How do You Even Bend the Stem On a Presta Valve?


If you ride a bike, you know what a Presta valve is. If you don’t know what a Presta is, then this article may not be very interesting to you. If you do know, you still might not care. That’s why my blog is called Esoteric Observations on Bicycles and Cycling.

For a long time I have been wondering how people manage to bend or break the little stem on the end of a Presta valve. Yes, I know, it happens attaching a pump or removing it. But really, is it that delicate? Let’s find out.

I attached a string to the nut of a presta valve on an old tube and exerted a lateral force (measuring the force with my Park Tools DS-1 digital scale of course).

At about 15 lbs the stem broke off, right where you’d expect it to, at the point of highest stress, where it enters the neck of the valve body. I was surprised that it broke rather than bent, based on the number of bent stems I’ve seen over the years. It must have been harder and more brittle than average.

I’m not sure what my advice should be based on this little experiment with a sample size of one. Be more careful attaching and removing the chuck from the valve? Spend more for a premium quality chuck than you did for your pump? That’s about all I have to say about breaking off the stem, but let’s keep talking about Presta valves.

Did you ever notice how you have to get above the pressure in the tire to start air flowing? Two things are at work here, both related to the little cone-shaped rubber seal inside the valve.

First, the rubber sticks to the metal, and you have to generate enough force to push it free. Some valves are worse than others. I’ve had valves that required 140-150 psi to unseat a valve with 90 psi behind it. The solution to this is to depress the stem by hand momentarily to unstick the rubber. This does let some air out of the tire, so don’t hold the stem down long. I like to bump the stem with my chuck. I get a satisfying “pffft”, letting me know I have unstuck the rubber without letting out significant air. And no, I have never bent a stem by doing this.

Second, because of the conical shape of the seal, the diameter at the inside of the seal is larger than the diameter at the outside. The pressure in the tire is acting on a (slightly) larger area than the pressure from your pump. So it takes a higher pressure from the pump than what is in the tire to initiate flow. After the valve is unseated and air is flowing, the differential area issue goes away.

There is no spring in a presta valve. (That’s why you want to screw the nut down after inflating your tire.) Gravity, pressure differential, and air flow determine the position of the stem within the valve body. I wonder if there is a preferred valve orientation for inflation. If the stem is pointing down (stem at the top of the wheel) gravity will reseat it after each pump stroke. If the stem is pointing up (stem at the bottom of the wheel) it will fall away from the seat, and remain there until backflow pushes it into place. I wonder if this matters at all.

If your tube has a broken stem, you can still inflate the tire and go for a ride, maybe. Just be sure you place the stem at the top of the tire (pointing down) so it will fall back into place after each pump stroke, instead of falling into the tube. The other day my friend Eric (aka Bobke) described getting this wrong once. Oops.

Also, a story is told of how in 1985 during a bicycle speed record attempt John Howard’s rear tire went flat at almost 150 miles an hour. Quoting from a 1990 LA Times article:

“Three-time U.S. Olympian John Howard was mounted on the prototype bicycle that featured a double-reduction gear setup and was nearing 150 m.p.h. when the rear tire went flat … The flat tire could have killed him … There were 1,600 Gs (the measure of gravity against an accelerating object) on the valve spring of the tire.”

I was wondering if the given reasoning is a valid explanation of the world’s fastest bicycle flat tire, so I did some calculations. Feel free to check my work.

I assume this was a Schrader valve like the ones on your car tires and not a Presta, because of the mention of a spring. Stirring in some realistic numbers like 30”(?) wheel diameter and a one gram valve stem weight (I measured it), I get about 1200 Gs and a centrifugal force of about 2.5 pounds trying to offseat the stem. Fighting that force are the air pressure in the tire and the spring in the valve itself. The valve seat seal surface is about 2.8mm diameter (I measured it), so 100 psi would exert a force of about a pound, and the valve spring exerts about a ¾ lb closing force (I measured it).

One and three quarters is less than two and a half, so, yes, the valve stem could really have been forced offseat causing this flat. There may also be some things going on with the distribution of air pressure within the spinning tire. I can imagine that the pressure is higher near the outside than at the rim, but, never mind.

Two final thoughts on Presta valves:

One – in this age of wonder materials, why are inner tube valves still made of brass?

Two – the cap, use it or throw it away? I don’t use the cap, do you?