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A technical question for Mr Tyre about pressures

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Just got MTs

Joined: 22 Feb 2009
Odometer: 279
Location: Hampshire - the county of green lanes

PostPosted: Tue Dec 21, 2021 10:22 am    Post subject: A technical question for Mr Tyre about pressures Reply with quote

I have found two websites that show how to calculate tyre pressures:

Both are for caravans but presumably apply to cars as well (or not?). Correct pressure is supposed to be (max allowable pressure in psi) x (actual load on tyre in kg) / (max allowable load on tyre). These parameters can be found on the sidewall of the tyre.

From the MOT certificate, the load on the axles of my 110 is 1230 kg on the rear and 1260 kg on the front. for the calculations, I'll make that 1250 kg each, or 625 kg per tyre.

I have two sets of four tyres:

4x BFG MT KM2, 255/85-16 on 16x8J rims, max pres 80 psi, max load 1550kg. The above calculations show a correct pressure of 32 psi front and rear.

4x Simex Jungle Trekker 34/10.5-16 (=280/85-16) on same rims (16x8J), max pres. 35.5 psi. Calculations show a correct pressure of 19 psi for both front and rear.

So that is quite a big difference (for road use), and also very different from the specs by LR, which are 29 psi front and 49 psi rear for the standard 235/85-16 tyres.

Mr Tyre does this make sense?

Manage the unavoidable.
Avoid the unmanagable.
(stolen from a traffic sign in Bangalore, India)

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Mr Tyre
Mr Tyre

Joined: 17 Jun 2002
Odometer: 4951
Location: Darlington Co Durham

PostPosted: Wed Dec 22, 2021 6:24 am    Post subject: Reply with quote

Does to me, but a word of caution, lots of tyres used/stlll are to be stamped with 'North America and Canada only', with a max pressure and load stated on them so be careful as you could end up running either too much or not enough pressure.Most but not all caravans in Oz are running heavy duty axles and usually wheels and tyres the same as the tow vehicle, so they can be swopped around if the needs arise.

I can remember when BFG and many other makes were first fitted to Land Rovers, BFG themselves stated standard handbook pressures + 6psi front and rear, that has of course changed but tyre dealers either listen to the manufacturers or just go by what people use, which is a strange thing to do as not everyone runs the vehicles at the same load, same supension set up even factory engines so weights vary in between vehicles.

You have worked out what pressures you need for your tyres and their intended use, so i would stick with it.Putting the vehicle on a weighbridge and doing at an axle at a time would be the only other way to do it.
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Adrian Flux 2022

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