this is really strange...

I put a new tube on my bike. within 24 hours it's flat. replaced that tube with another new one. 24 hours later, flat again. Remove the tube, inflate it and submerge in water - no leaks. Inflate the tube without the tire and it holds air just fine. 4 days and counting...

my only guess is that someone is playing a joke on me

 

Valve core.

 

go over the tire thoroughly and make sure you dont have anything puncturing the tire, which in turn could puncture the tube.

 

Using a Presto valve? I've forgotten to tighten down the valve core before putting on the valve cap and it forms a nice seal that will let the air out, but you wont hear it.
Could be a buddy letting your air out though... that trick never got old at the bike shop.

 

I was a bike tech for 9 years and saw that prob. alot...check for burs on your rim, sand em' down...I assume you're using rim tape too eh? Are you using presta or schrader? I haven't hit the trails in a while...I work in the bush and that's the last place I want to be on weekends...the job has taken away from the outdoor experience and has a whole other meaning now. Still love it though. You guys have awesome trails out there...I'm jealous! Hope that helps!

 

presta valve, valve core is tight. i have turned the tire inside out. there is nothing sticking through at all.

if the tube has been punctured by something on the wheel or stuck in the tire, why does it seem to hold air fine when the tube is out of the tire???

 

I would say put in a new tube... if it continues to happen check rim/rimstip, and tire.

 

My buddy had a reoccuring flat on his Spec.Big Hit every time we went out. We were riding some DH national trails and he got a flat, sometimes a slow leak other times fast at least once throughout the day. I hardly got any and I was riding my P3 hardtail. We kept on checking rim,tire,tubes...trying different tubes (I kept telling him he needed to pick better lines lol!) and eventually he switched rims and problem solved (more or less). My other buddy had a similar problem on his Trek with Mavic 517's...always got a flat and he was a good rider, switched out rear rim and problem solved. I spoke with the OGC/Mavic rep. about these chronic flats and he said it either just "bad luck or a small defect naked to the eye"...whatever that means. As for why the tube holds air when out of the tire...mystery to me. If you got the loot, change tube, tire, rim tape...or borrow all of it off a buddy and see if it happens again with your rim....hmmmmm very sneaky.

 

I agree with the first part.

If it continues, start kicking some ass.

 

hijack -

weird, i used to get flats on my 517's all the time too! i wonder if that was a characteristic of those hoops?

end hijack

 

Its the TIRE

You most likely have something sharp lodged through the tire and penetrating enough to hit the tube.

 

Another possible place to check is where tube and valve meet. Had the same thing happen with a bad batch of tubes. One even passed the water test....until I moved the valve around and found a slow leak.

 

This happened to my BMX bike after I put a used chrome rim on it. The chrome on the inside was puncturing the tube everytime it got weight on it. All you have to do is take some electrical tape and coat the inside of the rim for a quick fix. If not take the rim off and take a dremel with a flapwheel to the inside of the rim and replace the rim strip so the spokes dont pop it.