[getmygoat]

Hi all,

I've got a significant vibration in my 1970 GTO over 60 mph... All the tires have been balanced, driveshaft rebuilt/balanced, motor/trans mounts new, and the bushings and wheel bearings in the rear are new.

The vibration continues when the clutch is pressed in at speed.

I called a shop today and asked them if they could look at it, and what their diagnostic process would be.

Basically, they said it would be to put it on the dyno and look at the various parts to see if they could see anything.

That sounds like a pretty lame approach, don't you think?

What I'm trying to avoid is spending $500 so they can find nothing.

Thoughts?
Thanks, Fred

[Jack Gifford]

Bent wheel? Bent rear axle?
If any drum brakes on the car, any pieces of drum ribs broken away?

[smagnotti]

If no vibration is present in the motor with the the trans in nutral and the car stationary then the problem could be a shifted belt in one tire.

[Derding]

I too had a vibration on my Firebird over 60 mph. Turned out it was a bent axle.

[David Jones]

Have the tires been balanced on one of the computer machines that can diagnose bad wheels and/or tires?

[getmygoat]

Originally Posted by David Jones

Have the tires been balanced on one of the computer machines that can diagnose bad wheels and/or tires?

I don't know.. it was balanced at least 2 (maybe three) times. Twice it was done by places that do "on car balances".... didn't really help.

The car is not enjoyable to drive, after sinking $$$$$$$$$$$$ into a major rebuild... frustrating.

[Keith Seymore]

a) Does the vibration "peak" at 60 mph, or does it continue to get worse as you increase speed?

If it "peaks", that might suggest a driveline imbalance, since they will vibrate at their resonant frequency but don't generate enough energy to continue the disturbance seperate from the resonance. If it continues to get worse that could imply a tire/wheel problem.

b) Can you put the rear axle up on jackstands and re-create the disturbance?

Again, if it is still there, then you have eliminated the front tire/wheel/brakes as a problem. Conversely, if it is not there, then it implies the problem is in the front somewhere.

c) Along those lines, where do you feel the disturbance? In the seat (ie, your backside)? In the steering wheel?

d) With the car stopped and trans in nuetral, can you run the engine up to the same engine rpm that you see at 60 mph? Is the vibration still there or gone?

K

[getmygoat]

Originally Posted by Keith Seymore

a) Does the vibration "peak" at 60 mph, or does it continue to get worse as you increase speed?
K

No it doesn't peak at 60 mph...the vibration starts around 60 but then varies all the way up.. things maybe smooth out a little around 85... but not completely.

The vibration also varies somewhat with throttle position. - but I can't tell if it really varies or its just my imagination.

Originally Posted by Keith Seymore

If it "peaks", that might suggest a driveline imbalance, since they will vibrate at their resonant frequency but don't generate enough energy to continue the disturbance seperate from the resonance. If it continues to get worse that could imply a tire/wheel problem.

b) Can you put the rear axle up on jackstands and re-create the disturbance?
K

This was done during the on-car tire balancing process.... and the problem didn't seem to be as bad when this was done - but then again there wasn't as much load on the driveline either, so I don't believe this isolates it to the front.

Originally Posted by Keith Seymore

Again, if it is still there, then you have eliminated the front tire/wheel/brakes as a problem. Conversely, if it is not there, then it implies the problem is in the front somewhere.

c) Along those lines, where do you feel the disturbance? In the seat (ie, your backside)? In the steering wheel?
K

The vibration is definitely in the seat of the pants... although if I pay attention I can also feel it somewhat in the wheel.

Originally Posted by Keith Seymore

d) With the car stopped and trans in nuetral, can you run the engine up to the same engine rpm that you see at 60 mph? Is the vibration still there or gone?
K

No vibration in this case. Also no vibration when in first gear at same RPM.

[Ollie]

I would say driveline. With the e brake on and trans in gear Jack up the car. Try to twist the drive line if there is any movement or if you see any red rust looking powder around the u joint they are going bad.

[getmygoat]

Originally Posted by Ollie

I would say driveline. With the e brake on and trans in gear Jack up the car. Try to twist the drive line if there is any movement or if you see any red rust looking powder around the u joint they are going bad.

Car is a 4-speed, u-joints are new.

[johnta1]

Take the driveshaft loose at the rear and rotate it 180º then bolt back together and try it.

[Keith Seymore]

Originally Posted by getmygoat

This was done during the on-car tire balancing process.... and the problem didn't seem to be as bad when this was done - but then again there wasn't as much load on the driveline either, so I don't believe this isolates it to the front.

Should still be a valid test, since the imbalance is not a function of how much load is on the system (not like a u joint angle, for example).

Sounds like tire/wheel/bent axle to me. I base this on the fact that it continues to get worse as you continue to get faster. Also, because it doesn't do it with an engine neutral run-up that means you can disregard anything forward of the trans. Doesn't sound like it's in the front, since you feel it primarily in the seat rather than in the steering wheel.

I'd put it up on jackstands again, let it idle and watch the rear tires/wheels for any wobble or bounce (ie, up and down vibration). I'd focus in on the rear axle, tires and wheels. Check the lateral and radial runout of the wheels and make sure they are in spec.

I could be wrong, so if later data points us back to driveline, I have some ideas there. But for now, it sounds like "not the driveline" to me.

Good luck -

K

[sts]

Basically, they said it would be to put it on the dyno and look at the various parts to see if they could see anything.

That sounds like a pretty lame approach, don't you think?


Sounds like a super lame approach. These guys are crooks, and you'd be best finding another mechanic.

I usually don't play hit or miss with troubleshooting, but the very first thing I would do is yank the axles and install a set of bearings. Anything rear with over 80k is on borrowed time for bearings. Toss in the fact that these cars get thrashed.

[Don66GTO]

Well I once owned a 96 dodge 1500 and it had a bad vibration at about 60MPH so bad it would make the doors vibrate enough to let air in around them. I greased the pitman arm and the idler arm and it would stop long enough for me to identify what the problem was and I ended up replacing both arms. after that there was no more vibration.
Plus the grease I used was way cheaper than taking it to some one.

[just501]

I had a similar vibration, started to come on at 60 and would go away at about 80 and stay smooth until I decelerated back to around 63-64 then would come back in. Could really feel it through the shifter and somewhat through the steering wheel. When I clutched it, the vibration would go away. If I slipped the clutch slightly, the vibration would not be as noticable. I too had the wheels balanced, but after changing the wheels with a new set, the vibration was gone. Hope this helps.

The new wheels were quite a bit taller than the old ones. Also, the rears are a little taller than the front, could have changed the drive line symmetry just enough to eliminate the vibration.

[getmygoat]

Originally Posted by sts

...the very first thing I would do is yank the axles and install a set of bearings. Anything rear with over 80k is on borrowed time for bearings. ....

Thanks... the bearings (both front and rear) - are new.

Originally Posted by don66gto

... greased the pitman arm and the idler arm and it would stop long enough for me to identify what the problem was and I ended up replacing both arms. ....

Thanks for the suggestion... but the idler arm and center link (and tie rod ends, ball joints, control arm bushings, etc) .... are all new.

Like I said, I spend major time and $$$$$ rebuilding this whole darn thing, that's why I'm frustrated the car still rides like a POS.

[fiedlerh]

A tire with a shifted belt, bulge or a stiff spot in it can be balanced, but will still vibrate.

Any missing weights on the rear drums? Usually you can mount a drum on a wheel balancer to check.

Drivetrain angles could be off. GM had lots of rear tranny mount part numbers, but most replacements are generic. Basically, the angle between the tranny and the drive shaft must equal the angle between the driveshaft and the rear end pinion. This goes for up and down and side to side. A common angle gauge can be used to check. Of course angles can change with bad motor mounts, bent tranny crossmember (floor jacked or twisted from thrashing it), bad or incorrect rear tranny mount, bad rear suspension bushings. Bad body mounts may not change the angles but can cause a resonanace.

Had one case where the driveshaft was slightly bent. It was balanced, but still vibrated. I suppose the same thing could happen with a twisted one.

Putting it on a chassis dyno and observing underneath sound like a good idea to me if the solution remains elusive.

[SS-TA]

As fiedlerh has suggested you may want to check your driveline angles. I had a GM A-body (Olds Cutlass) that developed a severe vibration at 55-60 MPH (~2500 RPM) that was clearly a driveline problem. At 2500 RPM in 2nd gear (T-350) no vibration at 60 MPH with gear in neutral same vibration. This all occured after a Engine/Trans rebuild. The motor mounts and trans mounts were replaced. The issue was the Energy Suspension urethane trans mount. This mount is way to tall for may applications ( I believe height is ~ 1.927 inches). I purchased three different rubber mounts (Anchor, NAPA and Pioneer) all had different heights the Anchor 1.88, NAP also about 1.86 and the Pioneer 1.78. I replaced the Energy Suspension mount with the shorter Pioneer mount and the vibration was gone. As indication on how far off the Energy Suspension mount is, the GM (OEM) trans mount for my 73 Firebird with a Muncie is only 1.80 inches even though ES claims their mount fits.

[wilcar]

I had the same problem with an ES trans mount, was too tall. I called them and talked to a tech rep and he said he knew that their mount was too tall in some applications but said he could not get anyone there to make a shorter one!!! Go figure!!!

[getmygoat]

Thanks..

I've been suspecting the trans mount as well... all the mounts are new (motor, trans)..... I'll check the angles.

If the rubber mount is too big, can I cut it down?... sounds like I should try the pioneer mount.