Outer Limits 4x4 Board

Posted: **Thu May 31, 2007 11:37 am**

Just wanted to find out what pressures people run in their tyres for various activities.

I have a new set of 31 Goodyear Wrangler MT/Rs on a hilux and have been told 36 front 40 back and no lower than 20 for offroading.

Specifically, how low do you go say for corrigated roads vs mud vs sand?

Thanks

Posted: **Thu May 31, 2007 11:46 am**

A) do you have a compressor on the car
B) Are you willing to reseat a tyre, or learn?

I would tend to say in terms off off road work if you haven't had a tyre fall off the rim you are running too much air.

as for road use, 40 seems pretty high for a 31 - are you always heavily laden? I would have thought 36 all round would have been plenty.

As for how low to go, it depends on use. If you are driving slowly on technical terrain, you will be surprised how low you can go even without beadlocks.

15 should be fine, at low speed, even 10 if you are sensible with cornering.

Generally, I don't air down when on corrugations, but I don't do the driving you do (i.e outback) so I don't know what others would recommend.

I will happily drive my cars around all day at low pressures though when on a trip - 10-12PSI in my 2500kg merc and 6PSI in my suzuki, with roads speeds up to 60 kph.

Just my 2C

steve.

Posted: **Thu May 31, 2007 11:53 am**

this is from another forum and the general rules seems to hold true

Use the 4psi rule to work out exactly WHAT pressure you should be running! I would guess that whoever fitted the tyres didn't check the pressures too much after seating the bead! You probly should check the sidewalls of the tyres to see what the max pressure cold should be, depending on the size it may be as low as 35psi!

But if you start with say 32 psi cold (check the pressure and set it before driving) one morning, drive them for about an hour. How long does it take to get to work? Then check the tyre pressure. If it has gone up by MORE than 4psi, then the start pressure was too LOW and the tyre has done a lot of flexing and squirming and heated up the casing and the air inside it. You can get a pretty good idea of the next start pressure to use if you work out what the difference is between what it IS and what it should have been. Starting at 32, it should have been 36, so if it is say 38 then the difference is 2 psi (38 take 36 = 2) Add 1/2 that (2/2 = 1) to your original start point (32 + 1= 33) and then use that as the start point tomorrow. You can add that much air after your hours drive, but it won't work so well then 'cos the tyre takes a helluva long time to cool down and even while adding the extra 1psi will help it cool a little, it won't be enough to drag the pressure and temp down significantly.

Now, if your pressure reading shows that while you started at 32 psi, after one hours driving the pressure has gone up by LESS than 4psi then the start point was too HIGH! The tyre has been fairly firm and hardly squirmed or flexed at all, so the temp and therefore pressure hasn't gone up much. Since 32 was too high, but there is no way of working out how much, the best guess that you can use is to drop the overall pressure by 2psi and then leave it at that for the rest of the day, using 30 psi as a start point tomorrow.

That all make sense? I know that it sounds difficult and convoluted, but it isn't really, and after only one or two tries you'll work out how easy it is and you will very quickly get YOUR pressure right for the driving YOU do under the conditions that YOU meet and with the vehicle loaded the way YOU like it and YOUR driving style!! What is right for YOU may NOT be the same for me, and in fact is very unlikely to be the same, 'cos we have different driving styles, conditions, ambient temps, speeds, braking techniques, and a whole heap more! You hafta do it for yourself, but you can do it for every type of road surface and for every load condition and temperature that you will encounter. It is the ONLY method that takes into account ALL the variables, because it is the only method that responds to the actual driving of the car under the influence of all those variables. The START point is the one that the Tyre and motor industry can give you on the tyre placard or thru their tyre pressure manuals, but you still hafta apply the influence of all the variables that YOU introduce by driving the car.

Give it a go and prove to yourself that the 30-32 is OK or otherwise for the driving that you do. Personally, I would think that if your tyres are around the 265/75R15 or 16's size then 30-32 wouldn't be too bad for an MTZ, they, like the Coopers, tend to need a lower pressure than most of their competition. So start with the 30-32, and see how it goes, I would think that it'd be pretty much around the right mark!

Enjoy!

Posted: **Thu May 31, 2007 12:11 pm**

I believe Pirelli recommend the 4PSI rule for highway use.

Steve.

Posted: **Thu May 31, 2007 12:16 pm**

yes sorry that was more relevant for general road driving.

Posted: **Thu May 31, 2007 12:41 pm**

Gwagensteve wrote:A) do you have a compressor on the car
B) Are you willing to reseat a tyre, or learn?
.

I have a compressor but hope never to have to reseat a tyre. One day I will have to know I suppose.

Gwagensteve wrote: as for road use, 40 seems pretty high for a 31 - are you always heavily laden? I would have thought 36 all round would have been plenty.

Yeh i was recommended 36 all around and 40 with a load on the back.

That 4psi rule is a good one. I might have to try it.

Posted: **Thu May 31, 2007 1:15 pm**

100 Series Cruiser 265/75/16

City - 42 front, 39 rear (sharper cornering, less front tyre wear)
Highway Touring - 38, 38-40 (often have load in back)
Corrugations - Till it "bags" - approx 25
Sand - 18 - 20
Sharp Rocks - debating that atm in another thread re. Sidewalls. 25 is my preference, but does expose sidewalls.

Posted: **Thu May 31, 2007 1:58 pm**

i always ran 32 in my lux front and back when i had no load on., always had good wear and ride over 10 years of owning them. luxs tend to be stiff enough in the rear without running around on 40 psi. off road is a different story which i'm not entering into but i'm confident you will have no probs 32 on road unloaded.

Posted: **Thu May 31, 2007 8:48 pm**

danos wrote: I have a compressor but hope never to have to reseat a tyre. One day I will have to know I suppose.

Yeah - don't fear it, it's no big deal. Your tyres will work best off road at about the same pressure as they want to start falling off the rims.... it seems to be just the way of things.

I went beadlocks in 2000 and wouldn't be without them. I'm running tubes only in my zook atm, but I am underrimmed (9" tyre, 4.5"rim

) and I;m not going as low as I want because I don't want to tear up a tube, so 6psi is it for me for now. My beadlocks for the zook are ready to go but the tyres are a bit too big to fit atm so I have to be patient and cross my fingers.

Posted: **Thu May 31, 2007 9:07 pm**

danos wrote:... and no lower than 20 for offroading.

Who told u that

Dunno what your driving, but I go to 12 straight away on sand and on rocks ran my simex's on about 5. Sometimes dead flat

But thats with a ~1500kg 4b so might want more in something bigger

Posted: **Thu May 31, 2007 9:21 pm**

RockyF70 wrote:
danos wrote:... and no lower than 20 for offroading.

Who told u that

Dunno what your driving, but I go to 12 straight away on sand and on rocks ran my simex's on about 5. Sometimes dead flat But thats with a ~1500kg 4b so might want more in something bigger

12 on sand lol you can get away without even letting tires down on some beaches

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