Wednesday, October 29, 2014

Trying to Fix That Which is Broken


Extracted Presta valve core caked with sealant

On the third tube with a Presta valve that wouldn't hold air for the length of one ride, I had to find out. These things are supposed to be air-tight when lightly tightened, but these would leak even when cranked down. Those nuts are not meant to be cranked down, either, which I demonstrated by breaking one off. This was about curiosity more than anything else, perhaps also a desire to stop throwing money at tubes that don't hold air out of the box. So what was the deal?

First thought was that the Internet forums that said that sealant tubes don't work well on higher pressure, 700c tires for several reasons. Same posts also mentioned clogging of the valves being an issue. Since these came with removable cores, I had a look.

Indeed, there was a bunch of congealed sealant around the valve that looked suspicious. But on cleaning that off, I found the base problem.


Not going to seal no matter how tightly it's closed with the nut

Correctly made Presta valves often seal just under the pressure of the inflated tire because the valve body is allowed to move freely upward and press the rubber against the valve face. In this particular core, though, the valve face is so uneven that the rubber can't make an effective seal. This valve let air leak through contstantly, which the sealant tried to seal and perhaps even did at times, but that's not desirable since sealant works both ways, making it more difficult to get air back in, too.

Of all the ways to spend my time...

Or so I diagnosed. Introspectively, I wondered, who really cares? I mean, there are other broken and unbroken things which I could spend my time on, why this? A pro would probably just throw this core away and use a new one, or just not even bother and use a new tube, or, even more likely, not buy a sealant tube in the first place. Why did this grab my attention?

Just because I wanted to try to understand what had caused three tubes to not hold air. Was the uneven valve seat really the root cause, or was it something else? To test this idea, I filed off the rough spot, cleaned up the valve core, and put it back in.

Result: the tube now holds air for a few days. Better, and probably the uneven valve seat was the cause. Looking at the valve interface, it would probably be best if the meeting edge were beveled, but I'm not going to go that far. The nut would have to come off first, and it's got a stop at the top of the screw meant to inhibit that, and I don't want to spend more time on it. There are other broken and interesting things that I should go and fix. But I think I'm convinced about high pressure tires and sealant tubes. Lower pressure mountain bike tubes seem to be more tolerant of sloppy valve quality control. 700c 100 psi requires better engineering. It seems. In any case, I prefer tubes that hold air out of the box.

 

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