If a tyre is fitted to a rim and then fitted to a vehicle without first being balanced, vibration may be noticed when travelling at speed from the steering, seating or floor.
Tyre balancing is the process of ensuring the combination of the wheel rim, tyre and valve spinning without vibrating aiding a comfortable ride as well as helping to extend the tyres life with even wear.
The tyre will be on the rim and inflated before it is put on the balancing machine. The balance machine spins the tyre and wheel combination to then identify any spots where there is uneven weight. The operator of the balance machine adds weights to the rim in areas identified by the balance machine. The operation is completed when the balancing machine can no longer identify any heavy spots which will cause vibration in the vehicle.
Rims may have small dents from previous use and the various brands and types of valves attached to the rim are not all the same weight. Tyres although very uniform in their design may still have areas which weigh slightly more than other parts for example where the internal belts are joined. When all 3 units are combined it is unlikely there will be complete uniformity in the weight distribution.
Yes, it is always recommended when buying new tyres that they go through the balancing service. Most retailers will provide this service whenever fitting new tyres to your vehicle.
There is no specific mileage to have tyres rebalanced however over time balance may change. This can be from a weight becoming detached from the rim, dents to the rim or uneven wear to the tyre. For the latter wheel alignment should also be checked. A tyre with significant uneven wear may not be able to be balanced correctly. Vibration can also be a sign of an issue with the tyre. The tyre should be checked for bulges or other kind of damage. If the vibration cannot be cured further investigations to the tyre and rim may need to be made but do not discount other vehicle components which are connected to the suspension brakes and steering.
No, wheel alignment focuses on the angle that the tyre makes contact with the road, whereas balancing focuses on vibration and a smooth ride.