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Here are some comments about driveline bending.

Anything that is spinning has centrifugal force. We have all felt it with one thing or another. Well, so does a driveshaft. The driveshaft is fixed at each end (not really but I may touch on that later) and spins. An MN-12 driveshaft is about 57" long and when it's spinning this centrifugal force is acting on it. The center of the driveshaft is spinning in a whipping motion, or the circle it makes when it spins is larger than the ends. There is a speed where this amplitude is largest. On a steel driveshaft 4.6L, this is about 5,000 rpm. In production a 15% safety factor is used so this speed is then lowered to 4,400 rpm. The speed limiter is set to the lower of this or max tire speed. 4,400 rpm is 105 mph in a 3.27 axle MN-12.

If you get rid of the speed limiter the driveshaft will not break right away. It's kind of like N2O, the more you are there (above 4,400 rpm) the more you are fatiguing the driveline. At best the bushing will spin in the transmission extension housing. I've seen trans cases break.

There are several things to do to eliminate this problem. The lighter the driveshaft, aluminum, the less force it has when spinning so the higher the speed the driveshaft can go. A composite shaft is best, very stiff and light. The ends of the driveshaft are not fixed, even though they seem like they are. They are also moving, so if you stiffen up the ends, then you will also raise the speed.

Let's use the MN-12 S/C as an example. It has a steel shaft and with it's 15% safety factor it can go 5,300 rpm. Why? It has what's called a structural oil pan. The oil pan bolts to the trans, stiffening up the whole powertrain.

You can't just get rid of the speed limiter without doing something about driveline bending. If you don't believe me, go ask a cop. We made an error on the 1999 cop car and let it go too fast. This became a huge problem in the field and I'm sure most cops have heard about the problem. The solution, rather that slow the car down, we put in a composite shaft. I'm sure some of you won't believe me, but this is real.

Jerry W.
 
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