The Stovebolt Site Forums The Shop Area Driveline Strange vibration

1958 Chevy. Under load the driveline runs smooth. When I start braking or push the clutch in and it is free wheeling I feel a vibration. It has new u-joints. I thought the issue was worn bearings. I just had all the bearings replaced in the differential and the axle bearings. I took it for a test drive and it was doing the same thing. Now I am stumped. I had a guy tell me to adjust the clutch? Could a clutch adjustment cause this? Any other thoughts?

If it has a 2 piece driveshaft it could be the drive shafts out of phase. The carrier brg could be bad but certainly that would be discovered when the u-joints were replaced. The pinion angle might be wrong or the spring bushings worn & letting the rear end shift.
If you sit still & race the engine in neutral can you feel it?
I can't imagine the clutch adjustment causing this.

George

One piece drive shaft and I made my best effort to align the two ends correctly when I replaced the u-joints.

If you put the transmission in neutral while driving and let it free-wheel while in neutral will it do the same thing? If so, you can rule out the clutch. If no...that is if this only happens when the clutch is disengaged it could possibly be something with the throwout bearing or something in the transmission. Long shot on the pinion angle I'd think but not an impossible thing. I've had transmission vibrations that vanished when under load and re-appeared when free-wheeling.

I will do another test drive and drop it into neutral to see what happens.

Put the rear axle on jack stands, run the engine at idle & in gear. Crawl under the truck with a piece of chalk & hold it close to the driveshaft in the center & towards the ends. The marks on the shaft will show any runout or bent shaft. Check the driveshaft for any dents. Sometimes guys will put the shaft tube in a vise while replacing U-joints & put a dimple in the tube. Those can cause vibrations.

George

Another good test, stand out away from the truck but close enough you can see under it. Have a helper start the engine and let the clutch out slowly while holding the brakes, you are looking for the pinion angle to change or spring movement, really anything that it shouldn't be doing. Try Reverse also, it's easier with automatic transmission, but should be able to " load " the driveline enough to see if it all stays in place.

I'm not sure I would recommend this. The NHRA viewed this type of activity so unsafe that they no longer allow vehicles in the pits to use jackstands in this way. You have to use a stand that you can pin to the chassis if you want to run the car in gear. A way to ensure the vehicle will not come off the stands. When you see them "whack" the throttle during tune ups, those vehicles jump around so much that this makes a lot of sense. I definitely would not recommend getting under a vehicle that is running and in gear! That's just asking for trouble.

I thought you and The Beach Boys were on to another song! My 2 ton with the SM420 will run smooth under load, bearing noise in the tranny when decelerating, push the clutch in while coasting at speed and it still has some noise.

I was a little apprehensive about sticking my hand into a spinning driveshaft, but hey, I took my life into my hands with my wife when I bought a second project truck. I already violated the safety rules once. Prior to rebuilding the differential I put it up on jack stands, put it in gear and throttled up using the throttle cable in the cab. I then got out with it revved up and went to the back to see if I could spot where things weren't right. I currently don't have the bed in so I was able to watch the driveshaft. It really didn't tell me anything. I was hoping for some flashing arrow pointing to the problem.

In case I wasn't clear, the truck was running at more than an idol. It was in gear and no one was in the cab to put on the brakes if things went south. I guess we have all done some dumb stuff. At least I hope I am not the only idiot working on these trucks.

I just got back from another test drive. Dropping it into neutral changed nothing. I am probably wrong, but I almost want to say the source is the front end but that makes even less since because it does not happen under load. I would think if I had a bad tire/wheel or steering component it would be consistent, even under load.

Have you considered motor mounts? They are up front and under load they would be under tension/compression, but not otherwise.

I had not but I will check.

Hmmm. I'm assuming you have the torque tube setup, yes? And this wasn't happening prior to changing the U-joint? Sorry if I've missed something here, Dave. Just trying to unscramble the egg a bit more. Thinking about this while shaving this morning, I did many years ago have a somewhat similar situation and it turned out to be the muffler (which was going bad). At some speeds it would produce odd vibration noises which would disappear once I slowed down. But this seems to have begun when the U-joint was changed, so I doubt it is something like that...although if you move the exhaust pipe I've seen those start to make odd noises sometimes...rubbing on the manifold. Another long shot.

No, open driveline. Yes it was happening before the u-joint replacement. U-joint replacement and differential rebuild have been in response to the vibration. I can feel the vibration in the whole truck, including the steering wheel. It is not OMG, what is happening, but it is noticeable. I have not checked the motor mounts yet as suggested by Fibonachu

Have you had the tires balanced?

I have not, but I would think it would be constant and would not go away when under load.

I see. Maybe a brake drum catching a brake shoe as it turns? I've had that happen before and it did cause an odd vibration until I had the drums turned. However it was a constant thing unless I was driving at less than 30 mph (didn't notice it then). If your front end is stock and in good nick, I don't see what might cause that sort of vibration except for brakes, wheel bearings, etc.

I had an Oldsmobile which suffered a failing motor mount and when accelerating the fan would rub against the shroud but that was a very obvious thing in terms of the noise it made and I doubt you have that same sort of setup.

I'll keep my thinking cap on here, Dave.

Are the U-joint yokes phased correctly?

Ed

Don't make assumptions about the effects of imbalanced tires. They do different things under different circumstances. Make sure they're all inflated properly , too.

Rotate the tires front to back , back to front, make sure they're inflated and see if there is any change when you do down the road.. How old are the tires?

Much wisdom is spoken here. I have been down this road before and seen some weird effects that ended up being tires.

A rotation and balance is quick, cheap, and easy.

U-joints are phased. I will try moving the tires around and confirm inflation PSI. I was also wonder if anything in the front steering components could behave like this. Bad tie rod or something?

When did the problem start? Underload means here you are pulling a trailer or a heavy load in back?
How much? If a heavy load makes the truck roll smooth ,then it might very well be the tires.

By under load I mean I am accelerating or moving under power of the engine, putting tension on the driveline. When I push in the clutch and I am freewheeling/coasting is when I feel the vibration start. As soon as I release the clutch and start to accelerate the vibration stops and the ride smooths out. No trailer or anything in the bed.

Ok, thanks. Clutch adjustment/engagement maybe.

flywheel imbalance?

My 2Cents: Check engine / transmission mounts real good,also check for debris ground out power train to frame or body.
In my former life I have seen several rigs get u-joints and way more tossed on them only to have me show them the very small rocks that I removed from the transmission mount.

Just to clarify: was this vibration present before you did the u-joints?

You said it was not affected by the bearings, but it is unclear to me if the ujoint replacement was related or not.

The vibration was there before I did anything to the truck. Before u-joints. Before differential bearing replacement. Before read axle bearing replacement. Before pinion yoke replacement. It is not the motor mounts either. One other point. I keep saying when I push in the clutch but even with the clutch engaged, if I am reducing speed it does it. It only smooths out when accelerating.

Maybe there’s a bad spot on the “coast” side of the ring and pinion engagement. It might be worth taking the cover off to inspect it.

I'd start taking frame/chassis measurements to see if your truck is going down the road like a three legged dog.

Is the rear axle centered on the springs?

You need to have the driveshaft checked for runout & balance at a drive shaft shop.

George

I talked to a couple driveshaft shops about checking the balance. They said they could but both asked my why. They said they did not balance or put weights on them at the factory. One of the shops was the shop I had rebuild my differential. Unless there was some damage to the driveshaft there would be no reason for it to be out of balance. That said, I am getting desperate and out of things to check. I might need to do it just to mark one more thing off the list.

"did not put weights on them at the factory " , must be talking about AD trucks? I've seen many modern driveshafts with balance weights. Do what George suggested , I'd get it checked and checked off as you've said. Please keep us informed.

I am going to call BS on that one. All of my custom driveshafts have balance weights, the stock ones from a 78 GM and and 85 F**d both had them.

It is possible that GM didn't balance them in the 50s with the expectation they would be spinning too slow to matter, but I have some doubts.

Take a long straight edge & hold it on the shaft in several places to see if it may be bent. I'm still thinking runout.
Does it have a slip joint or does it slide in & out of the trans.? Check to see if one of those are worn.
Look the shaft over closely for any dings or flat spots

George

I lean away from the driveshaft but it you think it is out of balance, here is a trick you can try. It will take some time and you may get tired of crawling under to do it, but it can work. Get a large radiator hose clamp and put it on the driveshaft. Mark where you put it (where the screw part goes) with some chalk or grease stick and start by putting the clamp around 6 inches back from where it connects with the differential. The large part of it will act as a weight. You may have to use 2 or 3 if the shaft is really out of balance, but I'd start with one. Then go drive it and see what happens. If it is worse or no improvement, rotate it about 30 or so degrees (does not have to be exact), make another mark and try that. Keep going until you see if anything changes until you work all the way around your "circle." Then move forward 6 inches and repeat. Like I said, painfully slow work. If you find a spot where the vibration goes away, tighten the clamp well, paint it black and leave it in place.

I could be wrong about factory weighs. I know very little about the driveline. Just what I was told.

That would do it!

The stock muffler mount on an AD clamps the muffler in place fairly close behind the crossmember with the large gusset plates just behind the cab. If that's in place the muffler will sit pretty close to the bed cross-sill. But it's worth a check.