Take the 2-minute tour ×
Bicycles Stack Exchange is a question and answer site for people who build and repair bicycles, people who train cycling, or commute on bicycles. It's 100% free, no registration required.

In order for a floor pump to work correctly, something has to make sure that when we pull the pump piston up, the air from the tire does not escape back into the pump. For that there should be a one-way valve between the tube and the pump interior.

My questions is: does the pump itself have its own integrated one-way valve (installed in the pump head, in the pump body or in the hose)? Or does it rely on the existing tube valve (Presta in my case) to keep the air trapped inside the tube between pump strokes?

In other words, when I use a floor pump to inflate my bicycle tires, is the Presta valve on the wheel supposed to open and close with each stroke of the pump? Or is it permitted to stay open at all times (pressed down by pump head), while an integrated valve in the pump provides the one-way functionality?

share|improve this question
    
The valve on your bike tyre is not one-way. Your pump must necessarily have a built-in one-way valve. –  meagar Jul 24 '12 at 19:13
    
I can see how OP could think it possible, since the valve is held closed by pressure from inside the tube. So, in principle, the pump could force the valve open briefly just by exceeding the pressure inside the tube ... I suspect it would be very inefficient and much harder work though. –  Useless Jul 24 '12 at 19:33
1  
@meagar: Valve on the bicycle tube is a one-way valve. That applies to both Presta and Schrader. –  AndreyT Jul 24 '12 at 23:22
    
@Useless: Regardless of the one-way valve design, in order to open the valve and push more air into the tube, the pump has to "exceed pressure inside the tube". That's the primary principle on which the whole idea of a "one-way valve" is based. –  AndreyT Jul 24 '12 at 23:23
    
Sorry, I meant it is not one-way in the way that the question implied. It's not a case of simply increasing the PSI outside the valve until air flows into the tube, the valve must be opened and then air is free to flow in either direction. –  meagar Jul 25 '12 at 13:24
add comment

3 Answers

When I press my pump head onto the (presta) valve, I can feel & hear air escaping until I twist the hose/head junction to lock it on. When I unlock and remove the head, some more air escapes until it is fully clear.

To me, this indicates that the pump head holds the presta valve open, and the pump head must have its own return valve.

This pump head is double-sided though (one side presta, one schrader), so it's possible that a dedicated presta head would be able to rely on the tube's own valve.

share|improve this answer
    
Yes, but at the same time I often observe a different phenomenon (and lots of people report it too): sometimes the air just doesn't want to go into the tire. The pump develops like 200+ psi of pressure, but the Presta valve just refuses to let it in. It remains "stuck" until at some point it decided to get "unstuck" and everything starts working as it should. This suggests that when the pump head is on the valve, the valve head is not depressed and the valve is not actually open. (If it were open, it would let the air in easily). What's causing this then? –  AndreyT Jul 24 '12 at 17:43
    
That happens when you don't have the pump head on the valve correctly. Probably what happened is the little pin inside the pump head which contacts the springed pin in the Schrader/Presta value, and around which air flows to move into the tube, has not made contact with the springed pin in the value or is pushing on it crookedly. –  Brad Jul 24 '12 at 19:16
    
Presta valves open and close using air pressure only. There is no pin required. When the air pressure in the hose exceeds the mechanical resistance and the air pressure in the tube, the valve opens. Sometimes, if the valve is corrode, or has been over tightened, it will stick a bit before it breaks free. That is a design failure, not proof that the design is different than it is. –  zenbike Jul 25 '12 at 12:18
    
I know the valve is held closed when the inner tube pressure exceeds external pressure, and opens if the external pressure exceeds the internal pressure. However, the OP asked if the pump relies on this, and my pump demonstrably does not rely on this, apparently using instead an internal return valve. –  Useless Jul 25 '12 at 13:30
add comment

There is a valve in the floor pump, yes, so it is supposed to press in the presta valve the whole time it is attached to it.

I too have seen the problem you refer to in the comment on @Useless's answer - diagnosis has always shown this to be down to the presta valve either catching on the side of the pump fitting or just being stiff and refusing to budge.

A simple solution I use is to give it a quick press with my finger before using the pump, just to ensure it moves cleanly and isn't stuck.

share|improve this answer
    
+1 I too press down on the press down on the presta valve to make sure that it is moving freely. –  DanS Jul 25 '12 at 11:28
add comment

The valve on the tube (Schrader or Presta) will stay open the whole time your pump head is attached. It knows nothing about the pump state. The valve is kept closed in normal operation by the pressure inside the tube + the threaded collar pushing it up (Presta) or a spring (Schrader).

The pump is where the one-way valve lives. Air is only supposed to leave the pump and never go in (via the connection to the tube, air gets in elsewhere). Air is supposed to go in and leave your tube whenever you wish.

share|improve this answer
    
If in fact your pumps check valve was defective the air pressure in the tube would lift the pump handle to the extended position. –  mikes Jul 24 '12 at 23:35
    
@Brad: I just tested two different pumps (will test more, once I get a chance). The pupms I tested don't open Presta valves. They don't lose air from the tire when I attach the head. They don't lose air when I detach the head. All I get when I detach is a short burst of residual pressure from the pump, but not from the tire. Regardless of how slowly I remove the head, the air never escapes from the tire. I think it is safe to conclude that the pumps I tested don't open Presta valves. They work by incoming air pressure only. –  AndreyT Jul 27 '12 at 15:27
    
That's a strange experience. When I pump my tires up air escapes as soon as I put the head on. I need to pump to force it the other direction. If I put the head on firmly enough and with a good enough seal this doesn't happen but it can. The tube valve is not the one way point. If the pump didn't have an integrated one-way valve the handle on the pump would rise up because the pressure in the tire is that much greater than atmospheric pressure. This doesn't happen so you know there is a valve stopping the flow of air in that direction. –  Brad Jul 30 '12 at 13:17
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.