the car vibrates pretty rough when Im going above 65. I was wondering if maybe my tires are unbalanced. Im not hearing any grinding sounds or anything so hopefully its just that. What are some other causes for vibration? bad struts?? bad transmission mount??

Move the tires front to back and see if it changes.

That is the fastest ,cheapest and best FIRST step you should take. It eliminates so much right off the bat!

Move the tires front to back and see if it changes.

Loose driveshaft bolts... just discovered that on my Z. Good thing I caught it in time, but it did cure my high speed vibration issues.

All of the above, but balancing should be your first act.

Had the same issue, made sure that all wheels were dynamically rebalanced, problem is gone!

Out of round tires can cause vibrations. Check the tires for bubbles, flat spots, and general roundness. Check for a bent wheel as well.

If you feel the vibration in the steering wheel, it's related to the front tires. If you feel it in the seat, it's related to the rears or driveshaft / propshafts.

Generally a tire problem on a 15" tire is going to give you a shake at 55mph or so.

I SUSPECT you have a bad u-joint or prop-shaft that is giving you your 65mph vibration. Put it on jackstands, remove the wheels, and run the engine up to 3000 rpm in 5th, or have a friend do so while you WATCH the propshafts for vibration.

Agreed - if the steering wheel is shaking in your hand at 65 - the problem is most likely up front.

The front suspension on the 240Z - 280Z's are very sensitive. Almost anything out of spec. can cause a high speed vibration. As everyone suggests tire balance, wheel problems are perhaps the first thing to eliminate. Past that - worn shocks, ball joints, tie-rod ends, worn steering racks and more commonly tension/compression rod bushings that are shot.

The problem with worn tension/compression rod bushings was identified very early on - and Bolt-On-Parts developed a T/C Rod Kit to solve it. They replaced the rubber bushings with an Aluminum socket and hard plastic ball arrangement. I've been running one on my 72 240Z since about 1976. I found the Kit only after I had replaced every other part, bought new tires and had them balanced several different times - but still had the front end shakes at high speeds.

FWIW,

Carl B.

Not a common problem, but funky wheels. When I bought a 75 Z in college, a shimmy problem drove me completely nuts. I finally swapped wheels with another Z, and the shimmy wasn't there. The problem? Off-center lug holes.

I had set of off centre tyres. Impossible to balance. Falcon brand. Crap.

Agreed - if the steering wheel is shaking in your hand at 65 - the problem is most likely up front.

The front suspension on the 240Z - 280Z's are very sensitive. Almost anything out of spec. can cause a high speed vibration. As everyone suggests tire balance, wheel problems are perhaps the first thing to eliminate. Past that - worn shocks, ball joints, tie-rod ends, worn steering racks and more commonly tension/compression rod bushings that are shot.

The problem with worn tension/compression rod bushings was identified very early on - and Bolt-On-Parts developed a T/C Rod Kit to solve it. They replaced the rubber bushings with an Aluminum socket and hard plastic ball arrangement. I've been running one on my 72 240Z since about 1976. I found the Kit only after I had replaced every other part, bought new tires and had them balanced several different times - but still had the front end shakes at high speeds.

FWIW,

Carl B.

Carl,

I have the same problem as the OP, after going through some of the archives it sounds like a ball joint problem to me but I have read in a number of places that the T/C aluminum/plastic combo caused more problems than it solved and that going back to rubber was the best all around solution. Would be interested in everyone's opinion on this.

Especially, those that have a well balance Z and what components you have that make up your suspension.

Thanks