Can't Fix A Flat Tire? :(
 

I have had another flat tire where the valve stem seems to get cut leading to rapid air loss. I feel like I'm an idiot or something. Do I have to pull the valve straight and out when I put in through the rim? That's what I suspect I'm doing wrong. Any advice?

After you put the tube in you should push the stem into the tire as far as you can, so the tube sits inside the tire, then let the valve relax out back and pump it up. It's possible that you've been leaving it pinched under the bead of the tire, which could lead to a flat.

It's also important to get the valve stem pointing straight out (toward the center of the wheel). If it's bent sideways it can tear the tube.

I would also double check the hole on the rims where the stem feed through. Make sure there is no burr or is sharp such that it cuts the rubber of the valve stem. If so, smoothe it out.

I have a set of wheels where this is prone to happen. My guess is that there's a sharp edge around the valve hole some place but I can't locate it or apparently do much about it. It's a double walled rim and I can't get a good look. So I took one of the ruined tubes and cut 2 sections out of it, - about 1.5 inches in length. In each one I poked a hole large enough for a valve stem to slide through. The hole is through both sides of the tube so I have essentially two layers.

I then took each of these sections and slid them over the valve stems on the tubes I intended to mount on the wheels. The idea is that they will protect the base of the valve stem from any sharp edges. One thing I no longer do is crank down that nut that comes with presta valves. I screw it down enough so that I can get a pump head on it, but not a ton more. Doing this has pretty much eliminated the problem.

I have a theory to the reason why we get 'stem' flats but no supporting evidence. I'd like to throw it out and see if anyone agrees. (I have never searched the topic, so I apologize if I'm repeating)

Is it possible that the stem nut which holds the stem in place while inflating, can be tightened too much? In the past, I would tighten the nut down snug with the rim, but I've since changed my mind.

Theory goes, the stem nut gets tightened on a 'fully' inflated tire (say 100psi). After a given time, the pressure drops to 60psi, but the nut is still tightened to the fully inflated tire and the reduced pressure in the tire causes the rubber around the stem to stretch and weaken.

I would like to ask those who get stem flats, do you use stem valve nut? If so, how tight is the nut to the rim?

I don't even use a stem nut. If you keep your tires properly inflated they are unnecessary.

A lesser quality tube is also more likely to have trouble with stem failure. If you get a chance, go to your LBS and have them show you the difference between a high-end tube and a cheap tube. The difference is very clear, especially at the stem.

A workaround that will sometimes work is to put a stem nut on the stem before you put it through the rim hole. There is not always enough room to do this but if there is it can solve the problem by keeping the tube from abrading on the rim hole.

The rim on my Scott CR1 cut the rubber on the stem right where it is at the radius of the tube. I used a Qtip and checked the hole and lots of cotton came off. I only had a triangle deburring tool and not a Swieval head tool at the time, so I decided to slip a 90 durometer Oring on the valve stem before slipping the valve into the hole on the rim. It lasted until I fixed the Burr after 1k miles.

I think part of my cut were also from me pulling the valve on my tire pump every time I yank it off the presta. Instead of a straight tug, I yanked it sideways and that gets the cuts at the hole of the rim.

Are you using a presta tube on a shrader rim?

Deburr the valve hole by swiping with a small piece of emery paper.
With Presta, use a valve of suitable length. Dont put a long valve into a normal box-section rim, long valves are for deep V section rims. Long valves can be levered about by pumping action.

I use the valve retaining nut to position the valve radially (towards the hub) when fitting the tyres.
I then slack off the nut and push the valve into the tyre so it doesn't obstruct the seating of the tyre bead. Once the tyre is properly seated I tighten it gently.