Essential guide to 4WD tyre pressures and related 4X4 driving basics
You’ve just bought your first off-road fun-tractor. What are the best tyre pressures and operating guidelines for maximising mobility for people starting out with 4X4 driving? Let me help you…
Starting Out
My attitude to off-road driving is somewhat at odds with that of your dirt, sand and mud-loving off-road fool. Off-roading is, like, a necessary evil. You might need to do it to get somewhere for a reason, or get out of somewhere.
But as a recreational pursuit in its own right - each to their own. It’s an excellent way to visit places where you can interact with this great nation’s incredible diversity of poisonous vermin, hours from the nearest hospital, and damage an otherwise perfectly functional machine while communing with nature in this way. Up to you. Free country. Except Victoria. Obviously.
A couple of caveats before we jump right in: This report is for novices. Someone who just bought a wanking tractor with real off-road ability - a 4WD ute or a Pajero Sport, Prado, LandCruiser, Nissan Patrol - that kind of thing.
Even if you’re misguided and ended up in a Jeep or Land Rover, this report might still help you. In addition to all other advice, just make sure you also carry a G-PIRB and a sat-phone.
But if you have full-blown CMD (compulsive masturbation disorder) and you’ve given your 4X4 a lift and fitted a set of Ron Jeremy 35-inch mud-terrain tyres (on the driest inhabited continent on earth) then this information is probably not that relevant to you. Get psychiatric help, because most people who do all that crap destroy their vehicle’s on-road handling and probably didn’t have the skill to exploit fully the standard vehicle’s off-road capability envelope. That happens a lot.
USEFUL 4x4 & UTE RESOURCES FOR YOU
FIRST GEAR
You don’t need much stuff to start-out off-roading. Everybody has limited resources so don’t buy yourself a whole bunch of crap you don’t need.
Here are the essential basics you need to start your off-road adventuring without blowing the budget. It’s no good needing this stuff and not having it when you’re out there.
Long handled shovel or spade - it doesn’t need to be branded, or collapsible/folding, and it doesn’t need to be fancy. Just get a good quality one from your nearest hardware store; it’ll do just fine.
The long-handled shovel is massively underrated for off-road recovery. You can dig your way out of sand, you can reprofile the exits of ruts, or even remove malignant crap stuck under the vehicle.
A little bit of manual labour will get you out of most situations.
A bottle jack with substantial load rating. You can mount it under a variety of positions on the vehicle like diff housings, suspension mounting points, chassis rails. Pro tip: Don’t forget the handle!
2.5 Get a piece of solid metal checkerplate to evenly displace the bearing pressure (stress) the vehicle applies on the bottle jack, preventing the jack from being pushed down into the dirt, sand or mud.
Air compressor. Have a look at the Drivetech 4x4 Compressor Kit for starters >> But even a decent robust compressor is a good idea. However, a bigger kit with puncture repair is highly recommended, including a deflation tool for dropping your tyre pressures for deteriorating driving conditions.
A DIY repair kit alongside an air compressor is an end-to-end solution if you’re 4 hours from the nearest tyre repair joint. It’s all in the box.
A hammer and/or mallet. For hitting things, obviously. I use the King Dick.
A battery jump-starter is also a good idea. And/or a decent first aid kit.
BE PREPARED
With that in mind - you probably bought your standard - or nearly standard - 4WD with touring and off-roading in mind. Tyre pressures are critical, and they’re not a ‘one size fits all’ proposition. As the terrain varies, so should the operating pressures.
For touring on bitumen - freeways, highways, B-roads - I’d be driving at whatever pressure the placard advises - maybe placard plus 10 per cent - for sealed roads and good dirt.
(For total disambiguation I mean: drive at the maximum loaded placard rating. It’s on that plate inside the driver’s door. If several sizes are mentioned, make sure you reference the size actually fitted to your vehicle.)
So, let’s ballpark that at, say, 36-40psi. I’m talking about typical OE-sized 4x4 tyres here, not those massive 35-to-37-inch tyres favoured by the hardcore Outback dickhead.
Chances are, you’re going to spend most of your time driving like that. Even if you’re crossing the desert or headed to the Gibb River Road - there’s more blacktop than anything else in most of these adventures, in the context of the total driving distance from home to hell and back.
QUICK TYRE PRESSURE GUIDE:
Placard + 10% for highway and good dirt.
Placard -10-15% for poor gravel roads with tread fracture risk.
25psi for general low-range off-road driving
16psi for soft sand and mud
12psi as a last-ditch resort to get out of a really boggy situation
BE CONSERVATIVE
When you do eventually hit poor-quality gravel, maybe come back about 10-15 per cent to (say) 30-32 psi for touring on rougher gravel. The risk there is a fracture from hitting a big, sharp stone square-on. 40psi is a bit hard for that, and you can potentially fracture the tread face with the ‘Goldilocks’ wrong hit. That’s bad.
The reason I say ‘10 to 15 per cent’ is that tyre pressures are notionally ‘cold inflation pressures’. Meaning, before you’ve warmed them up by driving on them. So, your 40psi cold pressure might be 44 (or something) at operating temp.
In that case, come back 5-7psi for that rough gravel. We’re not on a mission to Mars here. 10 per cent of 44 is about 5psi (ballpark) and 15 per cent is about 7psi. This will just make the tyres slightly more compliant.
But the thing which compulsive beard-strokers and aggravated outback dickheads generally forget here is to reduce your speed as conditions deteriorate. Lowering the pressure is not some licence to press on at 100 or something as the road deteriorates. It’s just not. You have to go slower.
ADAPT TO CHANGING CONDITIONS
If you slow to 80 and hit the wrong rock, pothole, washaway, shelf or other shit ‘isolated geometric deficiency’ in the road, you’re only travelling 20 per cent slower, but you’re going to hit that thing 36 per cent softer. If you’re doing 60, it’s more than 60 per cent softer, compared with 100. Speed matters.
Impact energy varies with the square of speed. Small changes in speed make big differences to impact severity. And for all you aggravated 4X4 Les Hiddins stupid hat-wearing wannabes who claim it takes too long to get there at (say) 80, versus 100:
If you have 200km of shit road ahead, it’ll take you two hours at 100, obviously. Or two-and-a-half hours at 80. The extra 30 minutes is worth it, weighed against the risk of spearing off the road after a blowout at 100. So there’s that. Drive conservatively, and get the pressures right.
After hours of 100km/h, slowing to 80km/h can feel like you’re walking.
But the relationship between speed and energy here is crucial. Small differences in speed make big differences to energy.
Energy is proportional to the square of speed. Double your speed and you have four times the energy, half your speed and you have a quarter the energy.
Thinking about those tread blocks, the severity of the tread face hitting the road is 36 per cent softer at 80km/h.
You’ve seen this sign. We’ve all seen it.
Which speed do you choose, 100km/h or 80km/h?
It’s easy to think you’ll never get there at 80 (lower risk) compared with 100 (greater risk). But think about it…
If you’re driving 200km at 100km/h, obviously that’s going to take 2 hours. Simple.
If you’re doing 80 and you drive for two hours, at the 160km mark you still have 40km to go, that’s going to take you an extra 30 minutes. It’s an episode of a show. You’re not in a race. You’re on holiday. You’ve created a greater margin for safety, you’ve lowered the risk, you’ve looked after your tyres and who knows - you might even get there feeling a bit less fatigued.
Pro tip: Scan the road.
There’s all kinds of defects in even good dirt roads - defects you don’t want to hit at speed. Big rocks, pot holes, sharp corrugations etc.
If you see one of these defects, DO NOT LOOK AT IT. Look around it, look at the solution.
Any time there is a ‘target’ don’t get fixated on it, look beyond it and the route around it. If you look at the problem, you’re going to hit it.
I suggest you practice this all the time when it’s not stressful. Practice looking at the course around the problem so you’re ready when it’s a more stressful moment.
More on this here: Safe Driving Tips Part 1 >> and here: Safe Driving Tips Part 2 >>
RISK MANAGEMENT
Note that I did not just say: drop the pressure the instant you see a dirt road. My approach to this stuff - and I’ve done a shitload of off-roading (occupational hazard), just never joined the stupid hat boy band - is to keep the pressures at wherever they are for touring and reduce them only if required, if the conditions deteriorate.
Good dirt road, drive at highway pressures. Rough dirt road, it’s time to lose that 10-15 per cent for greater impact resilience. It’s not like: ‘Bitumen ends here; must deflate’. It’s like, ‘This road’s getting a bit rough - we might stop and take six pounds out all round’. That kinda thing.
Now, when you get into the really rough stuff - low range off-roading - in general, 25 PSI is okay for that on most OE tyres - if you need to. People tend to think of off-roading as severe on tyres - and it’s really not, at least not if you do it properly and conservatively. Not severe on tyres, anyway. Highway and dirt road driving is severe on tyres.
RADIAL TYRES GROW LENGTHWISE:
Dropping from 40 to 25 psi means letting 40 per cent of the air out.
Radial tyre tread conforms to the surface better and offers slightly more area. (Better manages bearing pressure; same reason for the plate under your bottle jack).
Really soft surfaces: beach sand and mud: use 16 PSI, but avoid lateral shunts on the tyre and beware of sidewall vulnerability.
CORNER GENTLY: Otherwise you risk breaking the bead on the tyre because you’ve let 60 per cent of the air out which is holding the tyre to the rim.
MONUMENTALLY STUCK: 12PSI is okay as a last ditch effort - and luckily you’ve brought your trusty My First Air Compressor along for the ride. Maximum area to reduce bearing pressure to critical level.
Point of order: Low profile tyres on big rims are useless for off-roading precisely because of this reason: you don’t have the same ability to reduce pressures and improve the tyre area’s bearing pressure.
Always - keyword ‘always’ - re-inflate if you’re going back to higher-speed modes of operation. Even just for a few minutes. Take the time - break out your compressor. Add the requisite air.
Like, if you deflated to 12 to get through some epic mud - because (I dunno) your stupid hat-wearing, beard-stroking mates’ beer guts told you this would be fun. But when you get through it and the sign ahead says ‘cock of a road next 200 kays’ inflate back to 32-35. Or go back to full pressures if you’re back on the highway.
Or if you’re doing some more low-range F-U-N because you want to break something expensive and help put the dealer principal’s kids through private school so they can become stoners with impeccable manners, get back up to 25. Whatever.
Never drive out of the forest at 25 PSI after some off-road fun and drive down the highway at 100 for half an hour because you are urgently in need of three burgers and two litres of Coke. (Of course Jeeps and Land Rovers protect you from this by breaking down out of mobile range. Think of it as a safety feature.)
SLOW GOING
One of the biggest risks of operating tyres at low pressures relates to rolling the tyre off the rim of the wheel.
It has massive implications if you get it wrong and can be very costly - many times more costly if you get it wrong at speed with people on board.
French physicist Blaise Pascal tells us that putting air in a tyre means equal pressure in all directions pushing against the tyre, forcing it outwards. That loud popping sound a tyre makes when it’s inflated at the tyre shop is testament to this, and tells you the bead of the tyre is seated on the rim.
Letting half the air out means you get about half the restraint force holding that tyre onto the rim.
Lurching sideways or enthusiastically cornering under low pressures means you’re particularly likely to dump all the air out by momentarily pulling the tyre away from the wheel rim.
As you corner, reactive forces opposing the wheel pull the tyre away from the edge of the rim and open the door, literally, to that air forcing its way out because Pascal told us it pushes against the tyre.
You must drive extremely conservatively at 16 or 12 PSI. Not only do you risk deflation, you also flex the tyre sidewall outwards, further exposing it to damage if it hits a sharp rock, stick or even the rim itself squeezing down on it hitting a rut at the Goldilock angle.
If you hit the freeway and increase speed at low pressures, you also increase the risk of operating that tyre sidewall beyond its optimum temperature range and there’s a strong likelihood of splitting the tyre or wearing a hole in it.
DON’T EVER RATIONALISE DRIVING ON LOW-PRESSRUE TYRES. No matter how short the distance is, re-inflate your tyres to their safe operating pressures. No matter what your circumstances, kids, time (being late), sun setting, whatever - get those pressures back up.
The risk here is: 40 per cent of the required air is missing. The sidewalls are forced to flex further than intended, 10 times a second. It generates immense heat. This is the classic recipe for a high-speed blowout. Never do this.
If you come off the beach at 16 and there’s a servo three kays up the road, okay - drive there at, like, 50km/h. Very gentle on the steering. And at the very least buy a burger. Otherwise you’re just lunching off a battling small business in a pandemic. They’re laying on the air for free. Spend some cash. It’s not like you’ll be taking it with you when you croak.
One last thing
As you go to re-inflate your tyres with your big beautiful air compressor, for fark’s sake, make sure you:
A. Connect the terminals correctly: Red to Red (positive to positive) and Negative to Negative (black to black); and
B. Keep your engine running: Keep it running the whole time you connect to the battery and during the entire time inflating each tyre. Otherwise you’ll risk exchanging energy in the battery for inflation of your tyres.
If you flatten the battery inflating four big off-road tyres, you might just find yourself a long way from help with dingos circling your stranded family, and the only help being a set of alien headlights on the hill.
And you really don’t want to join that guy at the dinner table.
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