How Stuff Works

Fuzz Townshend goes wheelie nuts on us

Wheel nuts

The humble wheel nut was an oft-overlooked motor vehicle component for many years. But as the key fixings, keeping wheel and car combined, they undertook perhaps the most noble of automotive tasks.

The type of wheel nut used always depended upon the type of wheel fitted and its mounting method, although tapered nuts ruled supreme in the classic car world for many years. The tapered section of the nut was used to assist in the accurate centring of each wheel onto its hub.

Early cars often used nuts made from brass, featuring a 60-degree tapered seat. The soft brass helped to prevent any work hardening of the nut location tapers in the steel wheel centres which, if affected, could split.

This was fine on cars that pootled along at a relatively pedestrian pace, but as vehicles became capable of ever greater speeds and braking temperatures increased, throwing more heat into the hubs and so onward into the ever-so expandable brass nuts, there was a danger of them becoming loose, releasing the wheels from their rightful locations.

So it was that steel 60 degreestapered seat wheel nuts took the place of brass, necessitating more robust steel wheel centres. This helped to overcome most of the splitting wheel problems and allowed greater tightening torques, which meant that vehicles’ wheels were easier to keep attached to the now-speeding cars.

This was progress indeed, but there remained another, as-yet unmentioned problem – namely mechanical precession, also known as epicyclic fretting precession.

In effect, this was the wheel nut, sitting in the taper of the wheel centre, rolling in the opposite direction to the rotational direction of the wheel; or, to put it another way, coming loose.

In order to overcome this effect, right flank vehicle wheels were fitted with wheel nuts featuring left-handed threads, a practice that has continued in many larger vehicles into modern times. However, lighter-weight wheels meant that most cars were able to dodge this thread confusion, although those fitted with splined wire wheels and centre spinners continued the practice, but the latter deserve solus description on another occasion. The advent of alloy wheels again gave rise to heat transfer, this time into the highly receptive alloy, but a further problem was the less robust nature of the lightweight materials being utilised, which resulted in the use of the mag seat-type wheel nut.

These featured a tubular extension from the nut, onto which the wheel was positively located, removing the problems associated with heat transfer. Another benefit of this type of nut was that a wheel could withstand greater radial and axial loads, with the elimination of the tapered seat and the incorporation of a flange or washer into the fixing arrangement.

An alternative fixing on both steel and alloy wheels saw the flat taper replaced by a spherical machined face ( ball seat), giving a greater frictional area, thus spreading the load through this.

Finally, the advent of the spigot-mounted wheel saw it mounted directly onto a positive location on the hub itself, to cope with radial loads. The axial load was managed by nuts featuring an inbuilt flange washer, which required lubrication at the point between it and the nut, so that accurate torque setting of the fixing could be achieved.

’Tapered nuts ruled supreme in the classic car world for many years’