What do you guys with rear drive shafts with carrier bearings doing when you lift your trucks? Just drop them via a bracket of some sort to straighten out the drive shaft? I would think you'd want to drop the carrier only a little so that it would be a happy medium on all U-joints but I've also been told the drive shaft should be exactly straight from one end of the drive shaft entering the carrier bearing to the other side exiting it otherwise you'll get a bad vibration. Any truth to that? I plan on going 12"-13" of lift...will the stock drive shaft even be long enough?

 

I'd take 4" blocks (from a junkyard dodge) over no blocks and a shackle flip. I've ran both and the former was more stable when loaded. Axle wraps really isn't an issue at stock power levels if you're not in low range.

 

Yes, lower the center bearing mount and set up like as if it was a one piece shaft. Stock DS will be too short.

 

How would it be more stable?

Axle wrap all depends on what you do with the truck, regardless of power. More prevalent with hard launches and power? Yep....a non issue stock? All depends.

 

Since stock DS is too short for the amount of lift I'm going for would there be any benefit to converting to a one piece DS (if even possible) and ditch the carrier bearing altogether or would it just be too long and heavy to have a one piece DS with no carrier bearing?

 

You could make a one piece out of some thick wall stuff, but it better sure as hell be perfect in every way.....if it's not straight it'll start "swooping" like a jump rope and vibrate you to death.

 

I mean when it comes time I'd have a driveshaft shop do it wether I lengthened the stock one or made a custom one piece DS...guess I just see what they think and which is more price effective.

 

Quote:

Originally Posted by 0500757Arf

Whatdo you guys with rear drive shafts with carrier bearings doing when you liftyour trucks? Just drop them via a bracket of some sort to straighten out thedrive shaft? I would think you'd want to drop the carrier only a littleso that it would be a happy medium on all U-joints but I've also been told thedrive shaft should be exactly straight from one end of the drive shaft enteringthe carrier bearing to the other side exiting it otherwise you'll get a badvibration. Any truth to that? I plan on going 12"-13" of lift...willthe stock drive shaft even be long enough?

With a two-piece driveshaft you want the front part of the ‘shaft straight out of the transfer case. There should be an angle in the U-joint between the two pieces of the driveshaft (this angle should match the angle in the U-joint at the rear diff).

So when you lift a truck with a two-piece driveshaft you should NOT lower the carrier bearing. This will keep the angle in the front joint the same as it was from the factory (zero) and will keep the angles in the other two U-joints equal (assuming you don’t rotate the nose of the rear diff up or down as you lift it).

Caveat: this will not work well with a big lift because the angles in the middle and rear U-joints get to be too high. In that case you can lower the carrier bearing some (and rotate the diff nose up some) to reduce the U-joint angles, but this will introduce some vibration. What you are doing in that case is trying to find the setup that’s least bad. You aren’t going to get it “good” (although it may well be good enough).

If you want to do it right with a big lift you need to use a CV joint at the transfer case, drop the carrier bearing and rotate the front of the diff up to match the new angle at the middle U-joint.


Boring Engineering theory:

Single Cardan U-joints (the normal kind on driveshafts and front steering knuckles) cause the speed of the driven shaft to change as they rotate when they are going through an angle. If one side of this is hooked to the engine (that doesn't want to change speed quickly) and the other is hooked to the tires (which also don't want to change speed quickly) you get a vibration.

The way you prevent the vibration it by putting a second single Cardan U-joint in-line with the first one, going through the same angle, but rotated 90 degrees out of phase with the first one. That way the speed change from the second U-joint exactly cancels out the speed change from the first U-joint. The shaft between the two joints changes speed, but since its inertia is low you don't really notice it.

A double Cardan U-joint is simply two single Cardans assembled together so they have to bend through the same angle and are 90 deg out of phase with each other. These are also called CV joints (for Constant Velocity) when used on a driveshaft (there are different kinds of CV joints used in steering knuckles).

You do the same thing when you set up a one-piece driveshaft correctly. You set the angles at the two ends so they are the same and assemble it with the two joints out of phase.

But two-piece driveshafts typically have three single Cardan U-joints. So to eliminate vibration you really do want essentially zero angle on one of the joints, with the other two going through the same angle (as each other) and out of phase with each other (it doesn't matter how the first one is phased). Theoretically it doesn't matter which U-joint is straight and which go through the angles, but practically it's impossible to get that to work out unless it's the front U-joint (at the t.case tailshaft) that's straight and having the U-joint behind the carrier bearing and the one at the rear diff going through the angles.

 

As '89F2urd said, if a long driveshaft isn't perfect it'll tear itself apart. I'd be shocked if a reputable shop would make a one-piece 'shaft for a crew cab long box truck.

 

Bob, if you lower the center bearing mount, how will the u joint angle at the t case be " the same as it was from the factory"?

When I said lower the mount and set up as if it was a one piece DS, I meant to set it up with no angle on the center joint and equal but opposite angles at the t case and axle.

And yes, a single CV,single U joint DS SHOULD have no angle on the single u joint...but it doesn't have to be that way to work fine. The front DS on a stock 350 is set up that way.

I have found that all the theory about the correct way to set up joint angles doesn't always hold true and you just have to find out what works through trial and error. Have seen plenty of "wrong" set ups work fine.

 

I said NOT to lower the center carrier bearing. So clearly with the transfer case and the carrier bearing in the factory locations the U-joint between the two will be the same as factory.

That can work until the suspensions moves. Then the center U-joint is no longer straight, and the angle at the rear joint changes.

But yes, this is one of the "good enough" ways it can be done.

Yeah, the front driveshaft in my '71 Bronco is like that too. I don't know how well it works in an F-350, but the Bronco only gets away with it because it doesn't get driven fast in 4WD. There is definite vibration in the front driveline that is tolerable at lower speeds, but I wouldn't want to drive it on the freeway like that.

In my experience the theoretically correct way always works unless it leads to other problems, such as if the angles get too high or in front driveshafts where setting the U-joints correctly would screw up the steering, which would be much worse. And in cases like that we're all glad that you usually don't get much vibration with a carefully set up "wrong" setup, such as you are suggesting with keeping the center U-joint straight in a lifted crew cab.

 

Bob, do you have a 12" lifted truck?

 

No, but are we disagreeing about anything? I'm saying that the "right" way always works best until you start pushing other limits. And when you do push those limits you can compromise and find "wrong" ways to do it that end up working well. Are you saying anything significantly different?

 

But if you don't lower the carrier bearing wouldn't you have CRAZY drive shaft angle after the carrier bearing? Yes it would be stock angles at the transmission and prior to carrier bearing if you don't drop the carrier bearing but after the carrier bearing the angle would be insane which is why I felt it best to just divide the angle up the best you can between the two ends while maintain as straight as possible at at the carrier bearing.

Also what about the front shaft because I want to keep my 4x4? I'm assuming all I can do is lengthen it as it's a single piece shaft and I'll have to deal with more frequent U-joint replacement and maybe some more vibrations?

 

Depends on how much lift. With the 13" of lift you are talking about, probably yes. Go through the discussion Briansshop and I had earlier in this thread. When you are going well beyond stock you can't do things the "right" way. That's when you have to find whatever gives you the best compromise.


Briansshop can probably give you an answer based on his similar experience. Based on my 3" lifted Bronco (I know, not the same...) I get a vibration in the front driveline that is pretty objectionable at highway speeds, but is OK where I usually use 4WD (I don't drive the Bronco on winter highways). So that's one way to address it.

Another I've seen (and am considering for my Bronco) is to put a double-Cardan U-joint at the diff end of the front driveshaft too (it already has one at the t.case, as does an F-350).

You MIGHT be able to make it a little better by rotating the axle forward a little (but not too much so you get your caster too far out of whack).

Or you can rotate the knuckles on the axles so you can point your diff at the t.case without screwing up your caster. Not a simple thing to do though.