68vxm

I’ve had my ’68 (small block, four speed, convertible) on the road for a little over a year now after a rebuild, and I’m still working out some bugs. For example, at between 55 and 60 MPH, I experience a fairly significant shake in what seems to be the front end. Not so much a vibration, but a low frequency, high amplitude shake (some kind of resonant) that can be felt throughout the car.

Further info:

• The car drives straight, and is very smooth above and below this speed.
• Behavior is not consistent – Sometimes not so bad, and sometimes really bad
• It is not engine RPM related.
• Front and rear suspension and steering components replaced / rebuilt during rebuild
• Drive shaft and half shaft balance checked during rebuild
• Wheel run-out measured (stock wheels), and found to be well within specs
• Wheels/Tires balanced
• Is not influenced by braking, other than by reduction in speed
• I had a four corner alignment done about a year ago by a shop that frequently aligns old Vettes
• Behavior does not change with tire pressure or by repositioning the wheels on the four corners

I would like to swap in a completely different set of wheels/tires, but I don’t have any to use. I can avoid this by applying +/-5 MPH, but I would like to find and resolve the root cause. While grasping at straws, I’m thinking I might replace the tires, but before I drop that kind of money on a guess, I thought I’d check with the brain trust.

Anyone have any other ideas?

MelWff

How old are the tires?

68vxm

Only about two years old.

U17

Just stay above 65 ?

bashcraft

I've had this same problem before and it was always the tire/wheel balance. You could try switching the tires from front to back and see if it helps, or have them balanced on a good spin balancer.

The Money Pit

I had a TransAm that ate torque converters for some reason......(Young kid with a heavy foot) The resonance maybe a ballooned converter. Try hitting 55-60 mph when you feel that shake, and shift into neutral, let off the gas and coast.

With the TransAm, everytime I changed converters, the problem went away............for a while.

dochorsepower

Convertibles have less chassis stiffness so many things affect this. Since you say front end rebuilt, check the condition of the steering dampener (right side frame) and try balancing with an on-car road force balance procedure.

doorgunner

The method above will really narrow down the area of the vibration on a manual 4-speed also.....if the shake goes away immediately when the car is placed in neutral at 60mph with the engine idling---the problem is in the drive-train.......if the vibration remains---the suspension/wheels are the problem (I think I said that correctly)

Have you jacked each wheel off the ground about 1/8" and rotated the wheel by hand to see if the rim is bent/warped or if the tire is out-of-round?

PeteZO6

Since your tires are pretty new, I'd ask what brand and sixpence are they.
My'69 convertible was always sensitive to tires, their construction and balance. The original Goodyear bias tires would shake the car at various speeds. The Dayton radials next put on shook the car no matter what I did, including several rebalancing, and shaving the treat in attempt to make them more round. Nothing worked.
Next I tried B.F. Goodrich Radial T/As That fixed the car. Goodyear GTs were also fine. Today I have T/As again, and they are fine. No vibration at any speed.
These convertibles are sensitive to all kinds of harmonics. Slight tire out of balance or roundness can set them off.

FireballXL5

I had a weird minor humming vibration at certain speeds... Felt most on the freeway... Fixed when I rebuilt my trailing arm bushings and bearings... I had a right rear bearing failure that was VERY prominent though once I located it down to THAT side... As it got worse(before I diagnosed it properly) it started making grinding noises when making 25-30 mph left hand corners... And had a constant humm at higher speeds... Jacking up the car corners and trying to wiggle the tire showed a LOT of play... All good and fixed now...

standswithabeer

had a similar prob with 2 different cars recently. one was solved by having the tires balanced at a different shop (i went back to the first shop and was told the balancer needed calibration). second one not solved by spin-balance at either shop, but was told we have a slightly out-of-round tire.

Kevin68

Assuming you've ruled out tires, suspension, drivetrain issues, do a search for "Cowl Shake" in Tech.

68 convertibles are well known for it. 68's did not have the bracing that goes from the trans tunnel to the firewall, that the later years do. Late 68's may have had it added, not sure about that.