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Why does a wheel need a centre bore?

Serhii Maslov Section: Spigot rings Updated: First published:

So why is a centre bore needed at all? After all, the wheel is drawn onto the hub by bolts (or nuts), which also have their own tapered seats for centring. And the joint between the wheel and the hub’s spigot itself, it would seem, should not carry any load at all? So why do car makers bother making this bore, which only increases the chance of dirt getting into the wheel bearing?

First of all, it is worth remembering that a wheel is one of the few car parts that manufacturers are forced to let motorists replace themselves. And once replacing a wheel yourself is allowed, there is no point at all in designing and producing special, complicated equipment for fitting wheels (even for tyre centres and authorised garages).

All the same, how precisely a wheel is fitted onto the hub matters enormously for road safety and for the service life of the suspension’s key parts. The article on the importance of wheel balancing has already described what defects and poor balancing do to a wheel. In this case, exactly the same kind of imbalance can arise not from poor balancing or a wheel defect, but from the wheel not “seating” correctly on the hub. Put simply, if the wheel’s axis of rotation does not coincide with the hub’s axis of rotation, the wheel will run out of true, creating huge non-standard loads on the suspension. The result is obvious: suspension parts wear out several times faster, with the risk of their sudden failure. What happens when you suddenly lose a front wheel at, say, 100 km/h is something I don’t think I need to spell out.

And it has to be said that the accuracy required here is maximal. There can be no deviation of even 1 or 0.5 mm here: the wheel’s axis of rotation must coincide exactly with the hub’s axis of rotation! So how do you achieve that kind of precision, especially when a motorist is fitting the wheel by hand, right there on the road?

Car makers solve this problem as follows: they machine a spigot on the hub whose outer diameter exactly matches the inner diameter of the wheel’s centre bore, which lets you first seat the still-unbolted wheel onto the hub fairly precisely.

Thus, the centre bore, and the centre bore alone, is what a wheel needs for fitting correctly onto the hub, not the tapered seats on the bolts (or nuts), whatever some sellers may claim.

More on this topic

What affects the quality of wheel balancingWhat has more effect on the outcome of wheel balancing, the balancing machine or the fitter’s skill? On the whole, both matter in roughly equal measure. What kinds of wheel imbalance are there?Imbalance can be static or dynamic. In both cases the issue is a mismatch between the axis of rotation and the axis of inertia (the latter running through the wheel’s centre of gravity). What level of imbalance is considered normal?What is the maximum permitted mass of balance weights, and what criteria tell good wheels or tyres apart from bad ones? How accurate does wheel balancing need to be?Every unbalanced mass on a wheel creates its own centrifugal force as the wheel turns, with its own point of application. Let’s recall school physics: the formula for centrifugal force. Why are wheels balanced?If a wheel is not balanced across all its axes of symmetry, unbalanced centrifugal forces arise with different directions of action, which creates vibration in the wheel and loads on the hub pulling… What happens when the offset a wheel was designed for changes?Shifting the wheel’s central axis inevitably shifts the steering axis too, changing the steering geometry the designer intended.

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