Hot Rods How much drive shaft clearance?

How much clearance should I have above the driveshaft, 6" behind the front U joint, with the vehicle at ride height? It looks like I will have about 1 1/2" clearance between the top of the drive shaft and the cab floor, about 6" behind the front U joint. I believe it would give me about 4" of upward travel of the rear axle from ride height before there would be any contact. Will that be enough on a street driven, never loaded pickup truck? Gene

 

Do you have bump stops?

 

Um...there should be bump stops....I should probably look...
Guess that would solve that problem. 4" of upward rear axle travel should be more then I will ever need, right?

 

Should be plenty for a street vehicle, that does not see a heavy load.

 

Well, I looked this morning, sure enough, there are bump stops above the axle. They look to be in great condition and appear to have never been tested or used.

I added 2" lower blocks between the axle and the springs a few months ago. I just finished building cab mounts, and looking from under the cab at the trans yoke I grew concerned, not even thinking about bump stops. There is about 3 1/4" clearance between the axle and the bump stops. I'm pretty sure I'm good.

Thanks for the reminder. Gene

 

...that part of the driveshaft isn't gonna move as much as the rear u-joint area, you'll be just fine.

 

If you can tell us the length of the driveshaft it's some easy math to figure out the movement of the shaft 6" behind the front U-joint, based on rear axle travel. If you need just a bit more it may not be too difficult to lower the rear of the trans a skosh, although that would affect the u-joint angles a bit.

 

Disconnect the shaft from the rear axle and lift it approx the max and see the clearance in the area you want

 

I could have probably figured it out by math, but that was way too hard. I don't yet have a driveshaft ready, so I stuck the trans yoke into the trans, turned the yoke so one of the ears was up at the highest point, slipped a 1 1/2" tube over the ear, and lifted it up until it came in contact with the cab, and measured the distance to the rear axle yoke with the ear at the top. I have 4" + clearance between the tube and the rear axle yoke, with the truck sitting on the ground. To me, that means the rear axle would need to rise up 4"+ before the drive shaft would make contact with the body.

There are bump stops above the axle, and the clearance between the bump stop and the axle is 3 1/4". To me, that means the axle can only rise up 3 1/4", or maybe 3 3/4" if the pulverizes the bump stops. The axle will contact the frame before the driveshaft contacts the body. I can't picture the rear end on this truck rising up 3" on the leaf springs, let alone the 3 3/4 it would take to kill the bump stops. This truck won't be doing any 4x4 off roading, I think I'm going to be OK., or at least I'm willing to take that chance. Gene

 

...that truck will probly be light in the rear, that axle won't move that much, on a big bump the whole truck will move...depending on how soft your rear springs are, and the shocks.

 

If you have 1 1/2 inch 6" behind the pivot point and it will only let you have 4" of axle movement the drive shaft must be very short.

 

Drive shaft will be around 47"- 48" long. Rear springs will be plenty stout for the weight of the truck, (shocks are yet to come) but it should be OK. Gene