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Old 03-04-2004, 09:56 AM
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Yeah, that RPM site is pretty much for the roundy-rounders.

You can get a little drag info at some of the other manufacturers' sites, but not all of it is good. In other words, even the people that make 4links can steer you wrong.

For instance, they'll tell you that, if you drive the car on the streets, the links (upper and lower) should be parallel. This much is true. Unless the axle brackets...on at least one side...are free to rotate on the housing (a "floating" mount), a non-parallel link arrangement will result in a "binding" of the linkage as the car corners. This is because the right side link pair is trying to rotate the housing in one direction while the left side link pair is trying to rotate it in the other. But, if the pivot point is at infinity (parallel links), no binding will occur.

But, they'll usually go on to say that the links should also be parallel to the ground for the street. This is NOT true! This will yield excessive squat (of the rear of the car) as you accelerate. If the parallel link pairs are angled up from the rear, you can lessen or eliminate the squat and still avoid binding.

At the strip, you can find all kinds of non-parallel arrangements being used by racers. So long as you remember to return to the parallel arrangement before driving home, those Heim joints and links can take quite a bit of punishment.

There is, however, a little-known "trick" that can be used which works effectively on both the street and at the strip. On the street, it allows you to drive hard through the corners without worrying about binding, and, at the strip, it cancels driveshaft torque, allowing equal rear tire loading as you launch. In other words, it's "set it and forget it."

I mentioned angling up the parallel bars to reduce squat. If those links are set at an angle, to the strip surface, which has a tangent equal to the center of gravity height divided by the wheelbase, the squat is entirely eliminated. Any angle greater than that will cause the rear of the car to rise on acceleration. So, here's the trick: Set the parallel right side link pair to an angle 5 degrees greater than the angle I've just described and set the parallel left side link pair to an angle 5 degrees less. Stop and think about what this will do. The right side wants to rise (slightly) and the left side wants to squat. The net result will be an increased loading of the right rear tire which is in opposition to the unloading caused by driveshaft torque. The 5 degrees is, of course, a starting point. You might want a little more or a little less, but, with the average car, that will get you pretty close to full cancellation of the driveshaft torque.

As the RPM site points out, any upward angling of the parallel link pairs will result in a certain amount of that which is called "roll oversteer." That rear axle assembly is rotated, when you go through a turn, in a manner similar to the front axle of the push kart you made as a kid. But, here's something to remember about roll understeer/oversteer (and many roundy-rounders don't even know this): The roll U/O DOES NOT CHANGE THE WHEEL LOADINGS. In other words, the car is able to negotiate a given curve at the same speed, with or without the roll U/O. All the roll U/O does is change the amount of steering wheel angle required. The psychological effect is, however, very strong and it's often very difficult to convince a driver that the basic handling hasn't been changed when roll U/O is introduced.
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