Tyre Pressure Dropping

magibob

Cycling on a Decathlon Hybrid. Had it for 4 years.

The tyres are Michelins, 700x35. Says 2.5 to 5 bar on the tyres.

I'm a hefty guy, so I have them at the max.

The rear tyre has always dropped a bar or so in a week in pressure, the front one, hardly any drop.

Recently the back one went really soft. Couldn't find a leak, so assumed it was just age and it had gone porous.

I bought a new inner tube and new tyre, and it is doing exactly the same thing, dropping a bar or so a week.

So, I'm guessing that all tyres drop pressure, and the rear one drops most as it does the most work, but is this right or .... Well Or what? Oh, the wheel was changed a year or so ago, and couldn't find anything sticking out that could have been causing tiny leaks so don't think it's that.

Cheers

Andy

kajjal

I weigh about 100kg and pump my tyres up every few rides but not by that much.

Note where the inner tube lines up in the tyre and take it out. Pump up the inner tube a fair bit and put you ear next to it. You should be able to hear it deflating. Once you locate the leak(s) check the shape or type. Then check the tyre in the same place by eye and by running your fingers on the inside. Make sure you bend the tyre a bit when you check it. Also check rim tape and the rim for any damage / rough edges etc.

magibob

Hi.

Thanks for the reply, but, if we assume that, as the tyre, the wheel and the inner-tube have all been changed,The tube and tyre at the same time, and apart from once, it behaved exactly the same way, dropping from 5 bar to 4 bar in 50 miles, and over a week's time, I don't think it's a puncture.

So back to my original question, is this normal for a tyre to drop pressure over time, and the rear one to drop faster as it does more work in both load bearing and driving the bike forward?

cheers

Andy

mr_evil

Rubber is slightly permeable, so it's normal to lose some pressure. Smaller tyres, higher starting pressures, and thinner inner tubes will all cause faster pressure loss. Since you have two of the same tyre and imply that they are pumped up to the same pressure, maybe you have an extra-thick inner tube on the front?

leeefm

Are you using a latex inner tube on the rear? Compared to butyl tubes, latex is very porous, so worth checking.

mpdouglas

What leeefm says.

Latex leaks like a sieve. The tubular tyres on my vintage bike have latex inner tubes and they can go from 160psi to 40 psi in a week.

But I'd be surprised if Decathlon are fitting latex tubes to a hybrid. They're really only for extreme weight saving because of the downside of the rate at which they depressurise.

Inflate your tube off the bike and put it in the bath to see if it's leaking from anywhere. It could be a sharp edge on the end of a spoke that's pushing through the rim tape and making a small pin prick in the tube.