Hi all, I just noticed something that I want to make sure isn't indicative of a larger issue. I was driving today on the highway, and I noticed that around 75-80 mph I started getting what felt like a skipping vibration from the rear end of the car. Below this speed I couldn't notice it. So I drove in the slow lane (as usual) at about 70ish where it went away. It wasn't super dramatic, but it was there. The only things I can think of is that I just got some new tires, so maybe they did a bad job balancing them, or I also just redid my driver's side drum brakes, maybe I screwed something up there. The only other time I notice something similar, is that sometimes when I'm pulling up my very steep driveway, I'll slip the clutch a little to get up, and I get a horrible up-and-down skipping vibration from the rear which is really noticeable and very loud and dramatic. I have an 83' GS with open differential if it makes a difference. Thanks in advance.

 

Tire balance issue.

Take it back the shop should re-check it for you.

 

Yea, and the reason I'm asking is because there's no way it should be a balance issue. I took the wheels over there the first time, they put tape-weights on that were already falling off, from the extreme grime and grit on my wheels. I took them back and had them re-balance the wheels with actual tack weights on the rim. They should not have to do this 3 times. Ugh, I hate dealing with shops, there's a reason I decided to do all the work on this car myself.

 

'Up and down skipping' at low vehicle speed isn't indicative of tire balance - it sounds more like a chipped or broken ring gear or pinion. As the chipped gear comes around, the pinion has partial slip as the next tooth engages which causes a significant thumping sound at low speed - less noticeable at higher speeds on level roads and steady state or decelerating cruising. Would be worst any time you're applying torque at low speed.

This is your worst case scenario, and I hope it's just tire balance,

 

Well it's actually two separate issues I think. The harsh skipping has existed since I purchased the car and only happens sometimes when I'm slipping the clutch driving up my driveway. The high speed issue has only existed since I got the tires on. So I'm going to drive a few more days and see if I'm sure it's a tire balance issue and I'll take it to a different shop I guess.

Any way to tell if my other issue is more serious short of taking my differential apart? Maybe put some good oil in the differential? I also dunno how old the clutch is, could be factory, my car only has 56k miles. Thanks.

 

Are you maybe experiencing clutch judder when going up the driveway? I had that issue once on my 80 LS and it turned out that the previous owner did not have the flywheel turned when he replaced the clutch. This would cause a little judder sometimes when backing up or pulling into a steeper than normal driveway.

As far as the vibration on the highway, have the tire balance checked a suggested and find a shop that can do a road force balance. Its a much more thorough balance done on a Hunter brand machine and it will tell you where to put each tire on the car for the optimal ride. You may also want to check u-joint play just as a precaution.

 

75-80 is the harmonic speed for a driveshaft shudder that can start up if certain bearings in the trans are worn.

Try driving in 4th gear and see if it goes away.

 

Possible it could be an issue with the clutch, but I don't have enough experience to say for sure. A family member told me that it may just be that the wheels aren't balanced for that speed, since I just had them balanced at a regular tire place. I think I'm going to have everything re-balanced at an alignment shop once I get my suspension done. Since I hardly drive that fast, it's not an extremely pressing issue right now. I try to drive on the highway as little as possible.

I'll give that a shot, thanks. I rarely drive this fast, since speed limit is usually around 65 mph in my area. Maybe I just never noticed it.