High school chemistry may be a distant memory, but getting the hang of tyre pressure will help you get the best out of your vehicle. We asked the experts at BF Goodrich, makers of the toughest all-terrain tyres, to help us decode the science.
It’s worth getting a handle on tyre pressure because it affects your vehicle’s handling, traction and the way the tread wears. In essence, the volume of pressure determines the size of your tyre’s footprint (the area in contact with the road) and its stiffness or ability to keep its shape.
Also read: Don’t let those punctures leave you stranded
Pressure and heat
In case you’ve forgotten the basics from science class, the point to remember is that gas (i.e. the air in your tyre) expands when heated and contracts when temperature declines. Recommended tyre pressure is based on cold inflation pressure, so follow the below guidelines to get an accurate reading.
• Check tyre pressure in the morning before the outside temperature rises.
• Direct sun on the tyre will significantly increase pressure.
• Driving causes friction and warms the tyre, increasing pressure. For this reason, you need to check tyre pressure before driving.
• If the pressure is correct while a tyre is warm, it’s likely to be underinflated once it cools down. Underinflation causes the tyre to continuously overflex, which could lead to premature failure.
Do all-terrain tyres need different pressure?
Your vehicle’s pressure guidelines (posted inside the driver’s door or found in the owner’s manual) are for the original equipment tyres. If you change the standard passenger tyres to more rugged tyres for off-roading, you have to rethink the tyre pressure.
“All-terrain or light truck (LT) tyres are made with thicker tread and bigger bead packages to cope with their heavy-duty use,” explains Dawid Harmse of BF Goodrich. “Because an LT tyre has more rubber and is heavier than a standard tyre of the same size, it runs hotter.”
A tyre’s load carrying capacity decreases when operating temperature increases. This means that LT tyres need more air pressure to carry the same load.
Also read: Basic vehicle prep for remote areas
Why lower your tyre’s pressure?
When you reduce tyre pressure, you increase the size of the tyre’s footprint. This spreads the vehicle’s weight over a larger surface and improves traction, extremely useful when driving on sand or rocky terrain. Lower pressure softens the tyre, which allows it to absorb the bumps of a corrugated track. On a fully inflated tyre, every bump is transferred through the suspension, making for a less comfortable ride.
Lowering tyre pressure is one of the biggest adjustments you can make to your vehicle’s performance in off-road conditions. It’s even good for the environment. At lower pressure, the vehicle’s weight is spread over a larger area, so it doesn’t dig into the surface as much. Digging in causes damage to the track and the track becomes progressively wider as other drivers try to avoid the damage.
Also read: Bush mechanic notes for solo overlanders
What’s the right pressure for the job?
The optimum pressure depends on the situation e.g. the surface you’re tackling and how heavily your vehicle is laden. In one trip you may have to decrease and increase pressure several times. For this reason, BF Goodrich recommends always travelling with a reliable pressure gauge and air compressor. Remember to air up when you hit the tar again.
How much you lower the tyre pressure also depends on the speed you’ll be driving and personal preference, so there are no hard and fast rules.
Bear these pointers in mind:
• Smooth dirt roads: stick to the usual pressure. If you’re driving at similar speeds to tarred surfaces, lowering the pressure would be dangerous.
• Corrugated tracks: reduce your tyre pressure by up to a third to make the ride smoother.
• Soft sand: lower pressure by almost half to distribute weight over a greater area and prevent the vehicle from sinking in.
Golden rule: The lower you go, the slower you go!
How have you dealt with tyre trouble on a trip? Tell us about your experience in the comments.