Help! Why does my front tire keep losing air?
 

So I had been having trouble with my front tube, which had slime, was going flat every couple of days. I assumed it had taken on one too many goatheads and was time to just be replaced.

So I replaced it with a regular tube, and all seemed fine for about a week, but then I noticed it didnt have as much air as it should. It wasnt flat, but it certainly wasnt full. My rear tire is still fine.

I aired it up last weekend, and then yesterday (two days later) it was low again. Not flat, but low.

I only commute 3.5 miles each way, so after only 14 miles it was very low. A section of my commute is steep uphill, and I know I put a lot of pressure on the front when Im climbing, but I dont think it would cause me to loose so much air.

So whats causing the fairly quick loss of air pressure if Im not actually getting flats?!

Have you felt around the inside of the tire (carefully!) to make sure there isn't something stuck in it? Some pressure loss over time is normal, though.

I would also check your rim tape. My GF bike had a similiar problem that went away once I put new rim tape on.

+1 on the previous comments - make sure there is nothing stuck in the tire and poking through that may have repunctured the tire (thorns are a popular culprit around here). Also make sure there is nothing on the rim that may cause the flat, like a burr, spoke poking through the nipple head and rim tape, rim tape not covering all nipples, etc. If it lost 10 PSI or so, its probably natural air loss through the porous rubber, meaning probably no puncture. If you are losing a lot of air in a short period of time, like 1/2 the air pressure over 2 days, it could definitely be a small puncture with a slow leak.

A good trick for finding exactly what caused the flat is to take the wheel off the bike, and with a piece of chalk mark 1) the sidewall of the tire at the valve stem, 2) the sidwall of the rim at the valve stem, 3) the valve stem on the same side that you marked the rim and tire. Then take the tire off, take out the tube, and pump up the tube. If the tube is flat, air will be leaking out somewhere, feel for or hear for the leak. Once you find it, line the tube up with the rim and tire using the chalk markings and inspect the rim and tire in that area for something that could have caused the puncture.

One other t hing to check is the valve stem core -- could be loose. I've had schrader valves where the stem core is just a little loose and they slowly lose air. Not sure if that can occur with prestas - I've never had it happen.

Thorn Resistant inner tubes are 3x as thick , so have more rubber to weep air thru,
so may be a good substitute.
made in 700c 32 with a presta valve, [as well as many/several other sizes with Schrader valves]
I used a pair on a bike tour from southern Ireland to Northern Scotland,
and never had a puncture.

Like others have mentioned check the tire and check the rim. What is the loss in PSI, because as others have mentioned some loss is normal. Also, did you replace it with a new tube, or a tube you had lying around you were not using? Is the valve set properly as well as the tire?

Rim damage can be hard to find. Run your finger around the inside of the rim, if there's even a small indentation, you could be cutting into the tube. If this is the case, I'd just replace the rim, it's hard to pound these dents out.

If there's a small chip, it might be sharp enough to poke the tube, just sandpaper it flush. I've ran into both problems plenty of times and these fixes usually take care of it.

+1 on what others have said, and one more possibility.

Once a bunch of years ago, I had a small slit all the way through a tire. Caused by a piece of glass, I think. I patched the tube but when ridden inflated, the tube more or less got a pinch flat from that slit. I patched the inside of the tire itself and that stopped the subsequent flats.

Was just going to suggest this... I had a presta fail about a month ago like this. The failure however was a bit more obvious, I just had not been in a quiet place to notice the low hiss coming from the valve. I have no idea what exactly failed in the valve; but I know when it was under pressure, in the tire, it slowly leaked. But out of the tire, when it wasn't "compressed" it showed no signs of leaking (even passed the water tank test).

I didn't troubleshoot it any more, just slapped a new one in and went on.

1. Take the tube out
2. Inflate
3. Put in bowl of water
4. Look for air bubbles
5. Check relevant part of the wheel/tyre: if it's one the rim side, you probably need new rim tape (don't use the plastic stuff, use the sticky sided textile-type);if it's elsewhere, you have, as others have said, something (glass/metal/goatheads) still in the tyre. Remove
6. Repair puncture

thanks for the help guys.
when i replaced the tube i checked the tire for any sharp things still stuck in there, but not the rim. so i guess will check the rim tape and then the tube with some soapwater.

I was mostly just confused as to why it was a new tube wasnt holding air for very long. it would have been one thing if it was going flat instantly, but it would take a few days of riding it before it would get super low. even after filling it up last night and riding it today it wasnt as "firm" as it was last night, but its still rideable.

Yes, it can. I was losing ~10lbs/day from a new presta tube. I finally tightened up the removable core and haven't had any problem since. Well, I completely unscrewed it once while unscrewing my Lezyne hose connector and it shot across the floor, but that time I tightened it up better.

newer presta valves almost all have removable valves. My LBS has been tightening all the valve stems when someone buys a tube. I'm pretty sure I had a loose valve, but I haven't bothered to check the tube, I just replaced it.

It's not uncommon to develop slow leaks in the tube itself over time for various reasons.

+1. If I were having the OP's problem, I would do something along these lines.

Been there. Did something like that. Those little removable valve cores can be a bee-otch to find!

seems to me that about 5% of tubes are slow leakers right out of the box, whether from a loose valve core or leak in a seam. Check the tube first, that's the thing that's leaking. AFTER you determine where the leak is on the tube THEN you go look on the rim for the corresponding offense, not the rim or tire first.