I had an email this morning asking the effects of changing the rear bearing carrier timing on a Z-link style rear. Lets be clear that we are not talking about a wishbone style rear, this only applies to Z-link or the traditional sprint car rear end geometry. I can tell you that the common person will tell you that more bearing carrier timing angle will give more forward drive. Usually some explanation of the birdcage wrapping up quicker will be the reason. Of course that explanation has no place or truth in the real vehicle dynamics world. So I put my mind to work wanting to know why, because I never accept an answer without a real vehicle dynamics principal to back it up. My initial thought was that the rear steer will change as the angles change. But I was not sure, so I modeled it up. Turns out, that the rear axle position front to back does change when you change bearing carrier timing, but only very very little. I changed my bearing carrier angle in my simulation by 10 degrees (or 1 leaning forward) and it only changed the rear steer from .215 (in a 4-1/2 rear axle vertical displacement) to .230. So thats only 1/64 difference. Not even close enough to make any effective change. Here is the reason dirt track racers came to the conclusion of why bearing carrier timing changes forward drive. When we change the timing, one of two things needs to be changed. Either change the length of the torsion arm or move the rear axle front or back by changing the length of the top link. Without changing the mounting points of the frame one of these two things will change when the bearing carrier timing is changed. Both of these changes will have an effect on handling. If you add more bearing carrier timing by shortening the top link, you will move the rear axle forward which will add more forward drive due to the increase of rear weight bias. If you add more bearing carrier timing by shortening the torsion arm, you are stiffening up the rear spring rate which will make the car looser. So I propose that changing the bearing carrier timing will provide more or less forward drive if the change is made by changing the top link length only. Which is the way most people make this change because it is easiest. This will definitely change forward drive. Bearing carrier timing in and of itself does not make any effective change. It is the other factors that really matter. Just be aware of how you are changing your bearing carrier timing, because how you change it will determine how your car reacts to the change. I am open to other thoughts on this subject if you would like to discuss further.
Posted on: Tue, 02 Dec 2014 15:18:23 +0000
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