Vibration on deceleration at highway speeds

[Coontail]

I've been pulling my hair out with this one for a very long time now

So as the title says I get a strong vibration on decel at highway speeds (above 60 mph). I can make it go away by giving it throttle but as soon as I let off it comes back. If I do allow it to decel it doesn't go away until around 40 mph

The pinion seems pretty tight, can't seem to find any play. Also pulled the diff cover off and didn't see anything noticeabley wrong

I've already tried to remedy this with a couple things
-Rebalanced driveshaft, new u joints, straightened spline
-New pinion yoke (Tabs were worn)
-Transmission mount was loose so that was tightened

Front driveshaft is ~unavailable~
14 bolt FF
Custom Double Cardan Driveshaft

 

[Wes Harden]

it going to be pinion angle under decell. If you have lift blocks on the rear this adds leverage to twist the springs, allowing the pinion angle to dip down. Check your pinion angle to drive shaft tube. If that is good with in 1° or less then maybe time for a traction bar to control spring wrap. I have a vibration from @ 50mph that I am sure comes from springs allowing the pinion angle to move. Might do the traction bar soonish

 

[Coontail]

That's something I haven't really considered, I'll definitely have to look into it

 

[Wes Harden]

Also just on the off chance re torque your spring u bolts. This can cause a chatter issue

 

[Coontail]

I'll definitely check them, I don't see how it could hurt. I haven't gotten around to checking the pinion angle yet but I'm also concerned that there might not be enough of a spline. One thing at a time though

 

[Wes Harden]

Yes the spline not having good engagement might allow wobble at that rpm

 

[Coontail]

Well I got around to checking it and just by looking at it without a gauge I can tell it's way off

Driveshaft is at 21° and pinion is at 11°!!!

 

[Wes Harden]

yeah that is quit a bit, looks like you've got some tall lift blocks too. Lift block normally have an angle to them. double check that they installed so the angle is not contributing to the problem

 

[Coontail]

They do have an angle to them but are in the correct orientation

 

[Blue85]

Having an angle in the driveshaft is normal. The first question is what the angles of the pinion and transfer-case output are.

 

[Coontail]

Driveline is 2°
Driveshaft 21°
Pinion 11°

 

[Wes Harden]

Has a cv slip ? at t case. So pointing the pinion at the t case out put would be reasonable next step. IDK if is acceptable to to use shim with lift block.
I am thinking 4" shackle flip with shorter lift blocks and 6" shackles

 

[Coontail]

Do you realize that you only have about 1 1/2” of upward travel before hitting the bump stop?

HA

You know, I never did realize that...

 

[rampage]

HA

You know, I never did realize that...

Actually, never mind. Just checked and it’s offset from the spring plate.

 

[Coontail]

Has a cv slip ? at t case. So pointing the pinion at the t case out put would be reasonable next step. IDK if is acceptable to to use shim with lift block.
I am thinking 4" shackle flip with shorter lift blocks and 6" shackles

Yup, double cardan. Definitely way far off for that

Tom Woods calculator says I'll need a 5° shim. I'll have to do some research on using them with a lift block, otherwise I was thinking the same with a shackle flip. Tbh though I'd rather not spend the money if I don't have to

 

[Wes Harden]

That isn't a lot of up, and looks factory stop and spring. Spring must be well used.

 

[rampage]

That isn't a lot of up, and looks factory stop and spring. Spring must be well used.

The bump stop comes in contact with the axle tube not the spring plate. So he’s fine. If anything the bump stop should have a spacer to lower it so the shock doesn’t bottom out.

 

[Coontail]

It seems that as long as I use steel shims I'll be alright. The general concern with them is using aluminum cast shims which can break in half. So I'll order those soon and give an update after they're installed, thanks!

 

[JoshHefnerX]

Shackle flip and you may not even need to shim it, just shorter blocks. Or, if you can weld, move the perches. When I did mine I cut off the old perches bolted everything together loosely, rotated the axle to correct and tacked in. Pulled it all back out and laid in a good bead.

 

[Blue85]

Yeah, on a K5 with a lift in that range a shackle flip usually gets the pinion angle pretty close for a CV. Plus you can get rid of those terrible blocks and if you need a shim you don't have to combine shim and block.

If the height isn't exactly right after the shackle flip, you can always do a "zero-rate" or "add-an-inch" type of thing and if needed you can get those with a taper.