Thread: Tyre pressures

Opinion varies a lot. many people drop tyres to around 20 - 25 and lower on sand, nut dirt and rocks are more contencious.

48 on anything but bitumen is going to be a bit high, and is possibly a but high even for tarmac.

I run about 34psi in my landy most of the time, admittedly though I don't deflate for regular off road conditions because i'm worried about the strength of my retreads.

i run 35psi on the front and 40psi rear
on tarmac and 4wd
with my standard tyres

mid to high forties isn't such a bad idea. drop them for sand and low speed rock crawling but for any hard surface that you are going to be travelling fast on (road speeds) keep them at highway pressures or you will overheat them and in the worst case you could have a catastrophic blowout, more likely you will suffer endless punctures along your journey.

this happened to us a few years ago on the oodnadatta track, mereenie loop rd, diamantina developmental rd etc, out of six vehicles, five wore bfg muddies running within their load range at 45psi and not one vehicle got one puncture.

the sixth vehicle was carrying the same load with bfg muddies on, but they were the big 35 by 12 on 15' rims that had a lower load rating and he could only run them at 36psi, the weight in his vehicle caused the tyres to bag like they were running at sand pressures. over the trip, the heat generated by his tyres made the rubber soft and susceptible to the little sharp stones that are strewn over theses roads.
this vehicle got 10 punctures. it was obviously a case of overloading but the result was the same as running at too low a pressure.
i used to run my hilux (20 yrs ago) at 35psi and never had a problem. less weight.
i now run my landy at 45psi, the ride is a bit firm but i have no power steering and if the tyres are lower it is impossible to steer on paved roads

Depending on the load I usually run 40-45 all-round in the 130 and 35-40 all round on the disco. (have run up-to 65 on the rear of the 130 with a full load).

drop down to approx 20 on sand if needed.

Cheers

...

750R16 Wranglers fully loaded I run at around 40 rear 36 front and using the 4psi rule after 150km they were at 45 and 40 so pretty much spot on.

750R16 Michelin XZLs seemed happy at around the same pressures but let them down to around 35 on hard dirt.

Lowest I've run any tyres on the defender is 18 over big red a few years ago.




Bushie

Originally posted by crump
Are the standard tyre pressures suitable for everyday use, 48psi in rears seems high, and any advice on what i should drop them to on the dirt?03 Extreme.

OK, here goes. It's important to understand the logic behind tyre pressures.

The correct tyre pressure is a function of your intended use, the weight of the vehicle and the volume of air the tyre can hold.

The greater the weight of the vehicle, the higher the pressure required. The larger the tyre, the lower the pressure, as it's the volume of air in the tyre that does the work, not the pressure per se. The faster you drive, the greater the pressure required.

Intended use; the more slippery or loose the surface, the lower the pressure.

Manufacturer recommended pressures (MRPs) cannot be all things to all people. Use them as a guide. Pressures are best expressed as a percentage of MRP. Never exceed the maximum pressure for a tyre which will be stated on its sidewall. My Kellys are 80 psi, for example.

Increase MRP if you are heavily loaded and have some freeway cruising to do. The weight of the vehicle will flatten the tyre, and the greater the deformity, the greater the heat buildup, and the greater the risk of damage. And the faster you go the more heat builds up as the tyre flexes more quickly. So increase, all the way to the maximum if necessary.

For dirt roads decrease. Here an increase in contact patch is important, as gravel is a slippery surface. I run mine at 24/28 on outback roads. This is quite low. That means the tyres will easily heat up at speed. So I don't drive quickly, eg above about 80. I do get the benefit of vastly improved traction and a smoother ride. If I had to drive quicker, for example if I was in a competition, I'd raise the pressures and just go for it. But I'm touring.

In low-range territory with a medium load go lower still. I generally run 18psi. On-road handling is noticeably affected, but the traction gain is huge. Remember, the tyre's contact patch is not directly propotional to the pressure reduction; the difference between 45 and 40psi is small, the difference between 25 and 20psi is much larger.

Below 20psi you must be careful not to roll the tyre off the rim. Beadlocks prevent this. However, they do not prevent tyre or rim damage caused by very low pressures. In extremis, I've gone down to 9psi.

Note that a larger tyre does not give you a greater contact patch. The contact patch is just a function of weight and pressure. The main advantages of a larger tyre are an increased ability to walk over obstacles and extra under-diff clearance.

Also note that contact patches only lengthen. Sidewalls may bulge, but that does not increase the tread width.

The difference between cold and hot pressures should be about 4psi.

Everyone has to figure out their own tyre pressures, but to do so some of the principles have to be understood.

hth

[quote="rmp"]

Originally Posted by crump

Note that a larger tyre does not give you a greater contact patch.


Also note that contact patches only lengthen. Sidewalls may bulge, but that does not increase the tread width.


hth

to clarify;
a larger tyre will give you a greater contact patch. *a larger tyre in diameter will give you a longer contact patch, e.g. say a 35' tyre over a 30' tyre, bigger circumference = longer footprint. * a tyre that is say 33' inch diameter and 7" wide, when presure is reduced will give you a footprint of a set width and length, if a similar tyre of the same diameter but maybe 10" wide, being at the same pressure, will give you a footprint of the same length but it will be wider as the tyre starts off wider and thus more surface area.
remember the question of what penetrates the ground more, an elephants foot or a womans high heel shoe? it's all to do with contact area versus weight. have you seen those photos of the landys in iceland? i'm pretty sure those big mother tyres are not for show.

i do agree with what you say about the sidewalls, they only indicate to you how flat your tyre is. i didn't actually mean to highlight this bit.