Originally Posted by
kingston
Multiply the total weight on the wheels in pounds by 77
Divide that number by the width of the tire in mm raised to the power of 1.6
So here's an example for a 260 pound load on 1.75" tires (1.75" is about 45mm)
260*77=20,020 (total weight times 77)
45^1.6=442 (tire width in mm to the power of 1.6)
20,020/442=45 psi
thanks
so here is what that equation gave me, which my real world experience shows is much much too low
135+50 rider and load x 77=14245
50 to the power of 1.6=523
14245 / 523 = 27 psi
Ive ridden a lot with this weight of bike, rider and load, and generally run 42f and 45r, and given that I have done trips where I am riding day after day for weeks, I've had lots of time to try diff pressures, and while I might have been a bit lower than this, no way that under 30psi would have worked, so I use this as an example to explain why I dont agree with this method
I redid it adding 35lbs of bike, and putting 60lbs load instead of 50=230x77=17710/523=34psi, which is still too low and I know even this would feel wallowy and slow and unsafe in cornering on my bike (squirmmy).
re judging tire pressure, I really do think its good to try diff pressures, starting high, riding over diff surfaces, then lowering pressures by 5 or 10psi at a time and evaluating how the bike feels over the same section of road, ideally with flat asphalt, some bumps and rough stuff too.
A good trial and error way to get a feel for at what point too low a pressure starts to feel wallowy, etc., and conversely, what pressures feel too harsh for no reason.
thanks again for the explanation