Wheel balancing, also known as tire balancing, is the process of equalizing the weight of the combined tire and wheel assembly so that it spins smoothly at high speeds. Balancing involves putting the wheel/tire assembly on a balance machine, which centers the wheel and spins it to determine where the weights should go.
In essence, wheels and tires are never exactly the same weight all around. The wheel's valve stem hole will usually subtract a small amount of weight from that side of the wheel. Tires will also have slight weight imbalances, whether from a joining point of the cap plies or a slight deviation from perfectly round, because that kind of perfection is impossible to achieve. At high speeds, a tiny imbalance in weight can easily become a large imbalance in centrifugal force, causing the wheel/tire assembly to spin with a kind of “galumphing” motion.
This usually translates into a vibration in the car as well as some very irregular and damaging wear on the tires.
Traditional Spin Balancing To balance a wheel and tire assembly, we put it on a balancing machine. There are several ways to manually balance tires, but they frankly do not compare to machine-balancing in terms of either ease or precision. The wheel goes onto the balancer's spindle through the center bore, and a metal cone is inserted to ensure the wheel is perfectly centered. The machine spins the assembly at very high speed to determine the heaviest point, and signals the operator where and how many weights to place on the opposite side to compensate.
The most important things to know about balancing are:
a. Balancing Is Necessary: A weight imbalance in every wheel/tire assembly is pretty much inevitable. Only once in a very blue moon do we see an assembly come out naturally, perfectly balanced, and discovering a balance is just as much a function of the machine as discovering an imbalance.
b. Balance Changes Over Time: As the tire wears, the balance will slowly and dynamically change over time. Most good tire places will want to rebalance when tires are rotated, or when swapping in winter/summer tires for a second season, for example. Rebalancing at least once over the life of the tires will almost certainly extend said lifetime.
c. Balancing Only Fixes Balance: Balancing will not prevent vibrations from a bent wheel, out of round tire or irregular wear. Balancing weights can't compensate for a problem that is actually physical in nature, they can only compensate for weight differences. As well, one of the first mathematical assumptions the balancer's silicon brain makes is that the wheel and tire are both round to within a certain maximum deviation, so if a wheel is substantially bent, for example, the balancer will probably end up getting the weight placement wrong in the first place.