A preload of hot air?
QIn the feature on bearings in the October 2018 issue, you recommended tightening taper-roller wheel bearings ‘until play is just eliminated and no more’. I was always taught to leave slight play. What’s your reasoning?
Lionel French, Shrewsbury
AWe’ve found that just eliminating play in wheel bearings has improved the stability and handling of our cars compared to the method you describe – which we’ve always been taught, too. Studies by manufacturer Timken show that bearings wear least when running with zero play and zero preload (ie, ‘play just eliminated and no more’) – or a very small preload.
There are two possible reasons for the old ‘little bit of play’ maxim. The first, we think, is to stop people tightening them up too much and causing rapid wear, seizure or collapse. The second is to allow for heat expansion. This would be appropriate to a piece machinery that runs 24-hours a day – but less so to a car. If you drive in such a way that the brakes are frequently hot for long periods, you could leave a little play – or set the bearings after a good run.
Note that this doesn’t apply to all classics with taper-roller wheel bearings. Some have a stated preload – so check your manual.