Wednesday, September 02, 2009

Equipment - Presta Valve Nut

Recently I had a chance to work on a friend's flat tire, someone I work with. It was a clincher, a mountain bike tire no less, and it had a presta valve on it. My friend Jill is very exact about her equipment, and she dutifully had the valve cap on as well as the presta valve nut.

When I showed her how to replace the tube, I pointed out that she shouldn't use the nut.

She asked why.

I explained, she understood, but I could tell she felt a bit disturbed because she wasn't using all the parts. Look at it this way - if a mechanic told you, no, you don't need this bolt in the engine, it's just kind of useless, what would you think?

Then I saw a similar question on the forums.

And that started the idea of this post.

Let's start off with one major rule:

Air pressure is your friend.

Whenever you ride your bike, and I mean whenever you ride it, you should check the air pressure on the tires. I'd skip this step if you've already checked the tire that day, but if you have latex tubes (they leak quickly), I'd check it if, say, more than 6 or 8 hours have passed since you last checked it.

You need good pressure in your tires. Typical road tires will be happy at 95-120 psi, even more sometimes. An under-inflated tire will flat easier, and a severely under-inflated tire will slide readily when you heel over in a turn.

Then there are the more subtle problems.

When you ride on a slightly under pressure tire, the tire has a tendency to rotate around the rim. A good indicator that this is happening is if the tire label, which you should line up with the valve, is moving. If you don't use the "label next to valve" mounting method, another separate indicator is the valve will start to tilt just a bit. The latter is more useful nowadays because guys like me like really long tire labels.

To wit:

No presta nut! This valve is perpendicular to the rim. Good thing. The label is so long that you'd lose the valve long before the label got moved out of position.

Note: the Krylion Carbon is my favorite all around tire - I use it for the trainer, outdoor training, and even races. It's only marginally heavier than the "race" clinchers, it's extremely tough, and wears well. I can't think of a tougher tire that I'd readily race.

Another Note: You can tell this is a front wheel because I used aluminum spoke nipples for both sides of the wheel. On rears I use brass nipples on the drive side.

Note the valve is slightly angled. This is a sign that you should rotate the tire around the rim until the valve straightens out. And then pump up the tire.

Note: this wheel uses brass nipples because I bought it pre-built. I have no idea why I did that, but it must have seemed like a good idea at the time. I kick myself every time I pick up the wheel now.

Another Note: I had to cheat and turn the tire a bit on the second picture. It is a track front wheel so unless I was doing something really odd, tires on track front wheels normally don't rotate around the rim - there are no brakes. The tire is tight so I only turned it a bit.

Final Note: since it's used on the track I can use the flimsiest, easiest to puncture, easiest to cut tires I've ever used, the Michelin Pro Light 2s ("PR2"), the absolute worst tire I've used in the last 5 years. Worse than even the hated Conti tubulars - at least those held air. Yeah, I really like the PR2s, can you tell? I recommend staying the heck away from those tires. Last choice. Throw away $80 or whatever for a pair (I bought 4 because I usually buy 2 pairs of tires at a time). Ugh. What a waste of money. All slashed within a couple rides each, on roads I've used the Krylions on with no problem.

This reminds me, as soon as my arms work again I should mount all much cut, bruised, and otherwise damaged PR2s on whatever other track wheels I have. I don't mind tire boots for track - no rain, no water, short distances, limited top speed (no 50+ mph descents, and I can barely break 34 mph), and spare wheels 100 yards away.

Okay, enough on my Krylion versus PR2 thoughts...

Both symptoms (i.e. the valve or label moving) indicate a tire sliding around the rim, indicating under-inflation or incredibly strong legs. Unlike drag-racing and off-road racers, we don't have "bead locks" for bike tires. We shouldn't either, except for super low pressure riders like 'cross and mountain bikers, but my understanding is that they're migrating to tubeless tires (the valve and tire are separate pieces, and no tube exists) or even tubular tires (the tire is glued to the rim and so won't migrate around its circumference).

This is where the presta valve nut thing comes in play. If you have the nut on the valve, it holds the valve in place. You don't see the valve tilt. However, the tire and tube are still migrating around the rim, pulling on the valve. Ultimately this will cause failure of the valve stem base.

Well, do the nuts do any good?

I have to admit they do at times.

For those with a tall rim and a valve that wants to disappear into the valve hole, a presta valve nut can hold the elusive valve in place. I will use the nut to get some initial pressure in such elusive tubes, then remove after I check the valve's vertical stance.

The nuts also help in keeping the valve in place when using a stiff gasketed floor pump, usually one that is new or at least has new gaskets in it. Except for elusive valves, noted above, usually you can just jam the valve down until it hits the tire. But the nut can help if you don't feel like jamming a wiggly valve.

And a good friend pointed out in a much earlier comment that the nuts help keep the valve from rattling around. I'd use some electrical tape and wrap the valve, but the nut would work well too.

So when you replace the tube on your clincher tire, forget the presta valve nut. And if you have them on now, take them off. Stick them in your gear bag, leave them on your spare tubes, but use them only when you or another rider needs them, not as a default measure.


Yaniel said...

And there you have it.

That would probably explain why my stem tore for no apparent reason on my Sunday ride. I'll be taking the nuts off when I get home. Thanks

jookey said...

i tend to leave at least one on. with deep dish wheels, it can be a pain to pump up at times for me. rather than put it in my seat pack, having it on one wheel wastes less time when i pump up.

Giles said...

Now, I know no one that reads this even remembers what a Schrader valve looks like, but:

It's with Schrader valves that I always have a problem with shy valves (that like to hide when the pump is coming). My floor pump is one of those that's made to fit a Presta but you can stuff it over a Schrader and it will work, UNLESS it's a short valve and a large tire, then sometimes you end up just stuffing the valve in to the hole while trying to get a good deep grip on it.

Unfortunately Schraders don't generally have threading down at the base, so there's no way to screw on a nut to keep it in place anyway!

Tire Tubes said...

I feel like its not always necessary to check your tires as often as you say. I also do realize though, that every time I do check them, they are under the appropriate PSI. Maybe I should be a little more careful.