This was done by simply multiplying the pressure used in the 2.3in tyre by the ratio of the circumference of the 2.3in tyre over the circumference of the larger tyre. (These tyres have a relatively square tread pattern and flexible sidewall, so they need higher pressures to avoid the sidewalls collapsing.) This is why a fat bike tyre at 10psi holds the rider’s weight like a 23mm road tyre at 100psi.įor these tyres I calculated the pressure required in the 2.6in and 2.8in tyres, which would provide the same casing tension as my preferred pressures in the 2.3in tyres, which I determined to be 24psi in the front and 27psi in the rear. So, if the tyre was twice as big, you’d need half as much pressure to get the same casing tension. The tyre pressure required to provide the same casing tension in each tyre is inversely proportional to the tyre’s circumference. The bead-to-bead measurements (which we’ll call the tyre’s circumference for ease, even though the tyre forms a C-shape rather than a full circle) of the tyres on test are shown below: Bead to bead circumference vs tyre width Quoted width (inches) This relationship between pressure, circumference and casing tension is based on Laplace’s law, which is more often used to calculate the wall-tension in pressurised pipes or blood vessels. For ease, we call this measurement the tyre’s circumference Seb Stott The width of the tyre’s casing from bead to bead was measured in this way.
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