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Old 09-16-05, 09:52 PM   #1
CycleCat
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Strange question regarding tire inflation

I have a bizarre question. I am new to the world of cycling, and have a road bike whose tires require 100 psi. I was talking to a friend of mine who is very (auto) mechanically minded who was in disbelief that a bike tire could even take 100 psi without blowing. His reasoning was that a car tire filled that much would have exploded. Could someone enlighten me as to what allows a bike tire to be inflated with so much air pressure? Thanks!
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Old 09-17-05, 02:31 AM   #2
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There's much less surface area inside a bike tyre than an auto-tyre, so the 100psi accumulates much less total force on the casing than an auto-tyre. Show him the label, show him the pressure-gauge on a pump as you pump it up to 100psi, then pump it up to 125psi to scare him! Some of my track-tyres run at 200psi.
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Old 09-17-05, 03:50 AM   #3
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The Max pressure is very conservative. You can put in a lot more. When proof testing worn-rims for failure, the recomendation is to put 50% above the max pressure . If the rim buckles or splits then its worn too thin by the brake blocks. If the rim holds up, then it is safe (then you reduce the pressure back to normal levels before riding).
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Old 09-17-05, 06:41 AM   #4
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Volume is key. The larger the tire, the less pressure needed. I believe that large tires on large road or construction equipment require less than 10 psi.
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Old 09-17-05, 06:44 AM   #5
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Quote:
Originally Posted by MichaelW
...if the rim buckles or splits then its worn too thin by the brake blocks. If the rim holds up, then it is safe (then you reduce the pressure back to normal levels before riding).
How long does *that* take to happen? I never heard of rims being worn so much by pads that they need replacing.
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Old 09-17-05, 08:05 AM   #6
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Originally Posted by MichaelW
The Max pressure is very conservative.
I used to think that but then I had a couple of friends have tires blow off of newish bikes so I'm a lot more conservative than I used to be.
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Old 09-17-05, 08:10 AM   #7
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Originally Posted by Nicodemus
How long does *that* take to happen? I never heard of rims being worn so much by pads that they need replacing.
I think that it's a lot more common on mountain bikes than road bikes. My subjective rule of thumb is that when the brakeing surface feels noticeably concave, it's time for a new rim. Actually, lots of new rims have wear indicators built into them. One style looks like a groove around the circumference of the rim, another looks like somebody took a spoon and dug a divot out of the inside of the rim extrusion.
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Old 09-17-05, 08:31 AM   #8
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It's called PSI, which stands for "Pounds per Square Inch"

There's a lot more "SI" inside a car tire. Bike tires need to be a lot higher pressure to support the rider because the contact patch is so small compared to a car. If he doesn't believe it, just show him the specs for a vedestein tire.
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Old 09-17-05, 10:30 AM   #9
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Yes, the bike tire can take more pressure simply because it's a smaller diameter.

I know, tires are fabric, and a tire is a torus, but for simplicity here's the equation for the hoop stress in a thin-walled cylinder:

Stress = (Pressure * Diameter)/(2* wall thickness)

(in English units, this is [psi] = [psi] * [in] / [in])

You can vary the pressure up and the diameter down, keeping the wall thickness constant and you can have the same wall stress.

If the wall stress gets to high, you will have failure.

Are you asleep yet?

DanoXYZ also has a good point about contact area. For example, presume the contact patch between the road and the tires on a bike is about one square inch (written 1 in^2). To hold up a, say 200 lbf rider on 2 in^2 of contact patch, you would need a pressure of:

Force / Area = Pressure

200 lbf / 2 in^2 = 100 lbf/in^2, or 100 psi.

Amazing, huh?

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Old 09-17-05, 02:27 PM   #10
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[QUOTE=eubi]Yes, the bike tire can take more pressure simply because it's a smaller diameter.

Umm...actually, bike tires usually have a larger diameter (as measured outer egde to outer edge, straight through the axle)...but they are much narrower, and not as far from the rim to the outer edge.
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Old 09-17-05, 02:40 PM   #11
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That would be the diameter of the wheel, and the "narrower" part would be the diameter of the tubular air chamber. Both numbers could be referred to as "the diameter of the tire," but it was pretty clear what he was talking about.
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Old 09-17-05, 06:09 PM   #12
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Quote:
Originally Posted by CummingsSM
That would be the diameter of the wheel, and the "narrower" part would be the diameter of the tubular air chamber. Both numbers could be referred to as "the diameter of the tire," but it was pretty clear what he was talking about.
Thanks for the clarification CummingsSM. I perform this calculation often and didn't think of how it could be misconstrued.

The formula addresses the diameter of a straight thin-walled tube. In this case, the narrow diameter across the tire.

We'll talk about stress in a curved thin-walled pressure vessel next week!
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