29" to 650b

stevekay
stevekay Posts: 1
in MTB workshop & tech
I have a Specialized Stumpjumper Comp Carbon 12 Speed 29er - i have been told that if i change my rear wheel to a 650b it'll handle a lot better, is this possible??

Comments

  • JBA
    JBA Posts: 2,852
    Yes, it's easy to do. Just Remove the 29" rear wheel and fit a 27.5" wheel.
    You will end up with what is known as a "mullet bike". There is a bit of a fad for them at the moment.
    As for 'handling a lot better', that is debatable.
    “Life has been unfaithful
    And it all promised so so much”

    Giant Trance 2 27.5 2016 ¦ Sonder Broken Road 2021¦ Giant Revolt Advanced 2 2019 ¦ Giant Toughtroad SLR 1 2019 ¦ Giant Anthem 3 2015 ¦ Specialized Myka Comp FSR 2009
  • steve_sordy
    steve_sordy Posts: 2,446
    The rear axle will be lower down by half the difference in the wheel diameters. This will drop the bottom bracket by about half that amount. This will drop the centre of gravity of the bike a bit and may improve the cornering a bit. But the pedals will drop by the same amount as the BB, so pedal strikes are more likely.
    Because the bike is being tipped back by the rear axle dropping, the head angle will get slacker. To give you an idea of the likely change, I had a quick look at an old trail bike of mine, size large and the head angle would reduce by 0.9deg.
    The formula is, change in head angle = Tan-1(x/y)
    (where x =rear axle drop, and y = wheelbase)

    A slacker head angle usual means more controlled and comfortable descending but less controlled and harder climbing.
    The seatpost angle will decrease by the same amount. It's a small change but your bum may notice and you may have to change your saddle angle as a consequence. A slacker seatpost angle is usually associated with harder climbing, due to the rearwards shift in your weight.

    You may believe that changes of less than one degree are not worth bothering with, but new bike launches have been built on smaller changes than that.
  • The Rookie
    The Rookie Posts: 27,812
    BB will drop by about 2/3 the rear axle drop because its much closer to the rear than front axle.

    Axle drop will be 0.75" for the same width (and therefore height) tyre, the rim diameter being 38mm (1.5") bigger on a 29 v a 650B
    Currently riding a Whyte T130C, X0 drivetrain, Magura Trail brakes converted to mixed wheel size (homebuilt wheels) with 140mm Fox 34 Rhythm and RP23 suspension. 12.2Kg.

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