Driveline Vibration
 

My truck has a resonant vibration at highway speeds thats driving me crazy. I've searched this topic and read all the responses to others having this problem and still have a question. Has anybody here had to adjust pinion angle after lowering 3.5/5? My truck is a 72 longbed with a two piece driveshaft. I took the driveshaft to a driveline shop and had all the u-joints changed as well as the carrier bearing, then had it balanced. The transmission has been rebuilt and so has the rear end. I know to always go back to the last thing worked on before the problem started to troubleshoot which would be the rear end. I found some slop in the pinion bearing that my mechanic is going to fix next week but I'm still wondering about pinion angle.

Re: Driveline Vibration
 

Yes, pinion angle should be set at ride height. On a two piece driveline the first driveline should have a 1* true u joint operating angle and the pinion shaft should be parallel with the first driveline to cancel out vibration. Some good info here http://67-72chevytrucks.com/vboard/s....php?p=3822835

Re: Driveline Vibration
 

why not take it to the drive line shop. if you already spent money there they should have some free advise.

Re: Driveline Vibration
 

some vendors sell the angle shims cheaply that go under the saddles

another thing to check is make sure the drivshaft tyoke isnt going too far into the tranny// when i lowered 4" i trimmed about 3/8" off the yoke to b sure

Re: Driveline Vibration
 

Thanks. How did you trim it? On a band saw?

Re: Driveline Vibration
 

Just curious, does the vibration go away in neutral?

Re: Driveline Vibration
 

just used a chop saw with a fiber cutoff blade then dressed the cut with a good file

Re: Driveline Vibration
 

It is significantly reduced when the driveshaft is loaded, either accelerating or decelerating. When I'm cruising and feathering the throttle to maintain a constant speed is when it's the worst. Most of the time above 55-60 though I have a light rhythmic vib.

After looking at a buddies trucks (3) and seeing how much his slip yoke was sticking out of the trans tailshaft (about 5/8" - 3/4") I'm pretty sure that mine is either bottoming out or close to it. I don't have any stickout whatsoever. I took it back over to the driveshaft shop to get shortened about 5/8" and have a brand new slip yoke put on it just to be sure that isn't the problem.

Cdowns, will it bottom out on the inside or the outside of the slip yoke? And just out of curiousity are there any shims or washers in between where the shafts bolt together?

Re: Driveline Vibration
 

So Im not hearing anybody here say they had to mess around with pinion angle after a drop. I also called ECE and they said I shouldn't have to mess with pinion shims with the drop I have so I'm guessing I need to look elsewhere to find the source of the vibe.

Re: Driveline Vibration
 

I've got one as well at about 58-64mph when I'm feathering the gas to maintain speed. Mine is sticking out about 1-1/4" which might be my problem. I'll be watching this thread.

Re: Driveline Vibration
 

What about the transmission? Could anything in the trans rebuild be the root of the problem?

I've got a similar problem. Just a slight vibration between 40-42, 50-52. It's barely enough to feel and make the mirror shake, but it's there. In my case, the engine was rebuilt, but the trans was so far gone that it was hard to drive at all much less feel any vibration. Trans got rebuilt and everything runs well, except for the slight vibration.

I haven't lowered it, I haven't gotten the driveline balanced, and I haven't checked the carrier bearing. My first thought is now that I have all this new HP hooking up to the driveline it's showing some wear in the CB and now it's possible to notice some imbalance in the driveshaft.

But I did notice it after the tranny rebuild...And you have a rebuilt trans as well...

Re: Driveline Vibration
 

I've considered that the transmission may be the root cause too. I'm putting a different one in on Monday. I'll let you know what happens.

When I talked to ECE yesterday, Stan (I think) told me that lowering these trucks actually improves the borderline excessive pinion angle the trucks had from the factory.

Re: Driveline Vibration
 

Driveline vibrations are often torque sensitive, so the fact that it varies based on load is a good clue, pointing to the driveline.

The light rhythmic vibration is called a "boom/beat", and that means there are two vibrations going on at the same time, canceling and adding in amplitude as they go in and out of phase.

It is very likely the driveline (u joints are a second order of driveline, that is, they give you a "kick" two times for every time the shaft goes around) phasing with engine firing frequency.

1) Does the rhythmic vibration go away if you chose a different gear (while maintaining the same speed)?

2) Did you say what engine you have? V8 firing is four times per rev (4th order), I'd have to check to see what it is for an L6.

K

Re: Driveline Vibration
 

It's a 350. I'm fairly certain the problem is either transmission or rear end. I'm leaning more toward the rear end though because it is the last thing worked on. I pulled a driveshaft out of another truck in the junkyard for troubleshooting purposes. I installed it like it was, old u-joints, carrier bearing and all, it didn't change the vibration at all. Yesterday I put a set of u-joints and a carrier on that old driveshaft and still had exactly the same vibration. I'll have my driveshaft back from the shop today. It will be 5/8" shorter to alleviate the yoke bottoming out and it will have a new slip yoke. I'll update this thread with the results.

It is RPM sensitive though, not speed. I don't have a tach but I can recreate the vib at lower speeds in a different gear. That tells me the problem is somewhere between the motor and the pinion gear. I'm pretty sure it isn't the motor or the driveshaft.

Re: Driveline Vibration
 

I had a vibration in my 72 GMC (leaf spring) that was lowered. I tried many things to fix it.

After I destroyed the pinion yoke at the drag strip from wheel hop, I upgrade to a 1350 pinion yoke and there was no more vibrations.

Re: Driveline Vibration
 

Drive line and pinion angles are not the same, dropping does improve driveline angle but a 5" drop w/o a 2" lowering block changes the pinion angle ~5.6* and a 3" drop with a 2" lowering block (5" total drop) changes the pinion angle ~ 3.3*. Assuming the pinion angle was perfect before the drop both of the above changes puts it out of spec (IMO). While it is apart would be a good time to at least verify your angles a cheapo angle finder would work for this.http://ep.yimg.com/ca/I/toolsplus_2097_350297053
All angles were calculated on cad layout with real word dimensions.

Re: Driveline Vibration
 

Ok - so that rules out driveline hardware, like u joints and/or a bent or imbalanced shaft...but not driveline angles.

This is interesting, because driveline would definitely be speed related. Can you re-create the problem by running the engine up in neutral? How about with the vehicle not moving, like with the rear axle up on jackstands? (play with the driveline load by gently varying brake pressure).

The fact that it is a "boom/beat" still implies to me that there are two things going on, not just one...

K

Re: Driveline Vibration
 

Are you accounting for rear axle housing rotation as the suspension travels, based on it's instant center location? Or just translating the axle up/down the desired amount?

Not throwing stones; a sincere question in the hopes of gaining understanding...:thumbs:

btw -I used to do this with masking tape lines spread out on the garage floor. Your way is better :lol:

K

Re: Driveline Vibration
 

deleted (sorry)

Re: Driveline Vibration
 

Also - I assume your truck has the control arm suspension (not leaf springs) -

Which trans? Auto or manual?

I assume it didn't do this before lowering?

K

Re: Driveline Vibration
 

I'm having the same problem. I even raised the carrier bearing by rebolting higher on the cross member. I took it back to the rearend guy, they were looking at the driveline and my rear and front driveline are almost straight, he told me to put about 6 degrees of angle under the saddle to the the nose down. The question i have for him was correcting the front drive shaft angle. He really didn't have an answer. There is only so much room to drop the driveshaft and rebolt it. I have changed trans, ring and pinion, u-joints. Checked the wheels and tires for splits and still have the same vibration. The real question is do we have to go to a one piece drive shaft. Then put the 6 degrees under the rearend and then the angles would be right. My father in law has a 63 long box with a one piece from the factory. The driveshaft is big and thick and rubs from time to time on the crossmember hoop. There is a reason why the factory went away from that. What we really don't know if both shafts are vibrating or if its only one?

Re: Driveline Vibration
 

I did make a few assumptions,
1, It is a trailing arm suspension with the axle fixed to the TA.
2, The operating radius of the TA pivot to the axle centerline is 51-1/4".
3, The instant center or TA pivot location was estimated (idk the actual location in reference to the axle).
4, The axle was moved to its new location (5" drop) by rotating the assembly at the TA pivot which does rotate the axle.
I realize the actual location of the pivot affects the amount of actual pinion rotation so I stayed on the conservative side in relation to the quadrant of the operating radius. Either way it illustrates how the pinion does rotate as the suspension travels. Let me know if you feel I missed anything my moto with cad drawings is "junk in junk out" if my assumptions are wrong than so are my results.

Re: Driveline Vibration
 

I had the same problem. changed, balanced, turned you name it. Have any of you changed motor or transmissions????? Check your flex plate and make sure it is for your motor. They are different for 350's and 400's and more. Some have counter balancers on them other have notches cut/drilled for correct weight. This was my problem and has been many other peoples as well. Just a thought. worked for me. Gary

Re: Driveline Vibration
 

Nope - that sounds right to me. So it sounds like you've comprehended changes in pinion angle based on the travel of the rear axle through it's suspension "arc".

Well done-

K

Re: Driveline Vibration
 

Thanks, yeah I based everything on the 51-1/4" operating radius. I built a set of trailing arms with a built in 2" step (drop) with the end section rotated for pinion correction, http://67-72chevytrucks.com/vboard/s...d.php?t=336430 once I get around to installing it I'll know the exact location of the TA pivots and will have the pinion angle changes figured out pretty good.