Originally Posted by
Billydog
I let the bike shop put air in my new tires..
On the box is says 120 max psi..
The bike shop guy asks me how much I weigh and only put in 90 psi..
Wh did he do that?
The tire must connect the ground with the entire weight of the rider and bike with an oval shape.
The size of the oval shape that the tires contact the ground is constant for the tire make, size, and purpose.
So, if the contact patch is measured in area, and the contact area is 1 square inch per tire (2 square inches) and you and the bike together weigh 200 lbs, then the contact patch will have a pressure of 100 pounds per square inch.
Then the tire must be inflated to 100 PSI to maintain that contact patch.
Lower PSI will cause the tire to squish extra increasing the area of the contact patch until it reaches equilibrium, so at 50PSI the contact patch will have to be twice as big 2 square inches per tire for 4 square inches). The area of contact must equal the weight it supports.
Higher PSI will have a smaller contact patch accordingly but would make a harsher ride, not much speed increase and greatly degreaded traction).