Reducing Tyre Pressures in 18 in + A/T Tyres

[DiscoWeb]

I was reading the thread on the new General Grabber A/T's and other tyre choices and thought I would ask a question of those on the larger rimmed A/T tyres.

"Other than sand driving do you generally drop tyres pressures when off road", I would exclude sand driving as I think it is widely agreed that reduced pressures aid in flotation and the risk of sidewall damage is lower in sand.

However if you are just out driving "normal" tracks do you drop your pressures or not, not so interested in if you go down to 24 psi Vs 18 psi say but if you bother at all.

I ask this as I have had conflicting advise for 18 in tyres and above, some say absolutely other do not bother ?

By way of reference I did 2 day in the VHC over Christmas on some pretty rough tracks on my Continental Cross Contact A/T (18 in) without dropping from road pressures with no problems at all.

I am sure there has been plenty of discussion on tyre pressures before but not sure if it has been specifically for us who run the larger rims !

George

[rocmic]

I do drop mine, but then I worry about how much. Any suggestions on this would be welcome from me as well.
Cheers
Mike

[Ozzy119]

Hi,

For a while I have heard opinions do/don't and why you should / shouldn't and the rolling balloon story etc.

For me my mind was settled on last mini break. A simple enough station gravel track, some ruts etc. I dropped from 37 to 30 on factory tyres. My companion didn't, again factory tyres. He was strongly of the opinion that you shouldn't drop down at all (except sand). He got a flat. We fixed it with repair kit at camp.

It could just be coincidence but I was pleased with the outcome, especially as we are still on factory tryres.

[Redback]

OK, for me I do, BUT, you have to be wary of the weight of the D3/D4, it's fine going down to 26psi in a D1/D2/Defender, they are only 2.4t, maybe 2.7t loaded, but doing those pressure in the D3/D4 is a risk due to the greater weight, so I drop mine to 30s from the road pressures of 44psi rear and 36psi front, to 30psi front and 34psi rear, this is only when exploring not when loaded getting to camp, if on gravel for long distances, I will take a bit out, maybe down to 40psi rear and leave the front.

Although a lot has been said about LT tyres v PT tyres, I think the LT is stronger on the shoulder than PT tyres(and I have a tyre tech who reckons they are too) you risk a flat more with letting PT tyres down, verses LT tyres, the bagging makes the shoulder more susceptable to staking or rocks.

I don't do a lot of sand driving, but going down to 18 to 20 odd would be OK I think.

Baz.

[boofdtl]

Bush i would go to 30to 25psi
Sand 22 to 18 psi is safe well thats what i run..

[DiscoWeb]

Although a lot has been said about LT tyres v PT tyres, I think the LT is stronger on the shoulder than PT tyres(and I have a tyre tech who reckons they are too) you risk a flat more with letting PT tyres down, verses LT tyres, the bagging makes the shoulder more susceptable to staking or rocks.

Baz.

Baz,

Potential sidewall damage has been the main reason I have started to move away from lowering the pressures for more general off road use.

My A/T's have been great grip wise but I think are pretty soft in the sidewall so bagging them out by lowering the pressure somehow does not make sense when travelling on rocky tracks.

George

[Redback]

Baz,

Potential sidewall damage has been the main reason I have started to move away from lowering the pressures for more general off road use.

My A/T's have been great grip wise but I think are pretty soft in the sidewall so bagging them out by lowering the pressure somehow does not make sense when travelling on rocky tracks.

George

This is my thoughts as well, these new ATs I have on the car will be the next test, as the last set of ATs (PT construction) were a dismal failure, these new ATs are LT contruction and have been a good tyre on our daughters Amarok and the reason we got them, oh and they are a 70 profile as apposed to the 60 profile of the old tyres.

Baz.

[CaverD3]

I found I got more punctures in dirt roads with high pressure. Also the ride was very harsh.
Lowering the pressures on dirt I think is a good idea but how much will depend on your load, speed and type of surface. You can go too low on rock tracks and do side damage.

[NavyDiver]

A 4wd mag recently discussed the brakedown rate caused directly by failure to reduce presures (& speed:D) on corrigated dirt roads for people travelling up the cape.

A sharp rock on a rock hard tire must make it more likely to fail or puncture in my mind at least.
How low to go is my main question. I run 20 to 25 on dirt in my D3 which is a little heavier than stock and have taken in down to 12-15 in really soft stuff. Had some slight sidewall damage on one tire (GG AT) but everyone who drove that goat track with me did as well! I do pump up after getting out of the mud when down very low and always before getting back on the black stuff.

Oonandatta track often at 110kph with 25psi presure and a trailer did not damage anything for me. A fellow Disco 3 driver I met in Coober Peddy left the day before us and had a puncture and felt the track was a bit rough. All in my car with lower presure thought it was very smooth. Perhaps it was just my driving.
The tire manufactures such as Micky Thompson have a video on the topic I think.
links to info One (http://www.4x4tirereview.com/airdown.html) Two (http://www.4wdworld.com.au/Driving-Techniques.html) three (good one) Simpson Desert suggesting at least 20% reduction (http://www.mtdare.com.au/index.php?option=com_content&task=view&id=3&Itemid=23), Cape york with several presures listed (http://www.guidebooks.com.au/FAQs.htm#TyrePressures)

The best way to look at your car and the tire spread foot print may be with a high lift jack, some white paper or sheets of newspaper and some old kids paint. Lift the tire and paint the bottom, Lower it down on a sheet of white paper. Lift again, lower presure 5psi, paint again and low on a new sheet, repeat it from your norm running pressure right down to 10psi. The footprint shows you how much the weight distrobution alters. A spray can of paint might be easier. A 4wd show I saw years ago did this I think. Fun way to view this on your car at the very least. the first is like a thin line accross your tire. The last footprint down at 10psi is clearly a huge improvement just go that low only when needed of course and pump it up as soon as it isn't. Side wall damage for me has only occured from rock crawling.

[NavyDiver]

Micky Thompson Guide to presure (http://www.4wdtv.com.au/index.html#news004)