Tyre 'air-down' conundrum??

Submitted: Tuesday, Jan 30, 2007 at 13:56
ThreadID: 41784 Views:2932 Replies:11 FollowUps:26
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Subtitle:'Things you think about in a Dentists Waiting Room'!!!

OK- you're getting away for the weekend, Friday arvo vehicle all packed and ready. Check the tyres at the local servo. (or with that u-beaut 12v compressor) with an extra pound or two for the load. So we've got (say) 38psi cool.
Get away Sat. morning, drive for 3 or 4 hours on the blacktop then turn off onto the gravel track:
Option 1. Get out-lock the hubs in- screw on the Stauns and let 'em down to 28psi.
Now with the tyres hot, we've really aired down to about 24psi??relative.
Option 2: Onto the dirt, find a shady tree- have a cuppa and wait til the rest of the convoy turns up. Ready to go, lock the hubs in- screw on the Stauns and let 'em down to 28psi with cool tyres. But, we know that lower pressure tyres build up more heat, so when they warm up, might have 32psi???

What do ya reckon- or should I keep away from the Dentist!!!!
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Reply By: Redback - Tuesday, Jan 30, 2007 at 14:10

Tuesday, Jan 30, 2007 at 14:10
In reality if you let your pressures down to whatever, as long as it's not to low, and continue they should remain at the temp that they were on the black top, given the lower speed on the dirt.

If they do increase heat wise, then i would think you let them down too much, but you should be able to tell this by the way the car handles, if your a fairly compotent driver that is.

AnswerID: 218594

Reply By: Im.away - Tuesday, Jan 30, 2007 at 14:17

Tuesday, Jan 30, 2007 at 14:17
If you're only on gravel, not sand or rocks, don't let them down at all. 38 PSI sounds a bit light if you're loaded. More tyres are lost to under-inflation that over-inflation so bump 'em up to 45 psi when traction is not required. It's a little hard on the bum but you'll get better tyre wear and fewer punctures.

I have BFG A/T tyres on my Pajero. I've done over 105,000 km in 16 months, 85% of which has been offroad, and I don't even know where the jack is in my car. I run 50+ psi when off-road, 18 psi in sand and river crossings and unlike many, I don't reduce pressures to climb over moderate rocky country....I get less slices in the sidewalls with more pressure.

just my 2cents worth.

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Follow Up By: Member - JohnR (Vic)&Moses - Tuesday, Jan 30, 2007 at 20:50

Tuesday, Jan 30, 2007 at 20:50
Thats contrary to a lot of advice here. Bitumen with mine 36 - 38 rears 34 front. For dirt roads 28 psi is pretty good. If it gets stonier down to perhaps 25-26. Directional stability will be improved too with a longer patch on the road.

High Contry tracks even down to 20 or 22 or even 16 for more grip in tougher (and steeper) conditions. Have seen so many spinning wheels over rocks with higher road pressures. Let the grip get to the rock and climb over it rather than bounce.

I agree with your post with sand but in softer conditions people should drop to what helps them through. The danger at lower pressures may be rolling off rims but don't turn too sharply or drive too quickly and that should be ok. I know some that have driven below 10 psi. I haven't got that low but nearly there on a Robe beach. 14-16 psi gives better grip in the soft stuff.

I really don't know where you get the idea about fewer punctures quite. I think you will find that balloons are a lot easier to pop at higher pressures than lower ones. Same principals operate.

Corrugated roads are another place where you can make travelling more comfortable and reduce the potential damage to your suspension. Why would you want to have a steel hammer belting your suspension into the car or trailer? That is what it is like at higher tyre pressures. Some like to travel at higher speed and go from top to top with the bumps. The noise and vibration is much lessened with softer pressures. I was talking to one fellow the other day whose brother had adjusting his pressures several times a day to suit the conditions. Long trip too. He would have nothing like your 50 psi anywhere. Aware of what the conditions were and reacted to them

I saw quite a few people crossing the last of the Simpson when it was damp two years ago getting bogged at many dunes because of high pressures. Really damages tracks for the people to follow. They were using dealer road reccomendations. I would just be a bit cautious of your higher pressures mate.
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Follow Up By: Off-track - Tuesday, Jan 30, 2007 at 23:18

Tuesday, Jan 30, 2007 at 23:18
Yup - 42-44psi on ours and got 105K from the last set of AT's. Actually rides better on the road and wear more evenly.
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Follow Up By: Im.away - Wednesday, Jan 31, 2007 at 01:17

Wednesday, Jan 31, 2007 at 01:17
Hi John,

My experience is from the "school of hard knocks". I find those pressures work best for me. Your point about the balloon is moot except.... my BFG's have nice straight up and down sidewalls at the higher pressure and as such, don't roll in a slicing motion over sharp rocks....or pick up sticks for that matter.

I've done more than a million kilometres in the Pilbara region of WA over the past 20 years and I go through fewer tyres all the time as I try different things. These are the pressures that work for me.

Your point about directional stability is not valid in my opinion. Look at the skinny, high pressure tyres used on elite cycles that the Olympians ride, and at the speeds we are talking...no more than 110 KPH...the directional stability is not a big factor. Additionally, any wheel spinning in a vertical manner such as on a vehicle, will try to stay vertical and drive in a straight line through that axis. That's why you can let go of the steering wheel of your car and it generally stays in a straight line. It's simple physics.

I once did an advanced driving course, run by a former racing driver, and his philosophy was to run the tyres as hard as the manufacturer's upper limit...on bitumen and on gravel.

As for tearing up the tracks...here in WA the Shires close the roads as soon as it rains any damage is likely, and we respect those closures. If the road isn't closed then we adopt a lower tyre pressure for mud terrain, but to be honest, it doesn't rain much here so I guess I haven't done it more than a couple of times.

Can't comment on high country tracks because we just don't have them but I agree that more tyre surface on the ground under those conditions would enhnace traction.

Like I said, these pressures work for me, and I need to avoid blowing/staking/slashing tyres because: I'm paying for them; I don't like being stuck in the places I work at; I HATE changing tyres; I only carry one spare. 16 months and 105,000 clicks without breaking out the jack tells me I'm doing something right.
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Follow Up By: Member - JohnR (Vic)&Moses - Wednesday, Jan 31, 2007 at 08:08

Wednesday, Jan 31, 2007 at 08:08
I guess as the original question posed is the conundrum. You have had your successes but I think there are a number of factors. BFGs are pretty good at defraying punctures I believe. The Coopers I have on at the moment seem to attract them, well one tyre anyway. Have had punctures at 40 psi just as much as lower ones. The tread depth of a tyre contributes too I have found. Both with higher pressures on SA roads - sealed sections of the Prince's Highway, straight through the tread.

I am talking about directional stability for the tread length mate. There are points at which the stability is lost at speed where the tyre distorts too much. In general running on dirt it can help quite a bit.

Your point on bicycles is hardly valid as they want the least rolling resistence and the directional stability is maintained by the angle of the bike to the surface too, leaning the bike over gives the length of the contact patch. Yes, I do understand the effect of gravity that needs to be overcome.

I am just like you in regard to paying for them and hating to damage them. Interesting that Willem is in the same position and going for splits and skinny tyres to overcome damage to tyres. No perfect position you could say.
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Follow Up By: Bilbo - Wednesday, Jan 31, 2007 at 08:21

Wednesday, Jan 31, 2007 at 08:21
I agree with "ImAway". I do a lot of bush work in the WA Goldfields and I run 'em hard.

I don't seem to have half as much tyre trouble as most folks on this forum. Perhaps the ground in WA is fair bit rockier than Eastern States. I dunno.

"Hard" works for me,,,,,,,

BTW - "ImAway" - have you ever been up around Bamboo Creek on Yarrie Station? I'm looking at going up around there this year. Is the old Shay Gap Boreline track still any good?

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Follow Up By: Member -Signman - Wednesday, Jan 31, 2007 at 11:15

Wednesday, Jan 31, 2007 at 11:15
G'day Im.away..
Thanks for your comments to my post- and note you've done a few clicks around the Pilbara area..
On the weekend I was looking thru some pics. of 'places I've been' and came across what I reckon would have to be the roughest track I've been on- and wonder if you've been there, and if it's still in the same condition. It was early '80s I travelled thru there....

It was from Wittenoon Rd- near Mt. Florance travelling NNE to Mallina on the West Cost Hwy. I think it was called the Nunyerry Mine Track, and crossing the Sherlock River (dry) the rocks were bigger than Besser Blocks for about 500m. The rest of the 200k track wasn't much better.

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Follow Up By: Im.away - Wednesday, Jan 31, 2007 at 12:05

Wednesday, Jan 31, 2007 at 12:05
Hi Bilbo,

The Boreline Rd was in really good nick when I went thru Dec '05 but they had a huge cyclone season following that so I'd ring the Shire and maybe even Pilbara Iron (as part of it is on their lease). Same applies to Bamboo Ck, Yarree and other roads between Shay Gap and Marble Bar.

It's a beautiful part of the world for sure, so I'd give it a go if I were you. If you have time, pop out to Carrawine Gorge (via Rippon Hills Rd) and spend a couple of days there. It's excellent.

Signman, we don't talk about the Sherlock River around here. She was flowing when I crossed her last year, well er...tried to cross her. Fantastic (if not slow) devices them winches.

I do know the track you're talking about and yep, it's rough, but I found a rougher one. I was trying to find a shortcut from the Dampier to Bunbury Gas Pipeline to the Northwest Coastal Highway near the Cane River. The track was shown on my Natmap Raster Map so I started along it. Well to cut a long story short, if I had taken the long way back it would have taken maybe 1.5 hours. I persevered and 5 hours later I finally finished the 15Km shortcut. Nice scenery, long day....
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Reply By: Ozrover - Tuesday, Jan 30, 2007 at 14:19

Tuesday, Jan 30, 2007 at 14:19
Gerday Signman.

If I find myself hitting the dirt mid-trip, after the tyres get hot & need to air down, then I usually call a smoko break.

After having a cold drink of something (not booze!) & relieve any internal pressure, I do a walk around of the car/trailer checking for anything amiss, by this time the tyres have cooled down a bit, I check all tyre pressures & lower them to 30 psi using a dial gauge. (I keep the Stauns for 17 psi for the rough stuff).

At every stop I have a feel of the tyres & if their getting a bit hot, then I slow down a bit or add a couple of psi of air. (I only travel at 80 kph max aired down).

Seems to work for me, no punctures or blow-outs since I started doing it this way.

AnswerID: 218596

Reply By: Rod W - Tuesday, Jan 30, 2007 at 14:21

Tuesday, Jan 30, 2007 at 14:21
Yep keep away from the dentist what way ya won't dribble.
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Follow Up By: Member -Signman - Tuesday, Jan 30, 2007 at 14:27

Tuesday, Jan 30, 2007 at 14:27
Sorry you couldn't understand what I was trying to say Rod...but you know what it's like with a gum full of Novacaine!!! I'll use single syllables next time.
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Follow Up By: Redback - Tuesday, Jan 30, 2007 at 14:36

Tuesday, Jan 30, 2007 at 14:36
Don't worry Rod i thought it was funny ;-))

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Follow Up By: Rod W - Tuesday, Jan 30, 2007 at 14:42

Tuesday, Jan 30, 2007 at 14:42
Me too
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Follow Up By: Gob & Denny - Tuesday, Jan 30, 2007 at 14:44

Tuesday, Jan 30, 2007 at 14:44
i suppose it beats thinking about what the dentist is going to do ??????

how ya going baz??
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Follow Up By: Redback - Tuesday, Jan 30, 2007 at 15:10

Tuesday, Jan 30, 2007 at 15:10
Good mate, still kickin, and you?

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Follow Up By: Mr Fawlty - Tuesday, Jan 30, 2007 at 16:30

Tuesday, Jan 30, 2007 at 16:30
I spose you don't want my response but you're gonna get it... A wise man once said that if we all drove as if we were 2 hours early for a Dentists appointment there would be very few road accidents...
And what time did the Chinaman go to the Dentist? Answer - 2:30

Tooth hurty get it LOL LOL....
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Reply By: V8Diesel - Tuesday, Jan 30, 2007 at 15:15

Tuesday, Jan 30, 2007 at 15:15
"Dentist bad" I'd say mate.

Tyre pressures become a fanatical obsession of paramount importance on internet forums IMHO. It's ridiculous. A pound or two either way is not going to bring on floods, pestilence and eventual armageddon contary to popular belief. I'd suggest going half way on the hot/cold calculation. Easy.

Apart from 4x4 clubs, I don't think I've ever seen either a local or anyone else airing up and down when going from gravel to bitumen. Good or bad, right or wrong it's still what the vast majority of travellers do.
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Follow Up By: Member -Signman - Tuesday, Jan 30, 2007 at 15:37

Tuesday, Jan 30, 2007 at 15:37
I'm with you there....But you know what it's like when ya sittin there, and in the room across the way there's a guy with a 5/8" drill in a brace & bit into someones gob.......and you're next !!! Gotta think of something for distraction.
BTW...how did ya go in the stink boat the other weekend???
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Follow Up By: Oldsquizzy (Kununurra) - Tuesday, Jan 30, 2007 at 15:40

Tuesday, Jan 30, 2007 at 15:40
lol....What i was thinking...When all else fails let some out ...But only enough to get out of the immediate predicament....
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Follow Up By: V8Diesel - Tuesday, Jan 30, 2007 at 15:56

Tuesday, Jan 30, 2007 at 15:56
Signman, I can smell the burnt tooth right now.........urgggghhhhh! I went to a dentist in Subi when I was a kid who had the old green enameled, belt drive drill - it wasn't a pedal job, but not far off it. I liked it because it worked at low RPM so it didn't make that smell and ultra high pitched whine.

Didn't head over to Rotto on the long weekend as I'm currently out of action due to a crook leg. The gang all went over though and Phil had his Hamilton Jet boat going which provides a ready made water fun park. I got my maritime fix however by spending my Saturday night rescuing a mates broken down runabout (I certainly could have used a sail then). It has an old Chrysler outboard which he bought secondhand from a bloke called Noah.
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Follow Up By: Off-track - Tuesday, Jan 30, 2007 at 23:15

Tuesday, Jan 30, 2007 at 23:15
I'm with you V8Diesel. You have to laugh when you see these prongs airing up and down as soon as they hit the gravel.

I run about 42-44psi in our BFG AT's for the road and didnt even bother 'airing down' (gee I cringe when I hear that) in the Cape when it was wet. People get overly fanatical when they are unfamiliar with their surrounds.
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Follow Up By: Member - Matt M (ACT) - Wednesday, Jan 31, 2007 at 17:36

Wednesday, Jan 31, 2007 at 17:36
So let me get this straight. If I choose to lower my tyre pressures when driving on gravel, despite a fair school of thought that suggests it is not a bad idea, I am a 'prong'? I looked up 'prong' in the dictionary and it actually sounds like something that might cause a flat tyre, rather than prevent one.

I never knew that I should become LESS careful in unfamiliar environments either. Useful advice.

Thanks for the useful input Off-track, a lot of help that.

Matt (who has recently discovered he is a 'prong')
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Follow Up By: V8Diesel - Wednesday, Jan 31, 2007 at 19:08

Wednesday, Jan 31, 2007 at 19:08
Matt, I think the message intended was that people who are entirely familar with the local environment would generally agree you'd look like a bit of a noong nail biting and teeth nashing over tyre pressures for a dirt road. Put it this way, it's not a common subject of discussion around the fire or at the pub in WA.

I've driven hundreds of thousands of km's at high speed safely on dirt roads throughout the Goldfields and Pilbara of WA and never have felt the need to alter tyre pressures. I can quite honestly say I have NEVER EVER seen one local, one mining company or one station employee ever touch a valve stem unless they're going into very boggy sand.

Right or wrong, it's for people who wear matching shirts to worry about, not people who drive out there every day.
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Follow Up By: Member - Matt M (ACT) - Wednesday, Jan 31, 2007 at 19:17

Wednesday, Jan 31, 2007 at 19:17

Very happy with the point you make and I am not going to pretend that I bother with tyre pressures unless I am doing a couple of hundred kms on dirt or I know that the road is in poor condition.

I guess my issue was more about the tone of the post than the content.


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Follow Up By: Off-track - Friday, Feb 02, 2007 at 12:15

Friday, Feb 02, 2007 at 12:15

The term was given in a very broad way and you are right in that if people want to do this then they are more than free to do it - I dont contest that. I also agree that inexperienced people have to gain experience somewhere so dont begrudge them at all. We were all inexperienced once in everything we do and are still all inexperienced at a lot of things yet to do.

I guess to narrow down the target of prong-ism it would be those know-it-alls that feel they have to show off their flashy equipment and 'knowledge' to perform a rather unnecessary task.

Broadly applied it may be but if we cant use a descriptor like 'prong' without upsetting people...
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Reply By: Member - Doug T (W.A) - Tuesday, Jan 30, 2007 at 17:11

Tuesday, Jan 30, 2007 at 17:11
Dentist NO Psychiatrist .......maybe
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AnswerID: 218641

Reply By: Niffty - Tuesday, Jan 30, 2007 at 18:10

Tuesday, Jan 30, 2007 at 18:10
Funny nobody has mentioned the effect that a fully inflated tyre used in a fragile environment has. I always let the tyres down when heading off road (coastal dunes)usually 18-20 psi.If more of us did this then maybe the powers to be would leave more of the fragile environment open for all of us to enjoy.I know its the pits having to put the air back in again but I enjoy the bush more than I enjoy bush bashing.
Just off to hug a tree!
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Follow Up By: V8Diesel - Tuesday, Jan 30, 2007 at 20:07

Tuesday, Jan 30, 2007 at 20:07
Keep hugging.
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Follow Up By: Member - JohnR (Vic)&Moses - Tuesday, Jan 30, 2007 at 20:54

Tuesday, Jan 30, 2007 at 20:54
You are right Niffty, but I would rather go even lower for my pressures to lower the damage.
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Reply By: Member - Brian (Gold Coast) - Tuesday, Jan 30, 2007 at 19:10

Tuesday, Jan 30, 2007 at 19:10
Hi Signman,

Just last Wednesday I arrived at Rainbow Beach after 3 and a bit hrs towing the trailer with 40psi all round and 35 in the trailer. Lowered the tyres to 18 while filling the tank and then off to Fraser for a few days.... then Sunday we towed back to rainbow where I aired up with our compressor whilst SWMBO bought brekky .... tyres were still at 18 all round.... I'm not convinced that the pressures move around all that much, I realize that the pressure increases from a cold tyre to a hot tyre, but if you air down to 18 at road temp, then it should stay 18 at road temp..... it may decrease when the tyre gets cold but I would think that once the tyres temp increases, the pressure should again rise to 18.

Just my opinion, haven't really conducted any testing on this....



AnswerID: 218669

Reply By: peterjs - Wednesday, Jan 31, 2007 at 08:21

Wednesday, Jan 31, 2007 at 08:21
As there is a lot of talk about tyre pressures I thought I would ask your thoughts on this, I recently purchased new BFG 285/75/16 for my Landcruiser 2003 V8. I was advised that the tyre pressures should be around 32 for normal load / road conditions, after reading above I wonder if I should have them a bit higher, your suggestions are most welcome. Thanks Pete.
AnswerID: 218769

Follow Up By: Bware (Tweed Valley) - Friday, Feb 02, 2007 at 10:54

Friday, Feb 02, 2007 at 10:54
Hi Pete,

I'll try to answer your question. 32psi is the pressure recommended for most modern tyres and you will find that it is correct using the 4psi rule. Start with cold pressure read of 32, drive till warm, check again. The warm read should be up to 4psi higher. If it's more than that, your tyres are under-inflated. Running at highway speeds it is not recommended to have under-inflated tyres.

Having said that, I think a little over-inflated is a good thing; less wear on the edge of the tread, less heat build-up, longer tyre life. The trade off is a slightly bouncier ride (but not much if you are increasing the pressure by 3psi to 35).

For a loaded up vehicle, the 4psi rule still applies. Try a starting pressure of between 36 and 40 and see how you go.

Hope this helps,

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Reply By: Im.away - Wednesday, Jan 31, 2007 at 12:28

Wednesday, Jan 31, 2007 at 12:28
As you've seen, it's difficult to get a consensus about tyre pressures but one thing I do know is that when my offsider and I were messing around with pressures last year, we set up a test.

We work together, carry the same gear, have the same kind of vehicle (NP Pajeros) with the same options...so the cars weigh the same.

We both set off from Carnarvon in convoy travelling on bitumen at 110 kph. He was running 35 psi in his tyres, I was running 50 in mine. We stopped every 30 mins and measured the surface temperatures of the tyres with temperature probe connected to a digital multimeter.

The 35 psi tyres were running 15 degrees hotter at the first stop and got worse each stop as the ambient and road temperatures increased. We gave up the test when the 35 psi tyres were 29 degrees hotter than mine....and inflated them to 50 psi. Now these tyres were BFG A/T 265/70 R16's, so you could probably get an idea from these results. I would inflate to at least 45 if I were you and I was carrying a load.

Just a side note. We purchased new Discovery 3 in December and the model we liked didn't come with wheels that I could get BFG's for. I told the dealer that unless he could supply the vehicle with 17" rims from the lesser spec'ed "S" model, and fitted them with BFG's, the deal would not proceed. Well they did it and the salesman said that he'd had quite a few requests to do this previously! I reckon you've made a good choice of tyre, now all you have to figure out is how to make them survive...
AnswerID: 218797

Reply By: Gronk - Wednesday, Jan 31, 2007 at 13:44

Wednesday, Jan 31, 2007 at 13:44
Why not everybody go out and do the 4psi rule test and then there would be nothing to discuss !!! Would be interesting to see how far some people are out with their pressures.

I have cooper atr's and run them at 37 but 35 or 39 is ok too, and if I put the CT on then they are still at 37 because I'm too lazy to whack some more air in them !!

I'm not interested in trying to get 100,000 k's out of them, just want to have the best pressure that gives me good road holding and a COMFY ride. Had a run in a patrol running 44 psi ( BFG's ) once in the forest and was like riding in a billycart. He then let them down to 35 and the difference was amazing.

Its alright to run 50psi for a short time to suit a particular situation, but why you would want to run that pressure all the time. Talking of dentists, have you any fillings left ???
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Follow Up By: Im.away - Wednesday, Jan 31, 2007 at 15:08

Wednesday, Jan 31, 2007 at 15:08
Why? Because where we go, it's really, really scary if you get stuck. Rather lose the fillings than wait a day or so for help to arrive...if it does. And I'm not just talking about blown tyres here....A guy I know lost steering and went bush when his tyre blew. Tipped the vehicle over, broke the HF antenna off. He was in a lot of trouble. Two days before they found him. That's why.
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Follow Up By: Im.away - Wednesday, Jan 31, 2007 at 15:11

Wednesday, Jan 31, 2007 at 15:11
I s'pose I should also add that I work out there. I don't do recreational 4WDing. It's all about staying shiny side up, getting where you're going reliably, keeping costs down and getting the hell out again.
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