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  1. #1
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    Can someone explain "flex" to me?

    I was talking about 29er bikes with a buddy of mine recently who in avid rider. He complained about the "flex" of the 29er wheels. I have heard others say similar things. Can someone explain what that is exactly?
    Also, I hear a lot about stiffness qualities in carbon vs alloy. Can you actually discern the difference in stiffness in the two materials without literally trying to bend the material?
    Just a couple of things I was thinking about while daydreaming today.

  2. #2
    Rogue Exterminator
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    Larger rims and forks allow more deflection or what your buddy is referring to as "flex"
    Deflection (engineering) - Wikipedia, the free encyclopedia

    Here is a decent article to read.

    Ending The Debate: 29er vs. 26" Mountain Bike | Bike198


    There are pros and cons to 29ers and to 26ers.
    The better choice really boils down to the rider.
    So where your buddy says he doesn't like the difference in flex you may find the pros outweigh the cons.

    Also, I think sometimes (and maybe not with your buddy) some of the differences noticed are nothing more than placebo. Somebody was convinced by somebody else that 29ers flexed more and therefore when they road it, it was noticeable. Also, maybe that somebody was on a 29er with lesser quality parts than their 26er.

    Ride the bikes and decide what you like best.
    Just stick it in granny and start grinding.

  3. #3
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    Thanks for the info. I already own a 29er and love it. I just never have noticed any "flex" in the wheels. Curious as to how it effects riding.

  4. #4
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    Gotcha

    I don't notice any either but I suppose if somebody convinced me there was, I would notice it.

    I would imagine there probably is a little and it effects the handling, but I seriously doubt I am or ever will be pushing the bike that hard.
    Just stick it in granny and start grinding.

  5. #5
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    Quote Originally Posted by GnarBrahWyo View Post
    I was talking about 29er bikes with a buddy of mine recently who in avid rider. He complained about the "flex" of the 29er wheels. I have heard others say similar things. Can someone explain what that is exactly?
    Also, I hear a lot about stiffness qualities in carbon vs alloy. Can you actually discern the difference in stiffness in the two materials without literally trying to bend the material?
    Just a couple of things I was thinking about while daydreaming today.
    Some riders can discern the difference, some can't. I have a pair of Spinergy 29" disc wheels, that a friend riding behind me claims he can see the vertical compliance in the wheel on impact, but I can't feel it. I can feel a smoother ride, so maybe I CAN feel it.

    All I know is that so far, after 2000+ miles, they have yet to need truing.
    Re-Cycled Person who rides a mountain bicycle.

  6. #6
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    I think the only way to really notice the whole flex thing is to take 2 completely different bikes and ride them back to back. The only time I've honestly experienced it was when I started upgrading wheels/frames/cranks/forks/etc. before I made the changes I never once thought "this fork flexes too much". Gotta compare the 2 most opposites, or both ends of the spectrum, to really understand since most of it is the placebo affect or one riders opinion.

    Flex is also not a bad thing. For example, a carbon seat post on a hard tail. More energy absorbed in the material, less energy transferred to your body. Also, a lot of it has to do with old technology like when 29ers first came out. The research and development has been done, and even the cheapest bikes have some of the trickle downed technology/design/geometry.

  7. #7
    Nouveau Retrogrouch SuperModerator
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    Quote Originally Posted by GnarBrahWyo View Post
    Thanks for the info. I already own a 29er and love it. I just never have noticed any "flex" in the wheels. Curious as to how it effects riding.
    There are many different types of flex. Some good, some bad. Most you will never notice unless you can ride a "stiff" version and "flexy" version back to back.

    Generally, good flex aids comfort and can help a unsuspended bike maintain traction.

    Bad flex contributes to imprecise handling and/or degrades the performance of the drivetrain, suspension or other parts of the bike.

    To make a blanket statement that 29ers are flexy is just wrong. Too many factors are involved. The stiffest and most precise handling mtb I have owned was a full suspension 29er with a 20mmTA fork, 36 spoke wheels with 36mm wide rims. A VERY noticeable difference compared to my 26" steel hardtail. But only when ridden back to back (or the same trails within a few days).

    There are some wheels (CB) known to be flexy, especially under heavier riders. A good reason to avoid the low spoke count 29er wheels.

    Maybe the best way to demonstrate bad flex would be to air down your tires until you have difficulty riding a straight line. A flexy frame or wheels has a similar feel.
    mtbtires.com
    The trouble with common sense is it is no longer common

  8. #8
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    I sometimes wonder how much flex my CroMo-Mongoose w\MotoMag's had ... I'd think very little !

    I agree that some flex is good, if it's the right kind of flex.
    I also agree that most people won't notice flex, unless they have a reference to compare against.

  9. #9
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    A stiff set of wheels for someone that's 140lbs might be flexy for someone that weights 240lbs....

  10. #10
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    I think skillwise you can get used to just about anything. I was practicing trackstands yesterday and had my front wheel pushed up against a brick wall while I balanced. On my 26 hardtail with very narrow extralite rims with a standard QR I could see the wheel itself flexing, the wheel in the fork and the fork itself as I turned the bars.

    On my tallboy LTc, I put carbon wheels on and on a couple of downhills that I am very familiar with, the bike tracks a lot better than when I had Al wheels on it. The sensation is that the bike tracks a lot straighter without a lot of bouncing around so that I feel much more in control. Otherwise I dont feel much difference except the reduction in weight.

  11. #11
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    I experienced "flex" with an older set of wheels that had looser spoke tension and were a bit narrow. Couldn't really tell whether it was mostly the wheel or the tire giving a wobbly feel during tight hard cornering.
    Now I have newer wheel that have much higher spoke tension and are wider and I don't feel hardly any wobbly action in corners, only the tire doing it's thing. (rear is also ss specific)

    So, I think it depends on what and how you are riding... And I never noticed anthing in the front really, only rear.

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