|Why do we use Presta valves intsead of Schrader?||speedisgood|
Jan 1, 2003 3:57 PM
|Does anyone know?|
|re: Why do we use Presta valves intsead of Schrader?||motta|
Jan 1, 2003 4:47 PM
|The narrower opening makes it easier to achieve higher pressures with a hand pump.|
Jan 1, 2003 5:09 PM
|I'm sure what you meant to say makes a lot of sense, but what you wrote is ridiculous.
A Schrader valve uses a spring to close the valve, so it has to be mechanically pressed in to open it. This arrangement makes it very difficult to fill the tire then remove the pump hose without bleeding back some air. A chuck could be made that does a good job but would be more expensive and less reliable than what is in use today. A Presta valve is actuated by air pressure alone. It forms a check valve assembly right at the valve. When you disconnect a pump from a Presta all the air you hear hissing out is from the pump, not the tube.
Of course there is the snob factor as well, serious cyclists use Presta, kids use Schrader. My wife has a hybrid with very nice 110 psi Specialized tires, but it has Schrader valves, because that is the market it is being sold into.
Jan 1, 2003 5:53 PM
|If what I wrote is so ridiculous than go out to the garage and pump your wife's schrader tire with a frame pump to 110psi and then pump up your presta road bike tire to 110psi and tell us all which was easier to do.
His question was why do we use presta valves? You explained this to all of us how???
|You can't be serious||53T|
Jan 2, 2003 6:41 AM
|Presta valves are better for high pressure tires, for all the reasons I stated in my post. None of which has anything to do with having a smaller hole. Simple fluid dynamics tells us that a smaller hole results in more difficulty in filling the tire. For example, as the hole size approaches zero it becomes increasingly harder to pump up the tire.
It is no more difficult to pump up a Schrader-valved tire; the advantage comes when you have to remove the pump. I have a Zefal frame pump in Schrader and it has a screw-on hose at the end (clever design) when you unscrew the hose from the valve you lose about 5 psi. The same thing happens when I unscrew my electric air compressor from a Schrader valve. This is no good for serious high pressure applications. A clamp on frame pump with Schrader head will also bleed back pressure, but to a lesser extent.
The other poster listed smaller hole in the rim as a plus for Presta, but while it sounds reasonable, when you run the numbers it probably doesn't add up to much. Especially with the very strong aero profile rims we all run to day.
|Smaller Opening => Stronger Rim (nm)||jamesau|
Jan 1, 2003 5:19 PM
|Presta=higher psi nm||DINOSAUR|
Jan 2, 2003 9:48 AM
|This guy is pretty smart, have a read...||53T|
Jan 2, 2003 10:35 AM
|From: firstname.lastname@example.org (Jobst Brandt)
Subject: Re: Schrader -vs- Presta: why?
Date: 16 Dec 1999 18:49:53 GMT
Roger (who?) writes:
> Presta is allegedly easier to inflate because the force required to push
> the air in is lower due to the smaller diameter. I would have thought
> the diameter of your pump barrel was more important though.
Roger, you keep on posting a compendium of myth and lore on technical
subjects that you don't understand. This one is typical where you mix
and waffle. The presta valve is preferred for two reasons. It's stem
has a smaller diameter and therefore cuts a smaller hole in a rim, and
it does not have a spring loaded valve so that simple pumps, like the
Silca Impero frame fit pump (that has no valve opening device) can be
used. The choice was made back in the days when racers had to change
their own tires and inflate them.
Neither the diameter of the stem nor the size of the opening in the
valve affect pumping force for any given pump. That is a function of
back pressure only, in the case of the Schrader valve, principally the
force to open the spring loaded valve.
|Jobst Brandt trumps all....||motta|
Jan 2, 2003 10:57 AM
|That's an explanation I can agree with.|
Jan 2, 2003 11:31 AM
|Perhaps I should have prefaced my reply with:
Motta, you keep on posting a compendium of myth and lore on technical subjects that you don't understand... ;)
|No respect, you didn't earn it||motta|
Jan 2, 2003 1:30 PM
|Brandt's reply through your theory out the window as well.|
Jan 2, 2003 11:04 AM
|It might not be a valid consideration due to the low mass of the Schrader valve components, but I always imagined that a Schrader valve might be prone to leakage simply due to the centrifugal force exerted on the valve as the wheel spins at a potentially high rate (if the rotational force can overcome the valve spring, obviously). If the Presta valve is screwed snuggly to the stem after inflating the tube, this cannot happen.
I'm no physicist, and the masses involved might need a much greater rotational velocity, but it makes sense to me, in theory.
Jan 2, 2003 11:29 AM
|Qualitativly you are correct, centripital acceleration does tend to open a Schrader valve. The numbers are way way off, however. Particularly, you neglect the force of the air pressure tending to close the Scharder valve, just like in a Presta valve. You are correct in observing that the mass of the stem of a Schrader valve is a couple of orders of magnitude too small to worry about.|
|re: Why do we use Presta valves intsead of Schrader?||mapei boy|
Jan 6, 2003 12:14 PM
|All I can say is that, in my many years of hand pumping through both presta and schrader, getting a bunch of air into a tire via a presta valve is a helluva lot easier.|| |