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Old 06-06-2005, 03:19 AM
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The driveshaft torque, on any car that doesn't have an independent rear suspension, tends to unload the right rear (and, of course, load the left rear) during acceleration. For best tire performance, you want those rear tires equally loaded and static preload is one way to do it.

Suppose, just to have some numbers to work with, your car weighs 3000, the rear track is 62 inches, the axle ratio is 4.11, and the effective rear tire radius is 13.5 inches. Now, suppose you're launching at 1.5 g's. The difference in rear tire loading, due to driveshaft torque alone, would be:

(3000)(1.5)(13.5)/(4.11)(62) or 238 pounds.

In other words, the load on the left rear would be 238 pounds more than that on the right rear. But, fortunately, it isn't as bad as all that, for a portion of the reaction torque, acting through the engine and transmission mounts, shows up at the rear to cancel some of the driveshaft torque. This reaction torque is distributed, front-to-rear, in proportion to the relative roll stiffness. Suppose 60% of the total roll stiffness is at the front suspension. That means:

(0.60)(238) or 143 pounds

of imbalance remains to be "fixed" with static preload. You can tighten the coilovers at the left front and/or the right rear until the right rear weighs 143 pounds more than the left rear. Then, as the car launches, the loadings will be equal.

But, some of these numbers are pretty shaky, specifically the acceleration and roll stiffness percentages, so this just gives you a rough idea of the magnitude of static preload that is necessary.

And, if you do use static preload, it's important that the ladder bar adjustments be the same, side-to-side. In other words, you either do it with ladder bar adjustments or you do it with preload, but not with both at once. Which, then? Well, even with the shaky estimates used in the calculation, I'd go with the static preload.

Don't worry about the inequalities during braking. When your 250 pound mother-in-law sits directly behind you, so she can yell in your ear as you drive, the inequalities are equal or greater. So long as you don't lock up the wheels, braking torques remain equal.

Finally, there are ways to dynamically cancel driveshaft torque, so the static loads can be equal, but I don't want to go into these other ways here.
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