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  1. #1
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    WW tube question: better to stretch 26x2.0 or 700x38-45c tubes into 29" wheels?

    OK, let's say I want to drop a few rotating grams by installing lighter tubes. Basically that means going with a tube that needs to "stretch" a bit (one way or the other) to fit inside a 29" tire.

    Is it better to use a real 26" mountain tube (forcing it to stretch lengthwise) or a "hybrid" 700x40-ish tube (forcing it to stretch width-wise)? So far I've had decent luck with the latter approach, but I'm just wondering which will hold up better, really? Something tells me that stretching a tube lengthwise won't weaken it as much.

    - Dan

  2. #2
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    um...

    Quote Originally Posted by GlowBoy
    OK, let's say I want to drop a few rotating grams by installing lighter tubes. Basically that means going with a tube that needs to "stretch" a bit (one way or the other) to fit inside a 29" tire.

    Is it better to use a real 26" mountain tube (forcing it to stretch lengthwise) or a "hybrid" 700x40-ish tube (forcing it to stretch width-wise)? So far I've had decent luck with the latter approach, but I'm just wondering which will hold up better, really? Something tells me that stretching a tube lengthwise won't weaken it as much.

    - Dan
    no 26" toob. Just use a 29er tube or a 700x40-ish.

  3. #3
    dirty hippy mountainbiker
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    Vulture?

    Does that mean you have your Vulture? Or that you're just being prepared?

    Anyhoo, I've read here about guys using 26er tubes of specific varieties in a 29er. I think it would work better since if you pump up a 26 tube outside the tire it starts go grow in diameter before it grows in width.
    Mike Henderson, Dirty Hippy Mountain Biker and part owner of Jet Lites.

  4. #4
    what a joke
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    26" has worked with no problems for me... infact the shop that built it up for me did not have any 29er tubes so they used 26". I got a pinch flat but that was due to not checking tire psi before an urban ride.
    blah blah blah

  5. #5
    WAWE
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    I vote for the 26".... the diameter only needs to grow about 10%.... the cross section of a narrower cx/hybrid 700x35-40 tube would need to grow roughly 25-50%.

    FWIW, I've been standard 26" tubes some of the time without any problems.

  6. #6
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    Quote Originally Posted by wolfy
    Does that mean you have your Vulture? Or that you're just being prepared?
    Nope, no Vulture just yet. Still awaiting the fork ... might be a couple weeks out. This will all definitely be worth the wait though. I can still ride the KM in the meantime.

  7. #7
    EPO
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    My Vote------Streach 26er tube

    I'm going to geek out here and it may meander a bit so here it goes....In any tubular pressure vessal, the hoop stress at a given air pressure will be twice that of the longitudial direction (think of a hotdog/brat when over cooked). This means nothing to us bikers though, the tire cassing is the pressure vessal. The tube is only a means to slow the diffussion of air.

    26er streached over a 700 rim= strain ~10% in the lonitudinal direction.
    700*40 tube blown up to fill 54 tire= strain ~35% in the hoop direction (assuming tube is circular in the blown up condition)

    OK to the point:
    You run a bigger chance of relative motion between the tire and 700*40 tube than a 26er 2.1 tube. Due to the manufacturing process of tubes, the wall thickness will vary, making stiffer/less strechy sections within the tube. Each time you hit a rock the tube/tire interface will move more in the localized section that has less tube wall thickness. This section will lead to micro abrasions & then to a hole. The fact that the hoop strain(change in length from unstressed condition) in the tube will be greater the 700*40 tube will abraid faster. Try taking a gummy bear and bend him over by pinching him, licking his exposed back (sorry if this is too ranchy for you, lick gummy bears all the time!) His back will crack due to the strain and the addition of water. The same is true with the higher strained 700*40 tube, more chances for a hole to propogate.

    I learned this the hard way when I was short the 26er tubes I noramally carry on the trail and took a X-cross tire along. I was able to ride 3 miles before flating on the road. I was lucky enough to get picked up.

    What ever you do use talc when mounting a tube that you know is going to streach to minimize friction.


  8. #8
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    How much do you want your tubes to weigh?

    Why not just use the Schwalbe SV19A tube?

    Only weighs 140g, and will go big, with no worries.

    A lightweight cross tube, such as Kenda makes, weighs in at 105G, but is rated to work from 35-43mm.

    Don't know if I would want to use them at 60mm, though.

  9. #9
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    Quote Originally Posted by EPO
    I'm going to geek out here and it may meander a bit so here it goes....In any tubular pressure vessal, the hoop stress at a given air pressure will be twice that of the longitudial direction (think of a hotdog/brat when over cooked). This means nothing to us bikers though, the tire cassing is the pressure vessal. The tube is only a means to slow the diffussion of air.

    26er streached over a 700 rim= strain ~10% in the lonitudinal direction.
    700*40 tube blown up to fill 54 tire= strain ~35% in the hoop direction (assuming tube is circular in the blown up condition)

    OK to the point:
    You run a bigger chance of relative motion between the tire and 700*40 tube than a 26er 2.1 tube. Due to the manufacturing process of tubes, the wall thickness will vary, making stiffer/less strechy sections within the tube. Each time you hit a rock the tube/tire interface will move more in the localized section that has less tube wall thickness. This section will lead to micro abrasions & then to a hole. The fact that the hoop strain(change in length from unstressed condition) in the tube will be greater the 700*40 tube will abraid faster. Try taking a gummy bear and bend him over by pinching him, licking his exposed back (sorry if this is too ranchy for you, lick gummy bears all the time!) His back will crack due to the strain and the addition of water. The same is true with the higher strained 700*40 tube, more chances for a hole to propogate.

    I learned this the hard way when I was short the 26er tubes I noramally carry on the trail and took a X-cross tire along. I was able to ride 3 miles before flating on the road. I was lucky enough to get picked up.

    What ever you do use talc when mounting a tube that you know is going to streach to minimize friction.
    Thanks! That's exactly the kind of insight I was looking for. 26" tubes it is.

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