Q. I recently purchased a new set of tires at a discount store for my ’91 Hyundai.
Ever since, I’ve detected a minor shake that hasn’t been fixed, even after two trips back for them to recheck it.
The manager tells me the tires are perfectly balanced and whatever I’m feeling isn’t caused by the tires.
I’m positive the car rode better than this before the tires were installed. Is one of us being unreasonable?
— Peggy Turnbull, San Jose, Calif.
A. Peggy, nobody knows better than you how your car feels and sounds.
There’s a good chance one of your tire/wheel assemblies is causing a shake even though the balance machine says it’s OK.
In addition to being properly balanced, wheels must be true (not bent or out of round) and tires must be true, round and have uniform thickness and cord positioning.
Let’s start at the wheels.
A good Hyundai steel wheel should have less than 1 millimeter axial and .6 millimeter radial run-out (wobble), and an aluminum wheel’s limit is .3 in both planes (other vehicles have similar but differing specs).
This is measured with a dial indicator, preferably with the wheel mounted to the vehicle. If radial run-out exceeds specs, I’d try repositioning the wheel on the vehicle hub prior to replacing it.
Now the tires. Most, if not all new tires come with red and yellow dots on the sidewall that correspond to the tire’s uniformity and weight.
The red dot should be aligned with a dimple or similar marking on the wheel.
On the wheels without marks, the yellow dot is aligned with the valve stem.
This is known as match mounting and helps reduce the effects of a slightly out of true tire and wheel.
The idea is to align the high spot of the tire with the low spot of the wheel, or the tire’s light point with the wheel’s heavy point, reducing the chance of vibration.
If the above efforts do not provide acceptable smoothness, RFV (radial force variation) might be the culprit.
This term refers to variations in tire sidewall and footprint stiffness as the tire rolls under a loaded condition.
Excessive RFV can be caused by mispositioned cords or belts within the tire’s casing as a result of manufacturing variables. This can result in uneven rolling of the tire and speed-sensitive vibration.
How can excessive RFV be detected?
By using a special machine, such as a Hunter GSP9700 Vibration Control System, that applies a specified load to the tire as it is rolled and tested.
Wheel and tire run-out also are measured.
If both tire and wheel are within reasonable limits, the tire is repositioned on the rim, or forced matched, to bring the assembled unit into specifications.
Excessive RFV might require tire replacement.
To find a nearby shop with this machine, click the locate link on www.gsp9700.com.
Brad Bergholdt teaches automotive technology at Evergreen Valley College in San Jose, Calif. E-mail him at under-the-hood@juno.com or write to him in care of Drive, Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. He cannot make personal replies.
