I went for a ride four days ago. During the ride I can feel my rear tire (Continental Gatorskin tubular) getting soft. It is a slow leak but I can feel it getting really soft a mile later, so I stop, fill it with my co2 cartridge and ride home (another 20 minutes). Arrive home, tire is still hard.

Next morning, the tire is flat, so I change it.

Well, I want to find out what caused the flat on my tire, so I mount the tire to a spare wheel, pump it to 100 psi and listen for leak or try to feel where air is coming out. No luck. I rub soapy water on tread. Still no leak. Two days later the tire is still pumped up hard as a rock.

So how can a tire go flat (twice) then stay filled like nothing ever happened?

two thoughts

one

it could be the valve
maybe it wasnt making
a proper seal once

two

co2 tends to
leak out much more quickly than
regular pumped air

You can use the old water bucket trick or what most car garages do to find leaks. For the bucket, just take a bucket of water with the tube inflated enough to have a pressure and dip it in the bucket to find and bubbles. If you see bubbles you found your leak.

Alternatively you can get a spray bottle with water and soap mixture, spray it on the tire and the leak will make bubbles again showing a leak.

Intermittant leaks do happen, but they're rare. Possible causes include a faulty valve or possibly a grain of sand at the seat, a leak at the valve/tube connection that might only leak if the valve were bent one way (these tend to get worse), a puncture caused by a fine wire, small enough to self seal if the tube moves slightly in the tire.

Since you're OK for now, you have a choice. You can ride for as long as your luck holds out, or you can replace the tube, and try to find the leak to satisfy your curiosity. If you do remove the tube, fill it to about double it's thickness and hang it up to see if it holds. If it makes it 48 hours, ist's probably OK to use.

Everything above - use the bucket/sink method, and in this case mark the leak in the tube if you find its not the valve, then try to correlate that to where it contacted the tire and look closely at the tire, inside and out. If a new tube goes out the cause is still in the tire i.e. fine wire wedged in.

Thanks for the replies.

I think I will remove the valve core, clean off and reinsert. Maybe insert some tire sealant just in case and to prevent a future small leak. I will pump it up at home on my spare wheel and if it continues to hold air I will put back on.

The tire is a tubular so I am not going to have access to the tube to find a leak.

Tubulars can be prone to valve/tube leaks. Most have the valve held to the tube via a flanged base and compression nut. Sometimes people who go crazy tightening the PV valve nut end up turning the valve slightly and loosening the flange nut inside the tire. You can test for this, or even fix it, by pinching the tire against the valve nut, and turning the valve to counter clockwise. If it moves at all that was the problem.

has the tire ever had sealant in it?

if not, if it were me, i wouldn't spend time trying to find a leak that may or may not have a cause that i didn't or couldn't fix even if i did figure it out. i would just ride it, if it would hold air, until it became more trouble than it was worth.

as it stands i have a similar tire problem. on one of my bikes the front tire deflates about 10 times quicker than the rear one. for now it just means that i have to pump a few more times to inflate the front tire before a ride. i don't plan on finding the problem ever, although by happenstance i may.