Originally Posted by
commuterbiker
I agree that it's important to keep air in my tires. However, I would say that loosing 30 - 40 psi over a period of 20 miles indicates some other problem. How do folks that take long rides handle it? I mean do folks really get off every hour (assuming 20 mph) because they're down to 20 psi? ...Thanks!
As with the last reply by DCB0, I pump up every few days on 28/32C tires. Maybe once a week on my 2 inch wide street tires. It's not a function of mileage, but time. Air slowly diffuses through rubber tubes. About twice as fast with latex than with rubber. So yeah, you might need to re-inflate with latex every other day. But normal butyl rubber (the lower cost tubes) hold air pretty well. And I'm talking about losing about 10 - 15% pressure over 3 days (e.g. 90 psi down to 80 psi).
It definitely sounds like you have a slow leak.
You need to do the tube dunk test. Remove your wheel and dismount the tire and extract the tube. Note the orientation of the valve and the tire on the rim. Pump the tube up so it's quite bloated. Put the tube in water. Find the leak. If it's on a place that is NOT the valve, then note the location, and then feel the inside wall of the tire for tiny slivers of sharp metal or glass or thorns. Often, if you have a very thin metal wire (e.g. a strand of brake cable wire) it will not put a big hole into the tube and partially plugs the hole when the tube is pressed up against it, and so the leak will be very slow.
If it's the valve, then you should see it as well. You can take some spit or soapy water and cover the valve and see if bubbles start to rise. that means you need to tighten the valve stem inside (they make a cheap tool to do that).