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Handling: BB height vs drop.

Discussion in 'Downhill & Freeride' started by poekie, Mar 10, 2013.

  1. poekie

    poekie Chimp

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    After building my 29er HT I've been thinking about bike geometry a lot and one thing keeps bugging me: the effect of bb height and drop on the handling.

    I'm pretty sure bb drop is responsible for how poppy a frame feels because it determines the 'tipping point', getting the front wheel off the ground when manualling or hopping. My Fit bmx used to have a 11.75" bb which is about 1.75" rise whereas my Tonic Howie has a 12.8" bb which is about 0.2" drop, at roughly the same position the bmx feels a lot easier to pull up. Not taking other factors into account this should support my idea. Would it be bb height (i.e. relative to the ground) the 26er should be more poppy - which it isn't.

    What I'm not sure about is what makes a bike more willing to turn: the relation between bb and axles or the height of the centre of gravity. Intuitively I'd say the latter.

    I got into this after building my Kona Honzo. The thing is a blast to ride - one of the best cornering bikes I've ever ridden but it's next to impossible to get off the ground. This got me thinking about 29ers: to me it seems to make for a compromise between jumpiness and cornering which 26" doesn't as you can have a nice c.o.g. without having to lower the bb so much under the axles it spoils the fun jumping it.

    Can anyone shed some light on this?
     

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  2. jonKranked

    jonKranked Press Button, Receive Stupid

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    seems like you have it figured out. bb height, bb drop, and wheel size are interrelated. 29'ers don't pop as much because they have more bb drop; otherwise they bb height would be way higher (and so would your CoG).

    bmx bikes have bb rise because otherwise you'd slam your pedals into the ground constantly; this is what makes them super poppy. your howie has the same bb height as your Fit, but one has bb drop and one has bb rise because the 26" wheels put the axle 3" higher.
     
  3. jonKranked

    jonKranked Press Button, Receive Stupid

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    also, regarding turning, there are other bike factors involved; wheel base, fork trail, head angle, reach, stem length etc.

    all those being equal, the bike with the lower bb and more bb drop will turn better, because they put your CoG lower. that matters because when you lean the bike in the turn, it helps keep your CoG closer to the vertical plane of your contact points (where the tires meet dirt).
     
  4. Verskis

    Verskis Monkey

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    What? The Honzo I briefly tested was a lot easier to bunnyhop than my 26" Ragley Blue Pig.
    It was also really easy to start a manual with the Honzo.