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Old 05-13-09, 06:48 AM   #1
timinator
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Tire pressure question

I've gotten into the habit of putting air in my tires before each ride, usually around 100 psi. I have a Fuji Roubaix 08. Is this normal or should the pressure last a few weeks or more.

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Old 05-13-09, 07:19 AM   #2
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It's never a bad idea to top up pressure before every ride when using high pressure road bike tires. Their thin tubes tend to lose pressure much quicker, and they have a narrower range of pressure within which they can operate efficiently. That being said, pressure doesn't have to be an absolutely exact number. You usually can go a few days without topping up. I'm thinking of 2 or 3 days. But it depends. You sort of have to see for yourself how long your tire and tube combination loses pressure.
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Old 05-13-09, 07:28 AM   #3
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I check it every day. I have a compressor and an air chuck with built in dial gage in my garage so that makes it easy.
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Old 05-13-09, 08:05 AM   #4
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Thanks for the info.
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Old 05-13-09, 08:41 AM   #5
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I've gotten into the habit of putting air in my tires before each ride, usually around 100 psi. I have a Fuji Roubaix 08. Is this normal or should the pressure last a few weeks or more.

Thanks
It's normal. It's even got a scientific law named after it. Called Fick's law. In layman's terms, think of the molecules of air sitting in the tube all bunched up on top of a hill. They don't like being at the top of the hill and would rather fly down to join the rest of the molecules at the bottom. But there's a gate-the tube-keeping them from flying down that hill. But the gate is a little rickety and while it does a good job of keeping most of them at the top of the hill, some get past. Wheeeeee! And off they go! Over time enough of them will get past the gate so that the top if the hill isn't so full anymore. You just have to keep collecting the molecules and put them back, i.e. pump up the tire.

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Old 05-13-09, 09:16 AM   #6
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If you search the forum for "tire pressure" you'll get plenty of comments. But my summary of it all is that it depends on your weight and on how comfortable you want the ride. If you ride a road bike, the tires will be thinner than a hybrid bike. That difference calls for different pressures too.

One thing I noticed. Every time I fill up the tires, when I disengage the pump from the valve, I hear a quick air release "squish". That means some air escaped and the "100 psi" really isn't 100 psi.
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Old 05-13-09, 10:04 AM   #7
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One thing I noticed. Every time I fill up the tires, when I disengage the pump from the valve, I hear a quick air release "squish". That means some air escaped and the "100 psi" really isn't 100 psi.
That would depend on the valve. A presta valve uses the internal pressure to hold the valve closed. Any air you hear would be from the air in the line from the pump's check valve to the valve chuck. In other words, no air loss.

Schrader valves have to have the valve depressed to function. Once you flip the thumb lock, you pull the depressor back and the valve seals. There's the possibility of losing a little air when that happens but I doubt that it would be much. The rest of the whoosh of air would be depressurizing the hose just like the presta valve.
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Old 05-13-09, 10:21 AM   #8
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Smaller tires leak faster. Higher pressure leads to faster leakage, too. Mountain bike tires can hold their pressure for weeks. Thin road bike tires can only maintain proper pressure for a day or two.
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Old 05-13-09, 11:40 AM   #9
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Smaller tires leak faster. Higher pressure leads to faster leakage, too. Mountain bike tires can hold their pressure for weeks. Thin road bike tires can only maintain proper pressure for a day or two.
Sorry but it's all driven by pressure. Mountain bike tires are run at low pressure which means less energy driving the diffusion of the molecules. Higher pressure tires means more energy driving the diffusion.
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Old 05-13-09, 11:50 AM   #10
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Thanks for that assertion. I'll go home and inflate two tires of different size to the same pressure and see if they leak at the same rate.
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Old 05-14-09, 08:21 AM   #11
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Also depends on what inner tubes one uses, some are more prone to leaking down than others. I currently have some latex tubes made by Michelin and they loose pressure overnight to the point where they need to be filled daily.
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Old 05-14-09, 08:42 AM   #12
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Sorry but it's all driven by pressure. Mountain bike tires are run at low pressure which means less energy driving the diffusion of the molecules. Higher pressure tires means more energy driving the diffusion.
But smaller tires have less to leak, so the same amount of loss would result in greater change in pressure. All other things being equal, the loss rate would be proportional to the tire section size, but the volume is proportional to the square of the section size...a tire half the size would lose air half as fast, but has only a quarter of the air to start with, so pressure would drop twice as fast as with the bigger tire. Of course, as I said, "All other things being equal." But they never are...loss is also proportional to pressure, and if the bigger tire uses half the pressure, it'll lose air about half as fast.
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Old 05-14-09, 08:58 AM   #13
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100 psi ? every 2 or 3 days is normal. tubulars...even more often...every ride...hell at the track and at 180 you gotta pump them every time.
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