vibration
 

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.

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

Any thoughts?

Replace the BFGs. Did you have them spin balanced? Switch tires side to side and see if the symptoms change.

vibration
 

might want to check for bent rim. have had this problem before. sometime hard to detect but just jack up the front end hold a steady pointer by the rim and give it a spin

I would replace the BFG's with Kumho's with a matching tread pattern. Short wheelbase cars do not really like different tread patterns. Sounds kind of wierd but that has been my experiance.

Do you have a rag joint in your steering. They take up some of the vibrations from the tires.

Try having the tires "road force" balanced or whatever your local tire dealer calls it. My local Discount Tire will match up the high spot on the tire with the low spot on the rim. It helped dramatically on one of my cars.

Your tires can be balanced but not concentric with the axle center line and it will feel the same as out of balance.

John

so far
 

"Did you have them spin balanced?"Yes, at two different places.

"might want to check for bent rim"
Already done this using my dial indicators, same ones used to find the .090 high spot on the one tire

"replace the BFG's with Kumho's"
Kumho doesn't have a tire in the size the front calls for, however if the next step doesn't work I will try different tires.

"Try having the tires "road force" balanced"
Today I was talking to a younger churchmember that also runs a repair shop. He has one of these machines so I'll take the tires to him next week.

You do not absolutly have to have the exact recomended tire size that is called for. You can change that depending on what you want. You can change the width and aspect ratio and probably come up with something so close that you would not even know the differance.

What size are your front and rear tires now?

After a short run with min. braking check the temp of your rotors. I chased the same problem with mine and finally checked and had about 60 to 70 degrees difference in temp. from a brake draging ever so slightly. Lubed the sliders and problem went away.

Lester

I had a severe vibration issue after I went with the GY Billboards bias ply. Shook bad at around 35 and then again at around 70mph. Balanced on road force, balanced while spun on the car, rims checked, balanced again. I never could solve the problem until I went with a different brand tire. The Mickey Thompson ET's radials made all that vibration and frustration go away. Changing tires just might solve the problem for you too. Good luck.

John

Lot of things besides the tires can cause a vibration. You've already checked and balanced the tires more than once - time to move on.

Get the car up to about 65mph. Put it in neutral and let it slowly coast down below the vibration zone. if the vibration changes, it's most likely the drive line. If it doesn't change, it's suspension.

Suspension problems could be a loose tie rod end, ball joint, etc. Check everything very carefully.

I think the short driveshaft in a Cobra is part of the problem.

Dwight

what do the tires look like when on the spin balancer, are they round? the balance machine will balance an out of round tire but being out of round will be a mess on the road. bfg i don't believe are on the high end of the tire list.

BFG tires
 

the BFG-T/A radial are crap but the BFG G-Force and Sports are great tires. I have bought several sets and love them.
Great in the rain and hold good on dry pavement till I heat cycle them a few hundred times:LOL:
I have a couple of pairs with lots of thread but are to hard because of burn outs.:)

Dwight

problem found
 

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 ?

done
 

Road force balancing made all the difference in the world. Even with one tire at a 31 all the shimy was gone, up to 85 mph. Will go somewhere to try higher speeds this weekend.

lower your tire pressure to 22 psig, 28 is high for a 2400lb car, are you wearing the center of the tire out?

50 to about 57 MPH is the typical speed for such (wheel) imbalance.
Your 0.9 inch out of round is too much, which you corrected now.

FYI, Google Hunter GSP9700 for local shops that have this machine. Having performance tires road force balanced on one of these machines will make all the difference in the world. These are required mandatory machines for alot of manufactures. I am the service manager at a local Hyundai store and we just paid $14000 for one of these things. I was hesitant at first that we were forced to have such an expensive machine but it makes tire diagnostics a breeze. It tells you everything about your wheels and tires including how much lateral pull each tire has. If you have ever had a car that pulls and gone back and forth with a finger pointing contest from the tire store, the alignment shop and the dealer, this one will tell you whats what the first time..Just in case you giggled at Hyundai try going for a test drive in the 429 hp 5.0 Genesis, I guarantee your giggle will turn into a grin..

I have MT S/R Radials on my cobra, 15'000 miles in 4 years, Vintage 15" pin-drive wheels. No vibrations, even tread wear, still look like new, running 27 lbs.
Runs very smooth at any speed and handles very well at high speed with great stability, tires feel like thet're glued to the ground in tight turns.
So i'm happy, Bill