Nearly everyone that has owned a car in the last century has dealt with aging tires, and the discomfort related to them. After all, tires are the one single thing that actually connects the car to the road, so having tires with issues will always be detrimental to your ride quality. There are a few problems that create tire noise, but the number one worst offender is something called cupping. Cupping (also known as scalloping) is when the edge of the tire is uneven, with noticeable high and low points along the inner or outer edge of the tread. A cupped
tire will growl like a bad wheel bearing, so it’s important to tell the difference between the two.
A bad wheel bearing will change sound as it changes direction. If you are going around a left hand corner and begin to hear a growling/grinding noise, chances are you have a failing drivers side wheel bearing. The unloaded wheel bearing will always be the one that makes noise in a turn.
If the growling of your wheels steadily increases with speed, then your tires are most likely cupped. When stopped, reach in your wheel well and run your fingers along the edges of each tire to feel if the tread blocks are smooth and even, or varied and uneven. Sometimes tire cupping can get so loud, the cabin noise of the tires on the highway can get nearly unbearable. If that’s the case, don’t just replace your tires. If you do, you are looking at destroying a new set of rubber quickly, and wasting a lot of money. Instead, take a look at your suspension and ask yourself the following questions:
How many miles are on the shocks/struts?
When were suspension bushings last replaced?
The main cause for tire cupping is worn suspension allowing too much freedom for the wheel to bounce and vibrate, specifically shocks/struts and bushings. If your wheels are going out of balance, this issue is greatly amplified. As the wheel goes over bumps, the spring causes the tire to bounce, but lack of solid dampening allows the tire to continue to bounce on certain areas of the tread blocks, eventually causing more wear on certain parts of the tire. If you have worn bushings, the play in the suspension caused by them will also let the tire move ever so slightly resulting in uneven tire wear. Suspension bushings are one of the most neglected maintenance items on a car, and should be replaced every 100-150k miles. The SAAB 9-5 is notorious for going through rear shocks, causing bad tire cupping. The wagon variation can often require new shocks in less than 100k miles.
Unfortunately, if you have unevenly worn tires, there’s no turning back. A set of cupped tires will only accelerate the uneven wear as mileage increases.
Before you get new tires, be sure to check and replace your shocks and bushings to avoid quickly ruining a new set, and don’t forget to finish off with an alignment. Take these steps to enjoy a smooth commute once again, free of the annoying song of unevenly worn tires and the tire noise associated with them.