Chassis FAQ

Q: Mike, you mentioned earlier that front stagger does not help you turn. If this is true, does it matter if both front tires are the same size? If the right front is bigger, is this more like adding cross weight? Please explain what the left front does. Thanks.
A: Because front tires rotate independently, unlike the rear tires, the difference in circumference (stagger) will only change the ride height, it will not make the car turn. I will show you how to calculate the change in ride height. Since stagger is circumference to find the diameter we must divide it by pi, which is 3.14, then we must divide the diameter by 2 to find the radius. Use this formula (stagger/3.14)/2. As you can see even an inch of front stagger will only result in a .159" (not quite 3/16") difference in ride height. On a coil car one turn =1/8" in height so this can be compensated by adding 1-1/2 turns to the side of the front with the smaller tire. On a torsion front car you would add about 1/3 turn. You're Welcome.

Q: What would be a good weight distribution to start with on a micro sprint? Front to rear and left to right.

A: I can't just give numbers without telling my whole story. Remember that scaling only tells us a small part of the story. It tells us how fast the car is when it is sitting still, like in the garage. It does not tell us anything about what the car is going to do once we turn the car, accelerate forward, or brake, this is when all the weight transfers and factors like center of gravity height, roll centers, tire offsets, shock dampening forces, and spring rates come into play. But if you want to scale the car this is what I recommend. Always scale the chassis with the driver sitting in it and in the spot of the garage each time. Also, keep the same amount of fuel in the tank, unhook the shocks if you have a torsion car, Make sure you have the same size tires on each time, same tire offsets, wheel offsets, and set the tire pressures to race ready levels. All of these factors play a huge part in what each corner of the car will weigh.

Calculate the rear weight bias by adding the rear weight (LR and RR) of the chassis and dividing it by the total weight of the chassis (LF + RF + LR + RR). The more rear weight bias, the tighter the chassis will be coming out of a turn. For the 600cc chassis, 61-68% works best. Typically we are at 63-65%. Calculate the cross bite by adding the RF and LR and dividing by the total weight of the chassis. Look for about 42%-48% in the 600cc. For a normal track we will start at about 45%. Increasing cross bite will tighten the chassis coming off the turn on small tracks. For winged racing more cross weight will loosen the car on entry or even the whole way around the turn due to overloading the LR and creating more LR than RR weight. If you're confused about this concept, read my "Rethink Dirt" paper in the tech department of this website. It will explain why we want to keep the rear tires equally loaded for maximum traction.

Scaling the chassis can be useful in detecting binds in the chassis caused by bent or bad rod ends, bent shocks, bent axles, bent frames, or axles out of square. First scale the new chassis after the set-up procedure is complete and record all the numbers and the exact set up you used. Then, after each race, put the chassis back to that set-up and rescale the chassis and compare the numbers, if they are more than 4 pounds off (per corner), then something has changed.

Most Hyper owners will set up their cars using blocks and adding or subtracting turns to achieve a starting setup and then adjust from there. Once you get a fell for how much a turn on a corner will change the car, you will forget about reading percentages and go by turns, it is a lot easier and much more consistent. The one thing you need to watch if setting up with blocks and turns or by scaling is ride heights. Ride heights control the center of gravity height of the car which is huge in dirt track chassis performance. It is the single biggest factor to consider when setting up your car. CGH is determined by many factors, one of which is spring rate. When the bars or front springs are changed to different rates, it will have a factor on CGH. So in combination with scaling or blocking, ride heights should also be measured and documented. When a spring rate change is made, you will want to consider changing turns to get back to a desired ride height, although many times with a spring rate change we will want to change ride heights as well.

Q: Mike, I noticed in one of your responses that your left rear tire didn't show much wear. My left rear tire wears excessively on the outer edge, but the rest of the tire hardly wears. Any suggestions?
A: The left side edges of both R.R. and L.R. will always wear quicker than the rest of the tire. This is because the R.R. is bigger than the L.R. and the tires are on an angle relative to the track surface, creating more pressure on the left side edges. On some tracks in the feature when it gets black and hard, my left rear does wear, sometimes a lot.

Q: What is the advantage of turning your shocks upside down?
A: Upside down we will define as running the shocks body side up. To me this is actually right side up. Turning the shock upside down reduces unsprung weight. Unsprung weight is the weight not carried by the suspension, such as wheels, axles, and bearings. Reducing unsprung weight is always a good thing.

Q: Hey Mike, is a .750 solid bar supposed to be the same as a .750 tubular bar? Looking at the formula it wouldn't seem so. Thanks.
A: You are thinking right, but the manufactures make the hollow bars about .010" larger in diameter to compensate for that. So they both end up with the same spring rate.

Q: My car wanders going down the straights, what’s wrong?
A: I would bet that the king pins are not angled back enough (caster). They should be angled back 9-10 degrees. Put an angle finder on the front of your spindle, straight up and down, and measure the angle. Turn the right front control arms to achieve the 9-10 degrees.

Q: I have a 1998 Renco Chassis with Hyper Components . From center of rear axle to center of front axle to make the distance of 59 1/2 inches my front shocks lay 2 inches forward at the top. What would this do?
A: I'll assume this is a 250 car. The angle of the front shocks won't hurt anything unless it exceeds 15 degrees, then the spring rate will start to decrease. The spring rate = spring rate x cos (angle) ^2. The cos of the angle doesn’t amount to much until it exceeds 15.

Q: Is it ok to run different makes of tires on the rear. Or do you prefer to run the same make. IE: RR Hoosier and LR American racer.
A: It doesn't matter what brand tires you have on the rear, you can mix and match all you want. Just make sure your socks match, never wear two different color socks.

Q: Are softer springs or stiffer springs going to make the car faster?
A: Softer springs can sometimes be better as long as you don't bottom out because they allow the tires to maintain compliance with the track surface. But stiffer springs in the front will make the car tighter. Compliance is a good thing because this means that the tires are always on the track. You can't get traction if the tires are off the surface. You can visualize a stiff spring making the tire bounce off a bump or series of bumps. A soft spring will allow the tire to move up and down more easily as it goes over bumps. Stiffer springs also have the advantage of keeping the center of gravity high for longitudinal traction (forward bite). So there you go,it seems nothing is a straight forward answer. This is the beauty of dirt track racing.
Tend to use soft springs all around on a wet track and run stiff springs all around on a small slick track (for the longitudinal weight transfer), run stiff front and a little softer rear on a big slick track.

Q: Can you explain the main advantage to the offset wings?
A: The advantage of the offset panels is that they keep the car tight in the middle of the turn without getting it tight going into the turn. With the panels offset there is more clean air hitting the left board once the car is turning. Also with our wing, the center section is very wide (48"). Wider wings generate more down force and less drag.