I'm missing something about how these presta valves work. There's a nut at the base of the value that is supposed to be tightend down against the rim. After a ride, it's loose. And the screw-down cap on the valve, that gets loose, too. How can I fix this?

Should just be able to tighten it down with your fingers to get it to hold. I have never had the valve work loose. I have forgot to tighten it. The nut that fits against the rims (you don't have to use) do work loose shouldn’t after every ride. But then I don’t know how much flex you rims have and how hard you ride. You could use a weak lock tight on it but you need to be able to get it off to fix a flat, so be careful how tight you make it.

Cheer,
Dusk

Just throw them away.They serves no useful prupose.End of problem.

I tighten the nut, but very loosly. It acts as a shoulder if I'm filling my tires with an air chuck. Most air chucks are made to fit schrade valves, so you will need a presta-schrade adapter. They're cheap at your LBS!

If that nut's too tight, it can pull the stem off the tube when you deflate the tire. That's VERY tough to repair!

Hmmm... Is your wheel true? Does your rim receive a beating during your ride? Normally your presta valve doesn't receive much stress. The little ring that tightens down to the rim isn't critical. I keep mine only slightly tight. If yours is moving, I'd be concerned with tire failure. Personally, I hate to stop and fix flats. Check your wheels carefully, likely an indication of another problem just waiting to happen.

Yep, exactly....

Almost every serious rider I know does not use the nut. I have never heard of a presta valve screw-down cap coming loose.

I'd generally agree, however they fixed a problem for me.

When I put a 37c tire on the front wheel of my cyclocross bike with the correctly sized tube I get a situation where the valve taps on the inside of the hole in the rim. It took me and the LBS a hour to figure it out as it only occured under load and could not be duplicated on the stand. Putting a nut on the valve stopped it. Perhaps there is some other reason for this tapping, but it happens with several different people installing this same tire and two different brands of tube.

I also like them for when I have a flat in the field to make it easier to put on the pump - sure in most cases there is one on the spare tube, but if you patch a tube it can be easier to already have one handy than to dig into you bag and unwrap the spare tube.

Of course one needs to be careful in the use of them not to pinch or cause the tube to be postioned badly during inflation. One also needs to be careful to to overtighten and stress the valve.

So in the end they are basically OK to throw away.

Al