shortest safe driveshaft length?

we've run into a bit of a snag trying to run a 460/C6 in an A frame, as we wanted to run the motor pretty far back. But, that C6 is insanely long. What's the shortest driveshaft (measured U-joint centerline to U-joint centerline) that we could run- assuming the trans tailshaft was at the same height as the rearend, so that there wasn't much angle there, only the several degree sideways offset of the rear)

how much shorter is a C4 than a C6?

thanks!

 

There isn't really a set length that is "too short". It depends on the range of motion you try to get out of a given joint. The other issue is that the ends of a traditional u-joint are doing some pretty dynamic stuff during their normal operation. They accellerate and decellerate twice in each revolution. This can set up some destructive vibrations under the right circumstances, which usually occur when you are maxing out their range of motion. One solution is a CV joint. They often allow more extreme angles without such destructive side effects and less harmonic activity. If you have room, you could probably get a shaft with 2 CV joints made up that would protect you from trouble that you might see in a U-joint shaft with normal suspension travel.

 

My old modified had about 8 inches center to center on the U-joints. It really is a function of the travel you need. With no suspension travel there are direct couplers, such as in a drag car with solid suspension.

 

The drive shaft in my roadster is a little over 18 inches long and has held up for for over 200 9 second passes. The limiting factor of any u-joint is the severity of angle you are going put it in and that will determine your range of suspension motion. I used to remember what the actual limits of u-joint angles were but I have killed way too many brain cells since then. Back in the dark ages when we were running dirt cars with leaf springs we would run as much as 7 degrees of downward pinion angle so we wouldnt over center the u joint under acceleration. Seems to me that as long as you didnt have more than 15 degrees shaft angle at any point of suspension travel the u-joints would live. As a side note, a u-joint does not know the difference between up and down angle and side angle. Just make sure that nothing binds during up and down travel AND roll travel.

 

on the CV joint note... its the only thing that saved my ass when the crossmember and 4-bar mounts on the willys got bent to shit hell and back... kept right on doing its job with zero problems and got me home without damage to any part of the driveline except where the shaft istelf was rubbing on the crossmember over bumps... them things do a HELL of a job for extream driveline angles, and if you are worried about yours, i'd say def get a shaft with a CV on the rear. lincoln mark VIII's had them, and i think the merc cougars as well, so boneyard pics are relatively simple to find and have rebuilt on your shortened shaft.

T