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).