58% Rear

[LateModel B23]

Anyone ever run a car at 58% rear? All the other numbers are good. 52.3% left, 24# of LR bite. Fuel level is 1" high on my "measuring stick", this car is 9" = 20 gallons and it is at 10" (teardrop cell).

By the way it is 4 bar Skyrocket, with USRA/USMTS spec motor.

[Dirt36]

That's what I always ran in my skyrocket 57-58%. Won several races at different tracks with it, but 80% of the tracks were intermediate to extreme slick condition. If your tracks have a lot of moisture or you feel down on motor 55% rear is probably better, JMO

[Anonymous24]

58 is way too much. Too much rear percentage robs the tires of FORWARD DRIVE.

[langdonracing48]

I run 58-62 all the time. I've got more forward bite than anyone on the dry slick

[Anonymous24]

F=L×CfF represents traction forceL represents the load upon the tire Cf represents Coefficient of friction for the contact patchLets say we have 60% rear. As the car enters the corner we must consider a couple things. The cars rotation on the vertical axis...YawThe cars rotation upon the longitudinal axis...Chassis rollWeight transferRule of thumb: maximum side forces degressively increase with load. So right away we know the rear tires will have to work harder to provide side bite.As the car enters the corner, we have weight transfer longitudinaly, laterally, and from chassis yaw.Using a tool called the friction circle, we can see that the rear tires will be using the available traction to keep the car from spinning out. When we over work the tire for side bite, forward bite suffers due to little traction capabilty being available to propel the car forward. Im trying to keep the post informative and short so please forgive me. There are many variables that work to dictate weight transfer. Most cars are not well balanced and are not configured to keep lateral weight transfer to a minimum. I personally shoot for a rear percentage in the 52-53 range for both U.M.P. and IMCA modifieds.

[LateModel B23]

Took the gas to 7.5" and that brought the rear to 57.2% and the car was good.

[25drtrkr]

The CoF theory only applies to racing on hard surfaces (asphalt). It does not take into effect the forces of soil shearing. CoF ALWAYS increases with soil shearing.

[SuperEight]

58 is way too much. Too much rear percentage robs the tires of FORWARD DRIVE.

I don't believe this for a second. Excessive rear percentage can cause a car to be tail happy or a loose entry condition, but if you not loose on entry the extra rear percentage will only plant the rear tires that much harder. You have to strike a balance to avoid the pendulum effect.

[Matt49]

Assuming cars of equal total weight, a car taking off in a straight line with 60% static rear will always have more available forward traction than one with 50% static rear. That being said, we aren't taking off in a straight line. The problem that people get themselves into when going excessive with rear weight is that the car gets very hard to turn (tight) getting into the corner. This forces the driver to either slide the car with excessive steering of use the gas pedal to spin the tires to make it turn. In either case, all available traction in the rear tires is used up and no traction is left available for forward bite. The classic "tight in, loose off" handling that so many drivers diagnose as just "loose" because that is where everybody is leaving them in the dust (literally).
I think this is what Anonymous24 is trying to explain.
And yes, CoF goes out the window in most dirt applications. In fact, it goes out the window in most pavement applications also. Basic CoF formulas doesn't really apply to materials like rubber as it has a tendency to conform to the surface area and not follow the "rules" of CoF formulas where surface area isn't part of the basic equation.

[let-r-eat]

So 50% rear weight car standing on the rear wheels v/s a 60% rear weight car with NO TRANSFER has less traction?

[Matt49]

So 50% rear weight car standing on the rear wheels v/s a 60% rear weight car with NO TRANSFER has less traction?

You're over complicating it. Re-read the first sentence of my post but start with, "all else being equal". Meaning same motor, same tires, same wheelbase, same ride heights, same suspension package, same VCG, same driver, etc. "All else being equal", more rear weight will always provide more forward traction. UNLESS you have used up the traction in the tires on lateral traction which (back to my point) is the point that Anonymous24 I believe is trying to make.

And quite frankly, "the pendulum" affect is also a bit of a myth. Meaning it isn't really due to the high rear percentage. The reason a car with higher rear percentage is harder to reign in AFTER you have managed to get it tailed out, is almost always because it has a high moment of inertia because of WHERE the weight was placed to get that high rear percentage.

[bob75]

How do you get a mod down to 52--53 percent tail

[let-r-eat]

I agree Matt49. I misread the post.

[powerslide]

How do you get a mod down to 52--53 percent tail

You don't.

[Lizardracing]

Mine is....updated 08 Impressive @ 2430/driver and 53 percent.

[Anonymous24]

How do you get a mod down to 52--53 percent tail

Relocate componets towards the center of the car when possible. Minimize unsprung weight. Use a smaller fuel cell. There are several ways to accomplish a low rear percentage. I usually have no problems getting rear percentage down to 52.

[Anonymous24]

Assuming cars of equal total weight, a car taking off in a straight line with 60% static rear will always have more available forward traction than one with 50% static rear. That being said, we aren't taking off in a straight line. The problem that people get themselves into when going excessive with rear weight is that the car gets very hard to turn (tight) getting into the corner. This forces the driver to either slide the car with excessive steering of use the gas pedal to spin the tires to make it turn. In either case, all available traction in the rear tires is used up and no traction is left available for forward bite. The classic "tight in, loose off" handling that so many drivers diagnose as just "loose" because that is where everybody is leaving them in the dust (literally). I think this is what Anonymous24 is trying to explain.And yes, CoF goes out the window in most dirt applications. In fact, it goes out the window in most pavement applications also. Basic CoF formulas doesn't really apply to materials like rubber as it has a tendency to conform to the surface area and not follow the "rules" of CoF formulas where surface area isn't part of the basic equation.

I used the CoF to help explain why we should run lower rear percentages. Yes the CoF may not be a concrete fact, but it does represent the theory on how to achieve what we are wanting from our race cars from a traction stand point.