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Why are my bicycle tires always going flat?
June 20, 2011 12:17 PM   Subscribe

Why are my bicycle tires always going flat? (Air pressure gauge question within)

I used to just pump up the tire until it felt solid and ride, and I would get a lot of flats. Turns out according to the inflation recommendation on the side of the tire I wasn't putting nearly enough air in.
So now I follow the instructions and pump them up with a pump with an attached pressure dial. The amount of air in the tire seems insane, but anyway... I did this a few months ago with new tubes in preparation for the summer and haven't rode anywhere yet. The tires are now soft to the touch (is that normal?). (I did due diligence checking the rims and tires for anything that could puncture the tubes).
I pumped them back up and checked the pressure with the pump dial as well as with a few different pen gauges I had laying around and they all gave wildly different psi readings. So one concern is that I am over or under-inflating due to these POS pressure gauges.
I ride around town rarely on my bike, mostly because I know that every damn time I have to pump up my deflated tires and it is a pain. I must be doing something really wrong because this doesn't seem like rocket science to have tires that don't constantly deflate on you... Where am I going wrong?
posted by dino terror to Sports, Hobbies, & Recreation (16 answers total) 2 users marked this as a favorite
 
If you're running at pressures around 100psi (normal for a skinny tire) they will go soft after a couple of months.

Pen gauges won't give reliable readings, go with the gauge on the pump.
posted by dolface at 12:21 PM on June 20, 2011


Bike tires do lose air. If you're talking road bike tires at 100+ PSI you might be losing a few PSI every day so if it's been a few months since you pumped them up yeah, they would likely be pretty low. I wouldn't worry too much about the specific gauge readings but err on the side of more inflation. You're very unlikely to over inflate your tires to the point they'll explode if you're using a manual pump.

Personally I top off my tires before every long ride, it's usually only one pump or so and it's worth it to (practically) ensure I won't get a flat *knock on wood*
posted by ghharr at 12:23 PM on June 20, 2011 [1 favorite]


Some tubes have wonky valves and will lose air faster. Are these schrader or presta values? You might also have a slow leak somewhere. It just happens sometimes.
posted by GuyZero at 12:26 PM on June 20, 2011


What size tire do you ride? 700x25 or something similar? What's the recommended pressure?

No road tire is going to hold pressure for a few months. If I don't ride one of my bikes for, say, two weeks, I have to top off the tires before heading out. On my daily commuter, I have to pump the tires every week or week and a half. Nothing's wrong; there are just microscopic holes in the tubes that happen to be bigger than gas molecules in the air.

What type of pump with attached gauge do you have-- one that looks like this or like this? I have the latter, and I don't trust its gauge as much as my floor pumps. (Also, you lose air every time you check the pressure.) It's also a living hell to get it up to 100 psi. If you don't have a floor pump, get one.

Proper inflation is your friend. You get fewer flats, and rolling resistance is minimized. Mushy tires feel like biking through quicksand to me. Obligatory Sheldon Brown.
posted by supercres at 12:26 PM on June 20, 2011


I add air to my tires once a week. It's normal.
posted by hydrophonic at 12:29 PM on June 20, 2011 [1 favorite]


It's a mountain bike, 40-65 psi. My pump is the first type you linked, supercres.

Well, I feel reassured. Thanks for the responses!
posted by dino terror at 12:42 PM on June 20, 2011


Schrader valves.
posted by dino terror at 12:43 PM on June 20, 2011


Schrader valves tend to be more leaky than prestas but it's a bit of a crapshoot. I've had MTB tubes that go flat over the weekend and tubes that stay at high pressure for months. It could be a very slow leak, something else I've had happen. If you feel like it, change the tube. if you don't feel like it, continue to pump the tire up every few days. Also, if you're road riding you could even go to 70 PSI probably. For trails, keep the pressure lower.
posted by GuyZero at 12:49 PM on June 20, 2011


I have a couple of bikes with the same psi range. They definitely don't lose air as fast as the skinnier road bike tires, but going soft after a month or two of sitting around is definitely normal. If you pump them up to about 60-65psi, you shouldn't have to worry about it for a couple of weeks.

Generally I don't worry about checking the exact pressure - if it has been enough time since I can remember filling them (a couple weeks for the 65psi tires, five days or so for the 120psi tires), I just take a minute and top them off.
posted by mikepop at 12:52 PM on June 20, 2011


Nthing the pressure drop over time- but: every time you check the pressure, you lose air. Also when you disconnect the pump! You gotta be careful and quick- the bigger the "sput", the more pressure you've lost. This can be frustrating.
posted by drhydro at 2:45 PM on June 20, 2011


A quick check every week or so is what I do. They don't drop much so a single stroke or two is all that's necessary. I've never pinch-flatted since starting to do this.
posted by bonehead at 2:59 PM on June 20, 2011


Have you try taking out the inner tube, inflate them a little, then dip them in water, to detect leaks with bubbles?
posted by curiousZ at 4:28 PM on June 20, 2011


If you had repeated flats you might check to make sure none of the spokes have come through the rubber liner inside the rim. I do not think this would cause gradual deflation but the nub of a spoke rubbing against the tube is a guarantee for repeated flats. Check the surface of the liner when you have the tire off orif you get a flat..
posted by rmhsinc at 5:43 PM on June 20, 2011


Any pressure gauge you might find attached to a bicycle pump is, at best, a rough approximation to the true value of the pressure in the tire.

The rate a which a tube will lose air is proportional to the gauge pressure in it. High pressure road bike tubes will lose air quite quickly when at 100+ psi, say. I routinely give mine a top-up every two or three days. If I leave it a week they might have lost 20 psi. Fatter mountain bike tubes will lose pressure more slowly but will be equally flat without any air added for months. This is normal. It's worth investing in a floor-standing track pump so that a top-up only takes a few seconds and a couple of strokes.

Most air is lost by air passing through the tube material, so the type of valve is irrelevant.

Well inflated tires are the first defense against flats.
posted by normy at 8:07 PM on June 20, 2011


try to minimize the hassle of inflating the tires. get a decent floor pump if you don't have one already and keep it in a readily accessible place near the bike, not somewhere you have to dig out and make a bigger chore than necessary.
posted by 6550 at 11:53 PM on June 20, 2011


I got fed up with faffing about with awkward little pumps and wildly-inaccurate pressure gauges, despite only needing to inflate my mountain bike's tyres every few weeks, and eventually bought myself a decent track pump. Worth every penny.
posted by malevolent at 2:38 AM on June 21, 2011


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