Jul 28 2010
I was about to set out to work and upon inspecting the bike I spotted a flat! Both cars were out of action having work done so I set about fixing the issue.
I removed the inner tube which was still the OEM tyre provided by Cube which was an iMPAC 26” 40/57-559 to hunt for the problem. I had suspected the valve to be the problem after initial inspection and indeed it was. See the images! The valve had become detached from the tube – something that I had not come across in 20 year! Below is a picture of the problem.
Fortunately I had a spare Henda Tube that I had been carrying routinely so was able to fit that to get me to work.
I had a quick search on the internet and I think that the problem could have been caused by tyre slip or valve tear off. This info Schwalbe website here. Hopefully changing inner tube manufacturer will help to prevent the problem in future. I guess only another 600 miles or so will tell.
What causes valve tear off?
A valve can be torn off if it has been installed under tension.
Another frequent cause is installing a Sclaverand valve tube into a rim with a larger valve hole. The metal edge of the valve hole can shear the valve stem off the tube.
Caution: Be aware that there are also rims that have the correct valve hole of 6.5 mm on the outside, but a larger hole of 8.5 mm on the inside, which causes the problem.
A nut that is excessively tightened just exacerbates the problem of tear off. The major role of the rim nut is to lock the valve in place at the time of inflation.
Torn off valves are often seen on mountain bikes. Continual improvement in brake performance and low inflation pressures often cause the tire to slip on the rim. The tire movement then drags the tube and this can lead to the valve shearing off.
What can be done to prevent tire slip or valve tear off?
A higher inflation pressure considerably reduces the tire moving. Of course a higher tire pressure is not always desired.
We implement Limited Slip Technology (LST) in Schwalbe MTB-folding and balloon tires. The tire bead is coated with a special rubber layer that produces a dramatic reduction in tire/rim slip.
The Schwalbe Downhill tube has an highly reinforced valve foot. Theoretically, the use of talcum powder is also helpful. This can reduce the friction between tire and tube so that the tire no longer moves the tube along with it. However in practice, if talcum powder gets between tire and rim, it will increase the problem.
Some rims have such slippery surfaces that even LST does not suffice one hundred percent. It will help if the rim contact area with the tire is lightly abbraided with sandpaper (180 grade). To considerably increase the friction between tire and rim it is sufficient just to remove the very smooth surface coating (Eloxal, varnish etc.).
The problem is very much reduced with disc brakes because the rims do not get hot through braking.
There is no valve tear off with tubeless tires. The tire can move without causing problems.