Due to manufacturing limitations of both the tyre and the hub to which it is fitted, very few (if any) tyre/hub combinations are perfectly balanced (i.e. don't impart a vibration through the bearing on which the assembly is mounted). At very slow speeds, any imbalance is negligible, but at the high rotational speeds which road wheels are subjected to in normal driving conditions, even the slightest imbalance can put stress and strain on the bearings. This can be significantly reduced, and practically eliminated, by the addition of small lead weights which are attached to certain points on the hub, usually the tyre/hub interface. These can be fitted on both, just one, or neither side of the hub as determined by the balancing equipment at the tyre fitting centre.