Suggestions for front end shimmy situation

[jerry5453]

I brought my '74 260 2+2 out of a 15year hibernation last year, going essentially through all the mechanicals, specifically for this problem - struts, urethane bushing all around, and new tires(alignment done also). Well, I remember that it always took two balancing's to get the shimmy out of the front end on each tire change when it was running in the '80s. But I've been through 3 balancings so far with 2 different places and still have a horrible shimmy above 60mph with no change after each balance. I did cheap out and get some high speed General tires, something that I now totally regret, adding to the injury of taking off the essentially brand new Michelin XZX that I had put on 15years ago. So the questions/things I've been told are -

1) I took off essentially flat spotted tires and put on genetically out of round tires.
2) Newer tire balancing machines are not as good as of old - newer cars are more tolerant - if so what does everyone else do ?
3) Do the urethane bushings make that much difference on coupling and should I go back to rubber for the rack and the steering coupling ?
4) Do I just need to go to a Michelin/Pirelli/???? for a higher quality tire ?
5) There are mechanical alignment adjusting devices that can be added to the strut tower tops for more/finer adjustments - would they help ?
6) Wheels possibly a problem ? I don't remember any problems when I put it up, and I really doubt I would have allowed new tires to be put on without getting the shimmy out, so it wasn't exhibiting this then - leading me to believe its a change I've done.

I'm at a loss in general here, any suggestions/data would be appreciated.

I'd love to find another Z car guy in Indianapolis who would swap some front wheels that don't shimmy on his car for a test drive on mine to try and maybe narrow it down.

Thanks in advance,
Jerry

[Bambikiller240]

Jerry: Your post doesn't say, but I'm wondering if you have a "stock" front valence on the car. Many early Z's experienced wandering at high speed due to a lack of downforce at the front-end. The solution was the "Spook" or "Air Dam" added to the front of these cars.

Another possibility is worn inner tie-rods (inside the steering rack).

Also wondering if your tires were static balanced or spin balanced.

If you installed the PU bushings correctly (and it's hard to mess this up) they should not cause this problem. You MAY feel the shimmy more due to the compliance of the bushings, but they should not CAUSE it.

Have you switched the rear tires/wheels to the front & checked for any differnce in symptoms?

[jerry5453]

Carl,
Thanks for the reply !

Let's see - yes stock valence; yes all spin balancing, yes done front/rear swapping and little/no difference.

I think I know what you mean by high speed 'wandering' and I wouldn't call this that. It still tracks straight, the steering wheel just vibrates horribly above 55-60, below that speed feels solid and normal.

I called a few places looking for static or balancing on the car. I didn't find anyone who did either. Do you have any problems with the standard spin balancing (seems like mostly Coates(?) machines) gotten at the standard tire places ?

Hmmmmm ..... the inner tie-rods is an interesting idea. How would I test/check that ? Just play in the rack ? The first thing I had done was an alignment. Is that a checked item ?

I'm just trying to get a feel for the 'amplification' effect of the PU bushings. If somebody says "No problems before, put on PU bushings then had terrible vibration" that would say maybe going back to rubber might be get rid of it since I don't believe I had this before.

The fact that front/rear tire/wheel changes doesn't seem to make a whole lot of difference leads me to believe its PU bushing amplifying what was always damped by the rubber or something else totally. But, the symptom is exactly as I remember it from tire balancing problems each time I changed tires so many years ago.

A large part of my problem is the time interval(15y) between usages. If I had driven it and had a good feel for things and then did the re-build, I could make some guesses. But with tire, suspension, bushing changes all at once with only old memories, it leaves me a little bewildered (not helped by age either I'm sure .)

Thanks again Carl

[Bambikiller240]

Jerry: Know what you mean about time (and age). Two years ago I got back into Z's after a 15 year absense & I'm still re-learning things that I already knew!

The play in the inner tie-rods can be measured, but it's a PITA and most folks will just bite the bullet and replace them rather than spend the time mucking about with feeler guages etc. They don't cost too much.

Another possibility that I've heard can cause the problem that you describe is "slightly" ovalled mount holes in mag wheels. I haven't experienced that, but there was a thread on the IZCC list about that a few months ago.

Did you replace Ball-joints and outer tierods when you installed the PU bushings? They can cause the wobbles you describe too. The front-end is a simple system, but I've been told that EVERY part must be well within spec for the system to function smoothly and safely.

Your idea about borrowing some "known smooth riding tires & wheels" sounds like a good place to start. If that didn't fix it, I'd do the outer tierods and ball joints next, then the inner tie rods if necessary.

I have PU bushing throughout on my 72 Z and I haven't had the symptom that you describe, although I can feel very minor cracks etc in the road much more so than with the rubber bushings. I replaced all wear items in the front-end when I bought my current Z.

Carl