# Bicycle Mechanics - tire air pressure ?

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View Full Version : tire air pressure ?

Pasa09
07-31-08, 10:44 AM
Hi Evryone

I am new here . I don't understand why my bike tire needs 60 psi while my car tire needs around 32 psi ? Car tire is a lot bigger . Thanks

demoncyclist
07-31-08, 10:50 AM
volume and pressure are 2 completely different things. Someone could write a whole physics dissertation on the topic. Not me, someone else. BTW, my bike tires need 120psi.

ctwxlvr
07-31-08, 11:03 AM
you could write a book on the subject ... many have .... what it boils down to is rolling resistance, tire contact area, width, and design of the tire, and weight carried. for bikers we strive for the smallest contact area possible that still maintains needed traction and will support the weight and give us the desired comfort when riding. bikes require a higher pressure because of their tire size to keep it from bottoming out on bumps, reduce rolling resistance, and heat build up, in general terms the narrower the rim/tire combination the higher the pressure.

Bill Kapaun
07-31-08, 12:38 PM
Hi Evryone

I am new here . I don't understand why my bike tire needs 60 psi while my car tire needs around 32 psi ? Car tire is a lot bigger . Thanks

Put 32 psi in your bike tire and it'll be self explanatory.

Think of it as how many square inches of the "contact patch" is supporting how much weight.

bjoerges
07-31-08, 01:03 PM
Car tire: 4 * (4" x 6") squares of tire touching ground at 32psi: 4 * 24sq.in. = 384 sq.in of contact. 32psi * 384sq in = 12288 lbs of support.

Bike tire: 2 * (1.25" x 2") squares of tire touching ground at 60psi: 2 * 2.5sq.in. = 5sq.in of contact. 60psi * 5sq.in = 300 lbs of support.

rumrunn6
07-31-08, 01:24 PM
I once asked a similar question regarding car tire comparison, meaning why do some tires say max 35 and some say max 44, and yet others, max 50. I remember the guy said construction was part of it. A thicker and stringer sidewall can support more weight. Other things affect construction, like ride comfort and traction.

For my bike, since I'm the motor, I crank those babies way up to their MAX pressure. My car and truck tires I play with depending on where I'm driving and what I'm carrying. meaning off-road = low pressure; road trip = hi pressure, daily commuting = owners manual suggested pressure. On a final note another thing that affected my truck tire pressure was if the tire was the stock size or larger. Messing with stock specs requires compensation with the pressure.

Ok, seriously - last note. One 4x4 magazine recommended that the best pressure for ANY tire on ANY truck is do draw a line across the tire with chalk, then drive in a parking lot a little bit and see how it wore off, and adjust the pressure so that the chalk wore off evenly. Wonder if you could do that with a bike tire too ...?

Al1943
07-31-08, 02:27 PM
Ok, seriously - last note. One 4x4 magazine recommended that the best pressure for ANY tire on ANY truck is do draw a line across the tire with chalk, then drive in a parking lot a little bit and see how it wore off, and adjust the pressure so that the chalk wore off evenly. Wonder if you could do that with a bike tire too ...?

No. A car tire needs a wide footprint for traction. A bicycle tire needs a small narrow contact patch for low rolling resistance. A car leans very little. A bike leans a lot. Pump your bike tires up to the ideal pressure before each ride.

Al

rumrunn6
07-31-08, 02:48 PM
reg: "pumping before each ride" I was thinking the same thing myself. I had mounted new tubes and tires last week and did a few 44 miles rides, and my commute Monday felt slow, so I pumped the tires yesterday and they were definitely harder. I wonder just how much air bike lose and how many people are working too hard and don't even know it.

DannoXYZ
07-31-08, 03:51 PM
Car tire: 4 * (4" x 6") squares of tire touching ground at 32psi: 4 * 24sq.in. = 384 sq.in of contact. 32psi * 384sq in = 12288 lbs of support.

Bike tire: 2 * (1.25" x 2") squares of tire touching ground at 60psi: 2 * 2.5sq.in. = 5sq.in of contact. 60psi * 5sq.in = 300 lbs of support.Actually it's the other way around with the buoyancy equations. The tyre will deform and compress based upon the total load vs. pressure:

Car Tyre:
3200lb car / 4 tyres = 800lbs on each tyre (assuming 50/50 weight-dist F/R & L/R)
800lbs / 32lbs/sq.in = 25sq.in. contact-patch per tyre

4000lb car / 4 tyres = 1000lbs on each tyre
1000lbs / 32lbs/sq.in = 31.25 sq.in. contact-patch per tyre

Obviously a heavier car at the same 32psi pressure causes the tyre to compress more and have more of it touching the ground. The actual shape of the contact-patch will depend upon the width of the tyre. Narrow tyres have more square-shaped contact-patches while wider tyres have rectangular patches.

Bike tyre:
175lb rider + 25lb bike = 200lbs total load
200lbs / 2 tyres = 100lbs load per tyre

100lbs / 60lbs/sq.in. = 1.67 sq.in. contact-patch per tyre @ 60psi
100lbs / 120lbs/sq.in. = 0.833 sq.in contact-patch per tyre @ 120psi

So by pumping up the tyre to a higher-pressure, it compresses and sinks less for any given load. This prevents pinched flats where the tyre sidewall compresses completely and the rim-edge cuts through the tyre & tube (results in snake-bites pattern of holes). There's diminishing returns with higher-pressure though and you end up with lots more road-shock and discomfort on larger tyres.

Someone posted a great equation last week on finding optimum tyre-pressure ranges based upon weight and tyre-width.

rumrunn6
07-31-08, 03:52 PM
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