce in the last half of the straight.
An adjustable screw type panhard...
An adjustable screw type panhard bar mount, shown here, is mounted to the frame and is infinitely adjustable. The entire mount can be moved vertically to adjust the range for a particular application. This makes tuning the balance of the car more efficient.
Even if the rpm max out a little early, the driver can throttle back just before letting off for the corner without losing time. Being easier off the throttle is a good idea anyway and tends to help the transition into the corner. A car that lunges into the turn entry is very hard to control and keep on the bottom of the racetrack.
Fast Start-Slow Finish-From my observations, most stock cars finish a race much slower than they start out. Winning cars are the ones that lose less speed than the others. Consistency wins races. A consistent setup is one where the car is more balanced with both ends of the car, and all four tires, are working together.
Tire temperatures are the primary way to tell if your setup is balanced. Usually the LF tire is the coolest on the car. That indicates an unbalanced setup. Common fixes include: a) softening the front springs-reverse split in some cases, b) raising the panhard or J-bar, c) stiffen the RR spring-only on banked tracks, and/or reduce the "soft RR" spring split, d) move the front roll center to the left.
I purposely did not mention changing the crossweight percent. Reducing the crossweight percent will make a tight car more neutral, but that change alone will not balance the car dynamically to make the LF work harder and the car more consistent. It only serves to make an unbalanced car neutral for a short time.
As changes are made to the setup, adjust the cross weight percent to keep the car neutral in handling balance. When the LF tire temperature more closely matches the LR tire temperature, the setup will be balanced.
The practice of recording tire temperatures is not that common for dirt track racing, so many of those teams look at tire wear to help judge how each corner is working. Use tire wear in a similar way to tire temperatures, the tire with little wear needs to do more work. Be sure that excess Ackermann is not artificially adding heat to the LF tire. That can give a false indication of a balanced setup.
Solve your handling problems in the order that we have presented them here-turn entry to end of straight-away-and your car will produce the consistency that wins races.