Tyre pressures

Submitted: Saturday, May 20, 2006 at 23:14
ThreadID: 34081 Views:2056 Replies:9 FollowUps:9
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OK, now I know this has been done once or twice, but I'm interested on how low some people will run their tyres on say, the GRR etc.

I used to be a fan of the high pressures to compensate for the load etc,......but on a couple of occasions I have been a bit lazy and run beach pressures on bush tracks on the way hame and been blessed with a great ride. I know I am more subject to sidewall cuts and overheating, but at lower speeds the heat shouldn't be a problem (within reason).

We need a graduate engineer (we use them a bit for this kind of thing at work) to work out all of the optimums for us. It wouldbe good to seee the risks of sidewall punctures graphd against the tread punctures from high pressures to find an optimum. I'm starting to go lower and lower lately, as I haven't stuffed a tyre yet, the ride is so much better, and the wear on other components must be reduced (suspention works less, not as many cans in the esky rub through, not to mention what corrigations do to boats and caravans.

Thanks in advance for any suggestions etc, 3 weeks to towing the boat to kalumburu, I will let all know what pressures I run and how I went, along with photo's of the fish!

Cheers Andrew
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Reply By: Exploder - Sunday, May 21, 2006 at 00:04

Sunday, May 21, 2006 at 00:04
I would be careful about going to low as you may weaken the tyres over time due to heat build up and excessive flexing then at speed at hwy pressure the tyre/ tyres could fail.

Personally I have never run below 30PSI except on the beach and sand, but it all depends on what you are doing, the conditions at the time at what you feel is right for you.
AnswerID: 173724

Reply By: Willem - Sunday, May 21, 2006 at 00:13

Sunday, May 21, 2006 at 00:13
Yeah well...it is a late night question.

I run a 3 ton Nissan plus trailer as follows all shod with 265/75/16's

35 on bitumen
28 on good dirt roads
25 on rough dirt roads like GRR
20 on tracks sandy or rocky
15 on sand or in rough scrub

On a recent trip I staked 2 tyres in the scrub. Bloke behind me picked up 9 stakes.

These pressures work for me but others may have a different view

Cheers
AnswerID: 173725

Follow Up By: P.G. (Tas) - Monday, May 22, 2006 at 08:17

Monday, May 22, 2006 at 08:17
Hi Willem.

What brand of tyres are they, and what is the maximum load rating and at what pressure?

Cheers,

Peter
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Follow Up By: Willem - Monday, May 22, 2006 at 10:06

Monday, May 22, 2006 at 10:06
Geez Peter, I had to go out into the cold with a torch to read the requested info...LOL

Cooper STT .. Maximum load rating 1550kgs at 80 psi cold

Is there a secret formula?

Cheers
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Follow Up By: P.G. (Tas) - Monday, May 22, 2006 at 11:15

Monday, May 22, 2006 at 11:15
Geez Willem, sorry about that, remind me to buy you a warming ale if we ever catch up lol.

The load rating @ maximum pressure is the key to every tyre. I'm no mathematician, but by my figures (assumed only), your tyres would be running a tad soft. This is how I work tyre pressures out, using your figures.

Tyre maximum load rating 1550kg, divided by 80psi = 19.375 kilo per psi.

19.375 kilo per psi X 35psi = 678.125 kilo's per tyre

678.125 kilo's per tyre X 2 = 1,356.25 per axle x 2 axles = 2,712.5 kilo's total vehicle load.

This presumes you have a perfect 50/50 load distribution between the front and back axles, which you won't have. A weighbridge when loaded will give you the right figures.

I always add 6psi to the load figure I get for highway running, and generally this will always give you about a 4psi heat build up after an hours running.

On dirt road, I just drop this 6psi and go back to my original figures. Mud, sand and rough surfaces are a different ball game, but the formula above will give you the best traction, mileage and fuel economy out of the tyres, the way the tyre designers intended.

I think this is where a lot of people get confused. The original Bridgestones on my GU III had the maximum load/ pressure @ only 36psi, not 80psi like the Coopers, but the formula works equally as well on Bridgestone and any other make of tyre.

Sorry about being long winded, I'll make up for it over that ale ;-)

Cheers,

Peter
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Follow Up By: Willem - Monday, May 22, 2006 at 18:12

Monday, May 22, 2006 at 18:12
Thanks for that Peter. It is mid boggling....lol

So 40psi should be the psi I should run while on the bitumen according to your calculations and the weight of my vehicle. My Tyre bloke and Coopers retailers suggested 35psi not knowing the load of the truck.

I find the STT's pretty good in all aspects of offroad and quite quiet on the black top. When driving longer distances(say 100km) on very low pressures the sidewalls do seem to scuff a bit and some lettering may be scrubbed off.

Time will tell if my methods are detrimental to the longevity of the tyre....:o)

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Follow Up By: P.G. (Tas) - Tuesday, May 23, 2006 at 07:21

Tuesday, May 23, 2006 at 07:21
Holy Toledo Batman ! Scrubbing the lettering off the sidewall :-o You have been doing some serious cornering ;-)

IMHO try 40psi in the front and 44psi in the rear when fully loaded and see how that goes. Remember, at these pressures the tyre is only at 1/2 load carrying capacity.

BTW, cold tyre pressure is messured at 20 degrees C.

Catchya on the track :-)
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Reply By: froomey - Sunday, May 21, 2006 at 01:19

Sunday, May 21, 2006 at 01:19
hi andrew

i've done the grr 4 times , and mitchell falls up to kulumburu once , run on 25 psi , have yet to get a puncture, i like a good ride
there is no wright or wrong trye pressure, every one is different.
4 vehicles into mitchell falls, me on 25 psi others on 35 to 40, me nissan gq others rodeo's, between them 5 punctures all tread puncture's, one with huge split, they all had bf goodridge a/t 3 ply walls , me yoko's 2 ply .
might have only been lucky,or good choice, but on way out and up to kulumburu we all ran on 25 psi , no more flats , even went into port warrander very interesting drive. . done the csr on 25 all the way , simular wieght 3.5 tonne, never got stuck and no puncture's.( lucky )
as i said , every one to thier own, there is no written right or wrongs, as you said , lower is better ride and not so hard on vehicle.
have a good trip
keep safe

froomey
AnswerID: 173730

Reply By: Dodgin & Weavin - Sunday, May 21, 2006 at 07:36

Sunday, May 21, 2006 at 07:36
Hi Andrew

LIkewise we've always run with the low pressure theory (pressures similar to Willems suggestions). While those around us running high pressure have had a number of flats we've ony ever had one (while 4wding). This was on the Tanami where we gave in to the high pressure believers and left our tyres inflated. Within 2 hours of the roughish stuff starting we had our first ever flat. Needless to say the pressures came down and we haven't had a flat since.

Cheers

Dodgin

PS I just remembered we have had 2 flats at once in sand on the west coast of Tasmania but this was because we had lowered the tyres until thay were almost flat to debog ourselves, didn't raise them again, continued on and got sand between the rim and tyre. End result 2 slow leaks at the one time. In other words driver error.Taught ourselves how to repair tyres after that.

AnswerID: 173737

Reply By: pilbaradisco - Sunday, May 21, 2006 at 10:33

Sunday, May 21, 2006 at 10:33
Tread lightly- Lower tyre pressure does less track damage.

Also depends on tyre size. On my LR Discovery with the Std 235/70/16 tyres I could only let them down to 28psi before they would start to bulge, but with 235/85/16's I can let them down to 18psi with mininum bulging. 25 psi seems good for most tracks with the 235/85/16's and I run 34psi on the road.

Glen
AnswerID: 173760

Reply By: Member - Nick (Kununurra) - Sunday, May 21, 2006 at 11:26

Sunday, May 21, 2006 at 11:26
Gday MEMBER Andrew,like your rig profile,need this need that,dont we all.
Your pics,are they out at Cape Domett.
By the way,when we done the GRR last year we ran 28-30psi all round,there were 6 vehicles with us and we only got 2 punctures,same vehicle BFG A/T's.
Leaving Wyndham along Parrys Creek Rd we came upon a Rodeo with one flat and another going flat.I mended(plugged) both tyres and asked what pressure they were running,reply 40psi what else?? pumped them up to 30psi and talking to them later they had a good trip afterwards.
AnswerID: 173764

Follow Up By: Member - andrew B (Kununurra) - Sunday, May 21, 2006 at 23:35

Sunday, May 21, 2006 at 23:35
Yep the camp photo is of dommett - can't wait to get back there again, the other is about 30k up river at 'sandy beach'.. as for the pressures, looking like going with the 25 psi give or take, but we'll be going slow. Good to catch up with you the other day, catch up soon

Andrew

PS - apparently the road to dommett is getting graded at the moment and is open.
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Reply By: Battlecat - Sunday, May 21, 2006 at 14:34

Sunday, May 21, 2006 at 14:34
Hi,

I find that it all depends on weight. If you have a full touring load and the rear of the car is carrying most of the weight then more psi would be an advantage and would avoid tyre damage.

I have a GU and run the following. Front/Rear psi

Road - 50/50psi - it is high but it saves fuel, handels better and is not causing uneven ware
Reccomended Road 36/36
Reccomended Road Carrying Load 36/40
Gravel - 20/25
Sand hard - 20/20
Sand soft - 16/16
Bogged - 10/10

All in all I guess you have to weigh up safety, fuel consumption and tyre ware and potential damage to your tyres and car. Always drive to conditions and ask the locals.
AnswerID: 173785

Reply By: Kiwi Kia - Sunday, May 21, 2006 at 15:22

Sunday, May 21, 2006 at 15:22
I use 16 to 18 off road and 24 on on the likes of the GRR. Just today, when I was lowering tyre pressure for off road trip I asked a friend what he used and he told me "knee pressure" ! He pushed his knee into the side of the tyre occasionally while bleeding air and when he can deflect the rubber thats the pressure he uses. I tried this and it worked out to be about 18 psi.
AnswerID: 173791

Reply By: Member - andrew B (Kununurra) - Sunday, May 21, 2006 at 23:41

Sunday, May 21, 2006 at 23:41
Thanks to all who have replied, I think I'll go with 24 front, 26 rear and 20 on the boat trailer. just a quick one - with the 4 lp rule and tyres increasing pressure as they get hot, are the pressures aboe in all of your most appreciated replies hot or cold, sorry for the extra question...

Thanks Andrew
AnswerID: 173883

Follow Up By: Richard Kovac - Monday, May 22, 2006 at 00:21

Monday, May 22, 2006 at 00:21
Andrew
I think you will fined that they are all cold temp settings
as it would be to hard to get the tyre to the same hot temp to test

Richard

When we travel from bitumen to dirt we will drop pressures 10 psi front & rear (but I run split and nower tyres) 50 psi rear 40 psi front
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Follow Up By: Willem - Monday, May 22, 2006 at 10:09

Monday, May 22, 2006 at 10:09
Mine were hot temps.

I find that the Coopers warm up 2-3psi only so that that amount off.
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Follow Up By: Member - andrew B (Kununurra) - Monday, May 22, 2006 at 15:27

Monday, May 22, 2006 at 15:27
Thanks Willem and Richard. I just needed a bit of conformation on the hot/cold measurements as I will be in the situation of leaving on the blacktop, and dropping the pressures into the journey, ie starting with cold highway pressures, then dropping to hot dirt road pressures.

Cheers Andrew
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FollowupID: 429943

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