Thread: Tyre pressures?

Originally Posted by JDNSW

Dave's discussion is worth noting - however, this method of determining the correct pressure is the optimum pressure from the point of view of adhesion, as I note above, there are other considerations.
However, I don't believe your comments about tyre size make sense.
Assuming we take Dave's method and adjust pressure to make the tyre contact area the ideal shape, the only part of the tyre size that makes any difference is the tread width and the overall tread diameter. The actual volume of air is totally irrelevant, as is the height of the sidewalls. In practice, when you get to light truck type tyres, the stiffness of the sidewalls makes a significant contribution to support, so different ply ratings will require different pressures to give the same contact pattern from tyres the same size.

Generally speaking the larger the diameter and the wider the tread, the lower the necessary pressure, provided everything else stays the same, but for example, the wider the tyre, in general the less stiff the tread, so that it relies more on pressure to maintain the contact shape, so the difference is less. And as I commented - there are other things to take into account. There is no "perfect" pressure!

John

Of course it makes sense, assuming 2 similar carcass constructions, say 8PR vs 8PR, the larger volume tyre will require less pressure to carry the same burden.
JD you have actually said the same thing.

And as you commented there are many variables, this is why I am not offering any suggestions of actual pressures; this is dependant on the tyre/load/speed/conditions/comfort expectations.

I recently installed a tyre pressure/temperature monitor in the Disco.
245/75-16 MTR's on a D2.

I originally always ran my tyres at 34/36psi but I found that from cold the pressure increased 6-7psi after only 1/2 hour and they were running at about 55degC.
I upped the pressure to 40psi all round and the pressure only increased by 3psi from cold and they run at about 42 degC.

When towing I need to run the rear at 45psi to keep pressure and temp the same as when not towing.

Originally Posted by JDNSW

Yep! Hubs and tyres every stop, and especially if the trailer has been sitting a while, or if you have done anything to wheels, bearings or brakes on car or trailer, stop to do it after the first ten kilometres on the highway.

John

I bought a brand new tandem trailer in Adelaide last year. First stop saw one of the hubs much hotter than the other three.
I rang the supplier and he wasn't too concerned. I kept an eye on it (ignored it) and kept going. It got me back to Alice OK.
Since then the trailer hasn't done much work (couple of rubbish runs) but I have done a springover. Maybe one day I will check the bearings...

Originally Posted by oldzook

.........
JD you have actually said the same thing.
.........

Sorry, no I didn't - I said overall diameter and tread width are all that matter - volume does not. - Consider the case of, for example, a tyre with an overall diameter of 32 inches and a tread width of 9 inches - it could be on a 15", 16" or 18" wheel, and have very different volumes. Or even the same nominal size tyre with the same volume could have different tread widths and require different pressures.

But as we agree, this is only a theoretical ideal pressure for maximum adhesion, and there are a lot of reasons why you may not want this pressure, although for most people and most tyres you probably won't go too far from it.

John

Thanks all. Lots of things to think about and try, so will have a play over the coming weeks.