| Speed limiter and Driveshaft Secrets |
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Here are some comments about driveline bending.
Anything that is spinning has centrifical 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 centrifical 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 5000 rpm. In production a 15% saftety factor is used so this speed is then lowered to 4400 rpm. The speed limiter is set to the lower of this or max tire speed. 4400 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 kinda like N2O, the more you are there (above
4400 rpm) the more you are fatigueing 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% saftety factor it can go 5300 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
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