Thursday, December 13, 2007

How To - Presta Valve Locknuts

You know those presta valve locknuts? The ones that you screw on to hold your presta valves in place?

Don't put them on.

I don't use them for a functional reason - when your tire loses a bit of air, you aren't being methodical about pumping up your tires, or you do some insane maneuvering which moves the tire around the rim, the locknut will prevent the valve from moving.

If it moves, and you noticed your previously 90 degree valve stem is now at 45 degrees, it's like your smoke detector going off. It may not indicate fire but it does indicate something is wrong. This gives you the chance to deflate the tire/tube, slide it back around so the valve is at 90 deg to the rim again, and reinflate. And think about what caused the tire to move around the rim.

If you use a locknut the valve will not move. If it doesn't move but your tire did migrate around the rim a bit, then your tube is under extreme duress. More importantly you can't tell because your valve is happily pointed up at 90 deg to the rim - it's like the smoke detector hanging on the ceiling with no battery inside. You find out only when the valve rips off the tube, goes shooting off, and you have a massive and sudden air pressure loss. Not good, esp if it's a front tire (typically front tire moves when you brake hard and have low pressure or a low tension tire). A slow flat will give you time to react. A fast flat, virtually no time.

If I need help in seating a pump, I'll put the locknut on the valve, start pumping, and when the tube has enough air in it so I can re-engage the pump without it, I'll remove the locknut and top off the pressure.


Sean said...

hi aki-
one more benefit would be less rotational weight, allowing for a faster sprint...

a question: what if the screwtop of presta valve opens and bends slightly. bad idea to try to bend it back, right?

Aki said...

If it's slightly bent I leave it alone. If it's bent enough that you can't pump it up, I try and straighten it out.

Worst case is that it breaks off. If it does, your tube will still hold air, but if you hit a bump just right (or something lodges into the valve) then the air will come right out.

The tip of a presta valve is there to hold it shut. It doesn't have much to do with holding the air in (that's the other end of the valve stem).

hobgoblin said...

Sean beat me to it. I was going to point out that I leave the nut and cap off mine because that's about 2 grams of rotating weight. Over 100 miles, that's equal to 100 pounds.

I just made that number up, by the way.