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 speed. Balancing involves putting the wheel/tire assembly on a balancer, 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 one side of the wheel. Tires will also have slight weight imbalances, whether from the joining point of the cap plies or simply from a slight deviation in the shape of the wheel. 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 place 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 then tells the operator where and how many weights to place on the opposite side to compensate.
The most important things to know about balancing are:
•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.
•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 their lifetime.
•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, only for weight differences.
Road Force Balancing
Because there are those other reasons than just balance for vibrations and strange tire wear, the “Road Force” balancer was born. This style of balancer, in addition to performing a traditional spin balance, also measures both the wheel and tire to determine if there are conditions that would tend to cause a vibration on the road. Generally, most balancers do this by pressing a large roller against the tire as it spins slowly, reading out tire pressure and radial runout (i.e. deviation from perfect roundness). This can detect conditions such as belt separations and match mounting issues.
Generally, both wheels and tires will have high and low spots in terms of their runout, because perfection is impossible. If you can imagine pulling one point of a connected circle (such as the edge of a wheel) just slightly outwards, you can see that some other point of that circle must move inwards to maintain the connection, creating an egg shape. These are high and low spots for radial runout. If balanced on a traditional balancer, this assembly will not only require more weight to balance but will still probably cause a vibration.
The solution is to measure both the wheel and tire, and then move the tire around on the wheel until the high spot of the tire matches the low spot of the wheel. This process is usually called “match mounting.” Most tires today have small dots on the sidewall to indicate the point on the tire that should match to the valve stem to get a decent match mount. Road force balancers do a much more precise job of this by measuring both the wheel and tire with rollers and then directing the operator to mark the points to be matched. The resulting assembly requires less weight to balance and spins straighter.
The solution was Tape-A-Weights. Strips of flat adhesive-backed lead squares, each square weighing one-quarter of an ounce, the weights can be cut to size with clippers and stuck to the inside of the barrel behind the spokes. The adhesive is quite strong, but wise tire techs will still clean the surface where the weights will go to make it free of brake dust and grease. This will prevent the weights from falling off. If there is any question of the adhesive holding, a strip of duct tape to cover the weights will hold through just about anything. Racing techs use duct tape to hold weights on wheels under heat conditions that would melt the weights' adhesive.
So that is why using bang-on weights on the face of an aluminum alloy wheel is a terrible sin. Always ask for adhesive weights when you have your alloy wheels balanced. Be suspicious of any tire place that does not use adhesive weights. Many places will use bang-on weights on the inside flange of the wheel and adhesive weights on the outboard side. (Bang-on weights are generally less expensive.) This is usually perfectly acceptable unless you have chrome wheels, since any break in the chrome can begin the flaking process and eventually be fatal.