Steering wheel shake

[diegokid]

When driving my Cobra I get a steering wheel shake at 50 to about 57 MPH. If speeding up or down it goes away. The tires have been balanced over and over to no end. I even used an old fashioned bubble balancer to check the balance.

While in storage the car was on jackstands on the lift unless it was being moved and this shimmy was an issue before the last teardown so it hasn't been sitting on the tires for a few years.

I started checking the tires for being out of round and found one of the front tires to be out by .090 and the other front to be at .045 both are BFGs. In comparision the Kumhos on the rear are out less then .010.

[souzape]

I've had this problem with every set of Radial TA's I've ever owned. Currently run Yokohama S/Ts and have been told by a very reliable source that Cooper Cobras are exactly the same tire. The BFG experience covers at least 4 sets of tires on Vettes and my Cobra.

[spd4me]

My BFG's do the same but at 60 to 65. This was a discussion some years ago on this forum and we all decided it was due the what's called harmonic balance of objects and materials. At a certain vibration or wave lenght, materials will begin to vibrate more radically. If you've ever sat in the back of a jet plane that has the engines mounted against the rear fuselage, you can hear a distinch pattern to the vibration.
Bob

[diegokid]

Guy at church has a road force balancer, I'm taking the two in question to him tomorrow. If that fails I'm changing them.

I read the old posts on this subject.

After 23 years of flying for the USAF I can tell you this. Jet engines are a lot like tires, if there noticeabily vibrating there's something wrong, call the crew chief!

[Aggressor]

Diego - Here's a copy of a response I made in 2005 to a shimmy problem.

"My car exhibited a severe shimmy when I drove to the Unique Homecoming (2002). It occurred at around 45 MPH and got worse at speed. I had the wheels balanced only two days prior to the event so it seemed likely that balance was my problem. In Gadsden I went to a Ford quick service shop and was lucky to find a person very skilled in balancing to close tolerance. Evidently there is a spring within the centering cone on spin balance machines and if it is not removed for the larger rim/tires it will offset your balance. My wheels were tape/weight balanced on the inside as close to centerline as possible."

The tires were BFG TA's and everything was fine after that. The tires wore evenly until there was very little tread left with no balance issues.

- Aggressor

[Naumoff]

Don't drive at those speeds

[spd4me]

Aggressor - When you say spin balanced, was that while tires on car or taken off and spun on a machine?
Bob

[Aggressor]

Off the car. Fronts are 245/60/15 TA's on 15X9 Compomotives. Rears 295/50/15 TA's on 15X11 Compomotives.

Good call Tony - Spin'em fast enough and you won't notice the vibrations. Problem solved!!

-Geary

[diegokid]

After taking the tires to Andy for the road force balance the culprit was found. This balancer assigns a tire a number depending on how much force it takes to make the tire spin without vibration. I was told that a good tire for a car should be 6 - 9. The tire I had that was .090 out of round had a road force of 48! He laughed and told me that he has seen mud tires for 4WD trucks with numbers not that bad. The machine then found the high spot/low spot of tire and rim. These marks were where I had marked them using the dial indicator. The tire beads were broken and the tire moved on the rim and rechecked. Best we could get out of that tire was a 31. He said it would help but may not clear it up, he recommended replacing it, no he doesn't sell tires. The other BFG scored a 13, the two Kuhmo's on the rear scored a 5 and a 6. Suprisingly he told me that tires that fell in the 9 or less range could actually be a little out of balance and still roll without vibration.

Its a sigh of relief and was actually glad it was a tire because after checking drive shafts, axles, rotors ect and the way the vibration was acting the tire was my last try. I just didn't want to believe that a tire with less than 750 miles was bad. Another interesting note was after writing this down in my log yesterday I found where I had checked this before tearing the car down for paint. The reading then was .120 @ 32 PSI vs .090 @28 so the extra PSI must have an affect.

On the upside this gives an excuse to buy a new set tires instead of just one, that way I can have spares! So who has the best deals on the Shelby rims and has anyone tried the Mickey Thompson Sportsman S/R Radial ?

[Slither]

I saw your thread over on CC, Jim. Word over there is that the Micky Thompson S/T is the way to go over the S/R... supposedly a softer compound than the S/R... see here:

http://www.clubcobra.com/forums/all-...bra-tires.html

Also, there was something about heat-treating the tires before installing them... see post #10 and following here:

http://www.clubcobra.com/forums/tire...811-tires.html

Anyone know about this heat-cycling on the tires?

[Naumoff]

Off the car. Fronts are 245/60/15 TA's on 15X9 Compomotives. Rears 295/50/15 TA's on 15X11 Compomotives.

Good call Tony - Spin'em fast enough and you won't notice the vibrations. Problem solved!!

-Geary

No shimmy at 120mph.

[diegokid]

Drove the car this weekend and the difference is nght and day. Even if looking for the shimmy it was hard to locate. So new tires will be on the way. I figure if it rides this much better with 6-9 being good and mine at 13 and 31 a better set will help even more.

I highly reccommend the road force balancing or doing a high low spot match yourself.

[eliminator]

We have beat this horse before, like I have said before sometimes the search feature is a GREAT TOOL!!!

We have also covered Mickey Thompson Sportsman Tires....I know because I have a set.

FYI

[diegokid]

Would that be the Sportsman SR/Radial, ST/Radial or the Sportsman pro? Three different brands of Sportsman tire. FYI

[Naumoff]

[QUOTE=eliminator;87558]We have beat this horse before, like I have said before sometimes the search feature is a GREAT TOOL!!!

We have also covered Mickey Thompson Sportsman Tires....I know because I have a set.

FYI[tQUOTE

Well we can cover it again. You never know when you will get a new tidbit of information .

[Aggressor]

Tony may be right this time. It's like panning for gold. Ya never can tell when a nugget will show up in your pan.

[traveler]

Thank you Tony

[diegokid]

I actually did that when we used to deploy to AK for the ATTF. Didn't strike it rich but found enough to have a good night out a time or two!