Presta Valves

Discussion in 'Cycling Equipment' started by Mak'em Lad, May 31, 2010.

  1. Mak'em Lad

    Mak'em Lad New Member

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    Hi, I'm new to cycling (but old to life :() and am having trouble with the Presta valves on my bike.

    OK I know it's a cheap inflater I got (it's getting binned) but even with a car tyre footpump & adapter air leaks all the time & I have to remove the hose while I'm still pupming to keet the tyre inflated.

    Once inflated the valve holds the pressure no problems.

    What am I doing wrong? :confused:

    I've just ordered a Lezyne Pressure Drive pump (read about it on this forum) to take with me when I cycle (just in case) but ideally I'd like to be able to use the footpump & guage to set presure at home.
     
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  2. tafi

    tafi Member

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    Presta - Schrader adapters are a poor substitute for a proper presta capable pump. The foot pump is also not designed to be used for bike tyres.

    Car tyres have much higher volume than bike tyres and pumps for filling them have short barrels with large diameter. Car tyres also run at quite low pressure (no more than 40psi) so a stubby foot pump will suffice.

    Trying to use this pump on a bike is asking for trouble. Filling a bike tyre would happen in only a few strokes (giving very little control over pressure) and it will be hard work to push such a big diameter piston against 100psi. Also given the pump probably isn't designed for the high pressure I'm not really surprised that air is leaking out.

    A proper bike pump has a long small-diameter barrel suited to pumping at higher pressures. It also gives much better control over how much air pressure you get in a small volume bike tyre. The one to keep at home is a track pump. It is designed for the application and won't need an adapter.
     
  3. Mak'em Lad

    Mak'em Lad New Member

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    Thanks Tafi.

    That explains some of my problems, the others are probably down to the quality of the first pump I purchased. The one on order comes reccommended on this forum so hopefully I'll not have a problem with that.

    Looks like I will have to get a track pump for the home as the main one & have the Lezyne Pressure Drive pump as the emergency one for when I'm out with the bike.
     
  4. Fonzarely

    Fonzarely New Member

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    I have a question, I have presta valves on my road bike. My tires go up to 160psi but it is so hard to remove the hose once they are filled to 160psi. By the time I remove it I loose about 10psi. Is there a trick to this?
     
  5. 64Paramount

    64Paramount Active Member

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    Sounds like you need to over inflate them by 10 psi, then they should be at 160 when pull the hose off.

    Are you sure they need to be at 160 psi? That seems like an awfully high pressure.....
     
  6. LB CYCLIST

    LB CYCLIST New Member

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    i picked up a pump simular to this one at a local bike shop it works great for presta valves,i use it at home before i go ride and i carry co2 in my sadle bag incase of a blow out.
    this pump is the best investment i made.

    [​IMG]
     
  7. alfeng

    alfeng Well-Known Member

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    Not that you care, but I know someone who thinks putting 145PSI into his Road tires is the right thing to do because that is the MAXIMUM PRESSURE indiciated on the sidewall ...

    I can't convince him that it is NOT the best pressure for his tires ...

    To actually answer your question, you should pony up for a SILCA pump head ... they simply slide on & pull off the valve stem.

    Although SILCA says that their pump heads are intended only for SILCA pumps, you should be able to cut the pump head off of your current pump and replace it with the SIlCA pump head of your choice.
     
  8. alfeng

    alfeng Well-Known Member

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    I do not believe that those foot pumps are not intended for the high pressure needed for most bicycle tyres ...
     
  9. tafi

    tafi Member

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    Actually I don't think this is a good idea. A presta valve is held shut by having a higher air pressure inside the tyre than outside. It will not let air out of the tube as long as this pressure difference is maintained.

    Once the gauge reads 160 it means the air in the tyre AND the hose is at 160 psi. The air you hear comming out as you release is only the pressureised air in the hose.
    On releasing the nozzle the only way air will come out of the tyre is if the valve core is pressed inwards. What you are doing is pulling the nozzle away so it is unlikely to release air from the tyre.
    The pressure difference around the presta valve forces it shut so your tyre pressure will remain at or very close to 160.

    The problem with measuring the pressure again afterwards is that when you connect the nozzle, you're filling the hose again but this time with the air from the tyre. Because you've now let air out of the tyre its pressure has dropped. Hence why the gauge now reads lower.

    In short the value of the pressure at your last downstroke in the pump is the most accurate and any air escaping on disconnection is from the hose and not the tyre.
     
  10. Mak'em Lad

    Mak'em Lad New Member

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    Hi Tafi, I know your last reply was not to me but I found the info useful and it is directing me to get the correct kit for the job in hand. So I'll be looking for a track pump in the near future. Any you would recommend (UK)?

    The problem mine was having in letting the pressure out was the fact that every time I connected the pump (the foot pump with adapter) the Pesta valve centre was being pressed allowing the air to escape. With the bike pump (reversible head components) the Schrader end pressed the valve centre, the Presta head would do the same to a lesser extend but the pump would not pump any air (all compressed in the tube).

    Your replies have taught me the errors of my (skinflint :eek:) ways. Legs slapped & sent to bed with no supper.
     
  11. 64Paramount

    64Paramount Active Member

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    As long as the pump nozzle or head is not holding the valve open as you stated, then yes I would agree that over inflating would be unnecessary.

    I'm not sure what is actually happening with his pump, he just indicated he was losing 10 psi.....
     
  12. tafi

    tafi Member

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    I'm not sure if we are talking at cross purposes but there are two different questions running in this thread: One related to pumps (by the OP Makem Lad); and one about a perceived loss in pressure when releasing a pump nozzle (by Fonzarely). I'm writing only about Fonzarely's issue first.

    This is not a problem associated with the pump, but with the procedure of pumping.

    What is happening is that the tyre is being pumped to 160, then as the nozzle is released Fonzarely hears air escaping. This is air escaping from the hose and not from the tube (so it is not a problem and does not result in a loss of tyre pressure). The air pressure in the tube is still at 160 even though you can no longer measure it.

    The key mistake is made when reconnecting the nozzle to measure the pressure a second time. In the process air escapes from the tube and into the hose (which was previously empty - or at atmospheric pressure) in order for the gauge to read. The volume of air lost to the hose results in the measured pressure drop (10 psi).

    It is the act of remeasuring which causes the measured pressure drop.
     
  13. tafi

    tafi Member

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    Schrader valves are spring loaded and so need to be opened mechanically. Presta valves have no spring and they are best operated as a one-way valve which operates like so:
    Once pressure in the hose is higher than that in the tyre, the valve opens and air will flow to equalise pressure (into the tyre). If the pressure in the tyre is greater than that in the hose or outside, the pressure pushes the valve against its seat, holding it closed. The valve needs to be able to seal between pump strokes so that the air doesn't come straight back out again. When I pump my tyres I can hear the valve clicking back and forth opening and then sealing.

    As someone else mentioned above, a Silca type nozzle allows the valve to function properly, and I've used a track pump with this type of nozzle for about 10 years with no problem. They usually consist of a hollow brass fitting with a rubber gland seal in the end to seal on the valve stem. They have no protrusions to force the valve open. Pressure alone should do this. You just need to exercise caution when removing the nozzle as it can be tight. Take your time to wriggle it free. Reefing it off will rip the valve out of the tube. There should be several manufacturers which make pumps with these nozzles.

    The best pumps have long narrow barrels to allow you to achieve high pressures. They are no good for car tyres because they have too low stroke volume. You would be pumping all day.
     
  14. Fonzarely

    Fonzarely New Member

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    Silca Pump Head, very interesting. I will give that a look.
     
  15. Fonzarely

    Fonzarely New Member

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    Hey, this is the same pump I have that bought from Performance bike. Do you find it hard to remove the pump head once you reach your desired PSI?
     
  16. Froze

    Froze Well-Known Member

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    You have to be careful when pumping up to max psi's over 120 because some rims won't handle it...not that the rim will explode, but the design of the rim won't allow the tire to go that high without blowing the tire off the rim. I have a set of Wolber's that will only allow 115psi, but my Torelli's will allow at least 125 which is the max I've ever put into a tire on that rim.

    You shouldn't be having any problems with your presta to schrader converter unless it's the plastic one, if you have the plastic one throw it away and get a brass one.

    The pump your getting is a great pump, you got it right.

    Indeed, the air you hear when releasing a pump is the air in the hose, the presta valve is designed to shut extremely fast and at the most may lose a pound of air from the tire.

    There's couple good track pumps, one has already been mentioned the Silca, the other is SKS, and Lezyne makes one.
     
  17. Mak'em Lad

    Mak'em Lad New Member

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    It is a brass adapter and fitting the adapter does not let the air out, it's when I fit the car pump to it the part that is designed tp press the schrader valve presses the centre of the Presta valve (it protudes slightly out of the adapter) and the hose does not seal 100% (or it leaks pass the threads as there is no seal on the adapter). Prehaps it is too short (are there different length adapters?) & needs a seal.

    Your right, it is.

    Not sure if this bit is in reply to my problem (I'm the OP) but if it is then it's not air from the hose as the tyre goes totally flat unless I take the hose off while still pumping.

    I'll look at these models, but thanks to all the replies I now know what to look for and cheap is not top of the list now :eek:.
     
  18. Froze

    Froze Well-Known Member

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    Without being at your home to see what is happening I'm not going to be able to tell you much other than I've used those converters for years and never had a problem till I got a plastic one. You can't let the air out of the converter easily without removing the converter then just let the air out of the presta valve directly.
     
  19. Mak'em Lad

    Mak'em Lad New Member

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    I could fit & remove adaptor without letting air out but the centre stem DID protrude (1/8") out of the end (hence Q about different length adaptors) & could be pressed to release air easily.

    Fitting the car pump would press the stem every time & it was impossible to get a good seal so air always leaked (quickly) & pressure gauge didn't register.

    I had to operate the foot pump like 'a man possessed' then pull the hose off once tyre seemed hard enough.
     
  20. Froze

    Froze Well-Known Member

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    Mine valves don't protrude out of the adapter at all, maybe that's the problem? I have never seen different length adapters, every place I've went to they were all the same length which is why I couldn't understand why the plastic one didn't work.

    Try using the adapter on some else's bike and see if it works any better, if it works better then it may be a problem with your tubes.

    It's too bad you didn't have a video of yourself pumping like "a man possessed", that would have been good for a few laughs!!!!
     
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