Thread: Tyre pressures

[quote="barney"]

Originally Posted by rmp

Originally Posted by crump

Note that a larger tyre does not give you a greater contact patch.


Also note that contact patches only lengthen. Sidewalls may bulge, but that does not increase the tread width.


hth

to clarify;
a larger tyre will give you a greater contact patch. *a larger tyre in diameter will give you a longer contact patch, e.g. say a 35' tyre over a 30' tyre, bigger circumference = longer footprint. * a tyre that is say 33' inch diameter and 7" wide, when presure is reduced will give you a footprint of a set width and length, if a similar tyre of the same diameter but maybe 10" wide, being at the same pressure, will give you a footprint of the same length but it will be wider as the tyre starts off wider and thus more surface area.
remember the question of what penetrates the ground more, an elephants foot or a womans high heel shoe? it's all to do with contact area versus weight. have you seen those photos of the landys in iceland? i'm pretty sure those big mother tyres are not for show.

i do agree with what you say about the sidewalls, they only indicate to you how flat your tyre is. i didn't actually mean to highlight this bit.

This could get interesting.

The stilletto/elephant foot analogy doesn't hold because neither is elastic, ie the contact patch is constant regardless of pressure. A tyre's contact patch varies with pressure.

What support a vehicle is the volume of air in the tyre. A smaller tyre requires the same volume of air as a larger tyre, but it must be compressed into a smaller space, hence the higher pressure.

Contact patch is just pressure and weight, with allowance made for the stiffness of the sidewalls. At one extreme a train's wheels are a very stiff sidewall!

As for the Iceland machines, their objective is to minimise downforce, so they want the greatest possible contact patch. Given that the contact patch is a function of weight and pressure, they must lower the tyre's pressure as far as they possible can. And given that a vehicle needs a certain volume of air to support it, no matter what the tyre size, a very large tyre makes sense, allowing them to have the volume of air they need, at a very low pressure, with a very large contact patch.

If they reduced a normal sized tyre to that pressure they'd be on the rims and it would be so deformed it couldn't be driven on. It'd be a massive contact patch though.

In fact, those trucks air down to less than 5 psi. At that point the sidewall is doing a lot of the work, so the tyre pressure equations and logic don't really come into it.

And narrow vs wide tyres; if we have a 33x9.5 and a 33x12.5 then both will have exactly the same contact patch, but different shapes. The 12.5 will be short and wide, the 9.5 long and narrow.

Originally posted by crump
Dont know if I'm game to ask, but what about tyre brands, any observations?

Kelly MSRs are good. And guess who distributes them....Tyrepower stores!

BFG AT KOs are bad.

Silverstone are good.

You need to look at a particular model of tyre, not just the brand. I think BFG muddies are fine but I won't touch the ATs.

The average suburban tyre and suspension retail outlet wouldn't have a clue about what the best 4WD tyres are for a given application and would just look at the manufacturer's brochures and go off that. The best place to find that out is to ask at clubs and on forums like this, or at specialist 4WD places. Of course, if you can find one that is run by Tyrepower or someone equally good, you're in business, but I find a lot of them specialise in fitting 18" alloys to Falcodores.