Now you have the tyres and wheels you want, what pressures should be in them ?

I will try to explain how to determine your  inflation pressure for on road use and off road use. Be aware that the difference between  on road  and off road pressures will be quite different and as such offer completely different driving characteristics in the opposite environment. A note on caution later. My thanks to Harry Lewellyn for the original article.

On Road Tyre Pressure.

First thing I want you to do is check your tyre pressures, and while your there have a look at the wear rate of your tyres and make a note of the max load weight on the tyre wall and at what pressure. Go on, humour me. I’m confident that most of you will have pressures in excess of 30 psi and notice that the centre of the tread is wearing faster than the outside edge.

When you work out the pressures you should have, the First thing you are going to do  is to say ‘nah, that can’t be right!’ You will be amazed at the answer you get.

For the purpose of this article, I will use my jeeps weight (CJ340-Brave), which is 1285kgs. Next thing to find out is what it says on your tyre walls. Now, my All Terrains state 1060kgs @ 50psi.

The maths to use is quite simple and goes like this. Add up the load rating (4 tyres) and calculate what percentage the vehicle weight is against the total load weight. When you have this percentage, use that to determine what pressure is optimum for you, based on the max psi rating on the tyre. If that didn’t make sense, follow this: 

Take the total weight of your jeep, write it down and put an "A" next to it (get your vehicle weighed).

Then multiply the maximum weight from the tyre sidewall by four {four tyres under the jeep}, write down the answer and but a "B" next to it.

Divide "A" into "B" and write the number down with a "C" next to it.

Next divide "C" into the maximum sidewall pressure "D", and you have the correct pressure for your tyres on your jeep.

The formula is shown again below to help.

Jeep weight =A,

Sidewall max weight x 4 =B,

B into A = C

Side wall max pressure =D,

C into D = ideal psi.

As a point of reference, I have it on good authority that all tyres (of our type) max loads are rated at 50psi. So ignore what pressure it says on your general grabbers.

4 Tyres (1060 x 4)                        4240kgs     (42.4kgs = 1%)
Max load pressure                        50psi           (0.5 = 1%)
Percentage of vehicle weight          31%           (1285 / 42.4)
My Ideal on road tyre pressure     15.5psi        (31 * 0.5)

Now that’s the ideal on road pressure if my jeep is empty i.e. with no one sitting in it and no load in it. Obviously the pressure must be increased to take this in to consideration. What you need to do is assume you are usually going to carry two people and a small load. Then do the calculation again. My tyres are actually inflated to 18 psi to account for my beer belly, the wife, kids, dog and any other heavy things I’m carrying (an all up weight of about 1550kgs). This provides a good ride and good grip and perhaps more importantly good tyre wear.

Off Road tyre Pressure.

As I don’t use my MT tyres on the road, these are set to a pressure suitable for off-roading. When deciding what pressure to run your tyres at when going offroad you need to consider what you are trying to achieve.

You are looking for grip, pure and simple.

You also have to accept that when you deflate your tyres for offroad use you will lose some ground clearance.

It’s a simple fact, the bigger the footprint,  the softer the ground you can drive over. Mother Nature knows it. Take a look at the feet of camels, polar bears and marsh birds. They are big and spread out to distribute weight over a larger surface area.

Lets take a look at tyre pressure and footprints and why it makes sense to do it to your tyres.

To determine your lowest offroad pressure, (for use on sand or mud) perform the following test on a flat, level and smooth surface.

Fully load your jeep as it would be for a day out in the dirt (full fuel tank, ropes etc and passengers included). Measure the distance between the floor and the bottom of the wheel (rim). This is your 100% tyre height (at the pressure we worked out for on road).

Now reduce this height (not pressure) by 25%. In other words, let out air until your tyre is 75% of the original height. Mine was about 6 inches at full road pressure so I let them down till the measurement was just over 4.5 inches. My tyre pressure was measured at 7psi. This is the lowest pressure I can take mine down to before the danger of unseating the bead becomes a major factor. I typically run mine at 10psi for offroading. This offers a good degree of grip against good protection from wheel damage and unseating the bead. More on that later.

As the illustrations I'm using are from the US of A, you need to change ‘street’ to ‘road’ and ‘sand’ to ‘mud’
 

footprint

Lowering tyre pressure changes four things:

  1. Footprint, As the TREAD FOOTPRINT figure above shows, this typically results in better than a 250% increase in the tyre contact patch with the ground. That is like having ten tyres at normal road pressure where before you only had four. This gives you more grip, it’s as simple as that.

  2. Ground clearance, Common sense and the TREAD FOOTPRINT figure shows the centre of the axle is lowered by the decrease in wheel height. This results in lower ground clearance and the softer tyres also flex and give more resulting in compression loss of ground clearance too.

  3. Rolling radius of the wheel, Rolling radius is part of the equation which contributes to your net moving force, your overall gear ratio, and your "stump pulling power" or torque. Think of it this way, you know how bigger tyres change gearing and slow you down up hills because the engine won’t ‘pull’ them, so conversely, smaller tyres mean you accelerate well but can’t get past 50 because the engine will blow up !! Flatter tyres act like smaller tyres and increase your pulling (moving) power.

  1. Obstacle Rolling Resistance.

rollover

Try this experiment. Face-off two jeeps (one needs a winch) a short distance apart, on a flat, smooth surface, take the winch and connect it to the other, put a piece of 2X4 wood in front of both vehicles front tyres and with all the brakes off and both out of gear, start the winch…… the jeep with the low pressure tyres will move forward over the wood while the jeep with the pumped up tyres will stay right where it is. A simple, lowly piece of 2X4 wood offers more of a hill than you might initially think. It is roughly 25° to 30°. Knowing this, it’s now easy to see why not so big rocks require the thrash and bash technique to climb with hard tyres. It takes approximately 40% more power to get over the same thing if your tyres are inflated hard. Throw this figure in to the mix with the gains in grip, and you can easily see that you are now getting the effectiveness of 15 tyres !!!! Sorry that’s a wild inaccurate guess but I’m sure you can see what I'm trying to say.

A note on caution

Use Caution! with low tyre pressure. The Obstacle Rolling Resistance factor works against you when speed goes up on hard uneven ground! Soft tyres will allow things to reach your wheels (because the tyres are soft) and can easily pinch the tyre causing a puncture or even bend and break the rim. If you crawl slowly over a house brick, you won’t even notice it and there would be no problem, but if you went over the same thing at 15 mph the brick will most likely pinch the tyre right back to the wheel rim and that’s where the damage and danger starts.

And last but not least, you obviously need some way to re-inflate the tyres back to normal on road pressure when you go back to the black stuff. DO NOT USE your vehicle on the road with deflated tyres. Catastrophic tyre failure will occur with the resulting horrendous road accident.

 

Tread Lightly, a note on ecology.

In our ‘green’ conscious world, It is my belief that lower tyre pressure is more ecologically compatible. With better traction you won’t spin your tyres as often and that means you leave Mother Nature unscathed. So you’ll be doing yourself and the environment a favour when you lower your tyre pressures next time you venture offroad.