Another Shraeder/Presta question, briefly

Discussion in 'Cycling Equipment' started by tripp013, Mar 31, 2006.

  1. tripp013

    tripp013 New Member

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    OK, guys. I'll make this brief. First of all, I have a busted Presta tube that says 700x18/25c, and I'm tired of searching for Presta stuff. So, will a 27x1" Schraeder tube suffice if I open up the valve hole a bit?

    Secondly, if I use a Shraeder tube to replace the Presta tube, how the hell does anybody know how much tire pressure to put in it? I mean, the tire says 130 PSI, which is obviously for Presta tubes. But the tubes themselves say nothing about tire pressure. Anyone wanna pitch some advice my way?

    Thanks for the help!!!
     
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  2. John M

    John M New Member

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    There are many reasons why Presta is now the standard for road tubes and they are readily available at any bike shop. I would suggest that you stock up on a supply and stick with Presta. That being said, if you do want to go with Schrader, see the advice below.

    You can drill out the rim to accomodate the larger valve stem of the Schrader valve, but you will be stuck with Schrader on that rim from that point forward unless you use some sort of grommet to reduce the hole size again should you want to go back to Presta. Be sure to file or sand down any burrs that may result from the drilling as they could puncture the tube.

    PSI is the same regardless of valve. It is the tire that determines the PSI requirement, not the tube.
     
  3. DiabloScott

    DiabloScott New Member

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    Searching for presta stuff? Where - Walmart? Go to a bike store or order on-line. Now finding prestas in 20" is a little tougher.

    Most road bike rims drilled for presta aren't really wide enough for Schraeder valves so you may ruin your rim if you try to enlarge the hole - if there's clearly enough room there then go ahead and try.

    John's right about pressure, and his answer about the burrs is a good one, but the part about the grommet and presta is hooey - I've got prestas in my MTB rims drilled for Schraeders and there's no problem at all. Used to have some old 27" wheels that I did the same thing for - I bought the grommets thinking they were necessary - threw them away when I realized they weren't.
     
  4. tripp013

    tripp013 New Member

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    OK. Thanks for the advice so far, and I'll definitely consider sticking with Presta before I do any reconstructive surgery. But what about the size issue? When I'm checking out tubes that have inches listed (26",27"), what's the equivalent to my original tube size of 700x18/25c (is that cubic measurement or what)?
     
  5. John M

    John M New Member

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    When I worked in the shop, I would occasionally see people bring in flats that were caused by running presta tubes in schrader holes--probably caused by running underinflated and the tube moving around. My guess is that an experienced guy like yourself always rides properly inflated.

    Yes, I have done as you have, but I think that the grommets help get the stem lined up straighter.
     
  6. hd reynolds

    hd reynolds New Member

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    Even though it says 130psi on sidewall, these hi-pressure bike tires are tested and can take up to twice the written psi. This is done on test by pumping water instead of air to prevent an explosion. The tubes outside the tire will just balloon until it explodes but will not develop the pressure. Only if you have something encasing it to prevent it from expanding will it get up to pressure. The tire test I mentioned is done with conventional tubes inside.
     
  7. tripp013

    tripp013 New Member

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    Awesome. I'll check out grommets if I go the Shraeder way. Can anyone help me out with the size question I posted about two back (700mx18/25c converted to inches)? -Thanx.
     
  8. hd reynolds

    hd reynolds New Member

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    700c is the standard metric wheel size for compatible rims, tires and bicycle frames. You cannot "literally" translate it from metric to English system because there is no English size standard for it. Other wise a 700cx18/25 will look 25.7in x (7/10)/1 which sounds peculiar. The 700c size as well as the 27" actually refer to the actual diameter of the tire and not the rim. Therefore a 700c road tire or rim vs a 27" English standard aren't compatible. Unlike car and motorcycle tire/wheel sizes where they refer to rim diameter, bicycle tire measurements sound rediculous to refer to the tire diameter instead of rim size but bike tire/rim diameter standards came from tradition.
     
  9. DiabloScott

    DiabloScott New Member

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    Yeah but 700c tubes and 27" tubes are interchangeable, just not the tires.

    So 700 20-23 tube and 27" 7/8 are essentially the same
    700 23-25 tube and 27" 1
    700 25-28 and 27" 1-1/8
    etc
     
  10. hd reynolds

    hd reynolds New Member

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    Yes you are right indeed. I was just talking about tires not tubes.
     
  11. Retro Grouch

    Retro Grouch New Member

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    I think that it's irresponsible to advise anybody to exceed a tire's rated capacity. I honestly don't know how they come up with that number but I'm sure there is a logic behind it that shouldn't be cast aside without more careful consideration than most consumers are able to give.

    This is a story that I know for sure. Bikee used to be a company that made recumbents. They produced a tandem model with a 20" front wheel. The tandems had a frequency of blowing front tires off of the rim even at the tire's rated pressure. The company's solution was to put a sticker over the tire's molded-in pressure rateing to use less air pressure. Bikee isn't in business anymore.

    The tire is only half of the equation. Your rim has to be able to handle the pressure too. Furthermore, the amount of pressure that a rim will withstand is a moving target. As the brake surface wears (and it does wear) it's ability to withstand the sidewise pressure from the tire beds decreases.

    As I've gotten older I've become more conservative with the tire pressures that I use.
     
  12. tripp013

    tripp013 New Member

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    OK, so just let me recap here. I can take out my original busted Presta 700x18/25c tube, widen and smooth the hole, and replace it with a 27x11/8" Shraeder tube.
     
  13. hd reynolds

    hd reynolds New Member

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    If you read my post carefully I did not advise to exceed the rated pressure. I said that tire companies test their hi pressure tires twice the rated capacity.

    It is the same industry standards that they do for, say, cars. A Z rated radial that's recommended to up to speeds of 200MPH are tested to speeds up to twice that. Even if that tire survive 400MPH speed, the safe recommend rating is only half that.
     
  14. Retro Grouch

    Retro Grouch New Member

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    I did read your post carefully but I must have missed your point.

    "Even though it says 130psi on sidewall, these hi-pressure bike tires are tested and can take up to twice the written psi."

    So what was your point? Why did you feel the need to tell us this?
     
  15. John M

    John M New Member

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    Yes, you can do that unless you are using a very narrow tire (700 X 19 or 20). An 18/25mm tube would generally correlate to a 7/8-1" width tire in English units, although tubes can stretch to fit larger tires. Sometimes there is a problem getting larger tubes to fit in smaller tires.

    A 1 1/8 tube may be tight in a 25mm or less tire, but would be ideal for a 28-32mm tire (or 1 1/8-1 1/4).

    Good luck. Still think that you ought to stick with good quality presta tubes obtained from a bike shop or online.
     
  16. dhk

    dhk New Member

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    Even if the tires can double the pressure, clincher rims are generally rated for 140-160 psi max. Check Mavic website for their max pressure ratings. Would never advise anyone to "test" inflate their tires to anything over the max sidewall pressures. There is a safety margin designed in, but the max limit is on the sidewall for a reason.

    Also, pretty sure Z-rated tires are not tested for twice their rated speed. Z indicates only that the tire is capable of more than 149 mph, not 200 mph. Higher speed rated tires carry a W or Y designation, in addition to the Z. There may be a 10-15% safety margin above the rated speed, but doubt any passenger car tire on the market is capable of holding together at anywhere near 400 mph.
     
  17. Insight Driver

    Insight Driver New Member

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    To stick with the orignial question, it make sense to just stick with a presta valved tube. Tubes are rubber, so anything that fits inside the tire works just fine. You don't want to drill out your rim to go to an older standard that is used on cars. Presta valves are just better to work with and less likely to leak. You simply push the pump head on the shaft and start inflating, simply pull it off when done. What could be easier? What I don't understand is why the poster is having any difficulty finding presta-valved tubes.
     
  18. hd reynolds

    hd reynolds New Member

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    The point my dear watson is that tires are safe enough even if you inflate them at the top of the written rating.

    Just a tip, when you read a post you just don’t take snippets out of it. Coz you will have comprehension problems. Read the whole post coz all of it are interelated then you will understand.
     
  19. fish156

    fish156 New Member

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    With all the discussion about tire sizing, I think it's worth posting this link:

    http://www.sheldonbrown.com/tire_sizing.html

    Everyone should read this article. (26x1.75) DOES NOT EQUAL (26x1 3/4). Read and find out why.
     
  20. Retro Grouch

    Retro Grouch New Member

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    Good idea about reading the whole post. So Watson, how does your post match up with the reality of Bikee's blowing front tires off of the rim even at the rated tire pressure?
     
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