DRIVELINE CRITICAL SPEED, ORIGINALLY POSTED BY JERRY WROBLEWSKI

MadMikeyL

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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.
 
Another note on driveshafts: the stock ones suck. They are two pieces of sheet metal, bonded together with rtv. The rtv fails over time. I have ruined 3 so far, lol. Apparently, speeds over ~80mph will cause it to come apart after a year or so. (My daily drive was high speed interstate).
The first symptom is vibration, that gets worse at higher speeds. This vibration ruined a Diff, a set of wheel bearings, and took out the transmission. All in 6 months. The only fix is a 93 Mark 8 driveshaft which is one piece, or aftermarket. I bought a 3.5" aluminum ds, and have never had another problem.PST gave me a great deal on the ones I bought. I'd recommend them in a heartbeat. Driveshaft lengths are:
94-97 3.8L 4R70W 57" driveshaft

94-97 4.6L 4R70W 58.20" driveshaft

93-98 Mark VIII 4R70W 58.20" driveshaft

89-95 3.8L M5R2 53.80" driveshaft

89-93 3.8L AOD 57.80" driveshaft

89-93 5.0 AOD 57.80" driveshaft
That is center to center of the u-joint locations, iirc.
 

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