Okay... I had another look and found a " knock '' sound in the steering housing ( rack ), when turning fast left to right on the steering wheel when parked. Als found play in the right side in the steering housing ( rack ). So I took the steering housing(rack) out for rebuild ( I guess it was never done since 1971. Hopefully this solves the problem. If not it should be the wheels or a hub that is bent I guess. I checked the brake discs and they are not warped.

What an interesting problem that I'm now intrigued to see wha the final result of this is.

Look on the bright side, chasing this problem "allowed" you to do a lot of refreshing of your Z.

Given it's fine until you hit 50 mph, it seems to point to a balance issue. As others have said, I suspect the wheels / tires.

I have vibration issues that occur at 75 and above, have checked wheels, tires, suspension and drivetrain, still no solution. Have not looked at steering rack or diff yet. Still want to swap out tires/wheels with buddy's Z to assure that is not the issue. Seems funny that it would occur at such a high speed (it is not rpm, but actual speed). My wife says stay under 75, we know that is just crazy ...

Hi Bart,

You can try my wheels. I have no vibration issues up to 80 mph at least. Don't drive it harder than that. I would drive up next weekend so we can change them for a test run, but Im having a fuel issue problem. See thread http://www.classiczcars.com/forums/fuel-injection-s30/51923-weekend-drive-engine-hesitating-then-gave-up-altogether-water-fuel.html

Maybe you could come to me?

Chas

Hi Chas,

Next weekend I allready have some other things to do. I can't go anywhere with the 240z now anyhow have to wait for the rebuild steering rack.. it will be somewhere next week for that to arrive. I could come over for a visit, but that will be in 2 weeks or so..

I made a video of the front left side wheel putting the camera on a fixed position, you can see there's a few mm height difference, not sure if this could cause the problem ? I have to wait for the steering rack to be back and drive to find out..

View My Video

Watch the rim...it also moves up and down like the tire. I can just barely make it out in your video but it is noticeable.

Yes, that amount of radial runout could cause the issue depending on the reason for the runout. When you try Chas' wheels, I'll bet the vibration goes away.

I'm going to try chas' wheels when I have my steering rack back and the problem is not solved It could be radial runout maybe..

The FSM also point towards that direction:

Shimmying: this is abnormal vibration of the front suspension group and the entire steering linkage, and occours when a specific speed is attained.

Possible causes: Difference in height of left and right tire treads.

Corrective action: replace tires

That would suck because I just bought for about 400 euro on Dunlop tires last year and they barely made km's..

I have vibration issues that occur at 75 and above, have checked wheels, tires, suspension and drivetrain, still no solution. Have not looked at steering rack or diff yet. Still want to swap out tires/wheels with buddy's Z to assure that is not the issue. Seems funny that it would occur at such a high speed (it is not rpm, but actual speed). My wife says stay under 75, we know that is just crazy ...

Does your steering wheel also shimmy ?

Yes, it does shimmy, but can't pinpoint whether the cause is front end or rear end issues. Replaced the tires with slightly different size, same wheels, still vibrates. Will try replacing wheels/tires and see if that makes a difference.

Although the new rebuild steering rack feels more solid, shimmy is less, but still there..yup

I'm going to try chas wheels next step..