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Dunlop on inflation pressure
- Subject: Dunlop on inflation pressure
- From: Jim Huber <jhuber@xxxxxxxxxxx>
- Date: Fri, 29 Sep 2000 11:14:12 -0700
http://www.dunloptire.com/tiretech/Welcome.html?article=inflation_pressure.txt
For those without web access, here 'tis without the pretty pictures:
A tire is a pneumatic system, which supports a vehicle's
load. It does this by using a compressed
gas (usually air) inside to create tension in the carcass
plies. It is important to realize that a tire
carcass has a high-tension strength, but has little or no
compression strength. It is the air
pressure that creates tension in the carcass and allows the
tire to function as a load-carrying
device. That's why inflation is so important. In an
inflated, but unloaded tire, the cords pull
equally on the bead wire all around the tire. When a tire is
loaded the tension in the cords
between the rim and the ground is balanced or relieved. The
tension in other cords is not
changed. Therefore, the cords opposite the ground pull
upwards on the bead. This is the
mechanism that transmits the pressure from the ground to the
rim.
In addition, a tire must transmit handling (acceleration,
braking, cornering) to the road. Cornering
forces are transmitted to the rim in a similar manner to
load. Acceleration and braking forces rely
on the friction between the rim and the bead. Inflation
pressure also supplies the clamping force,
which creates friction.
A tire also acts as a spring between the rim and the road.
This spring characteristic is very
important to the vehicle's ride.
Too high an inflation pressure causes the tire to transmit
shock loads to the suspension and
reduces a tire's ability to withstand road impacts.
Too low an inflation pressure reduces a tire's ability to
support the vehicle's load and transmit
cornering, braking and acceleration forces.
Finding the optimum inflation pressure requires extensive
engineering efforts on the part of tire
and vehicle manufacturers.
Under-inflation can cause many
tire-related problems. Since a
tire's load capacity is largely
determined by its inflation pressure,
under-inflation results in an
overloaded tire. An under-inflated tire
operates at high deflection
resulting in decreased fuel economy,
sluggish handling and may result
in excessive mechanical flexing
and heat buildup leading to
catastrophic tire failure.
Correct inflation is especially
significant to the endurance and
performance of radial performance
tires. For example, because of
a performance radial's aspect
ratio and design, it may not be
possible to look at a radial tire
and actually see under-inflation of
5 psi. However, under-inflation of
5 psi can reduce a performance
tire's tread life by 25%. A
typical tie may also lose 1 to 2 psi a
month, if not checked and
adjusted.
Temperature Effects: Air pressure
is affected by temperature.
The air under pressure in a tire
is no exception. Typically, an
inflation pressure can change by 1
psi for every 10 degrees
Fahrenheit of temperature change.
Higher temperature means
increased pressure.
For example, if a tire is inflated
to 35 psi on an 80-degree July
day, it could have an inflation
pressure of 23 psi on a 20-degree
day 6 months later in January.
This represents a normal loss of 6
psi over the six months and an
additional loss of 6 psi due to the
60-degree temperature change. At
23 psi, this tire is severely
under-inflated.
Safety Note: For safety and vehicle performance, Dunlop
recommends that tire inflation pressure
be checked at least once each week and as often as possible
when tires are cold (ambient air
temperature and if the vehicle has not been driven for
several hours) Repeat or excessive
inflation loss (more than 2 psi); visible damage such as
knots bulges, punctures, cuts, cracks,
irregular wear; experiencing impacts, vibration or pulling;
all require removal, expert inspection of
tire and rim to determine reparability, or the need for
replacement. Damaged tires may fail
suddenly or burst upon re-inflation, resulting in serious
injury.
* * * * * * * * * * * * * * * * * * *
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