It's okay, the pressure's off

"I told you we should have lowered the tyre pressure some more."

Some 4WD trainers reckon deflating tyres is one of the worst sins an off-roader can commit. But go out with some tough club guys and anyone running more than 6psi is a pansy. The truth, as usual, lies somewhere between the extremes. Trainers often argue that dropping pressures at the start of a trip potentially uses your "get out of trouble free" card before you even get going. Extra grip offered by the bulging and elongated tread may lure the driver into trickier situations than his vehicle and/or skill level can handle. And with the pressure already dropped, it may not be safe to lower it further when running tubeless, as most 4WDs are. The tyre may come off its rim, causing more trouble than the vehicle and its occupants were already in. Trainers are also aware that many 4WDs come with fairly low profile tyres whose "short" sidewalls are likely to part from the rim after a much more modest pressure drop than, say, "tall" 75- or 85-series rubber.

Hard-core enthusiasts, on the other hand, want maximum traction and will run the lowest pressure they can get away with. They will normally run tubes and maybe beadlockers to achieve this. I've never had beadlockers, but used to run my tubed tyres as low as 6psi on the Land Rover when conditions warranted it. The grip was amazing. Usually, I found 16-17psi satisfactory for a rutted and muddy track. Many drivers, myself included, also like to drop pressure for off-road comfort. Rutted tracks can give a jarring ride at street pressures; going down to about 24 is wonderfully smoothing.

Dropping pressure for sand driving is common, but only once or twice have I needed to go really low, like below 12psi. Running in the 20-24psi range usually does the trick and often it's not necessary to go anywhere near as low. However, there's more to good sand driving technique than just losing air.

It's okay, the pressure's off, II

 
Had a bit of a play in some mud today. I kept my street tyre pressures, partly because I was feeling lazy, but the others dropped theirs by varying amounts. Turned out we all got through okay but it prompted me to dig out and post this diagram showing the effect of lowering tyre pressures. This one was stolen from a Mickey Thompson brochure, as if you can't tell from the "Baja Claw" tread pattern. When you look at a part-deflated tyre, it seems from the "bulge" that the extra contact area is lateral, but the diagram shows that the big footprint gain is in added fore and aft contact.

Tyre pressures between 16 and 28psi are often recommended for mud, although some drivers go much lower when using tubes and/or beadlockers. On the other hand, you may not even need to lower pressures. The most suitable pressure depends very much on the type of mud, the steepness of slope and what sort of base lurks under the mud. That's both my experience and what the Mickey Thompson brochure says. If it's thick mud, with a loose, deep base, lower pressures and less wheelspin with some decent momentum may get you through best. If the mud is watery with a solid base, you can maintain higher pressures, maintaining momentum without going fast enough to either lose control, hit something or wreck the track. Higher pressures will be required when carrying heavy loads. 

Politically correct disclaimer: Remember, whenever you reduce pressures, re-inflate as soon as you drive back to the road.

Here's another take on tyre footprints, this one from Andy Cockroft's book, 4WD North Island: 102 Off Road Adventures (Shoal Bay Press, 2002).

Effect of tyre pressure on length of footprint. (Measurements taken from Land Rover Series IIa with Dunlop Mud Rovers fitted.)
1. Over-inflated to 35psi, Reduces the footprint to 7.5 inches (-6.25%)
2. Normal inflation of 25psi. Footprint measures 8x8 inches.
3. Halving pressure to 12psi increases length of footprint to 11.5 inches (+43.75%).
4. Halving again to 6psi increases footprint to 15 inches (+87.5%).

'You got gum …?'

The latest statistics that lurk behind this site note that Americans are the second largest group visiting 4wd Newz. They now comprise 15% of visitors, after the local audience at which it's aimed. I'm guessing most are lured here by Google after searching for "Jeep", but maybe there's just a groundswell of interest in off-roading Down Under! Whatever your reason for visiting, welcome to this USA friendly corner of the 4WD world, guys. Just to save any confusion, your Liberty is our Cherokee (but in Australia the Liberty is our Subaru Legacy!); and a ute is a pickup.

Take a second to say 'Hi' in the comments section, especially if you own a huge 4x4 discount shop and have killer shipping prices to NZ!

4x4 Action site plugs out of the bog

It's been under construction for, well, ages but now the new 4x4 Action magazine website is up and running. It's still a work in progress but shows lots of promise and I like the extensive coming events listings. For anyone looking for some great 4WD trips this summer, it's worth checking often. One of the things I like about the revitalised magazine itself is that it reads like those involved have put some effort and enthusiasm into it.