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Old 02-21-08, 01:19 PM   #1
Strelnikov
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Tires "equally" losing pressure

Hello,

My tires will equally lose pressure. I will pump them up to 80 but they will both go down to 40 after a few weeks. Is this normal for tire pressure? How often should I be looking at needing to "top off" my tire pressure?

Thanks
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Old 02-21-08, 01:38 PM   #2
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Well, barring the odd chance that you may have caused a minute puncture in both tubes equally, which I find really quite unlikely:

Are you using Schraeder valves? I would suspect that you may have troubles with your valve cores. They may be loose or stuck, either caused by a defect on purchase (are they new?), or by the pump you used to fill them being misaligned or damaged.

Thats my take. You would have to troubleshoot to figure out if it was accurate though.
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Old 02-21-08, 02:16 PM   #3
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All tires lose air. I generally top mine up before every ride.
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Old 02-21-08, 02:17 PM   #4
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Originally Posted by Strelnikov View Post
Hello,

My tires will equally lose pressure. I will pump them up to 80 but they will both go down to 40 after a few weeks. Is this normal for tire pressure? How often should I be looking at needing to "top off" my tire pressure?
Thanks
From what I have been told, and read... you need to top off your tires; as you say, everytime you ride... Most will agree that it seems to just be the nature of bike tires to lose air. My self, if I am going to be riding every day: I top mine off about every two days.(I'm lazy) But, if it is a week or more in between rides, I always top the tire pressure off...

P.S. Your car tires lose air as well, but they hold a lot more air, so it takes much longer, and you don't notice it as much until they get really low.
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Old 02-21-08, 02:25 PM   #5
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80-40PSI over a couple of weeks is par for the course.
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Old 02-21-08, 02:32 PM   #6
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Originally Posted by Bionicycle View Post
From what I have been told, and read... you need to top off your tires; as you say, everytime you ride... Most will agree that it seems to just be the nature of bike tires to lose air. My self, if I am going to be riding every day: I top mine off about every two days.(I'm lazy) But, if it is a week or more in between rides, I always top the tire pressure off...

P.S. Your car tires lose air as well, but they hold a lot more air, so it takes much longer, and you don't notice it as much until they get really low.
On the car tyres- Wife complained that the steering was heavy- Checked tyre pressures and all 4 were at 23psi. They should be at 30psi and I was the last one to pump them up to that pressure about a year ago.

Not so much the tyres that lose pressure- but the tubes. Latex tubes will lose pressure fairly quick and I have known a 120 psi in the morning be down to 100 in the evening after a long ride- and a week later to be at 80.

I Use butyl tubes and these hold pressure longer but 20 psi loss in a week is about normal. And then there is the Inflator. CO2 users have noted a big decrease in pressure the day after inflating a Flat tyre on a ride.

And then the valve- Schraeder valves are not normally classed as a high pressure valve and will lose pressure easily. What the limit is for these valves I do not know- but is worth changing to Presta if 80psi is your normal pressure.
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Old 02-21-08, 04:18 PM   #7
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Before every ride is an overkill for me, but I top up my tires once every ten days or so. Less often if it's wide MTB tires, more often if it's narrow high-pressure tires. Leaving them for a few weeks (what's "a few"? three, five, eight?) seems like a bit too long.
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Old 02-21-08, 05:39 PM   #8
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Thanks, everyone. I am using Schraeder valves, and a few weeks would be three weeks. I think I will start checking them every week. If I notice that they go down about 5psi in that first week's time, I think I will start switching to filling my tires every other day...and so on. My tires are 700x38c and I ride every day in wet/cold/dry/warm weather.
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Old 02-21-08, 05:59 PM   #9
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Originally Posted by Strelnikov View Post
Thanks, everyone. I am using Schraeder valves, and a few weeks would be three weeks. I think I will start checking them every week. If I notice that they go down about 5psi in that first week's time, I think I will start switching to filling my tires every other day...and so on. My tires are 700x38c and I ride every day in wet/cold/dry/warm weather.
It's simple diffusion of the high pressure gas (air in the tube) out into a low pressure area (the world). The rate of diffusion is defined by Fick's Law. If you start losing more pressure than your normal 5 psi, check for a leak. Some tubes are 'holelier' than others too. Although they are all made of the same material, there is some variation in the rubber. 5 psi over a week is pretty good.

Valve type shouldn't make a difference. Type of gas will make a difference. Carbon dioxide has a higher diffusion coefficient through rubber than does nitrogen. Tires filled with CO2 have been reported to experience dramatic pressure losses overnight. Helium, if you were so inclined to use it and flush enough with money to afford it, would probably diffuse out rapidly too.
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Old 02-21-08, 08:32 PM   #10
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Tubes are made of butyl. Butyl is not completely air tight material. Leakage is normal.
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Old 02-21-08, 09:46 PM   #11
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Generally a 10 Fahrenheit change in air temperature will cause your tire's inflation pressure tochange by about 1 psi (up with higher temperatures and down with lower). Bike tires have a small amount of air so the change is noiticed quickly.
For those of you that have cars with TPMS systems check the pressure on a cold morning before driving and then when you get to destination check the pressure again. You will see that when the tire warms up from driving the pressure is higher.
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