Wheel Shake

[CWizard]

I've read several posts here about various front wheel shake problems, but none of them seem to match this. My new Superformance has a pronounced front wheel shake that starts at about 70 mph and lasts to about 80. The steering column is actually moving, not just the steering wheel. All of the vibration seems to be coming from the front (for example, my butt isn't shaking), so I'm pretty sure it's a front end issue. Had this problem from day 1. So far, the dealer has replaced both front wheels and tires, and rebalanced the new front tires. Still shakin. Could a front wheel alignment problem cause this? Could the rear wheels cause this? Help!
And thanks very much.

[Bob In Ct]

This problem has been discussed at length on this site and others. It's the tires!

Get them "trued" and then dynamically balanced.


Bob

[CWizard]

Bob - The wheels were dynamically balanced on a Hunter GSP 9700. Haven't checked for trueness yet. I'm looking for a place to check that and shave the tires if necesary. Thank you.

[driver]

Bob is correct ! Because of their light weight and huge rubber, Cobras are very sensitive to vibration. If those same tires were on an Expedition you probably would not notice any vibration . I have heard that Goodyear can give you trouble right out of the box. Shaving them true will probably solve your problem.

[CWizard]

Thanks guys. I didn't mention this, but the tires are Yoko Avid s/ts. One more question please: Can I trust the Hunter GSP9700 to give me accurate results on tire roundness? If the answer is yes, then I do not have a roundness problem with the front tires. It says they're perfect circles. Haven't yet done the rears. Is it likely that a rear tire could cause a dramatic steering wheel shake? (sorry - sneaked in a 2nd question on you).
Thanks a lot!

[driver]

there was a monster thread on this subject a while back and if I remember correctly the latest Hunter was not accurate on the out of round problem. There are a lot of cars out there with this problem and have had success with shaving as a last resort

[coosawjack]

Find someone who can "Road Force" balance them!! This procedure puts pressure on the tire while balancing as if the weight of the car is on it and defines "Hard Spots" in the tire as heavy spots.

If done properly the car will be very smooth but as stated before a "Bump Steer" type of shake will always be apparent in light weight cars with wide tires and rack & pinion steering!!

Good Luck!!

Coosawjack

[CWizard]

Update: Front wheels have been trued and dynamically balanced (Hunter GSP9700, which does the road force thing). Still shakin. Had no effect at all. Suspension and steering are tight. Four-wheel alignment was done 6 weeks ago. So now I'm thinking brake rotor problem or hub is off so that the wheel isn't centered or maybe I'm jinxed.
Any other suggestions will be appreciated.
Thanks.

[wtm442]

Originally posted by CWizard
Any other suggestions will be appreciated.
Thanks. [/b]

I am interested in results because I have a very slight steering wheel shimmy at speeds above 60 MPH on my SPF with Goodyears and 7000 miles. Nothing like your violent shake. Mine is hardly noticable. Tires and rims have NEVER been off the car since new.

I know vertually nothing about aligment, but I'll throw out my 0.02 cents. I had a radial on another car (not a Cobra) where the belt had a slight shift in one spot and caused a slight shimmy at speed. The tire was not out of round. If you looked at one of the grooves when you spun the tire when off the ground, there was a slight wobble in one section of the tire. I have no idea if the Hunter GSP9700 would notice this. I would think so, but ???

Also, just for grins, if you have a good dial indicator, jack up the car off the groung (use jack stands) and spin each tire with the indicator on the back side of the rotor. Maybe you can narrow it down to one side or the other. Or my suggestions may be all wet !!!! Your comment about bad rotor sounds plausible. Hopefully someone will chime in with some "expert advice", cause I aint one!!!!

Good luck and post an update when you find the problem.

[niles]

It sound like the vib prob I had on a 67 Porche after changing to some after market mags.
Assuming that you have changed tires and had multiple unsuccessful balances. You have to go technical; ie, one varible at a time.
What wheels do you have? Pin drive? or tapered lug nut/hol;e??

Have you been able to pin point a side? Try switching left to right, etc see if it causes a change(I doubt it)

If all negative, take tire off and put rim back on and torque down. Put rim back on the spindle and take dial indicator reading for roundness off the tire bead mounting surface(not the rim edge), thats what centers the tire mass. At the same time check the rotor for out of roundness in it mounting holes.

This is hard to describe in a few minutes in writing. If you don't follow , send PM.

Bottom line on the Porch vib, it was cause by the mounting lugs being engineered poorly, they did'nt pinch/self center, but were a cylinder that went thru the wheel. This cause the tolerance stackups between the wheel /lub combo to go up to .015 -.020 in. That was enough to cause the vib on a "light " weight Porche
The solution, as it was, was to mount the rims hand tight, spin the wheel with an indicator and find the high spot, rotate that spot down,,lower the car to weight that spot. This took out the out of round tolerace of the crappy nut disgn and torque down.
Most of the time the vibe was gone after that. I found that it was amazing that .015 in could cause that! I was told by dynamicist that what .015 was causing a "resonance" at a specific wheel frequency. Thats why it sppeared suddenly and would go away. Mine would appear at about 64 mph and be going away at 70 mph. It set my cruise speed alway at 70 or above!!

gn

[CWizard]

Warren & Niles - Thanks. Wheels are Superformance pin drive knock-offs. I don't have a dial indicator, but I'm sure the dealer does. He has been working very closely with me on diagnosing this problem. My next step will be to get the car back to him (dealer is 150 miles away) to get the hubs and rotors checked, and whatever else his mechanic wants to check. I'll post results.

[Pete Wolff]

When you say that you had the tires "trued" did you actually have them shaved to make them perfectly round ? And then have them balanced ?

[niles]

Pin drives have two interfaces which can have tolerance problems which can cause the out of round condition.
Dont rely on the dealer to correct; even if he dail indicates .015 in he will probably say thats OK. But it can cause the problem; which masks as an out-of-round.
You can get a cheapo dail indicator at harbor freight for about $10. that what I did. With it I found my bell house off by .012 in. Of course the eng builder said it was fine when it left the shop.
BTW, if your going to have one of these antique machines you have to learn do do/check/verify all these mechanical chores, or else you will quickly get frustrated by the never ending list of problems. Its not fun to chase these grimlins paying someone $50 an hr.

In this case you would want to meaure the trueness of the pin drive hub as it turns by setting up the dail and turning the wheel.
gn

[CWizard]

Pete - Yes, the tires were shaved to perfect circles and then road force balanced. The shaving took only a very small (less than 1/16 inch) amount of rubber off the highest spot, so the tires were not terribly off to start with.
Niles - Very good point about doing my own work on this antique beast. Actually, (and feel free to call me weird) that's one of the things I like about the Cobra. It has relatively simple systems that are accessible and can be fixed (usually) without plugging in a computer to find out what part to replace. But at this point, the dealer is paying the mechanic, not me. And so far, I'm pleased with his standards and troubleshooting. So I'm happy to have that support.
And tomorrow I'm gonna go buy a dial indicator!