Tyre Pressure

[Skipper]

OK, so you get a roadside puncture. Stop. Fix it. Pump the tyres up hard. Five minutes later they seem to have lost half the pressure, and you either have to stop and whack a bit more air in or carry on with sub-pressure tyres. Why do tyres lose pressure after about 10 mins? It's common practice when I service the bike indoors now to go back to the wheels later and stick more air in, even though I pumped them up to within an inch of their life the first time. Confusing?

 

[ianrauk]

Maybe because you have not fixed the puncture probably is my guess.
If it was done properly then a patched tube should stay inflated.

 

[Skipper]

The tyre doesn't go flat, it just goes down a bit, and needs a couple more pumps after 10 mins. Then it's fine.

 

[HLaB]

Your patches don't sound perfect Skipper, have you thought about carrying a spare tube and patching at home (I leave them inflated overnight as a test before I use them) no loss of psi in that case due to dodgy patching

 

[VamP]

I have never had this happen

 

[HLaB]

The tyre doesn't go flat, it just goes down a bit, and needs a couple more pumps after 10 mins. Then it's fine.

That still sounds like a dody patch, often air will force its way out at a certain psi but the hole in the tube will close a bit at a lower psi.

 

[boydj]

Are you using a track pump with as gauge?

 

[youngoldbloke]

Never happened to me either. Pump mine up to 110/120 psi, and maybe have to top them up after a week as they've dropped 5 psi or so, but thats all. Any more and I begin to suspect a slow puncture. Mysterious

 

[Globalti]

I can only achieve about 60 psi with the bike pump, I have to find a track pump to get properly hard.

Where's Fnaar?

 

[wheres_my_beard]

I had this problem a few weeks ago, and eventually found a tiny piece of flint buried deep in the puncture proof layer of my tyre, that I couldn't see from the outside of the tyre or feel on the inside. It accounted for three flats, 2 of which were within a few minutes of each other, with some squidginess (!) between flats.

 

[DWiggy]

I pump mine to 120psi and top them up half way during the week with about 10-20psi (after about 60miles)

 

[AndyxBennett]

If you are noticing a fall in tire pressure after a few minutes and then it stabilises (so therefore not punctured or leaking), it is quite possibly a phenomenon called Adiabatic Heating which occurs when the pressure of a gas is increased from work done on it by its surroundings;

As you inflate the tire you compress the gas in the pump / tyre / inner tube, this in turn increases the temperature of the gas in the pump / tyre / inner tube which then causes the gas to expand which in turn causes the pressure to increase.
(Basically - the greater the pressure increase - the greater the temperature increase - the greater the expansion of the gas - the greater the pressure increases...)

I hope that makes sense.

What will then happen; you start riding, after a few minutes the temperature of the gas in your tires cools to the ambient temperature and the pressure in the tire falls.
Don't get me wrong we are only talking a few degrees, but because of the pressures involved & the volume of the tyres it will be noticeable.

Andy

PS - Those who use CO2 inflators will notice that the CO2 cylinder cools when they fill they inflate their tyre - the reverse of the logic applies (the pressure in the cylinder falls rapidly causing the remaining gas in the cylinder to cool - this in turn causes the gas to contract thus causing the pressure in the cylinder to fall further)

 

[Mapster1989]

I have to find a track pump to get properly hard.

 

[edindave]

I've had Conti tubes lose 10-20PSI overnight. Pinpointed it to loose valves on each one in a multipack.
Just used needlenose pliers to tighten the inner part of the valves up and problem solved.