In Part One of this series, we learned about the early research in chassis dynamics and something about the individuals and companies that were involved. In this final part, we will take a look at modern-day setups and see where we can apply the technology we have learned.
We have presented all of the elements that influence the chassis setup as far as balancing the two suspension systems. We discussed the moment center (previously called the roll center), how the moment arm works, and what the differences are between the front and rear suspension systems.
We explained how to make the car balanced by predicting what the front and rear suspension systems desire to do and then changing components to allow them to work together. A balanced setup-one in which both ends are working together-is what we have always tried to find by using trial-and-error setup techniques.
Let's now take a look at four types of commonly raced stock cars and see how we can produce a more equally balanced setup for each one. The primary influences on the base setup of each car are the front moment center location, the rear moment center height, and the four spring rates. The modern trend for asphalt racing is the big bar and soft spring setups. This brings the larger sway bars into the picture, and we now have one more primary influence on the setups of asphalt cars.
As we discuss the setups in these cars, follow the reasoning and methodology so that when you think about making changes to your car, you can move in a direction that makes sense and causes a more positive result. This is important because every race car is built a little differently than another within the same class or division.
Setup information in the charts will include the four spring rates (LF is the left-front spring, shown in the left upper corner of each chart, RF in the right upper corner, LR in the left lower corner, and RR in the right lower corner). The front moment center is shown first as height off the ground and then as width from a centerline that is halfway between the centers of the two tire contact patches (a negative number represents an MC that is to the left of the centerline).
The Panhard bar is listed as left/right mounting heights off the ground in inches. For the metric four-link, we show the moment center height. The sway bar diameter (if shown) is in inches, and we will assume an average wall thickness for a hollow bar. The banking angle is shown in degrees, and the last number shown is the estimated g-force.
The top numbers represent common but not necessarily balanced setups. The bottom numbers represent changes we have made to the car to make it more balanced.
Many of these cars are the metric type with a four-link rear suspension. The rear moment center in these cars is very high, usually 14 inches or higher. The original front MC is too far to the right, the spring base is narrow, the front springs are too stiff, the rear springs are too soft, and the rear roll desire will be much more than the front.
This car would be very tight and would require a lower amount of cross weight in order to make the car neutral. The balance problem would still exist, though. The two ends of the car are very different in what they desire to do. The difference in roll angle is approximately 2.2 degrees.
To balance this car, we could do several things:1. Soften the front springs and reverse split the rates to: LF= 50,RF= 002. Decrease the rear spring split to: LR = 185, RR = 1753. Make changes to the rear ride height or trailing link mounting points to lower the rear MC to 13 inches or so4. Move the dynamic width of the moment center 2 inches to the left of the centerline by increasing the upper control arm angles (inner mounts lower than the ball joints), which will also raise the MC. Installing taller ball joints is one way to do this.
With these changes, the two suspension systems are now much closer to being balanced. The car will turn much better, and we can use more crossweight to load the LR tire and promote better bite off the corners. Our roll angle desires are now within less than 1 1/2 degrees.
This car also has the metric four-link rear suspension system. This would be called a Street Stock or Hobby Stock. As with the dirt stock clip car, this setup is a tight one. Because of the higher g-forces encountered on asphalt, the front and rear roll angles differ by 3 degrees, resulting in an unbalanced setup. We need to match the desires of each end more closely.
To balance this car, we could make these changes:1. Change the front spring rates to: LF = 850, RF = 950.2. Change the rear spring rates to: LR = 175, RR = 2253. Move the dynamic MC to 2.5 inches in height and 5.0 inches to the right of the centerline. This improves the camber change on the RF wheel and makes the front end more efficient to work harder to turn the car.
These changes will make both ends of this car more balanced, and the performance will be consistently fast because both ends will work together.
With the low g-forces, the small moment arm (caused by the MC being located too far to the right of the centerline), and the stiff front springs, this car is unbalanced and much too stiff for dirt racing. We can make changes similar to those in the stock clip dirt Late Model, which include the following:
1. Soften and reverse split the front spring rates to: LF = 750, RF = 650.2. Reduce the rear spring split to: LR = 175, RR = 1503. Raise the Panhard bar to 9/10.4. Move the dynamic MC width 2.0 inches to the left of the centerline.
The changes served two important purposes. By softening the front springs, raising the Panhard/J-bar, reducing the rear spring split, and moving the front MC to the left, we were able to bring the roll angles of the front and rear closer together to better balance the two suspension systems. We have also maintained the rear spring split, which gives us more bite off the corners.
The asphalt Modified experiences much more g-force, not only from the improved grip that the asphalt gives, but also because of the 14-degree banking of the racetrack and low center of gravity that these cars have. Nonetheless, this setup is also very unbalanced.
The MC is too far to the right of the centerline, and the Panhard bar (rear MC) is too low. There is about a 2-degree difference in how far each suspension wants to roll, with the rear out-rolling the front. Here is what we might change to correct the setup:
1. Leave the front and rear spring rates alone2.Raise the Panhard bar (rear MC)to: 10.75/11.75.3. Move the dynamic MC width 2.0 inches to the right of the centerline.
With these changes, the car is now balanced with both ends (suspension systems) wanting to do exactly the same thing in the turns. This car will now turn well, drive through the middle faster, and exit the corner much faster.
As you make these changes, don't forget to change the weight distribution to add more left-rear weight or crossweight percentage. As the left-front tire begins to carry more load, the car will turn better and become loose if cross is not added.
This car is set up from recommendations that were published by some dirt Late Model builders about seven years ago. Since that time, most manufacturers of dirt Late Model cars have changed their view of a starting setup. Many now publish starting front spring rates that have lower overall rates and a LF spring that is stiffer than the RF spring. This reverse split helps the car on turn entry.
The older base setup makes the car so tight that the driver would need to throw the car sideways in order to have any chance at all of setting up a good line for coming off the corner. We can make the following changes to help the car turn better and make it more neutral in the middle of the turns so we don't have to be nearly as sideways.
1. Soften and reverse split the front spring rates to: LF = 375, RF = 350 for dry-slick only2. Reduce the rear spring split to: LR = 200, RR = 1753. Change the J-bar to 12.0 on the left and 9.5 on the right.4. Move the dynamic MC 2.5 inches to the left of the centerline5. For tacky/tight track conditions, we would raise the rear MC even up the rear springs, and level out the J-bar
The plan for setting up all dirt cars is to narrow the gap between roll angles, front and rear, but not necessarily match them exactly like we do on asphalt cars. This method provides enough front grip so that the car will turn well, but still allows adequate rear bite so that the car has enough rear grip to get off the corners.
On tracks where there is plenty of moisture and the grip level is high, we can set up the dirt Late Model more like an asphalt car with even spring rates in the front and rear and a higher average J-bar. This is not theory, but reality, as some major dirt races have been won with even spring rates front and rear.
This car has a front MC that is too far to the right of the centerline for a flat asphalt track. It also has stiff front springs and too much rear spring split. The roll angle difference is 1.3 degrees, and that makes for a very unbalanced and inconsistent setup. This car might be fast for a few laps, but in the long run, the lap times will fall off quite a bit.
These changes will balance the car while still providing sufficient bite off those flat corners. A high crossweight percent range works best to help with exit performance.
1. Change the front spring rates to LF=350, RF=3252. Change the LR spring to 185. The split is reduced to 10 pounds,versus the original 25 pounds.3. Raise the Panhard bar to: 10.75/11.754. Move the dynamic MC width to the left to end up 2.0 inches to the right of the centerline.5. Use a high crossweight range (a range more equal to the left-side weight percentage)
The reverse spring split on the front helps the car on entry to the corners. If we were to run a stiffer RF spring than the LF, the car would begin to roll to the left when braking into the corners because of the softer LF spring. As the car continues to drive further into the turn, the front must reverse and roll to the right. This gives the driver a very uncomfortable feeling and can best be described as a "flip-flop" sensation.
When we use a stiffer LF spring, the car will begin to roll in the same direction as we brake into the corner that we would normally see at mid-turn. The transition is smooth, and the driver will have more confidence on entry.
The rear spring split with the softer RR spring helps promote traction in the rear by increasing the crossweight percentage as load is transferred to the rear upon acceleration. Usually, a 10- or 15-pound rate split is sufficient to provide the desired effect, whereas a split of 25 or more is often too much.
The front MC change (moving the MC to the left) makes the front suspension feel softer and more efficient and makes it want to roll to a greater roll angle to match the rear suspension's desired roll angle.
For the big bar and soft spring setups at flat tracks, we would change the springs to look something like this: We reduce the front spring rates, increase the size of the sway bar, and increase the right-rear spring rate (but not too much). The original setup has a right-rear spring that is much too stiff. This is not necessary and may unbalance the car, with the front out-rolling the rear.
There are special shocks that are built in conjunction with these BBSS setups, and we recommend that you consult your shock manufacturer so that the setup matches the shocks. Be careful when trying to run soft front springs at high-banked tracks. The car may bottom out and cause damage.
Many teams who mostly run the flatter tracks try to set up their cars the same way at the higher-banked racetracks. The higher banking produces much more downforce, and the arrangement of springs that works best on the flatter tracks won't work well at all on the high banking. The higher the banking, the worse it gets. Rear spring split with the LR stiffer than the RR cannot be used because the higher g-forces cause more downforce and magnify the effects of the spring split. This will make the cars very unbalanced.
At the high-banked racetracks, we get more chassis dive and less chassis roll, which is similar to the effect experienced due to running the BBSS setups. Excess camber change is a factor, but traction loss is less obvious because of the high amount of downforce and available traction caused by more load ending up on each of the four tires. Here are the changes that we would make to the setup to make the car balanced for a 14-degree track.
1. Reduce the front spring split to: LF= 350, RF=3752. Reverse the split in the rear springs to: LR = 175, RR = 2003. Raise the Panhard bar by 0.50 inch.4. Move the front MC to the left so that it is 4.0 inches to the right of centerline5. Use the low crossweight range (about 50 to 52 percent)
The following is a setup we would want if the track banking angle were much higher (e.g., Bristol or Salem, about 26 degrees). The car will have a lot of traction due to the high amounts of downforce and, therefore, the cornering speeds will be very high. The resultant force, which is a combination of gravity and the lateral cornering force called centrifugal force, will be in a direction that points between the front tires. This results in a lot of vertical chassis travel and less chassis roll. Bite off the corner is almost never a problem unless the setup is very unbalanced. Here is an example of a balanced setup for a highly banked racetrack for an asphalt Late Model.
Note that the front spring rates are much higher and the rear spring rates are both higher and split to a greater extent. Both of these could be even higher if the radius of the turn is smaller. The front MC is farther to the right, the average Panhard bar height is higher, and the g-forces are up considerably to 2.2 g's.
The front suspension travel will average in excess of 3 inches, and the rear suspension will travel more than 4 inches. If your car normally has a 4-inch ride height, then the car must be raised or it will make contact with the track surface.
The starting cambers might be: LF = +4.0, RF = -1.0. This is because of the high amount of chassis dive the car will experience, which will reduce the positive LF camber (become less positive) by 3 or more degrees and increase (add to the negative amount) the RF camber by about the same amount.
Again, let me say that the changes we have made to these sample cars are the same changes we have made to real cars in actual competition. That is not to say that you need to run out to your shop and put these setups in your car. Rather, remember the directions in which we have gone and trends we have described related to the different types of race cars. The hot tire temperatures as well as the tire wear will be an indication of how close you come to a balanced setup.
The differences that probably exist between these cars and yours indicate that these setups might not work for you. Typical differences include: a) center of gravity height; b) front moment center (roll center) location; c) front-end motion ratios for spring mounting; d) weight distribution front to rear; e) front and rear spring location and spring angles
All race teams need to know certain basic technical information about their cars. Learn the tendencies that make a car's suspension want to roll more or less, and try to match the ends for a more balanced setup. Know your front geometry settings, especially the moment center location and camber change characteristics. If you do, finding that winning chassis setup will be a much easier process. Above all, be willing to take control of your chassis and do not be afraid to make whatever changes are necessary to improve the geometry, the alignment, and the balance of your setup.