Correct tyre pressure formula
I have read a lot on recommended tyre pressures and would like to add my own
personal experience on the subject. One thing you have to remember is that there
are a few factors involved. One to remember is that it is the air volume and not
the pressure that supports the weight, so to get the same amount of air in a
small tyre will require more force and therefore pressure than the same amount
in a larger tyre. i.e.. 35 psi in a 30"x9.50" tyre may have the same amount of
air in it as a 33"x12.50" with 30 psi. I used to have a lot of punctures on my
outback trips from stones getting forced into the tyre casing through the sips,
(the cuts that are put in the tread blocks). It turns out that having too much
pressure opens up the sips and makes it easier to get the stones in. After 7
punctures on that trip and finding that the centre of the tyres had worn I knew
that I had to find a better way of getting the right pressures. The old chalk
line method I had used was just not working where they say to drive with a chalk
line across your tread and see if it all wears off evenly. But this does not
take into account the increase in tyre pressure you get by the heat generated
from the friction and sidewall flex. As it always happens after the fact, I read
in a 4x4 mag after I got back that to find the right pressures for different
temperatures, loads and conditions you take front and rear pressures cold, and
then again immediately after at least 20 minutes at highway speeds or more and
there should be a 4 psi increase. If there is more than 4 psi, than you had too
little to start with and if there is less than 4 psi, than you had too much. I
have used this technique for the last 5 years and didn't suffer one puncture
from stones again, ( had a steel railway spike go through the trailer tyre on
the old Ghan railway line ) on the same roads, temperature and loads. The tyres
have also lasted longer with very even wear. You can use this technique for your
trailer as well. For my tyre size (275x70x16) and load, I run 33 psi in the
front and 32 psi in the rear. My mate in his XJ ended up with 35psi front and
rear with 30"x9.5"x15 's and later with 33 psi front and rear with 31"x10.5"x15
's. I ended up with less in the rear because I don't carry much in the back of
the car, most of it is in my trailer.
I have also read that the best rim width for a tyre is the
actual tread width of the tyre, (not the side wall width quoted on the tyre). This will
give you the flattest tread on the road and longest tread life and handing. They did say
that a rim that is mainly used offroad should be narrower for better rim protection and to
help stop the bead coming off at very low pressures.
When I was
curious about tire rotation patterns on my BFG's , I found out from Michelin
(the manufacturers
of BFG's) what their recommended rotation pattern was. They have a database with
different patterns for different rigs, and they recommended a 'modified cross'
pattern for our XJ's:
Rears straight forward to the front, and fronts cross to the opposite side on
the rear. (if you're doing the recommended 5-wheel pattern, the spare goes down
on the right rear, and the tire that would have gone to the right rear becomes
the new spare.)
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