I have a vibration in my car slk 230 at speeds up to 40mph. I brought it to mechanic and he said it is the left front tire and rear right tire. I brought it to discont tire and they said tires are fine. they took off car and put on machine. The mechanic still thinks it is the tires. Any ideas? The tires still have lots of tread, there is no grinding noise, no pulling. the rotors and pads are brand new in the front.

It is unfortunately quite common that many tires produced today are "out of round". What this means is that the tread is not concentric with the bead. When the tire is mounted on the wheel, the tread is not running true with the axis of the wheel. This should be apparent on the tire balancing machine, if it gets excessive, because the amount of weight added will be 3/4 oz. or more.

You can also get a sense of how much the tire is out of round when it is on the tire machine. Get a piece of chalk and very carefully move it in and just touch the tread of the spinning tire. If you can barely contact the tire and the chalk marks the tire over 180 degrees or so, then the tire is probably not the culprit.

However, if the chalk marks the tire for only about 90 degrees, or less, then you probably can see the runout of the tread. If this occurs, a M/B suspension is going to feel it. You need to take the tire off the wheel and rotate it 180 degrees and remount it. Perform the chalk test again. If the chalk marks the tire in the same way, the tire is at fault. If the chalk marks the tire 180 degrees from the original point, then the wheel is at fault. The concentricity of the wheel is not acceptable and the wheel needs to be replaced.

You can have a perfectly balanced tire and wheel combination that will perform very poorly on the vehicle.

Typically tire vibrations START at 40MPH and higher, not END at 40MPH.

That said, tires can do some really screwy things. Tires exhibit vibration for one or both of two reasons, imbalance or Road Force Variation(RFV) which basically means stiff spots. If the belt is not laid in the tire properly it will have a stiff spot in the tire.

What you need to do is find someone who is competent with a Hunter GSP9700 machine. This machine not only tests the balance of the tire, but has a pressure roller that can analyze the road force variation.

To find one of these machines near you, go to: www.gsp9700.com

Good luck,

Thanks for the info, I will try that. I will post when I find out.

They found that the rear rim was bent! brake rotors had some rust also... I replaced the wheel and rotors..and still get a bit from the front... So I am going to buy tires next. sheesh!