How does rider weight affect the pressure you choose? (I hope you're not going to say heavy riders must use less pressure as it adds up or something like that...
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No heavy riders need more pressure and lighter riders need less. Isn't it something like 15% widening of the tyre, i.e. 7.5% per side as you look down between rider off and on the bike? Obviously a very rough guide but you are aiming for some give in the tyre but not too much. This gives optimal grip as well as suspension effect. Obviously a simple test of applying your weight quickly to the bike while stationary to see how the tyres respond to mimic road bumps etc is effective too. Many very heavy riders exceed the stated pressure on the sidewall of the tyre slightly in order to get the right suspension effect and quality of the tyres and tubes becomes more critical for heavy riders. Although saying that some of the most expensive tyres can be the worst tyres for heavy riders if they have pursued speed and lightness in their design philosophy.
Personal preference comes into it too. A lighter rider may prefer higher pressures for less rolling resistance over comfort but heavier riders should always ensure the tyre pressure is ample so the tyres don't bottom out and get damaged. Not forgetting running tubes at higher pressure often means air leakage is higher and needs more frequent pumping. Same as those ultra thin road bike tyres that have recommendations to pump up the pressure every day where as a low pressure mountain bike tyre could be good for a week or so. So heavy guys have to get the pump out more often