MORE ON MICRO BALANCING While tire quality and structural integrity is paramount, it has less effect on tire shake than you'd think. A grossly out of round tire can still be properly balanced to remove almost all effects of the out of round condition. Unless traveling over 140 mph frequently, a tire with 3/16" of out of roundness, if properly balanced, can provide acceptable ride and performance. Side to side tire wobble in the range of 1/4" is still consider well within the tire balancer's domain of repair.
When you have a light car with lots of rubber on the front in relation to it's weight, tire shake can be aggravated more so by two slightly out of balance tires, rather than one significantly out of balance tire. The reason that is, is when the two out of balance tires have their heavy spots inline with each other (both heavy spots at the same rotational position in relation to each other), the front end my shimmy slightly, but not to the point of major concern. However, as you continue to drive the car and deviate from a perfect straight line course, one tire will spin slightly faster than the other and the heavy spots in the misbalanced tires oppose themselves, now the one tire's heavy spot will attempt to pull the steering slightly to one side, and then the other tire's heavy spot will come around and attempt to pull the steering the other way. Once this oscillation sets itself up, the shake can be violent. As the two tires align themselves again, the shake can then simply disappear… until the tire synchronization once again changes. In more severe imbalances, the shake will simply come in at a certain speed (usually 50-70), and stay there.
When balancing tires, a high quality and very accurate high speed balancer must be used. The balancer needs to be able to read at least down into the 1/10th of an ounce. Many machines refer to this as "Fine Balancing." Another concern is to "Dynamically" balance the tires. This means location compensation weights on both the inboard and outboard sides of the rim. Dynamically balancing a tire not only reduces the up and down "bounce" of an imbalanced tires, it also reduces the side to side "warble" of the tire in motion, which greatly affects front end shake. While many refuse to locate weights in visual view on the rim face itself, there is an alternative. This is typically refered to as "Mag Balancing."
Mag Balancing is a special feature of certain balancing machines which the machine is programmed into it, the width of the wheel, the inside edge AND the center wheel location. Then it calculates the amount of weight needed to place on the inboard lip. It also indicated the amount of weight needed to place on the outboard lip and THEN calculates the corrected amount of weight to place just behind the center of the rim to mirror the effect as if it was placed on the outboard exterior lip. This yields similar results without the unsightly presence of the weight on the rim face.
Many balancing shops do not striver for perfection, but use the dreaded word "good enough" and stop short of a perfect 0.0 balance. There are two reasons for this… 1) Most cars do not feel the difference between 3/4 ounce off and perfect 0. The SPF certainly does. 2) Many times in order to achieve a perfect 0.0 balance, it takes almost as long to get that last 1/10th of an ounce corrected than it took to get there in the first place. Many shops simply do not have, or wish to exert that additional time. This is the key to eliminating tire imbalance as a contributing factor to the dreaded tire shake.
In achieving a perfect 0.0 balance within less than 1/20th of an ounce, unorthodox methods may be required to reach that goal. Modifying production weights to achieve the EXACT amount of applied correction to a rim requires imaginative options, such as using coins of specific weight values. Sometimes cutting washers in half and grinding them lighter and lighter until perfection is achieved is necessary.
HINT: Keep in mind, the more perfectly the tire is balanced the first time, the longer it takes to loose it's balance. In some cases, a perfectly balanced tire can actually "correct" a physical out of round tire over a period of time, by purposely wearing the tire more on the heavy side than the other.
In any case, tires should be rebalanced every 6,000 miles regardless if they feel fine… because if you wait until you feel the imbalance, it's already too late.