Cat3

Tire Pressure Isn't What You Think – 29er And 650 Wheels In Stock.

In cycling, there are a number of things that don't seem to make sense.  Spoke tension is one of them.  With stainless steel spokes, wheel stiffness isn't dependent on spoke tension.  A very high tension spoke is equal in stiffness to one with only a minor amount of tension.  It's the nature of the steel that determines tension.

Tire pressure is another one.  What we really care about is casing tension – how "hard” is the tire.  We all know cars have relatively low tire pressures (25-30 psi) but they support a car.  Most people don't make the mental leap to understand how that works on their bicycle wheels, and understandably so.  It's never mentioned that casing tension is important, nor any mention of what it is or how it's measured.  So we use the saying "Hard enough so you don't get pinch flats.”  There is nothing wrong with that except it doesn't give you an understanding of what is going on.

The new trend toward wider rims and bigger tires has brought this to the forefront.  The wider rim gives more air volume and therefore affects tire pressure.  The key component to this understanding is that casing tension is very much affected by air volume.  Here's a great link to the detail.

In the chart below, the casing tension is equal. So, if you were running a 23 mm tire and switched to a 25, you would go from 104 psi to 96 and still get the same casing tension.  Since there is more air volume, you can go down even more in pressure without getting pinch flats. Lower pressures give you more rubber on the road – better traction, a more comfortable ride, and better performance – the sidewalls flex over bumps. So if you put a bigger tire on make sure to reduce the tire pressure.

Tire Size	PSI
20	120
21	114
22	109
23	104
24	100
25	96
26	92
27	89
28	86
29	83
30	80