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This only applies to bitumen or highway work
I often see the "4psi rule" quoted in tyre discussions on various forums, but nobody seems to realise what it actually is or how it works. So the following is my understanding of what the 4psi rule is and how it works. I’ve been told by senior tyre industry blokes that when Michelin was developing the Steel Belted Radial tyre in the 1960s they came up with the 4psi rule during tyre testing and it applies to handling, braking & tyre wear (I don’t know if this included a tube or not).
The best cold pressure for highway work with Steel Belted Radial tyres (normal tyres) is when at operating temperature (after the tyre gets warm from running down the bitumen) the tyre goes up 4 psi.
Here is an example;
So you are at 36 psi cold with your Steel Belted Radial tyres. You head down the bitumen highway for an hour or so. You pull up for fuel and check your tyre pressures while they are warm & they are now 40 psi. That means with that load & all the other conditions 36 psi is about right for tyre wear & handling; not rocket science is it!
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