lowering <span class="highlight">tyre</span> <span class="highlight">pressures</span>......

Hi Rob

If your doing slow track work then your tyres will tend to cool down so your adjust closer to your target pressure.

i.e. if initially 32 and you wish 20 then drop current reading by 12 (32-20)

if your not sure of initial pressure then drop to 4 psi above target i.e. 24 on basis that they will cool down a bit.

Robin Miller

Generally all measurements that are referred to are 'hot' measurements. So 24psi hot for dirt as a rough guide or maybe drop to 20psi if it's starting to get really muddy. There's a bit of trial and error in it as some tyres bag more than others when you air down.

Leroy

Re Tyre pressure, I have never have the pressure below 30psi on the Bridgestone 693's on my Cruiser, but after talking to friend who did Cape York in his Prado was advised to lower the pressure to 16psi and had no problems, this is what I will seriously consider next time I am in the bush on rough tracks.

I also though that using the higher pressure the wide tyres would be less prone to staking on the walls and rock fractures on the tread. My Prado friends advisor a bush mechanic rejects this view.

Towing a large Van on the black top I have reduced tyre pressure from 50 to 40 on the back and the front remains at 30, Ditto on the tandem van from 50 to 40. I suppose I have been sucked in reading on the tyre the maximum load is at the higher pressure and looked at this as a safety issue.

Of course the higher pressure provide less rolling resistance , less load on the engine, but especially in the van more vibration on rough roads. How low do you go with Van tyres on rough tracks 20psi?

Tyres warm up when 4wding too, depends on what you're up to.

If you lower your pressures to say 20psi on a freezing cold Simpson desert morning, you'll find they'll be up to 25-26 by lunchtime, and you'll need to let more air out. Then the next cold morning they'll look flat.

The pressure I use is whatever the reading is when the tyres are warm and working.

Rob,

Assuming the tyres are hot when you wish to adjust them, use hot pressure readings. I agree with the comments above that a few psi difference won't matter a great deal, unless you are driving on really soft sand, then it may.

What your "target" pressure will be will depend on what sort of surface you are driving on.

As a ballpark, try 28psi on firm, or gravel based tracks.
18psi on sand. If the going gets really tough, you can deflate them further, but I have never been below 12psi on soft sand and then I am driving fairly slow.

Now removing all technical issues, consider this.
On a trip along the Oodnadatta Track and down the Borefield road, I was towing my camper and had the tyre pressures at 28psi. Our friends in a same vintage Jack, but not towing anything, left their tyres at normal highway pressures.

Guess who got the puncture and totally destroyed a tyre?
Yep....It was ME.
Just goes to show, bleep happens:-))

Hi Rob, I don't worry too much about whether the tyres are hot or cold because I reduce the pressure "by" rather than "to". eg for shingle/rough roads -8psi, for sand -18psi.
Airing back up is the same and the pressures tend to be within a psi or two.
Keep it simple.
cheers
T.R.