In this final segment of our explanation of racing dynamics, we will show how to apply the technology previously presented. We have learned that understanding racecar chassis dynamics helps us set up our stock cars. We have come to understand how all of the forces work in our suspension systems, and we can now adjust the components on the car to create a more balanced setup.
In Part One of this series, we learned about the early research of chassis dynamics and something about the individuals and companies that were involved. The thread of technology that had been pursued in our past only went so far. We have shown how that early technology has been refined and perfected.
A balanced setup is one where...
A balanced setup is one where two suspension systems in the car are working together as we drive through the turns. This means that they are trying to roll to the same angle, or what used to be called having equal roll resistance, although that terminology is lacking in totally explaining the process. This balance causes the setup to be both fast and consistent, both of which are qualities of a winning setup.
Racers have, from day one, had to go about sorting out their setups using trial-and-error testing. Even in Detroit, new cars are taken to a skid pad, a circular test pad, in order to determine the handling characteristics. Numerous books have suggested that racers should use that very same method to tune the setups in their cars.
Previous technology that refer to roll axis and roll couple distribution did not present the entire picture of what was happening with the racecar chassis. What the early research did that was needed was to define certain characteristics of suspension systems and tires that would lay the ground work for further research.
In the '90s, a new thread of technology was pursued, and out of that research and extensive development came a method that addressed the goals we racers have for analyzing the chassis setup. It gave us a way to predict what the racecar's suspensions desired to do. We found that once we knew the measure of each suspension system, we could then change components to cause the suspension systems to work together for the benefit of performance. A more balanced setup, we have learned, helps a car to be fast and consistent and to maximize the use of its tires.
A balanced chassis-where both ends are working together-is what we always tried to find by using trial-and-error setup techniques. In the past, when we accidentally found a really good setup, we didn't really know how we got there or why it worked so well. Often the performance could not be duplicated when we built, bought, or otherwise acquired a new car.
The moment arm length determines...
The moment arm length determines much of the front suspension dynamics, and it's overall design can be the difference between a car that turns well and is fast in the turns, and one that pushes and must slow down to get through the middle of the turns. Knowledge about the true picture of the moment arm and its relationship to the moment center has increased dramatically.
Part Two of this series explained 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 themselves. We also showed how the forces combine to move the chassis.
We explained how we can make the car balanced by predicting what the front and rear suspension systems desire to do and then changing components to cause them to work together. That is the most important consideration related to handling performance that we can concentrate on for our racecar. To do that, we need to have some kind of idea of what changes to make in order to accomplish the goal.
Now we will take a look at three commonly raced types of stock cars and see how we can produce a more equally balanced setup for each one. The primary influences on the base setup of the car are the front moment center location, the rear moment center height, and the four spring rates. As we discuss the setups in these cars, follow the reasoning and methods 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.
Every racecar is built a little differently than another, even in the same class or division. The examples given are for a specific car (yes, these are real setups that have been used in real racing situations) and probably will not work exactly right for your racecar. We are trying to show tendencies for how to balance your setup and directions to go in making positive changes and why we are doing them. For setting up your car, apply the reasoning, not the specific changes or setups.
Setup information will include the four spring rates (LF, being the left front spring, shown in the left upper corner of the 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 as height (off the ground)/width (from a centerline that is halfway between the two tire contact patches. A minus number represents a MC that is left of centerline). The Panhard bar is listed as left/right mounting heights off the ground in inches. The banking angle is shown in degrees and the last number shown is the g-force exerted on the car at mid-turn. Red is the starting setup, green the balanced setup.
This car would be very tight and would require a very low amount of cross weight percentage in order to make the car neutral. The two ends of the car are very different in what they desire to do-approximately 2.5 degrees difference in desired roll angle.
To balance this car, we could do several things:
1. Soften the front springs and reverse split the rates to: LR = 850, RF = 800 or softer.
2. Reduce the rear spring split to: LR = 185, RR = 175 for a leaf spring or Panhard bar car. For a metric four-link car with a high rear MC, the split needs to be around 50 pounds with the RR softer.
3. Raise the Panhard bar to 11/12. The metric car's MC is between 14.5 and 16.0 and not adjustable.
4. Redesign the front suspension points to move the dynamic width (after dive and roll) of the front MC to 2 inches left of centerline.
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 cross weight percent that will load the LR tire and that will promote better bite off the corners.
As with the dirt stock clip car, this setup is a tight one. Because of the high g-forces encountered on asphalt, the front and rear roll angles are more than three degrees different resulting in a very unbalanced setup.
To balance this car, we could make these changes:
1. Change the front spring rates to: LR = 850, RF = 850.
2. Change the rear spring rates to: LR = 175, RR = 175.
3. Raise the Panhard bar to 11.5/12.5.
4. Move the dynamic MC to 3.0 inches in height and 4.0 inches right of 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 perfectly balanced and the performance will be consistently fast because both ends are now working together.
With the low g-forces, small moment arm caused by the MC being located too far to the right of centerline and the stiff front springs, this car is both unbalanced and much too stiff for dirt racing. We can make changes similar to what we did to the stock clip Dirt Late Model, which are:
1. Soften and reverse split the front spring rates to: LR = 700, RF = 650.
2.Increase the rear spring split to: LR = 200, RR = 150.
3.Raise the Panhard bar to 10/11.
4.Move the dynamic MC width to (-) 2.0 inches left of centerline.
The changes did two important things. By softening the front springs, raising the Panhard/J-bar, and moving the 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 increased the rear spring split which gives us much more bite off the corners.
The asphalt Modified experiences much more g-forces, 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. None the less, this setup is also very unbalanced. The MC is too far to the right of centerline and the Panhard bar (rear MC) is too low. There is about a two 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.Change the front spring rates to: LF = 750, RF = 800.
2.Raise the panhard bar (rear MC) to: 10.75/11.75.
3.Move the dynamic MC width to 6.0 inches right of centerline.
With these changes, the car is now balanced with both ends (suspension systems) wanting to do the exact same thing in the turns. This car will now turn well, drive through the middle faster, and exit the corner much faster.
This car is setup from recommendations that were published by leading Dirt Late Model builders years ago. Since that time, most manufacturers of Dirt Late Model cars have changed their view of a starting setup. Most now publish starting front spring rates that show the LF stiffer than the RF spring. This reverse split helps the car on turn entry.
The older "base" setup makes the cars way too tight so that the driver many times 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.
We always try to move the upper mounts of the rear springs on a four-bar car out towards the wheels as much as possible to widen the rear spring base and help control the excess rear roll tendencies that these cars have.
1.Soften and reverse split the front spring rates to: LR = 400, RF = 375.
2.Reduce the rear spring split to: LR = 200, RR = 175, or in the case of a high grip and/or high-banked track, running a stiffer RR spring to a 225 or 250.
3.Raise the right side J-bar to 11.50.
4.Move the dynamic MC to (-) 4.0 inches left of centerline.
5.For tacky tight track conditions, we would even up the rear springs or stiffen the RR spring over the LR spring 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 will still allow 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 setup the Dirt Late Model more like an asphalt car with even spring rates in the rear and a higher average J-bar. The World 100, as well as the Dream Dirt Late Model races run at Eldora, were won using setups that used even springs in the front and at the rear. One car had a pair of 375 pound springs in the front and the equivalent of 175 pound springs in the rear (this was a swing arm car with 350 pound rear springs mounted on the trailing arm - the motion ratio causing the car to feel about half the installed spring rate).
This car has a front MC that is too far to the right of centerline for a flat asphalt track, stiff front springs and the rear spring split is too much. The roll angles are 1.8 degrees different 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 cross weight percent range works best to help with exit performance.
1.Change the front spring rates to: LR = 350, RF = 325 for conventional setups or under 250 for BBSS setups.
2.Change the LR spring to: LR = 185 (the split is reduced to 15 pounds vs. the original 25 pounds) for conventional or run a 175 LR and 250 or more in the RR for BBSS setups.
3.Raise the Panhard bar to: 9.50/10.75 for conventional or lower it to 9.00/9.50 or less for BBSS.
4.Move the dynamic MC width to the left to end up at 2.0 inches right of centerline.
5.Use a high cross weight 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 on braking into the corners because of the softer LF spring and then as the car continues to drive further into the turns, 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 change to running a stiffer LF spring, then as we brake into the corner, the car will begin to roll the same direction as we normally would 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 loading the cross weight percent as weight is transferred to the rear upon acceleration. This is considered old school nowadays, and almost no one runs a softer RR spring. What is common is a stiffer RR spring combined with softer front spring rates. This forces the LF down more so in the turns. The Panhard bar must be mounted low for this type of setup to allow the car to be balanced.
The Panhard/J-bar can remain relatively low because of the wider rear spring base that is a characteristic of these cars. On a solid axle rear suspension like we use in stock cars, the wider the spring base, the more resistance to roll the car will have at the rear suspension. So to balance this type of car verses a stock type of Late Model, we need to run a lower rear MC (which produces a longer rear moment arm) to compensate for the wider spring base.
The front MC change (moving the MC to the left) makes the front suspension 'feel" softer and more efficient and want to roll to a greater roll angle to exactly match the rear suspension's desired roll angle.
Many teams who mostly run the flatter tracks try to set up their cars the same way at the higher banked racetracks. With the BBSS setups, teams are unpleasantly surprised when their cars bottom out hard on the high banks. It doesn't take long for them to start installing much stiffer front spring rates at high-banked tracks.
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 make the cars very unbalanced. The popular BBSS setups are great on the high banks due to their high RR spring rate that is already installed.
At the high-banked racetracks we get more chassis dive and less chassis roll. Excess camber change is a factor, but less obvious as to traction loss because of the high amount of downforce and available traction caused by more load being exerted 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: LR = 275, RF = 300.
2.Even up the rear springs: LR = 200, RR = 200 for more conventional setups, or keep the stiffer RR spring that is popular on the BBSS setups.
3.Raise the right side Panhard bar to 11.50 (which raises the rear MC and increases the split in the two ends of the bar to compensate for the increased travel on the RR corner due to the added downforce.
4.Change static cambers on the front; increase the LF positive camber and decrease the RF negative camber. Try to design a front suspension that will have less upper control arm angle while still maintaining a decent MC location.
5.Use the lower cross weight range (from about 50 to 53 percent depending on the front to rear percent. More rear percent = more cross weight).
Example 8 shows where we would want to be if the track banking angle was much higher, at say 26 degrees. The car will have a lot of traction due to the high amounts of downforce and therefore the cornering speeds will be high. The resultant force, a combination of gravity and the lateral cornering force called centrifugal force, will be in a direction that lies between the front tires. This results in a lot of vertical chassis travel and little chassis roll. Traction is never a problem unless the setup is very unbalanced. Here is an example of a balanced setup for a very highly banked race track 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 turns is smaller. The front MC is farther to the right, the average Panhard bar height (rear MC height) is higher, and the g-forces are up considerably to 2.2 gs. The front suspension travel will average in excess of 4 inches and the rear suspension will travel upwards of 4 to 5 inches or more. 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 well be: LF = +4.5, RF = (-) 1.0. That is because of the high amount of chassis dive the car will experience that will reduce the positive LF camber (become less positive) by 3 or more degrees and increase the (add to the negative amount) RF camber by about the same amount.
Again let me say that the changes we have made to these sample cars are the same as 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 direction we have gone and trends we have described related to the different types of racecars. 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 between these cars and yours that might cause these setups not to work for you include: a) center of gravity height, b) front moment center (roll center) location, c) front end motion ratios for spring mounting, d) weight distribution, e) and front and rear spring location and spring angles.
All race teams need to know certain basic 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 fine tune all of these, finding that sweet spot related to a 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 changes to improve the geometry, the alignment, and the balance of your setup.