I'm Picking Up Bad Vibrations.....

UKjohnnycobra · #1 (**permalink**) 09-05-2008, 04:19 PM

...at between 60 and 63 mph. Its coming form the rear of the car. It runs great at all other speeds. It feels like a wheel out of balance (they are Image wheels with a significant offset at the back) so I took it to the local shop and they balanced it up and told me that one of the tyres/wheels is slightly out of true (ie not circular) so maybe I have a flat spot.

So a question to the cream of mechanical intellect that posts on this board is: could a slight flat spot cause this? If I roll the wheels over a flat smooth floor they don't seem to hang up anywhere. All my UJ's and wheel bearings are good and everything is tight. The front end is vibration free. I really don't want to buy new tyres just to test if this is the cause.

Anybody have any experience with this?

Apologies to the Beach Boys BTW

RICK LAKE · #2 (**permalink**) 09-05-2008, 06:15 PM

UKjohnnycobra If the tire has a flat spot, even balanced, the faster you go the worst the vibration should get. It should not be in a narrow mph range. Putting the car on a lift and going through the rpms should show something if the tire is out of round or the drive train. Keep the speed to only where the problem is. You could damage the rearend on some cars when you have the tires hanging. You could have a driveshaft angle problem, an unbalanced rotor for the brakes, Go to a good tire store that can do a force test to see how far out the tire is of spinning true. If it is in spec, you are looking at the drivetrain for the problem. I am going under the fact that the tires are new and the rims spin true on a machine. 90% of the time a new set of tires fix the vibrations. Could be a belt shift inside the tire too. The tires will still balance up OK and zero out, but you will get a vibration. You feel this in your A$$ not your hands. Rick L.

Bob In Ct · #3 (**permalink**) 09-06-2008, 08:48 AM

UK:
Many tires are not round. There is a place in Bridgeport that will "true" the tires on the wheels that they are on. I had my tires done a few years ago. Todd of Guilford (Blue FFR) also had his done with some success. Any interest?

Let me guess, BFGs?

Bob

UKjohnnycobra · #4 (**permalink**) 09-06-2008, 11:47 AM

Ricks comments sound logical......up to the problem speed I have no problem at all. The wheels and tires are new, with less than 1500 miles on both. However I live about 30 mins from Bridgeport, so I would be interested in trying that place out, Bob. In the meantime I think I will take them somewhere else for balancing again and see if that works. Thanks for the help guys.

paulbarrick · #5 (**permalink**) 11-16-2008, 04:31 AM

I have the same problem with my Goodyear F1's, 17 in and I have discovered when I don't drive the car for a week or so, the tires get a flat spot from sitting. After I drive for 10 miles or so, they heat up and then the vibration goes away. Don't notice mine til I get to 50 mph or so.....just a thought...

wtm442 · #6 (**permalink**) 11-16-2008, 05:24 AM

You need a "Road Force Balance".

Bill Bess · #7 (**permalink**) 11-16-2008, 05:41 AM

Could be a lot of things....use the process of elimination.
If it's possible after you varify that all the balance is good, tires are not out of round, put the front wheels on the back to see if the vibration shifts to the front, or put some junker wheels and tires on the car to determine if the wheels or tires are the problem.
Next as mentioned check the drive shaft for balance and angle, then the rear end for possible bad bearings. Also check the front end for loose steering u-joints, loose rack and wheel bearings, its possible for a vibration to telegraph to the rear.

Anyway, just take it to a good shop that does front end and suspension work and have them check it over.

Best of luck, Bill

Chilibit · #8 (**permalink**) 11-16-2008, 06:15 AM

When did it start? Did something change then?

Was there ever a time when it was not an issue?

Out of true usually means that the tyre/wheel has wobble (side to side) rather than out of round which is what has been addressed. But this is only from my experience and you were there to watch his hands when he said it.

Out of round, out of true and flat spot can all be checked reasonably well by merely spinning the tire slowly and measuring run out.

Does this persist once tires are up to temperature? Let's say after ten miles.

What brand/model tires? How tall are the sidewalls? Or, better, what size are the wheels? Very short sidewalls exacerbate most problems.

Vince

UKjohnnycobra · #9 (**permalink**) 11-16-2008, 03:53 PM

The tires are 235 45 17 so are fairly low profile and I am sure the tire guy said they are "out of round". It hasn't always been a problem its just that one day I noticed it and it hasn't gone away. It does not improve with warming up either. The rear bearings are good and the rear brakes are drums (don't ask!). The drive shaft is balanced although I have not tested it to see if it is "true". I can t really change out the front and rear wheels as they are significantly different widths. I am going to get the wheels checked again.....

thanks for the comments everyone.