Monday, 30 January 2012

Letting The Air Out

So do you air down your tyres when you go off road, a lot of folks don’t! Airing down is the best thing you can do for off-road performance, second only to running lockers in your axles. The difference between aired-up and aired-down on the trail can be staggering. So why does airing down make such a difference, people always think that by airing down you increase the width of the contact patch of the tyre, this is true but it also increases the length of the contact patch, so it puts more rubber in touch with the ground, more rubber in touch with the ground means more traction. The table below shows how the contact patch of the tyre grows as the pressure goes down.





Another advantage to airing down is the way the tyre will deform to follow the contours of the ground, the photo below illustrates this very well. If the tyre can conform to the obstacle it is trying to climb over the rolling resistance will be reduced, which means a lots less effort is required.





So now that we have established that airing down tyres is a good thing how low can you go? Here is an extract from JP Magazine
“Keeping-It-Low Cappa
Almost everyone I meet runs too much air pressure in their Jeep's tires both on- and off-road. Like Christian, I cringe when I see beadlocks on a Jeep where the tire barely has a sidewall bulge. The owner generally considers 25 psi aired down since he runs the max sidewall-listed pressure of 35-50 psi on the street. Not only is he prematurely wearing out his tires, but he's rattling his teeth loose and giving up a ton of off-road performance. This is even more widespread now that 16-, 17-, and even 18-inch wheels have become common on small Jeeps, whereas before these wheels and higher-pressure tires were typically only found on 3/4- and 1-ton trucks.

Running the correct tire pressure starts by picking the correct tires. Most Jeeps only need a load range C tire. The D- and E-rated tires are for heavy-hauling trucks. With a C range radial tire most Jeeps can run about 20-25 psi on the street. Most bias-ply tires can hold up a Jeep with about half that. Off-road I consider 10-12 psi a good starting point for a radial tire and I'll often go lower than that if the terrain is really soft and there are few tire hazards. On a bias-ply tire you really don't get a good traction bulge until you hit the single digits, so I'll start at around 5-10 psi and go down from there if need be. To some people these pressures sound unreasonably low, but they increase traction significantly and improve the ride as well. You can either adjust your driving style to keep the tires on the beads or step into beadlock wheels if you decide to run at the lower end of these pressures.”

On my yellow Wrangler TJ with 33x12.50x15’s without beadlocks I normally drop them down to 15psi. Many people have different opinions as to how much to air down when off roading, so find what works for you and your setup. But be sure to be careful, it must be noted that we are only talking about and recommending tyres pressures being lowered while off road and at lower speeds, NOT driving on the road or at higher speeds.

15104.53So how do you air down your tyres, if you have every tried doing it will a a match stick pushing the valve down you will know how long it can take. There are a lot of devices out there to make life easy, but the one I favour is the Rugged Ridge adjustable tyre deflators, once you have set them up to the pressure you want, you just screw them on to the valves and go have a cup of tea. Once they have finished hissing unscrew them, and you are god to go. The Rugged Ridge tyre deflators are available from FTE 4x4 Specialists Ltd.

Don’t forget that once you have aired down at the end of the day you need to pump the tyres back up.


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