At different times an owner will be confronted with vibration and or shaking whilst driving. Most people self diagnose this as a tire balancing problem and they take the vehicle in to a tire store for a check up. Invariably the wheel is removed, the weights are also removed and the wheel is balanced. Sound familiar? Many of you have gone through this exercise.
You road test the car and the vibration has gone. That's great and you were lucky! Many times, especially with older cars, there is little improvement and in some cases the problem is actually worse. Now what do you do?
Let's review the wheel concept. Ideally the rim/tire combination should be round, a concept that is lost on many tire stores. Anything can be balanced. An egg can be balanced beautifully, but it will not give you a smooth ride.
The first step is to make sure the rim/tire is round, so we check for run out. Drive the car for a few minutes to warm up the tires and remove any "flat spots" caused from sitting. Jack up the car and spin the wheel by hand, eyeballing a point between the outer diameter and a fixed point on the car or ground. Placing a pointer or a tool close to the tire will work also and serve as a measuring datum point.
As the wheel turns you may see an up and down motion. All wheels will have a certain amount of run out, but if it's more than 1/4" you have a problem. For the drive wheels you can actually put the car in gear and view the show at speed, just make sure that the car is well secured and with a person inside doing the "driving.". If everything is round you have passed the first check and can take the car in for balancing.
Here comes the tricky part. Make SURE that you ask the store to CHECK the balancing first. This means placing the wheel on the balancing machine WITHOUT removing the weights. This is Crucial to the test. If all the wheels are OK then you have to look elsewhere for the source of your vibration .- the driveshaft being the next possible culprit. If one or more wheels show to be off spec then the weights can be removed and the wheel balanced.
Had the weights been removed initially, as is so often done, you would not have known were the problem was.
Let's go back to the run out test and assume you got an out of round wheel, not that uncommon on older cars. Tire cords shift with age and a tire that outwardly may seem to be OK can have a wow in it. Some tires were defective from new and ended up out of round. Rims can be bent or become out of round from sitting or hard driving. It is difficult sometimes to know if it is the rim or the tire that is at fault without taking them apart. One solution is to have the wheel shaved, a method many of us who race have done. The wheel is placed on a lathe type machine and a special blade shaves off the high side of the tire so that the whole unit is perfectly round. I had to have a couple of new race tires shaved 3 years ago due to run out problems caused in the manufacturing. I was given new tires a month later, but at the time this was the only way to get going. The great advantage of this method is you can save the rim and tire. If the tire tread pattern is rare and maybe the rim is very special it's well worth doing. Shaving tires is done predominantly by garages associated with racing. So don't expect any help from your local tire store.
The numbers on the tires determine the size but are NOT always interchangeable from make to make. In other words even though the numbers may be all the same, a Pirelli may be 1/2" taller than a Michelin, or a Goodyear. To be safe it is best to use a tape measure and check the width and diameter. You may be offered a good deal on a matched pair of tires that have the same numbers that your other tires have. When you mount them your car is not level and it is probably due to a different profile. Always measure first and remember that the height difference is only half of the actual measurement. On 14" rims this is not usually a big problem but it definitely is on the larger sizes