In our dirt test, we placed...
In our dirt test, we placed the rear trailing rods in all the combinations of holes to record the results. Lap times did not differ much between the runs, but the driver reported likes and dislikes associated with the different locations.
The driver liked the feel of the car, but traction off the corners was not necessarily improved. The surprising result was that the lap times did not fall off or get better. The car remained as fast as before and was consistent. The one difference noted was that the LR tire was working more and getting hotter. We will be doing more with this ongoing dirt testing and reporting the results in future issues of Circle Track.
The point here is that whatever the trend, we have to make it work to be consistent and the car must be balanced with all the qualities that will make it fast throughout the race. Don't be afraid to experiment. Our asphalt car did not have the low left-front attitude of the BBSS setups, but it was as fast as those cars.
Front Spring Rates The trend in asphalt racing is to run very soft springs in the front of the car. But there is a limit to how much benefit you get with very soft springs, and the top professional teams have discovered that full coil-binding is not beneficial to good handling.
Again, this is a product of going too far with a good thing. The front end must be low for maximum aero effect, but if the springs are bottoming out or the chassis is contacting the track surface, the car won't handle properly. Handling is the key to going fast.
If you are going to experiment...
If you are going to experiment with spring changes at the track, make sure you preplan your changes so that the car's dynamic balance does not change.
Dirt track racers have softened up quite a bit over the past five years or so. Street Stock cars that used to run 1,000 to 1,200-pound springs on the front are now down to 850 to 950-pound springs. The norm for springs in the touring dirt Late Model cars used to be 500 LF and 550 RF. Now we see those rates down to 400 to 450-pound springs, sometimes with a stiffer LF.
Dirt teams that have softened the front springs to as low as 600 pounds on Street Stock cars and 300 to 350 pounds on coilover-type Late Model cars have found those rates to be too soft when the car bottoms out or there is too much camber change. This is yet another case of having too much of a good thing.
The Front Geometry Front-end geometry is one area in which changes across the board have made a huge difference in all forms of racing. From Formula One to Street Stock, much research has gone into the value of proper geometry and success in making refinements.
The neat thing about front-end geometry is that what has been tested and proven works with all the setup trends. Good dynamics and proper camber change characteristics are needed in dirt and asphalt (for all classes in each) and helpful for BBSS or conventional trends.
The reason that MC design is valid for all race cars is that the front moment center location controls much of the dynamics of the front suspension. This invisible intersection point is the bottom of the moment arm, and its lateral and vertical location regulates the stiffness of the front end like no other setting. The MC should be designed correctly for each type of race car and for each particular track.
We experimented with front...
We experimented with front moment center adjustments on a dirt Late Model and found that the location of the MC was critical to the performance of the front suspension.
Moving from a moment center width of 0.0 to 10.0 inches right of centerline is like stiffening the front end by an average of 175 pounds of spring rate per side. That's like going from 275-pound springs to 450-pound springs. If you are trying to go soft on the front end, you must consider the MC design along with using softer springs.
The Rear Moment Center Height Many teams don't appreciate the value of the rear Panhard/J-bar as a tuning tool. Some classes don't have a choice in rear MC heights. The stock classes that run metric four-link rear suspensions must live with a very high rear MC that is very hard to adjust. To compensate, they must run a large spring split with the RR spring rate 50 to 100 pounds less than the LR spring rate.
Just like the front MC, rear MC is the bottom of the rear moment arm. If the Panhard bar is set too low or too high for your springs, the rear of the car will not be in sync with the front suspension. This causes either a tight or loose condition.
The bar angle has an effect on load transfer due to its reaction with lateral force. On dirt Late Model cars with bars mounted on the left side of the chassis and a lot of angle pointing the bar toward the RR tire contact patch, a lot of load ends up on the RR tire and the rear of the car rolls up while trying to flip over the rear end. As a result, the rear end steers excessively to the right.
This arrangement will help a tight car get around the corner, but we can't help thinking that this is a crutch. What if we worked on our setups and our geometry in the front end so that the car turned better? Then we could reduce the Panhard bar split, lower the rear MC, reduce the rear steer to the right, and maybe move the Panhard bar over to the right side of the chassis.
Many dirt cars have the Panhard...
Many dirt cars have the Panhard bar attached to the left side of the chassis and mounted high. In experiments done over the past 10 years, we found that some conditions prefer a right-side-mounted bar.
The CrossWeight Percentage Having an excess amount of crossweight, or wedge as it is referred to in dirt racing, causes too much weight to be supported by the left-rear and right-front tires and can cause a car to be tight in, through the middle, and off the turns.
We can work within two or more crossweight ranges to make our cars neutral in handling. Within each range, there is a percentage that balances the handling (not the dynamic balance) so that the car will be neutral at midturn.
The ranges and numbers depend on the front-to-rear percentage of weight distribution. The more rear percentage the car has, the more crossweight percentage you need in each range to make the car neutral. If I move weight in the car and go from 50 to 51 percent rear weight distribution, I need to go from 52 to 54 percent of crossweight to keep the car neutral.
Think about this. A team has trouble with a tight-loose condition in which the car is tight and the driver oversteers into the loose-off condition we described earlier. They add rear weight to tighten the car, and all of a sudden the loose-off condition is gone. They think they've added bite with the added rear weight, but in reality, they have loosened the car to make it more neutral in the middle of the turns by not adding crossweight with the front-to-rear percentage change.
Crossweight that is too low is a distinct indication of a tight car. If a car really needs 51.9 percent of crossweight to have proper weight transfer and is able to run only 49.8 percent, we know the team had to take cross out of the car because the setup was tight.
There is an optimum percentage of weight distribution that will make the car neutral if the setup is dynamically balanced. Remember, a neutral car is not necessarily a winning car. It must remain neutral throughout the entire race, and having dynamic balance makes that happen.
Conclusion The use of excess steering input, low crossweight, rear steer, or any of these crutches intended to make your car turn are indications that other problems exist and need to be fixed. What the car needs is an arrangement of spring rate layout, front geometry design (including moment center placement and camber change), Panhard bar height (rear moment center height), and correct weight distribution that will work in combination to provide a fast and balanced setup.
Once these areas of setup are correctly identified and made to work together, the car will give the team what it wants, which is a fast and consistent handling package that the driver can use to win races. Being able to identify the crutches and knowing which areas to look at to correctly solve your handling problems makes the process much easier.
If you can identify your driving and setup crutches and find the proper fixes for your handling problems, then your program will move forward and your team will be much more successful. If you've ever wondered what makes a championship team able to adjust and run well anywhere they go, it is largely because they have learned how to identify the difference between meaningful adjustments and crutches.