1989 6 speed. All stock suspension (no poly, rod ends, etc.)

I have a pretty strong vibration that seems to come from back of the car...

I've read through a few threads here and so far tried...

1. Jack up the rear and put it on jack stands.

2. Ran the car through the gears - see what/when it shook.

3. In 1st at idle, I noticed both rear tires appear flat spotted, so I removed one at a time. Vibration got better each time, but still there when I gave it some throttle.

4. Took a glance at the drive shafts - didn't see any obvious vibrating. Not too in depth at this point I know, just looked for any obvious trouble.

5. Ran the car up through the gears to ~2,500rpm - fairly smooth in 1-5, but in 6th at 1,800rpm ~83mph - jackpot!

I'm going to take a break from it and go back and try different speeds, different gears - see if I can narrow it down.

Any ideas, thoughts, avenues to pursue would be appreciated.

Could be more than one thing. If things got better w/o the tires, you'll have to eliminate that i.e. check their balance and see if the flat spots go away when hot. They may be aggravating the whole situation and you won't know until the balance is checked.

Then you can move on to the rest of the diagnosis.

Hi Jeff, welcome to the Guru

Hopefully we'll get warmer weather for the next autocross!

Thanks Brian...

I lurk here a lot...great people and information. Reminds me of the F-body board I used to frequent.

Oh, and it can't get much colder than the last (first SCCA) event! In case anyone's wondering, mid-40's, stiff, relentless wind all day and at one point - a light sleet shower. No worries however, in 6-8 weeks it'll be 90+ at Devens...

Bill..I'll get the tires checked. I ran it through the gears, again with out the tires. Vibration seems at its worst right around 80mph in 4th, 5th, and 6th.

While the car is up, I might try to pull the prop shaft and give it a look-see.

What is the condition of the ujoints? Are they rather new? or really old?

U-joints are new-ish, 3 years maybe.

Dealt with a few other items I found underneath - exhaust hangers, grease joints and bushings..squirrel!

Road tested the car to pay closer attention to when the vibration occurs. About 30-35 on up...little worse in 6th, but definitely seems to be related to final drive speed, not engine RPM. Pretty bad vibration.

Sorted the other items because I'd like to take a good look at the drive shafts - maybe remove them and take them to a shop for inspection. That won't happen for a couple of weeks.

Would anything in the diff. cause bad vibrations?

So, balance the tires, pull and inspect drive shafts...

It is possible something in the diff is causing this... but I like your first approach.

Balance the tires, and go from there.

Can you rotate the tires? If the vibration moves...

and what brand are they?

Check the wheels at the inside & outside lips for lateral & radial run out = .030 maximum on each plane.

If the rear jack stands are placed under the body jack points = to much half shaft angle. Place the stands under the shock mount bolts, retest.

Inspect all u-joints for signs of wear. Any redish dust = a failing joint.

Car in gear/park engine off have someone try to turn a wheel forward/back while you inspect each joint. There should not be any play/movement between the shafts & companion flanges/front yoke.

If all checks ok have the tires balanced on a Hunter road force balancer.

GL

Pulled the main prop shaft. Front u-joint at the slip yolk is smooth on one axis, notchy on the other. I'm going to replace the u-joint and continue on...

Make sure that the joint fits right!

I had an issue just like this last summer. Damned thing kicked my ass hard, too!

What I learned is that over time, the aluminum yoke will compress a bit. That and debris build up in the e-clip groove.

Clean that groove out, and relieve it a bit with a dremel tool if you must, but make sure the joint fits evenly and isn't binding in any way.

Also, replace the grease they come with with a good quality high temp high pressure grease.

What kind of shape are your wheel bearings in? That could be a factor, and since the vibration got better when you removed the wheels that might be something to look in to.

Wheel bearings new on April 4.

I read the thread on u-joints. I chucked a wire brush in a drill, cleaned out the bore and groove and re-installed the e-clips. Took some measurements with a machinists rule and discovered the yoke was spread slightly. Put some light machine oil on the cups at re-install to make them go in a little easier, and a little work with a 3-pound hammer got the u-joint to rotate freely.

Both u-joints looked good, but I replaced the front joint for good measure. I didn't see the tip about replacing the grease until I had re-installed the shaft.

Re-installed the prop shaft and noted the vibration was improved - didn't shake as much from the center, but still some serious shaking out back. Seems a bit worse under accel around 50+ - a little better under steady state, gets worse again under decel. Not a big change in vibration, but enough that I think I am feeling a bit of a difference.

Next I'm going to try one half shaft at a time. At this point, it's easier for me to mess with u-joints than get the tires balanced.

Feeling a little silly...

Bushings on the left side strut (camber) rod are SHOT!

(inner bushing rubber torn, outer bushing metal sleeve out of round, i.d. opened up - lots of play)

Yeah that will help magnify a wheel vibration.

I ran flatspotted tires on my vette for a few trackdays because I'm too cheap to replace otherwise good tires. Car would shake like hell at 120. It has to be hard on bearings too.

The hood would literally shake. I had em on front because I'd rather it cause loss of front traction than rear traction.

Time to call Banski!!!!

He's runng in the SCCA stock class, so he needs to keep the stock rubber.

Real PITA sine you can't buy some of that stuff separate.

you can't buy any of the rubber separate. GM only sells it as an assembly, the aluminum bit with rubber already in it.

Grand Imperial Pooh-Bah Can you PLS post the part number and price if possiable.

yuck...

no one list the part... lots of aftermarket options.

Try calling vette2vette.com on Monday and see if he has a clean one...

Sorry I haven't posted lately - life (work, fam etc) kinda gets in the way sometimes, lol...

Vibration is worst at around 50mph...get it to 60-65 at least is't bearable. Sorry officer, you see, I have this vibration at 50....

Probably won't have any time available to do more serious work for a while, but I still need to get the tires balanced.

Brian's correct - no Banski for me, although I'd love to run Tom's stuff. Maybe, maybe poly and jump to BSP depending on the quality of used stuff available, since that seems to be the only option.

Thank you again for all the suggestions...still working.

Are you sure the main drive shaft is indexed correctly?

The half shafts don't matter, but the main prop shaft does. If it's 180* out, you are going to get epic vibration.

FWIW I've had my driveshaft out a number of times and never had any reference points with which to reinstall it. Never noticed an issue from it.

Tires.

BillH nailed it in post 2!

Put a different set I had lying around on and went for a ride. Noticable difference as soon as I pulled out of the driveway.

It's not silky smooth, but vibration is mostly gone.

Thanks again!

tires can do it! a bad carcass and all bets are off. Glad you found it!!!

fyi see the loose stub axle thread he has going

http://www.corvetteguruforum.com/modu ... hp?topic_id=19158&forum=1