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Why are there tapered seats on wheel bolts and nuts?

Serhii Maslov Section: Spigot rings Updated: First published:
The short answer The tapers are not there to centre the wheel on the hub, but to hold it securely in that position!

A common myth put about by wheel sellers: the centre bore plays no special part, because the wheel is centred on the hub by the tapered wheel bolts (or nuts).

In fact, the tapered seat on wheel bolts (or nuts) is not designed to, and cannot, guarantee precise centring of the wheel on the hub, however precisely and well it is made! And do not listen to sellers or mechanics who tell you otherwise. The thing is that a taper could theoretically settle into an absolutely exact position in the wheel’s seat only if three conditions were met at the same time:

  1. The surfaces of the taper and of its seat are perfectly smooth.
  2. Those same surfaces are perfectly true and made to match each other absolutely exactly (lapped in).
  3. The bolts (or nuts) are tightened very slowly and virtually simultaneously.

As you will appreciate, all of the above is, in ordinary conditions, practically impossible, since there are countless different manufacturers of fasteners and wheels (each with its own equipment and its own tolerances), and nobody ever bothers cleaning bolts and their seats. And absolutely (ideally) correct tightening is only possible on a rig, where the bolts (or nuts) are tightened simultaneously and at the same speed.

So the tapered seats on the bolts (or nuts) always carry some tolerance in how precisely they position the wheel on the hub. Whether that is hundredths, tenths of a millimetre or even whole millimetres depends on how worn and how dirty the wheels and the fasteners are, as well as on the quality and precision of the tightening when the wheel is fitted; the main point is that these tolerances exist and are not stable under ordinary conditions of use. Combined with the “tight” fit of the wheel on the hub’s centring spigot, the risk of the wheel’s axis of rotation deviating from the hub’s axis of rotation when the wheel is fitted and used is not just greatly reduced, it is practically eliminated.

So why are these tapers needed at all, you may ask? Quite simply: the tapers are not there to centre the wheel on the hub, but to hold it securely in that position! It is the tapers that take all the load created by the car’s weight in the vertical plane, which is exactly why the hub’s spigot and the wheel’s centre bore do not wear, serving purely to seat the wheel correctly on the hub.

So an exact match between the diameter of the wheel’s centre bore and the outer diameter of the hub’s spigot is of exceptional importance for the wheel’s correct “seating” on the hub, while the presence and the quality of the tapered seats on the wheel bolts (or nuts) matters for keeping the wheel correctly positioned while the car is in use.

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