Nothing is more disconcerting in driving a car than feeling a vibration on the steering wheel or sometimes, in the seat of your pants. While vibrations in the steering wheel can come from many sources like out of true hubs or brake rotors, these causes are rare, compared to vibrations and steering wheel judder caused by wheels and tyres that are out of balance.
Every time you drive your vehicle, you are affecting the balance of the tyres little by little. However minutely, changes occur to the tyres as they rotate at speed. Centrifugal force and temperature combine to affect the tyre's structure. Steering wear, bouncing on the suspension, bad roads and road debris all combine to unbalance a tyre. An imbalance equivalent to 7 grams will not be noticed by the driver but over time, the small imperfections add up and that is when the driver notices the vibration. But at that point, the small, unfelt vibrations and imbalance are already affecting the longevity and wear pattern of the tyre.
Wheel balancing is required whenever you change tyres or, as some recommend, every 9-10,000 kilometres. The logic behind rebalancing your tyres every 10,000 km is that the wear, flexing and the bumps you hit will have affected the roundness of the tyre, to the extent that a high speed run will show the imperfections as vibrations on the steering wheel. Aside from that, hard braking can cause flat spots which will affect the tire's roundness. Hard acceleration can also cause wheel rim/tyre bead slippage, which will affect the balance of the wheel & tyre assembly.
Some experts in fact recommend a tyre rebalancing every time tyres are rotated in front-wheel drive cars. Anytime the tyre is removed from a wheel, such as when fixing a flat, it is recommended that a rebalancing should also be done. However, veteran tyre techs will put chalk marks across the tyre and wheel in case they need to remove a tyre, so that there will be no need to rebalance later. This can work especially if only a small patch will be applied to plug a flat. But vehicles subjected to regular high speed runs would be better off with a rebalance or at least a check on a road force balance machine to make sure that the wheel/tyre combo is running true.
There are many ways to balance a wheel and tyre assembly, which began with static balancing, then spin, or dynamic, balancing, on-car balancing and road force balancing, which is the state of the art. Static balancing uses a bubble balance to balance a wheel and tyre assembly. The wheel and tyre assembly will need to be removed from the car and placed on the balancer. A bubble indicator on the balancer will show where the heavy part of the assembly is, and wheel weights are added to the wheel assembly until the bubble is cantered on the bubble indicator.
Dynamic balancing uses a specialized machine to spin the tire and wheel, where a digital readout will then indicate the location and weight needed to balance the tire. Lead weights are commonly used to compensate for the imbalance, and will be the clip- or stick-on type, depending on the wheel design. On-car balancers use an electric motor to spin the wheel and tire assembly. An electronic pickup is used to determine where the wheel weights are needed. On-car balancers are sometimes recommended, because the wheel covers, brake disc, and lug nuts are included in the balancing procedure. All the rotating components are balanced as a unit and this can sometime cure pesky vibration or imbalance issues.
Currently, the most common form of balancing makes use of the spin balancer and it is the machine you usually see in tire and service shops. As with any specialized tool, it takes a trained and dedicated technician to fully utilize the potential of a spin balancer. Note that not all wheels use a hub-centric mounting method. There is also lug-centric mounting, and Toyotas use this type of mounting procedure. An extra adaptor needs to be mounted on the machine to mimic the mounting on the car and lazy technicians will just use the standard cone to mount the wheel. Sometimes, this works but it is an indication of the quality of the work that a shop will give.
Spin balancing works for most cars and 4x4's, but high performance and off-road vehicles have specialized balancing needs that are not always satisfactorily met by standard spin balancers. Part 2 of wheel balancing discusses the state of the art road force balancing machine and some methods off-roaders use to keep their huge, heavily lugged tires from wobbling when used on the road.