Tip 4 - Inflation Pressures
Have you ever thought of what you are asking of those two new tyres
you just put on the front of your truck?
So you'd like to see them deliver, say, around 120,000 kilometers
on the steer, then maybe run them out on the trailer with a couple
of caps for another modest 80,000 kilometers each. No worries mate!!
But just think what these tyre have to endure to achieve these
expectations before you throw them away, because make no bones about
it, truck tyres will eventually die through fatigue. Would you
believe that the average 11R22.5 steer tyre will complete around 38
million rotations during those above first life 120,000 kilometers?
Then two caps on the trailer position will contribute another
160,000 kilometers or 50 million revolutions to the total. Thus a
massive 88 million times each part of the tyre has to hit the road,
deflect under load through the footprint, and resume its normal
diameter out the other side!
Now consider the performance of your deep tread drive tyres.
175,000 kilometers sound reasonable? Work on approx. 50 million
revolutions in its first life alone. Why not give the tyre a break
when recapping, and request a shallow cap on it as it continues to
spin it's way to a huge number of flex reversals. You could do
yourself a favour and place the tyre in alternative applications,
i.e. urban work, as it approaches it's hard to predict end of
working life. That way you won't get caught somewhere between Sydney
and Melbourne wondering why this goddam tyre has failed --- and
after only three caps, too!
This end of working life fatigue condition is hard to predict
since it depends on a number of factors, not only on rotations. The
state of the road, heat, inflation, all contribute. Ever wonder why
a jumbo airliner tyre can accommodate up to ten retreads? Because
the tyre casing does very little work. These airliners do
comparatively few kilometers on the ground, but obviously scrub the
tread rather prodigiously each time they land. So the tyre casing
completes relatively few revolutions and can handle a multiple
number of retreads. Surprisingly, the taxiing of the aircraft with
it's attendent heat build-up has more affect on the tyre casing than
the actual landing.
Tyre designers try to minimise the flex, or deflection, of a
truck tyre to achieve a high fatigue capability. Obviously there is
a compromise here, since ride and comfort factors, impact damage,
etc., need to be taken into account. They calculate around this
deflection figure to achieve the ideal contact patch and obtain
optimum treadwear, braking and handling performance. This is where
inflation pressures become important to maintain the tyre deflection
within these design parameters.
Just remind yourself that it is not the tyre that carries the
load, but the air inside it. Remember, a tyre is basically a
flexible air container, and without air it cannot hold it's shape
let alone carry any load. So it is imperative that the correct
inflation pressure is in the tyre to match the load it is carrying.
Too little pressure will lead to over deflection of the tyre, which
makes the tyre run hotter and may cause premature tyre failure. The
service life of the tyre is shortened roughly in proportion to the
reduction in inflation pressure, (Rule of thumb --- a tyre reduced
by 20 percent below the recommended inflation figure equals 20
percent reduction in service life). Higher rolling resistance is
generated with under-inflated tyres, which leads to increased fuel
consumption. Lastly, irregular tread wear can be produced by reduced
Over inflation of tyres can also lead to difficulties such as
increased impact damage potential, as well as irregular treadwear.
Of course, other factors may come into play in choosing your
ideal inflation pressures. Factors such as stability, wear
characteristics, operational conditions, etc. We recommend that once
these ideal inflation pressures are established, high mileage fleets
should be checked regularly each fortnight or so with a reliable
tyre gauge (not with a boot!). Missing valve caps and leaky valves
should be replaced immediately. And if you can't reach the inside
dual valve to check it, don't ignore it, organise some valve