Chassis FAQ's
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, this is when all the weight transfers and factors like center of gravity height, roll centers, tire offsets, 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. Also, keep the same amount of fuel in the tank, unhook the shocks if you have a torsion car, and set the tire pressures to race ready levels.
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 going in and coming out of a turn. For the 250cc and 600cc chassis, 61-65% works best. Calculate the cross bite by adding the RF and LR and dividing by the total weight of the chassis. Look for about 42%-49% cross bite in the 250cc and 42%-46% in the 600cc. Increasing cross bite will tighten the chassis coming off the turn. Add corner weights by adding 1/2 or taking out 1/2 turn to each corner, ex: add right front and left rear weight by adding 1/2 turn to right front and left rear and -1/2 turn to left front and right rear. Read the article called Rethink Dirt-Advanced Dirt Track Theory found in the chassis section of this web site.
Cross bite decreases when we move the right rear in, yet we know this will tighten the chassis. Therefore we cannot look at this number without taking all other factors into consideration. Left side weight bias or percentage add the LF and LR and divide by the total weight of the chassis to calculate the left side weight bias. More left side weight bias will tighten the car on exit and loosen the car on entry (usually). This is affected mostly by moving the right side wheels in or out. Start with 53% left side weight bias in the 250cc and 56% in the 600cc.
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.
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 tracks like Lanco and Linda's, 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 .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. 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: What effect does moving the right front out have?
A: Moving the right front out does the same thing to the front as moving the right rear out does to the back. It will result in less front side bite making the car under steer (push). The right front needs to be offset the right amount to get the car balanced, not to loose, not to tight. If your car does four wheel drifts you may need to move both the right front and the right rear in. If your car bicycles a lot you may need to move both out. Hyper cars generally run the right front in and the right rear in further than an RTS or PMP. Both setups are balanced and work, in general the further right side offset cars will work better on tracks with more bite (tacky), and less offset will be better on slick tracks.
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 lateral traction (forward bite). So there you go, I confused you more.
Run soft springs all around on a wet track, run stiff springs all around on a small slick track, run stiff front and soft 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. I have seen wide wings that do not have a deep airfoil shape (big belly) but are dished. This will create as much down force but much more drag, a bad design. The center also keeps the air cleaner on the panels.
Q: What do I do to my car if it is bicycling? And what do I do to fix this?
A: Bicycling is caused by too much lateral load transfer. It can be reduced by moving the right side tires out, lowering the center of gravity, and/or raising the roll centers. If it is just the left front that is coming up in the air, usually you can fix it by reducing left rear weight (take turns out of the left rear) and raising the rear roll panhard bar, and/or move the right rear out.