3

Its quite intuitive that tires have maximum pressure figure - inflate it more and something could explode. Furthermore, it is common knowledge that under-inflated tires result in snake bites.

What puzzles me is that all tires indicate minimum pressure.

This could be somewhat justified about DH tires, which tend to be produced with thick sidewalls and thick soft thread pattern, resulting in reduced pinch flats (among other advantages). However, on different cross-country tires I have seen minimum pressures from below 2bar to 2.7bar at least. Is this just marketing or does the number mean anything?

Furthermore, how can the manufacturer know what the riding terrain and rider's style is? In my opinion those affect chosen tire pressure a lot more than sidewall thickness.

  • As others have indicated, riding underinflated causes the sidewalls to flex excessively and can very rapidly cause the tire to break down and develop cracks. That, pinch flats, and poor handling are the main hazards. Plus, of course, an underinflated tire takes more effort to ride. – Daniel R Hicks Sep 30 '15 at 12:25
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Even if the bead remains seated and no pinch-flats are suffered, an underinflated tyre will over-stress its sidewalls whenever there is a torque difference between the bead and the contact patch.

This problem would be visible as rippling in the sidewall.

This stress results in the carcass grain becoming visible through the sidewall rubber over time.

Many fatbike tyres, designed to be used below 10psi, will develop visible carcass grain over the life of the tyre. I would not expect a standard MTB tyre to experience sidewall rippling at all if maintained above a minimum pressure appropriate for the load (i.e. a higher minimum pressure for heavier, faster or rougher riders).

2

The minimum pressure is an estimate that will most probably work for the majority of people in generic riding conditions. Not going below the minimum pressure may prevent:

  • loss of control due to insufficient traction
  • pinch flats
  • burped air in case of tubeless
  • tyre unseating
  • rim bending on harsh hits
  • tyre carcass premature wear
  • rotation of the tyre/tube combo within the rim (can cause tube valve failure)

Of course, that number just an estimate. An aggressive rider, a heavy rider, or a really rough terrain may require higher than the minimum suggested values.

1

There is a minimum amount of pressure required to keep the tire bead seated on the rim. If the pressure is too low the tire could unseat especially while cornering.

  • I have noticed (and read) that rim outside diameter and tire bead diameter are subject to significant tolerances. For example, from the same rim one tire could require tire levers and significant force to remove while another could be removed just by hand. How do those tolerances correspond to minimum pressure, especially when it is not know which rim will be used? – Vorac Sep 29 '15 at 10:25
  • @Vorac That is correct the manufacturer does not know what rim you are going to use or riding terrain or rider's style. The pressure range is based on A standard rim. – paparazzo Sep 29 '15 at 12:04
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    @Frisbee - . "The pressure range is based on A standard rim" - you forgot the rest :) .... being ridden by a standard rider, riding a standard bike on a standard track at standard speed.... Its a "get out of jail" card for the manufacturer if the tire rolls off the rim. – mattnz Sep 30 '15 at 2:28

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