Tyre pressures

Submitted: Wednesday, Apr 20, 2011 at 21:13
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No doubt this has been discussed many time already but it would be good to get some advice on tyre pressures from experts. We have just fitted some Bridgestone Dueller 683's to our Hilux and have been given numbers from 32 psi to 38 psi as the tyre pressures for fully laden highway driving.
For fully laden travel on dirt and gravel, one outback expert has recommended front pressure down to 26 psi with rear at 30 psi, but I've been quoted higher pressures by others.
Any input will be very helpful. Thanks, Poitr
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Reply By: Member - Brenton - Wednesday, Apr 20, 2011 at 21:49

Wednesday, Apr 20, 2011 at 21:49
Never a silly question I say.. The old story is though - it depends, It depends on conditions, weight of vehicle, how fast you are able to travel (conditions) etc. Measure pressure when tyres are cold preferably (not always possible).

Having said all of that, if it is good gravel road conditions (as in near bituman standard) and you are travelling with heavy loads, then maybe a little higher. If the roads are badly corrugated, soft in parts etc, then 26/30 might be about right. When you start talking about loose stuff, like real "dirt" roads go softer again. The best advice I can give, is be prepared to change pressures and drive to the conditions, your experience and comfort level. Alll the best..
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Reply By: Crackles - Wednesday, Apr 20, 2011 at 22:08

Wednesday, Apr 20, 2011 at 22:08
What sze tyres Poitr?
Cheers Craig................
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Follow Up By: poitr - Thursday, Apr 21, 2011 at 07:27

Thursday, Apr 21, 2011 at 07:27
Craig,
Sorry for not mentioning that ; 235/70R16
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Follow Up By: Crackles - Thursday, Apr 21, 2011 at 08:59

Thursday, Apr 21, 2011 at 08:59
Being a skinny tyre (on a split rim I asume?) I would run 35 front, 38 rear fully loaded (that is carrying 1 tonne). Offroad I wouldn't let them down as much as say a wide tyre on 15" rims but use 26 front, 32 rear with a reduction in speed. Running empty I'd have the same front & rear.
The figures suggested to you already are in the ball park.
Although it doesn't specifically cover your type of vehicle, have a look at this guide for outback pressures. Possibly the camper in the middle is closest.
Tyre pressure guide
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Reply By: Tim - Thursday, Apr 21, 2011 at 01:00

Thursday, Apr 21, 2011 at 01:00
I was wondering about this a few years ago. On my travels I stopped an asked a bloke, a ranger working in the Kimberley. My theory was he would know one way or another.
Anyway, his answer was to leave the tyres alone. Dropping the pressure may make the ride a touch smoother however it exposes the sidewall of the tyre to the same nasties which the tread is designed to handle. The lower the pressure, the more likely you are to damage the sidewall which is the weakest part of the tyre.
I liked his theory and live by that rule.
Tim
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Reply By: vk1dx - Thursday, Apr 21, 2011 at 02:13

Thursday, Apr 21, 2011 at 02:13
We bought our car in Dec 2009. I did the same read, read and read. In the end I thought I would try what Dad told me. I did so for the highway and good dirt roads. Pump them up and have about 1/2 inch bulge on the side. And after the first big drive feel them. Warm = leave it at that. Hot = too low. Cold = too hard. Put 35 in each one. On the drive the rear were hot so they went to 40. For the rocky bush I drop them all to 25, as suggested on a 4wd driving course. I am not interested in beach driving so no data there. But I would use 15. Maybe less if a problem. But watch out for breaking the bead.

I did not rotate either set until early this month. I measured the wear and it is spot on equal across the width of each tyre in each set. Therefore by trial and error, I have found the correct pressures for our setup. They now get rotated.

Travelling weight about 2900Kg. Tyres are Cooper STT or AT's. I swap them for the bush trips.

Phil
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Reply By: TheMightyMoose - Thursday, Apr 21, 2011 at 06:44

Thursday, Apr 21, 2011 at 06:44
"Experts" are a dime a dozen, and each one has a different "expert" opinion.

I will state up-front that I ain't an expert but I have travelled quite a bit and so I'll offer my practical based opinion. But don't forget that opinions are like a@$eholes - everyone has one!

Anyway for the highway pump them up to say 35 then after travellling for a while and the tyres have warmed up take the pressures again. If the pressures have increased by more than 4-6 psi you were too low to start with. If the new pressures are less than 4-6 psi than the starting pressure you had too much in there in the first place. Adjust accordingly and repeat again after an hour of so.

Unfortunately this works well on long outback type drives but not so good if the elevation changes significantly or the ambient temperature changes a fair bit. But it will give you an idea as to what you should have in there. A few psi here or there will not matter that much.

Once you hit dirt it all depends on the road surface, speed etc - you'll figure it out over time. I reduce the pressure for dirt roads.

The main thing is to enjoy what's out there to see and travel safely.
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Reply By: Mick O - Thursday, Apr 21, 2011 at 08:06

Thursday, Apr 21, 2011 at 08:06
Poitr, the accepted wisdom from those who mend the tyres on the track is a reduction of 10 to 20 percent from your highway pressures for driving on gravel roads. This is accompanied by a suitable reduction in speed and being cautious, particularly around dips and creeks etc.

Cheers Mick
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trip would doubtless be attended with much hardship.''
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Reply By: Member - John and Val - Thursday, Apr 21, 2011 at 08:20

Thursday, Apr 21, 2011 at 08:20
Poitr,

There is no simple answer like use 32 psi. As indicated by Phil and Mightymoose, the 4psi rule is a good place to start. If the pressure rises above the cold pressure by about 4 psi once warmed up, it's about right. (The change in pressure results from the change in temperature due to sidewall flexing.)

Too much flexing due to too low pressure results in excessive heating. Ideally the flexing in all tyres should be about the same, so those carrying most weight will need a higher pressure. Heat buildup due to flexing also depends on speed - faster speed, more frequent flexing. When you drop your pressures you gain a bigger footprint and a softer ride, useful on corrugations, important on sand and mud, but you must drop your speed.

The load carrying capacity of the tyres is also reduced at lower pressures. This subject is dealt with in some detail Rob D's recent post, now ablog here.

One simple way of checking that your inflation is about right is to draw a uniform chalk line across the tread, roll the vehicle a few metres on a uniform surface and check the chalk line to see where it has worn off. Overinflated it will wear more in the centre of the tread, and underinflated more to the outside.

There's really no simple answer to your question, but for what it's worth, we typically run our loaded Troopy on bitumen at 30 front, 35 rear, dropping to about 26/30 on bad gravel, and down to about 18/22 or sometimes lower for sand. All subject to loading and the need for traction. All at appropriate speeds.

HTH

John



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Reply By: River Swaggie - Thursday, Apr 21, 2011 at 20:28

Thursday, Apr 21, 2011 at 20:28
"I will state up-front that I ain't an expert but I have travelled quite a bit and so I'll offer my practical based opinion. But don't forget that opinions are like a@$eholes - everyone has one! "



Ive met a couple of people on my journey that i believe have more than one......



Anyway its all about the environment your in,Rocky/Shaley areas you want the tyre to mould around these surface's as well as having good grip,without ripping lugs off as well...I usually drop the rear (heavier) to about 26psi and the front to about 24psi....

Muddy/Clay areas is a diff ball game and i usually leave above the same pressures and lower when required...Even lowered tyre pressures right down to get out of a situation then blow back up one recovered...

Sand driving ive never done and was supposed to be in Robe this weekend but due to an ol injury i'm still none the wiser about sand driving....

Every vehicles different and it will be trial and error,oh and if you havnt got one get an ARB onboard compressor or something similar to play around with ya pressures.....

Goodluck
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Follow Up By: poitr - Thursday, Apr 21, 2011 at 22:40

Thursday, Apr 21, 2011 at 22:40
Thanks everyone for the info. Really helpful. Obviously takes a bit of trial and error but it looks like the reduction for gravel and dirt is the way to go.
Poitr
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