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Flat Tire questions and issues

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Flat Tire questions and issues

Old 11-21-17, 07:35 PM
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Flat Tire questions and issues

Hi folks,

Since early this year, I've been volunteering at our local co-op. My primary job is refurbishing/rebuilding bikes for the disadvantaged. Sometimes I'm pressed into service to do flat repair which is actually installing new tubes. In this assignment, I've encountered a couple of weird issues.

There seem to be a lot of valve stems that have pulled away from the body of the tube and leaking at that point. What causes this? Is there anything I can do or recommend to prevent this?

The other issue is tubes sticking to the inside of the tire. It is almost like they are glued in with contact cement. Back in the day, I remember tubes and/or the interior of tires being coated with talc. I also vaguely remember a bike maintenance manual recommending keeping spare tubes in a baggy coated with talc. Is this still a good practice? Or, have the recent health concerns with talc usage stopped this.

Any feedback or recommendations will be greatly appreciated.

Thanks and regards,

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Old 11-21-17, 08:31 PM
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I've had valve stems break off, but only on Presta tubes, never Schrader. There have been discussions of this, and folks have suggested a number of causes: 1) Rough edges in or around the valve hole in the rim. 2) Improper technique attaching or detaching the pump head. 3) Inferior tubes. 4) The Presta design itself, though some folks report that they have never had any problems.

My own observation is that some pump heads are easier to get on and off the valve than others, and a combination of a fussy pump head and bad technique could shorten the life of the tube. I certainly can't rule out that my own technique is the problem. Another issue is that I pump my Presta tubes more often because they tend to be on the bikes with narrower and higher pressure tires. So it creates more chances for damaging the tubes.

As for tubes sticking to the tire, I use corn starch, which seems to work as well as talc, and I happen to have it in the pantry. If I were to buy something for the purpose, it would probably be baby powder which comes in a convenient container. I don't worry about storing them powdered, since there's usually enough powder left on the inside of the tire itself when I change a tire while on the road.
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Old 11-21-17, 08:48 PM
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I suspect that many of the bikes your encounter with inner tube valves torn are from the bike being ridden with very underinflated tires usually by people who don't/can't maintain their bikes. The tire can slip on the rim and pull the valve with it. Tell the new owners to keep their tires inflated. Using talc or cornstarch can help delay a tube from sticking to the tire but after several months it usually doesn't make much difference IME.
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Old 11-21-17, 09:03 PM
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Stems tearing away:
bike ridden with low PSI, never been inflated.
many people who aren't enthusiasts (ride by necessity rather than fun) aren't inclined to learn basic maintenance. I get a lot of surprised looks at the shop when I tell people they need to be inflating at least every 2 weeks, not every 2 months

tires rely on air pressure to hold the beads tight against the rim
without it, they slip and migrate as the wheel rolls
taking the tube with it, and tearing the valve at the base

also, be sure the pull the valve stem out fully as you install, so the base of the stem rests flat against the inside of the rim and comes out straight. leaving a gap inside can cause the tube to rupture there, tube needs to be supported
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Old 11-21-17, 09:32 PM
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The incidence of valve separation is way up and I trace it to (cost saving) design changes some years back.

The problem is different for Presta and Shrader, but the effect is the same.


In the past these had large base flanges, and the tube was trapped against them with a nut. Some later versions, replaced the base flange with a molded rubber cone with it's own wide base which was bonded to the tube.

The current design is simpler, the tube has a thicker area with something of a rib molded in. The valve has a narrow base, with a groove to accept that rib. The base is coated with glue, and popped into the tube and that's all. The advantage is that the narrower base fits narrow rims better, but there are two issues. Whereas, air pressure pushes the flanges and tubes together, it provides no assistance in this design. Add that there were large numbers produced in a way that allowed the valve stem to oxidize leading to bond failure.


They've been molding flanged cones onto brass valve stems for almost half a century, so that's not the problem. But in the past, the brass valve stem extended all the way to the tube. Sometime in the last decade or so, someone got the bright idea to save some brass by ending the valve 1/4" or so out from the tube and depending on the rubber cone finish the job. So, the brass no longer extends beyond the rim, and provides no support against the movement you cause when pumping. These valves break off outside the tube with decent consistency.

I use both PV and SV tubes, and valve failure, which had been very rare in the past, now accounts for almost half my flats. I've pretty much solved the PV corrosion problem by using O-rings both above and below the rim to keep water out.
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Old 11-21-17, 09:39 PM
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Originally Posted by Senior Ryder 00
valve stems that have pulled away from the body of the tube
For presta valves, it is often caused by over tightening the knurled jam nut.

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Old 11-22-17, 02:50 PM
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I have seen some that don't put the nut back on the presta stem. Or maybe they are putting it inside. The obvious thing if it is not on the outside is that when attaching the air hose you might push too much on the stem and over time weaken that bond between the stem and tube.

Similarly, the fitting on my floor pump got real hard to detach from the stem and I got a leak in a tube for the same reason as above but the reverse. A replacement hose and fitting fixed the issue though.
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