Tire Pressure?

[Steampig]

Been out 3 times in the last 4 days but cant seem to keep the pressure in the back tire.
All three times the back has been pumped to 120 but when i get back after an hour/hour n half its bearly 80?
i pumped the tire up last night and checked today before i went out and it was still up at 120. an hour later its back to 80?!? is this normal?
i can feel it going down within the first 20 mins of the ride.
Now im heavy at 15st 4 so im guessing this has something to do with it cos im mostly sat over the back wheel.
Could it be a dodgy tube, dodgy valve or just a big fat rider???
Any thoughts?

 

[tyred]

Your weight shouldn't have anything to do with it. Look at the valve maybe. Schrader or Presta? If you really want to be sure, take the tube out and pump it up and place in a bucket of water and watch for bubbles.

 

[black'n'yellow]

you have a slow puncture - easiest thing would be to change the tube..

 

[Boris Bajic]

If it's going from 120 to 80 in under an hour it isn't the rider's weight and it's unlikely to be a slow puncture.

The valve sounds like the culprit to me.

Are you tightening the thread on the presta (I'm assuming it is one) after inflating?

At 120 psi, a puncture is normally fairly sudden and catastrophic. I've never known a tyre that tight to go down 40 psi gently over an hour.

Tubes do lose pressure over a week or so, but not that much.

Only investigation will tell you, but my (virtual) money is on the valve being U/S or not screwed back up after inflation.

I hope this helps.

 

[black'n'yellow]

presta valves are self-sealing once pressurised, so it won't be that he hasn't re-tightened the thread - that's only to stop it being physically deflated from the outside.

My money is still on a slow puncture but for the sake of a price of a tube, it's not worth worrying about - just change it.

 

[Steampig]

If it's going from 120 to 80 in under an hour it isn't the rider's weight and it's unlikely to be a slow puncture.

The valve sounds like the culprit to me.

Are you tightening the thread on the presta (I'm assuming it is one) after inflating?

At 120 psi, a puncture is normally fairly sudden and catastrophic. I've never known a tyre that tight to go down 40 psi gently over an hour.

Tubes do lose pressure over a week or so, but not that much.

Only investigation will tell you, but my (virtual) money is on the valve being U/S or not screwed back up after inflation.

I hope this helps.

Yea its presta.
Im tigtning the little valve back down after ive pumped it up?
I have noticed i have to put the pump head further down the valve to get it to inflate? so im guessing its maybe a problem with the valve.
Guess its time for a new tube!

 

[HLaB]

You have a problem there Steampig, as other suggest a faulty valve or slow p'ture. I pump my patched tubes up to 90-105psi and after a week my pressure is only down by about 10psi.

 

[youngoldbloke]

Maybe you have a presta valve with a removable core? If you have you will see flats on the threaded bit that the dust cap screws onto. If so see that it is tight - they can work loose.

 

[Garz]

I have had out of a box of 20 tubes a couple that had valve leaks below the screw-tip where you can replace the valve unit. Remove the tube, inflate and then check in a tub of water for the leak.

 

[TonyEnjoyD]

I have had out of a box of 20 tubes a couple that had valve leaks below the screw-tip where you can replace the valve unit. Remove the tube, inflate and then check in a tub of water for the leak.

As it seems to get down to 80psi and stay there it would seem the valve is able to cope at that pressure and will no longer leak.
Personally, I wouldn't attempt to get it to that pressure outside of the tyre!
Apart from it would need to probably inflate to the size of a tractor rear wheel, it it found a weak spot it'd probably blow and injure you.

Try a new tube and bin the iffy one.

 

[Ticktockmy]

I would pump the tube up and check the Valve (wobble it around under water)where it joins the Tube, because i had a similar problem, and I found that when the valve was moved slightly as it might when in use under braking etc, there was the faintest stream of air bubbles from the joint

 

[Scilly Suffolk]

There is clearly a fault or damage: replace it.

 

[AncientWyvern]

I'd say the same as others - change the tube.

I might also try simply re-fitting the tube, i doubt you'll find the real cause of the problem due to the pressures you're refering too.