Help with flats at tube stem

[Bob s.]

I've been riding for many years, and have never encountered a problem like this. On a bike trip last fall we used our mountain bikes and just before the trip put on new smoother tires and new tubes, since this was all on-road. We had three people riding, and encountered like 5-6 flat tires across the three bikes where the tube popped right at the valve stem itself. The tubes by the way are presta valves. We at first thought we had bad tubes, but then realized that the last two flats were on tubes bought at local bike shops, and could no way be the same 'lot' of the tubes we bought through bike nashbar. On some of these, we tightened the metal nut on the stem pretty snug, on others we either left it off or tightened it less. All tires were hand-pumped with a pressure guage pump. The tires were hand-installed, I've changed hundreds of tires, and never encountered problems like this. Since they were across several bikes, it couldn't be a sharp rim edge or anything like that. any ideas? Please help!
--bob

 

[Vo2]

Hmm...sounds like a job for the X-files people.
Let's think. Since it happened across a range of bikes, there should be a common factor. You say you installed new tyres, new tubes on the affected bikes? I hear what you say, the fact that you have installed 100's of tyres and tubes before, but are you sure that you inflated to the correct pressure? That's really the only thing that can cause this, apart from a defunkt tube (which you say it wasn't) and apart from badly installed or defunkt rim tape (which you say it wasn't).

 

[rek]

Were you using a hand/frame pump or a track pump? I've once wrecked a valve pumping with a hand pump .. on each compression the valve was wriggling around and with all that sideways pressure on it, I bent the valve stem

 

[Bob s.]

I agree: The one common factor is me! What can I be doing wrong? I'm careful to not move the stem too much, so I doubt it's that. These were mountain bike tires, so not much force was used to mount the tires. Either way the lever would have been used only for the last little bit of tire mounting, 180 degrees away from the valve. What is causing this has to be something I'm doing, and it's got to be connected to valve treatment. Either I'm pulling it too hard to get it seated fully in the rim, or I didn't pull it out enough. If a valve were in a little too far, I'm thinking that in theory at least the reinforced rubber part would be away from the rim. That would leave the normal thickness rubber trying to hold back the full pressure at the hole in the rim. What do you guys think? I'd swear I always make sure its fully seated, but this is the only think I can think of. Thanks everyone for their input!
--Bob

 

[dirtrider]

Three things... I never use the nut on the stem (either road or mountain), I never pull or push hard on the stem to extend it out the hole (relying mostly on letting it seat naturally via air pressure), and I always finish mounting at the stem, not 180 degrees out. The latter is probably the most important technique. When you're getting the last bit of bead over the rim and the pressure is the greatest, the bead wants to drop down into the center of the rim everywhere else around the rim, especially at the opposite side. Depending on how tight the bead is, that could end up putting a lot of pressure on the side of the stem. Plus, by finishing at the stem, there is much less chance of pinching the tube because the stem tends to hold it up in place. My experience with tubes has been that some tubes are made very cheaply regardless of brand or batch and are much more prone to splitting at the stem or along the seams. I've been using Kenda Pro tubes for years, both road and mountain, and have never had one split.

 

[nelfinwoods]

Dirtrider,
I know this is October '04 and you last posted on the subject of tube stem leaks back in May '03 but, I was doing some research on the internet for help with valve stem leaks (because I just bought two new Continental Race 650 tubes (road bike) for $10 apiece and got flats in the stems on both; one after 20 miles and one immediately after installing and I was fit to be tied) and it linked me to yours and other posts in the Cycling Forums. I really liked your advice and will use it since I seemed to have been ignorant to the 3 steps you outlined. I am curious however, and have one question for you: Why do you not use the nut for the valve stem on the tube?
Thanks.
Sincerely,
nelfinwoods

Three things... I never use the nut on the stem (either road or mountain), I never pull or push hard on the stem to extend it out the hole (relying mostly on letting it seat naturally via air pressure), and I always finish mounting at the stem, not 180 degrees out. The latter is probably the most important technique. When you're getting the last bit of bead over the rim and the pressure is the greatest, the bead wants to drop down into the center of the rim everywhere else around the rim, especially at the opposite side. Depending on how tight the bead is, that could end up putting a lot of pressure on the side of the stem. Plus, by finishing at the stem, there is much less chance of pinching the tube because the stem tends to hold it up in place. My experience with tubes has been that some tubes are made very cheaply regardless of brand or batch and are much more prone to splitting at the stem or along the seams. I've been using Kenda Pro tubes for years, both road and mountain, and have never had one split.

 

[dirtrider]

By leaving the nut off, the stem is free to tilt a bit if the tube drifts (circumferentially) out of alignment, which can happen on mtb tires rather easily because they run at such low pressures that the bead can slip on the rim (but is a rare occurrence on high-pressure, clincher road tires). Allowing the stem to tilt reduces the amount the rubber tugs at the stem thereby helping to prevent the dreaded separation at the stem. The only useful purpose the nut seems to serve is to hold the stem up so one can get the pump head on it, but this is easily accomplished by holding it up with thumb pressure directly down on the tire from the outside (BTW we're talking presta valves here). So the nut just slows down the tube changing process and adds useless weight. Hope this helps. BTW, I recently had a Kenda Pro split on me. Clearly defective. Drat! Still a very low failure rate with them, so far, anyway.

 

[Fat Hack]

Iencountered , it couldn't be a sharp rim edge or anything like that. any ideas? Please help!
--bob

You may have had a batch of **** tubes, or it could've been you.

Any way, I HAVE THE ANSWER: you wrap a small strip (about 5mm wide) of sticky cloth tape quite a few times around the base of the valve stem before you put the tube in. This acts as a stopper, preventing the valve puuling throught the hole.

 

[nelfinwoods]

Spoken like true professionals. Thanks for the great advise from all. I ended up having a great ride out here in the farm country in-and-around Michigan State University yesterday with no flats! 'a windy morning and a bit brisk, however, but had great company and stopped at a nice breakfast nook midway.
nelfinwoods.

 

[gclark8]

I recently changed from MTB tyres/Schrader valves to Slicks/Pesta valves and the LBS fitted rubber gromets to the rims as the valve hole was then too large for the presta valve.