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AS with many aspects of recreational vehicles, their tyres take a greater
beating than those in general use. In fact a major US industry report
some years back noted that RV tyres are subject to more abuse than any
other tyre in any other form of use – including trucks used full-time
in quarries. In particular, stated the report, RV tyres tend to be chronically
under-inflated and overloaded. Virtually all RVs sold in Australia are
fitted with LT (Light Truck) tyres which, by virtue of their heavier
carcass, are better suited to running at or near their designed maximum
loading at all times. It does not follow however that heavier construction
enables them to carry a heavier load.
The maximum load that a tyre can legally carry is very much a function
of heat build-up. The latter is greater with light truck tyres because
their thicker side walls and tread support generate more heat as they
flex. To minimise this effect they are inflated to pressures higher than
for the equivalent size passenger tyre. In recent years there has been
a move toward larger rim diameters, lower side wall heights, and greater
widths. The more extreme examples resemble bicycle tyres.
There are various reasons for this change. Increasing rim diameter provides
better brake cooling and/or creates space for larger diameter brakes.
The low and wide profile provides more responsive handling and more precise
steering. It also reduces flexing and thus heat build-up. As less energy
is lost in heat and rolling drag reduced, fuel economy is consequently
increased.
Particularly in their Light Truck form, these newish wide tyres need
a fair amount of air pressure to prevent their collapsing in the middle – like
a failed souffle. Pressures of well over 750 kPa (over 100 psi) may be
needed to maintain that profile. This can be a major problem in outback
areas where few service stations have compressors that work above 75
psi or so.
Whilst wide tyres perceptibly improve vehicle handling they are not nearly
as effective in soft going as their high profile predecessors. Late model
OKAs, for example have 19.5 inch low profile tyres. This improves their
on-road performance but reduces their capability in sand. There, the
early 900 by 16s doughnut-like tyres are better suited. Tour operators
prefer the 19.5 inch tyres because they are cheaper and more readily
available. And, when they do get bogged, they have 13 passengers available
to get out and push.
With most laden RVs it is rare for tyres to need much lower than their
maximum permitted pressure, not least because vehicle makers’ suggested
pressures are usually the lowest recommended. Except when travelling
over dirt roads, it is usually better to increase recommended pressures
by 10%, however for dirt road use, lower pressures by 20%-25% and keep
the speed below 80 km/hr.
If you feel that pressures may need altering, bear in mind that, with
wide profile tyres, the old ‘rule’ of setting pressure
such that it rises by about 30 kPa (about 4 psi) as the tyre warms up
is not applicable. The only safe way is to take the vehicle to a weighbridge
and ascertain individual axle loadings (ideally individual tyre loading – see
below) and ask the Technical Department of the tyre maker concerned.
Collyn Rivers, W8054.
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