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gords

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Tyre pressures
« on: July 21, 2005, 12:02:59 »
I've seen mentions of and references to people adjusting tyre pressures when tackling different terrain.

Is this common practice? If so, please explain :)
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Offline Barry Scott

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Tyre pressures
« Reply #1 on: July 21, 2005, 12:05:08 »
I presume lower pressure will put more tyre on the ground, but I would not want to guess at how the terrain type affects that.

Sorry, not the most helpful response.
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Barry

Offline Eeyore

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Tyre pressures
« Reply #2 on: July 21, 2005, 12:13:35 »
You're about right.

For example, for day to day driving I have approx 28-30psi in the front tyres.

For play sites and the like I can air down to about 15-16 psi to increase grip (broader and longer tyre contact patch). It can be good for playing on boulders too - the tyre is more squashy and moulds to the shape of the ground better and increaes grip. However, your rims take bigger hits (and mine have had a battering).

Any less than 15 psi and my tyres start loosing air all by themselves and I end up completely flat in about an hour.

Some tyres will run lower pressures still, but the problem then becomes holding the tyre on the rim.

The next problem is inflating them again as 235/85s take a lot of air.....

HTH
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gords

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Tyre pressures
« Reply #3 on: July 21, 2005, 12:23:31 »
wow, 15- 16psi ... that seems pretty low!

I assume at that pressure, you'd need to keep your speed very low to avoid continually banging the rims!?

I assume the relationship between grip and pressure is a gradient, but how linear is it? E.g. if you go from 30psi to 25psi, is that half the gain of going to 20psi?

How do you know what's best, or is it a case of "as low as possible"?
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Offline Digsit

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« Reply #4 on: July 21, 2005, 13:05:28 »
My Discovery owners book has the figures in the front :D  I will look when I get home.


Personally I have never bothered. Seems like a lot hassle to me :?  Surely there is also a pay off between tyre pressures and ride height too ie if you lower pressures then you put the axles closer to the ground :?
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ChrisW

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Tyre pressures
« Reply #5 on: July 21, 2005, 13:13:53 »
I was under the impression it was more of an issue if rock crawling or desert running - not much need in our clime tho  :?:
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Offline Xtremeteam

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Tyre pressures
« Reply #6 on: July 21, 2005, 14:41:51 »
the limit for ARC trials is 12 psi & i regularly air down to that or less :oops: (depends whos does it )
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Offline Eeyore

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Tyre pressures
« Reply #7 on: July 21, 2005, 17:13:13 »
Rocks and sand yes, but it can also be blinder for bogs and mudruns on play sites. Extra contact area = greater floatation.

Yes, you idealy need to keep your speed down. Although you'd be surprised at how well 16psi can keep a BFG MT up, it's really no fun on hard surfaces (oooooh squirmy :shock: ). Ever driven with a puncture? The effect is exactly the same. I certainly wouldn't commend runnin' on tarmac with underinflated tyres.

BFG recommend a minimum pressure of 16psi in their tyres, but given the right rims, I'm sure you could air down to less - I just wouldn't like to say how long the tyre would last!

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Offline Guy90

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« Reply #8 on: July 21, 2005, 17:17:58 »
I'm with Cliff. Can't be bothered to air down. The only time I have done it was in The Sahara and the sand was very soft. You got better floatation.

in the UK i wouldn't bother but it can help on most surfaces.
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Offline LandRoger

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air down
« Reply #9 on: July 21, 2005, 21:48:54 »
i have attended a quallity off road school thruxton hants  one of the first lessons to be learnt is the importance of tyre pressures , i thought airing down was common practice ,the benefits are huge more footprint etc , i find that the peeps who dont practise this simple mod are the ones spinning out on wet grass etc , portable air compessors of half decent ability are cheap as chips  ,much cheaper than a winch anyway  . happy spinning.
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gords

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Re: air down
« Reply #10 on: July 21, 2005, 21:55:22 »
Quote from: "jeepster"
i have attended a quallity off road school thruxton hants  one of the first lessons to be learnt is the importance of tyre pressures

So, did they specify particular situations and pressures?

I think it was seeing a compressor that prompted me to ask the question :)
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Offline LandRoger

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tyre pressures
« Reply #11 on: July 21, 2005, 22:19:13 »
yes they did give pressures for different terrains - i will find hand book and uploadit for you , as a rule of thumb  18-20 psi on slippery climbs muddy runs, and the effect is really noticeable and pleasing , as you get further with less effort  ,12-15psi on rocks and sand  because you are only crawling allong you can keep your diffs aimed off line of rocks obstacles etc ,at these low pressures tyres rolling of rims , and self-deflation of tyre can be a problem  hope this helps. by the way its possible to buy smart little tyre air downs that you can set at desired presh just screw on and have a cuppa as they do the work  11 pounds for 4 .
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gords

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Re: tyre pressures
« Reply #12 on: July 21, 2005, 22:25:51 »
Quote from: "jeepster"
by the way its possible to buy smart little tyre air downs that you can set at desired presh just screw on and have a cuppa as they do the work  11 pounds for 4 .

Hey, they sound groovy :D  Where do you get them from? Do you have a link to a web site that sells them?
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Offline davidlandy

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« Reply #13 on: July 21, 2005, 22:27:39 »
Devon 4x4 do some made by a company called staun

http://www.devon4x4.com/products/18/Accessories/358/Staun+Tyre+deflators
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Offline LandRoger

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tyre deflators
« Reply #14 on: July 21, 2005, 22:44:09 »
i bought mine in aus -down under but i think that scorpion-racing do them, ask for STAUN tyre de flators  mine are 6-30psi ithink they also come in other psi specs for quads etc .www.scorpion racing .co.uk.
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Offline discomummy

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« Reply #15 on: July 22, 2005, 08:59:34 »
having driven a lot in australia - i used to air down a lot and carried those funky little things for letting the air out of the tyres and a decent compressor to reinflate ( i have a bushranger)

I used to go down to about 16 for sand, if you didn't you would get stuck!  i also used to air down for various rock climbing situations - but there you had to use your judgement - let your tyres down too much and you lose clearance!  

when driving with tyres down, it is essential to keep your speed controlled and also not to turn corners sharply as you could roll your tyres off the rims.  i drive with bgf ats and never had a problem with airing down - when we hit the beach it is the first thing you do - it is always a nice sight 20 odd 4wds all with people pottering around discussing which tracks are going to be the most awkward and who will rescue the idiots who didn't air down(cos they knew better) and would get bogged!!!

i have not yet found a situation in the uk where i need to air down, but still carry my compressor just in case!!!

Ela
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Offline Bishops Finger

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« Reply #16 on: July 24, 2005, 12:00:50 »
My understanding is you air down for rocks sand and some mud

Its actively encouraged if you want to do the Rubitong rock crawling trail at Tong/Parkwood near Bradford

For re inflating I've got a £29 air compressor from Halfords whichplugs into a cigarreet lighter and works fine
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Offline Range Rover Blues

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« Reply #17 on: July 24, 2005, 13:27:50 »
Someones darling offspring let my tyres down for me the other weekend and I killed my Halfrauds compressor trying to pump up the first one, they are just not up to tyres this size.
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Offline Bishops Finger

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« Reply #18 on: July 24, 2005, 21:50:06 »
They do say let them have a break now n then

I had no problem re inflating a 235 75 R15 from exceedingly flat to 33
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Offline davidlandy

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« Reply #19 on: July 25, 2005, 09:34:15 »
Quote from: "Bishops Finger"
They do say let them have a break now n then

I had no problem re inflating a 235 75 R15 from exceedingly flat to 33


what about the other three?

must have took hours! :wink:
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