Wheel/tire combinations can be balanced in many different ways, including static and dynamic, on and off vehicle, as well as employing various types of permanent balancing systems.
Also known as "bubble balancing" uses a fairly inexpensive machine to balance the wheel/tire assembly at rest using a bubble level as an indicator. This technique takes some operator skill to perform good balancing as you need to carefully split the balancing weights on the inside and outside of the wheel to avoid dynamic imbalance.
Also known as "spin balancing" can be done either on or off the vehicle. The majority of tires are probably balanced on computerized spin balancers. After clamping the wheel on the machine, setting the wheel dimensions, it spins up and calculates the locations and amount of weight to apply to the rim to correct the balance. Most spin balancers center the wheel on a cone-shaped mounting device. This works fine for vehicles that locate the wheel on the vehicle via the hub (i.e. "hub-centric"). For vehicles, like Toyota, that locate the wheel on the hub via the lug nuts (i.e. "lug-centric") a special lug-centric adapter should be used to properly balance the wheel. On vehicle balancers avoid this problem, spinning up the wheel in place.
In either case, there are several types of balancing weight that can be used, depending on the application. The most common weight is a clip-on lead weight attached to the lip of the rim. As mentioned above, clip-on weights on the outside of the rim are prone to being scraped off on curbs and rocks. They also may not be suitable for certain alloy wheels. Adhesive backed weights are another option and have the advantage of being placed in the center of the wheel, if needed. But once again, they are prone to being scraped off in mud, snow and sand.
Weights on the wheel have an inherent problem due to the wheel/tire geometry. Since the imbalance is probably located out at the tire tread, it has more affect on balance than an equal weight located at the rim radius. For example on a 33x15 tire, an ounce of imbalance at the tread (16.5" radius) would require over 2 ounces of weight on the rim to correct (16.5/7.5x1oz.=2.2oz.). A more effective (and expensive) method of balancing a tire involve the application of heavy rubber patches to the inside of the tire. This has the advantage of needing less weight and eliminating the risk of falling off due to rocks and tire spinning, but the tire must be repeatedly mounted, spun, and dis-mounted to verify balance.
Another form of dynamic balancing involves shaving rubber from the tire to achieve balance. This is very helpful in cases where tires are physically out of round. However, it can involve the removal of significant amounts of tread and is more expensive than balancing with weights. Some tire chain stores offer this service, but only up to about 31" diameter tires. Shops that deal with large trucks and tractors often have on-vehicle tire shavers that can be used for this purpose.
Several options exist for permanent wheel balancing. These include liquid and dry powder weight added to the inside of the tire and external weighted balancing rings the clamp between the wheel and hub. Both these options are commonly used for over-the-highway trucks, where tires can last 100,000 miles or longer. Frequent off-vehicle balancing costs could add up over the lifetime of the tire. One popular dry powder balancing product is known as Equal. Installation of the balancing material is done on a deflated tire, using a special tool to inject the powder into the valve stem. Alternately, it can be placed in the tire prior to mounting. Proper technique must be observed to keep water out, including being careful with liquid tire mounting lubricant and use of dry air for inflation. For a tire used in off-road situations, where frequent air-down/up cycles are common, user's may want to consider adding an air dryer to their on-board air system. Both liquid and powder in-tire balancers can cause problems with clogged valve stems, too.
The other option for permanent balancing is the external, or wheel-mounted balancing rings. There are two designs common in North America, one is Sun-Tech Innovations and the other is Centramatic. Sun-Tech uses liquid mercury as the balancing medium while Centramatic uses steel shot in oil. The Ecology Center strongly urges fleets to not use mercury containing balancing devices and is calling for Sun-Tech to immediately phase out this mercury use. In either case, the balancers work by making use of centrifugal force to distribute the weight inside the tube to compensate for dynamic tire balance as it rotates. Assume some excess tread weight is present at one point on the tire. As it rotates, this causes an acceleration of the wheel and tire in the direction of the heavy spot. The balancing medium in the tube will flow away from this acceleration until such time when the out of balance situation is corrected. The centrifugal force holds the weight against the outside of the balancing tube. Since the balancing tube is located inside the rim, it is closer to the center of the wheel. These balancers require a certain speed threshold to activate, usually around 20-25 MPH. Below that speed, tire balance is probably not an issue. Several advantages of this type of balancer for off-roading is that they automatically compensate for tires that spin on the rim. They also compensate for tread that gets chucked on sharp rocks, and is tucked away without causing trail damage inside the rim. Drawbacks are that the balancers have a fixed amount of balance medium and can only correct balance up to a limit of about 12 oz. of lead. Also, like external weights, the balancer operates at a smaller radius than the tire, making it progressively less effective as the tire diameter increases for a give wheel diameter.
Excerpted from an article by Roger Brown.