Originally Posted by Psimet2001
Formulas:
Only the intercepts change for tires between 20-28 widths. Larger widths and the slopes change. Obviously these are generalized and assumed to be linear. While not 100% correct they will give you a great place to start.
Tire Width=20: Pressure(psi) = 0.33 * Rider Weight in lbs + 63.33
Tire Width=23: Pressure(psi) = 0.33 * Rider Weight in lbs + 53.33
Tire Width=25: Pressure(psi) = 0.33 * Rider Weight in lbs + 43.33
Tire Width=28: Pressure(psi) = 0.33 * Rider Weight in lbs + 33.33
Tire Width=32: Pressure(psi) = 0.17 * Rider Weight in lbs + 41.67
Tire Width=37: Pressure(psi) = 0.17 * Rider Weight in lbs + 26.67
Example: You are 150lbs running 28's
Pressure (psi) = (0.33*150) +33.33 = 82.83psi (rear)
Front Pressure = .9*Front Pressure = .9*82.83psi = 74.55psi front
You could start by running 85 rear 75 front and adjust from there. For your weight I would step down to 23's which you could run at 92F - 102R. That would be nice and comfortable and have less rolling resistance than the 28's.
Wow, that seems low. The tires say 120 max.
I'm running 28's cause they came with my bike (Trek Pilot 1.2). I have considered going down to 25s. Not sure if 23s will fit on my rims.
I assume that the upshot of all this is that the lower PSIs (as adjusted for weight) give more comfort without sacrificing performance or durability. Do I have this right? I'm a little confused about the comments that lower PSI helps protect against flats (crossing fingers: I've never had one going back to 74 when I got my Stella 10 speed; which is still a pretty functional bike, by the way). At least on car tires, they say higher PSI gives better durability. Is this apples & oranges? Thanks for the info.
Dan