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  1. #1
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    Why do seat angles and stem lengths change for different size bikes?

    I understand why sometimes really small bikes or 29'ers will steepen the seat tube and push the fork forward- to preserve the reach and fit the wheels under the rider. But a lot of times even on the medium-xl sized bikes they will keep lengthening the stem and futzing with the angles through all the sizes. Why? It makes for a bike that handles differently in each size.

    Klein and K2 are two examples of companies that do it, but i know there's more.
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  2. #2
    Bicyclochondriac.
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    Quote Originally Posted by scottzg
    I understand why sometimes really small bikes or 29'ers will steepen the seat tube and push the fork forward- to preserve the reach and fit the wheels under the rider. But a lot of times even on the medium-xl sized bikes they will keep lengthening the stem and futzing with the angles through all the sizes. Why? It makes for a bike that handles differently in each size.

    Klein and K2 are two examples of companies that do it, but i know there's more.
    First of all, larger bikes handle differently, anyway. You are lengthening the top tube and wheelbase. Also, your COG is higher, and when you climb, with a similar length CS and seat angle, you are farther over the back wheel.

    As far as the stem, if you are taller, and on a longer bike, and kept the same stem lengths, then more of your weight is farther behind the steer tube. A longer stem shifts the balance back forwards a little. This is one of the reasons I shake my head when people talk about the best stem lengths without taking about how big they or their bike is.

    As far as the seat tube angle, 2 possibilities come to my mind. First, as you get a longer and longer seat tube, the saddle is farther and farther back over the rear wheel, so maybe the steeper seat tube is to alleviate that. But also, I wonder if tall people are not proportioned slightly differently (femur / feet / tibia) on average from shorter people?

  3. #3
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    You answered my question, but in a strange way. I think every XL bike i've ridden (well, both) that had the geo tweaked from the medium have kinda sucked. Much too nervous and over the front wheel. I guess you can't ever really know how something will work for someone else, you can only make an educated guess.

    I've been thinking about geometry and fit a lot lately.
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