The Lowdown, Off-road Tire Pressures

Words & photography by Mark Allen

It’s an often-argued point: does your tire’s footprint get wider or longer with lower tire pressures and what advantage does this lower pressure provide while driving off road?

While it’s easy to see that most tire side walls ‘bag or bulge out’ as you lower tire pressures, try viewing these side walls from ground level and you’ll note that the side wall bagging often doesn’t touch the ground, so its effect on traction and floatation is very minor when compared to the increased footprint length.

What generally goes unnoticed is the change in length of the actual footprint of your tires as the tire pressure decreases. It’s this increased footprint length that is the biggie in calculating the overall area of the tire footprint touching the ground. This area provides gains due to the reduction in pounds per inch of force that the vehicle exerts on the ground surface.

There are a few other advantages of lowering your pressures while off road – so read on and be sure to keep your thinking caps on as we dive into a few calculations to prove the point.

Procedure

The ink images you see on the following pages have been made by jacking up one wheel of the Land Cruiser, painting ink over the surface of the tire tread and slowly lowering the tire onto the cardboard. At no time did we roll the vehicle forwards or backwards or drop the tire quickly on the jack.

We then measured the length and width of the ink image to calculate the pounds per square inch the Cruiser is exerting at 5PSI increments to show the differences in tread width and length.

The only ‘guesstimating’ we have done is to quarter the overall weight of the Land Cruiser (4938 lbs) – half for equal front and rear, then half again for left and right to give 1234 lbs. While technically this is not the exact weight distribution, we used the same ‘quartered’ weight in all calculations, so the actual overall weight makes no difference in proving the point elongation of the tread surface in contact with the ground.

If you should try to simulate this at home, you should take into account the added load of camping gear, tools and all luggage you take with you, as the overall weight your 4×4 is carrying will change the weight per tire and hence the pounds per square inch of pressure on the ground.

Warning

It must be noted that we are only talking about and recommending tire pressures being lowered while off road and at lower speeds, NOT driving on paved roads or at higher speeds. Please consult your manufacturer’s handbook to be sure you are inflating your tire pressures to a safe pressure for on road use.

How low can you go?


There is no single answer to cover all 4x4s. The larger and heavier your vehicle and the more load you are carrying, the more care should be taken not to roll a bead off the rim.

As an example, with quality aftermarket tires, I often run 34-38PSI in the Cruiser when empty on road and will initially drop down to about 20 to 22PSI when off road. If the going is still tough, I’d be happy to drop down to 10 to 12PSI to get out of trouble but would be extremely careful not to drive too fast or turn to sharply to prevent tire damage or rolling a tire off the rim.

With competition 4×4′s fitted with bead locks, 5-10 PSI is achievable but don’t try that on your overland 4×4 when driving at higher speeds.

Gauges


Don’t try guessing the pressures in your tires with a quick glance or kicking them with your boot. One brand of tire will look different to the next even though they may be set at the same pressure. Different side wall construction, profile and weight in the vehicle cause different brands of tires to look different in the side walls.

Keep a quality gauge in your 4×4 at all times. The more often you check and adjust tire pressures the better. Don’t rely on the gauges at service stations; they don’t exactly get treated with respect and their readouts can vary greatly.

This is the bit that requires concentration and a huge thinking cap to follow our fast tapping fingers on the calculator.

As can be seen from the table above, the tire length has increased from 19cm (7.5″) at 40PSI to 33cm (13″) at 15PSI – an increase of 14cm (5.5″), which is just shy of double the tread length. Comparatively, the tire tread width that actually touches the ground stays the same at 23.5cm (9.25″) right through the pressure range.

Using the length and width measurements combined with the weight of the 4×4 at one wheel (1234 lbs), we’ve calculated that the pressure the Land Cruiser tire is putting on the ground has increased from 1.254 to just 0.722kg/cm, representing a 42.4% decrease in pressure being applied to the ground. This is what allows us to seemingly ‘float’ over sand, climb slippery surfaces and crawl over rocks with improved floatation, more grip and less tire damage….cool huh?

Fatties vs Skinnies


So how about skinny tires? Do they exhibit similar tread changes when aired down?

You bet they do. We made ink imprints of a 7.50R16 inch tire on a split rim. This time the length increased from 19cm/7.5″ (at 35PSI) to 27cm/10.5″ (at 15PSI), while the width stayed at 15.5cm/6″.

Side Walls

As can be seen from the side wall photo, the width of the tread actually touching the ground remains unchanged, although the  side wall does flex (if it actually touches the ground) it may help a little with reducing the pressure of the whole tire footprint –  but only marginally when compared to the increase in tread length in contact with the ground.

It’s this side wall bulge at low pressures that can allow a tire to be staked or cut from sharp rocks or tree roots, so be careful about wheel placement when picking your way through an obstacle.

On the other hand, this ‘softness’ of the tire can allow rubber to mould or conform to an obstacle’s shape, reducing the chances of damage.

My advice would always be to steer around an object if possible – no point chancing your expensive tires. Remember that the inner side wall also bulges out, so watch out for both sides of your tires.

Cricket Ball Sized Rocks

Here’s another way of showing that the lowering of tire pressures can help allow the tread stay in touch with the ground.

We placed a cricket ball (just larger than an a baseball) on our test board; at 15PSI the tire completely wrapped around the ball, allowing the tread to touch the ground. At 40PSI, the tire hardly deflected and sat on top of the ball.

What About the Trailer?


While you can find guides for your 4×4′s tire pressures for each given road surface, there’s not a lot written about the poor old camper trailer’s pressures.

Here’s a rough guide to get you started:

 

  1. Lower your 4×4′s tire pressure to suit sand, rocks, mud, high or low speed gravel…whatever your terrain.
  2. Measure the length of your 4×4′s tire tread touching the ground at the lowered pressure (see photo below for an easy way with two sticks and a tape).
  3. Duplicate that measurement on your trailer’s tires by lowering its pressures until the same length in Step 2 is achieved.
  4. Measure the pressure of your trailer’s tires that has given the required tire contact length.

 

Now you’ve got a good starting pressure to go off road while towing a trailer, but don’t be afraid to alter the pressures a bit. This trick will help greatly when towing your camper through deep, soft sand.

Remember that this is for off road applications only and you should inflate back up to on road pressures when you hit the paved surfaces.

Downsides

Yep, there are downsides to lowering your tire pressures – you lose ground clearance as your tires are deflated, but the advantages far outweigh the loss of a few centimeters of clearance. Driving too fast on low tire pressures will overheat your tires, so keep the speeds down.

Sand Only?

No way – you can lower your tire pressures on any off road surface to give your 4×4 the best chance at getting through an obstacle. Lower pressures in sand provide superior ‘floatation’ and allow your tires to remain on top of the sand instead of sinking.

Lower pressures also provide increased grip and less risk of punctures in slow rock driving scenarios. Same goes for any slowly driven, uneven surface. Lower tire pressures (but not as low as for slow rock crawling) can also be of benefit when driving on faster gravel or dirt roads by allowing the tread to conform to the individual stones or sharp protrusions to help prevent puncturing. Remember to keep in mind that the lower your pressure are set, the lower your speeds should be.

 

More Grip, Less Punctures

Allowing your tires to flex over the ground underneath, conforming to the shape of the rocks, bumps and hollows of the track will help your tires maintain as much contact with the ground as possible. This provides plenty of grip and forward drive, helping you to keep your 4×4 going where you’ve planned more easily. This is most evident in steep uphill climbs on loose, wet or rocky surfaces. With lower pressures, you’ll also do less trail damage.

Be sure to reinflate your tires to recommended road pressures as soon as you leave the test site – you’ll save on wear and tear on all surfaces.

 

 

This article originally appeared in ARB's 4x4 Action and was provided generously by ARB USA

{fcomment}