BROOKLINEBIKER

Hi,
I got a strange flat tire a couple days ago. I ride a bike with Schwalbe Marathon tires sized 700x32 and use Specialized tire tubes 700X20-28. The cause of the flat was that the valve stem had torn away a bit from the tube. I kept the tube well-inflated so it was solid. What would cause such a flat?

IslandTimePE

Could just be a manufacturer defect...if it only happens once I generally chalk it up to that. If it happens again then I start looking at my wheels and tires.

However, I notice you are running 20-28 tubes in 32c tires....may want to move up to a tube with a bit more volume.

50PlusCycling

Could be the valve stem wasn't centered in hole, or that the hole in the rim liner was also not centered, or that the tube was defective. The latter is the least likely scenario, but still happens sometimes.

TimothyH

The jam nut is too tight.

Overtightening the jam nut pulls the valve and tube up through the hole in the rim and rips it.

The jam nut is not needed. Just leave it off.


-Tim-

DrIsotope

I had this (sort of) happen to a tube in my front tire, back in the gory days of riding Gator Hardshells. Though mine didn't tear-- the threaded stem actually developed a crack in it, right where the stem passed through the hole in the rim-- because I hadn't gotten a flat, or done anything to that front tire other thad add air, in over 4,000 miles. Stress fracture. Every piece of metal has a fatigue limit. I found that limit on cheap Diamondback tube. Now I don't use tubes at all, so problem solved.

79pmooney

I"m going to guess that using an undersized tube was at least part of the problem. A tube that will be fine for its designated tire sizes might not tolerate the additional stretch at the valve. (Now you could try different manufacturers' tubes to see if there are better tubes and lesser tubes when used with over-sized tires. Share the results if you do.

Ben

veganbikes

Undersized tubes can be a cause of it, also poor pumping will cause it as well especially on Schrader valves. Typically on presta when people are wiggling and jiggling the pump head off they bend the valve but with Schrader they can tear more often. It could be a defect but I will rarely go to there unless it is on the seam and quite new.

Gresp15C

I've had Presta tubes fail at the base of the stem. After a string of two or three failures, I realized that I was stressing the stem when I was flipping the lever on my rather cantankerous pump head. I've been meaning to find a better head to put on that pump, or dedicate a pump to Presta use. Meanwhile, I've just been really careful when inflating Presta tubes, and have had no more problems.

TimothyH

I'm telling you guys.

Jam nut.

Get rid of it.

Retro Grouch 

I think that's the most likely answer too.

I once had a customer come into the shop demanding a replacement for a defective inner tube. After we replaced his tube he pulled out a pliers to tighten the jam nut on his wheel. Both me and my mechanic explained to him that what he was doing was wrong. After he left the shop, I watched him pull out his pliers to tighten the nut.

FWIW, everything has a failure rate so I assume faulty bicycle inner tubes do exist. In all my experience, however, (and I'm old) I have never seen a flat that I could attribute to a defective inner tube. And before you even say it, bicycle inner tubes don't have seams. What looks like a seam is actually the molds parting line. Defective inner tubes, if they exist, are rare.

tagaproject6

This. I see it happen all the time.

indyfabz

What was is that Forest Gump's momma said about stupid?

Retro Grouch 

That's the bike shop business. There is a body of customers who assume they know more than you about what you do all day, every day. Some of them can even be right now and then.

fietsbob

Riding under inflated will do it ,
Typically rear tire slips around the rim, pulling the inner tube , sticking to the tire casing, with it,

And since the hole in the rim will not move , a shearing force is applied to the bonding of the rubber to the stem.

pdlamb

I don't buy the jam nut theory (unless the user tightens it with pliers. WTH?). I've had a few stem failures with, and a lot more without, the nut. The nuts are pretty handy for certain pump heads that need to be pressed firmly onto the stem, and finger-tight nuts always (in my experience) end up proud of the rim after inflation.

More probable causes include:
Manufacturing defect
Rim defect or damage (sharp edge)
Pump head malfunction, doesn't release stem when you try to pull it off
User error (try to pull off a locked pump head)

TimothyH

An overtight jam nut may not be the cause of this particular failure but tearing the tube by overtightening the jam nut actually happens. It is not theoretical.

In 45 years of riding, the only stem/tube interface failures I've had ever had were when I was using and overtightening a jam nut. I've had zero since omitting the jam nut and have never had a problem with pump heads.

A good friend who taught me mechanical skills, how to use tools and how to troubleshoot, once said, "Always do the easy and cheap thing first."

Jam nuts cause more problems than they solve. Leave it off an see what happens.

tagaproject6

I only use the jam nut to keep the valve from sinking while I pump the tube with air after replacement.

billyymc

I agree with you. I leave the nut on. It holds the stem in place while inflating, especially with small pumps while fixing a flat on the road. It's easy to not overtighten it.

PdalPowr

i was wondering about what lookes like seams on inner tubes.
Thanks for clearing that up.

WNCGoater

Put me in the "leave nut on" crowd. I personally feel they help stabilize the stem when fiddling around with the pump head, trying to lock it on. That especially when first inflating a newly installed or otherwise un-inflated tire. Having said that, I make it just snug with finger pressure and oftentimes find they have loosened off the rim. I suspect most could get by just fine without them but I don't buy that using them will "cause" problems UNLESS they are grossly overtightened. Then absolutely yes, they could cause problems and probably will if torqued down tightly.

Flip Flop Rider

I believe it. Thanks!

BlazingPedals

Good thought. All of the other stuff is possible; but unless I hear that no jam nut was involved, I'm going to guess that was the cause. It's easy to pull the tube right up into the valve stem hole because that darned nut never gets tight.

BROOKLINEBIKER

Hi everyone,
Thanks for all of your responses. I used the nut that came with the tire to hold the stem straight. That sounds like the probable source of my troubles. In changing the damaged tube, I was able to loosen the nut using just my fingers.