Second only to electrics, motorhome tyres are an ongoing source of misunderstanding and concern.

Some years ago, a survey of thousands of private US motorhome owners showed that almost all of their vehicles exceeded tyre manufacturers' maximum weight loadings by 20% - 30%. Many exceeded 50%.

As if this were not bad enough, most users were running at tyre pressures well below that specified for maximum loadings. Thus tyres were not only grossly overloaded, but under-inflated as well.

The major conclusion of that survey stated: 'With gross overloading, and all-but universal under-inflation, US recreational vehicle tyres are subject to more abuse than any other known form of tyre usage… including mine, quarry and exploration vehicles."

Even casual observation makes it clear that overloading such as this is common also in Australia.

It is the air inside a tyre that carries the load. If there is insufficient air (within the maximum limits set by the tyre builder) then the sidewalls will overflex and the tread will squirm around on the road. Both effects will cause the tyre to heat excessively, with a risk of tread separation. Both will accelerate tyre wear.

Take your vehicle to a public weighbridge (listed in Yellow Pages) and have the operator check front and rear axle loadings. The technical department of all the tyre manufacturers will supply recommended pressures. The maximum loading (per tyre) is always noted somewhere on the tyre sidewall, as is the maximum permitted pressure. So if you over the maximum weight (and by definition, illegal) try to reduce what you carry, or at least run at the maximum permitted pressure.

Tyre Ratings

The load that a tyre can carry is, amongst other factors), a function of its 'ply rating'. Whilst originally an indicator of separate layers, ply rating is now an indicator of wall thickness, or ruggedness. You may be able to bring tyres back to safety and legality by going up one or two ply ratings. Any competent tyre supplier will be able to advise you re this.

Dual Wheels/Tyres

Two-axle trucks are generally designed such that, when laden, the rear axle carries about twice the weight of the front axle. As it is convenient to have the same size wheels and tyres all round, this can only be done by having single tyres (with the front being run at half-load) or twin tyres at the rear.

This works fine for on-road vehicles. It is usually avoided in off-road vehicles as, in rough going (particularly over rocks), one or the other tyre of a pair is likely to carry most of the load. There can also be problems with rocks trapped between the inner sidewalls. Their performance in sand is reasonably good because it is footprint elongation that does the job, not sidewalls bagging out. Almost all serious off-road vehicles use large single tyres – even to the extent of having dual rear axles rather than dual tyres on a single rear axle (except where massive carrying capacity must be combined with short overall length).

Wide Singles

In recent years there has been a move toward low, wide profiles. For car tyres, this reduces centrifugal forces, and by enabling a 'tighter' running profile, enhances handling.

The motives for wide truck tyres are different. The major one is that a wide single tyre is much narrower than the overall width of a pair of dual tyres. This enables the chassis rails to be set correspondingly wider apart, and this increases truck and trailer stability. It also reduces scrubbing forces on multi-axled trailer tyres whilst cornering.

Wide tyres generally run cooler and last longer. Dynamic performance is changed however and drivers must adapt to the differences.

Road surface studies however increasingly show that wide single tyres are (unexpectedly) harder on road surfaces and, as a consequence there is a current move to reduce maximum axle loadings for trucks using these tyres. If this happens it may well spell a general return to duals.

Off-road, wide tyres are not as effective in soft sand: their low profile inhibits footprint elongation. Whilst many outback tour operators have switched to using them this is because they are preferable to off-road tyres whilst travelling long distances on hard roads. They accept their lesser sand performance because they have 14-20 passengers to push if bogged down (for the same reason they rarely reduce tyre pressures in sand).

A further problem is that wide tyres rely on high pressures to maintain their correct profile. The pressures are often much higher than available from service stations outside the major cities.

Tubed/Tubeless

An inner tube squirms around inside a running tyre generating heat in the process (Michelin tyres in particular should always be used with Michelin inner tubes). By doing without inner tubes, tyres run cooler and last longer.

The downside of tubeless tyres is that they need an initial blast of air to obtain the essential seal against the wheel rim, and this is difficult to do in the outback. The clever thing to do is to carry a spare inner tube or two to get yourself out of trouble, but to remove it once the tyre has been properly repaired. Do not fit inner tubes routinely to tubeless tyres.

Failures in Rarely-Used Tyres

Peter Kelleher's experience of blowing out several old and barely used tyres (Wanderer, April 2002) is common also amongst caravanners. A chemical engineer will be able to give a more thorough explanation, but briefly tyres oxidise, and this oxidisation is normally 'pumped out' by flexing when the vehicle is driven. But if the tyre is unused over for a long time, oxidisation progressively weakens the tyre, which may then blow. Almost any tyre becomes suspect after seven or so years, and it pays to be cautious with any tyres that are unused for more than three or four. It's a good idea to drive the bus around every now and again! (Also be wary of buying new tyres that have been sitting around for years).