Air pressure

[johnfin1]

Is the pressure higher in a bicycle tire then a car tire because the volume/area is less in the bike tire.
P=F/A higher the area the less the pressure, right?

 

[btrueblood]

Reasonable arguement.

Same thing stated differently: the bike tire has less "stroke" or radial distance, before a bump might impact the rim, thus needs higher spring constant, therefore more pressure.

 

[rb1957]

i'd think it also has something to do with the side-wall construction ... a bicycle tire is alot less rigid than a car tire, and needs pressure to stabilise it.

also higher pressure = smaller contact area = less drag

 

[MiketheEngineer]

It's higher because you loaded it higher. That simple.

Now the reason for higher pressures are as stated above.

As a kid I can remember struggling to get to a gas station for "free" air because my tire was running flat.

 

[ivymike]

ramblings -
the pressure is not higher in all bicycle tires, only some of them. I've had car tires which asked for 45psi max inflation, and my son's bicycle asks for 25psi max.

bicycle tires are typically tubed, whereas car tires are typically not... so the car tire requires some pressure to maintain a seal...

The air pressure in the tire does affect the contact patch, but does not seem to directly carry the load from the road to the wheel - it seems from eyeballing it that perhaps the load is carried from the ground to the wheel primarily by a difference in tension in the sidewalls, and the pressure helps to maintain the shape of the tire so that the tension appears?

 

[drawoh]

johnfin1,

Bicycle tires and car tires do not do the same thing. Bicycle tires must create a minimum of rolling resistance. Road bicycle tires have pressures well above 100psi.

Car tires must make a maximally large contact with the road to minimize skidding. You need enought pressure to keep the tires from rolling off the rims and to keep the tire from bottom to the rim.

JHG

 

[johnfin1]

Load is not the answer. Look at the tire pressure of a riding lawn mower, mine is around 10psi per time(x 4 = 40, my 10 speed is 65psi x 2). What is heavier a bike or mower.

 

[zekeman]

Because, if a 200 lb person sits on top of the tire, you would need
200/psi contact area to support the load.
for 30 psi, that would be
7 in^2
So if the tire is 1 inch diameter, you would need a 7x1 in flat area, which would be close to the rim.

At the higher pressures you would get proportionately less contact area.

 

[rb1957]

john,

it's pressure*area ... your lawnmower tires would have a much larger footprint, and you'd want a low pressure tire (so you don't leave tracks in the grass), and the tire sidewalls would be stiffer than a bike's (ie some load would be reacted by the sidewalls).

your kid's bike may also have low pressure tires 'cause they're much fatter (the tires that is) and lower pressure also gives more shock absorption. a racing bike will have high pressure tires to minimise the tire area, and maybe to improve the life of the tire (higher pressure would mean less flexing of the side-wall, less fatigue)