Custom Geometry and Sizing Theory- Mtbr.com
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  1. #1
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    Custom Geometry and Sizing Theory

    While I have the latest Paterek Manual, I'm looking for some more discussion of the finer variations on frame geometry/tubing adjustments for factors normally considered by custom builders.

    Is this all Black Magic or is there some theory in print?

    Thanks.
    I would rather have a bottle in front of me than a frontal lobotomy. W.C. Fields

  2. #2
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    Quote Originally Posted by DirtZilla
    While I have the latest Paterek Manual, I'm looking for some more discussion of the finer variations on frame geometry/tubing adjustments for factors normally considered by custom builders.

    Is this all Black Magic or is there some theory in print?

    Thanks.
    Magic huh? Let me 'splains some more.

    Most of us know that if you go to three different builders--maybe even three using the same basic sizing system-- you are most likely to get three slightly different frames. Some differences will be the builders own preference and the others will be how he/she translates what the rider wants or thinks he wants.

    I reckon.
    I would rather have a bottle in front of me than a frontal lobotomy. W.C. Fields

  3. #3
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    Okay, no builders here.

    Wonder where I might find some framemakers hanging out?
    I would rather have a bottle in front of me than a frontal lobotomy. W.C. Fields

  4. #4
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    Quote Originally Posted by DirtZilla
    Okay, no builders here.

    Wonder where I might find some framemakers hanging out?
    http://frameforum.net/

    Go here.
    or the frame builders mailing listserv at
    http://www.phred.org/

    I'm curious about longer wheelbase / longer toptube frames married to a shorter stem.
    Such as sizing up on a frame say going from a 15" to a 17" and then putting a short stem on like a 60mm or so.

    I've got long legs/short torso and more modern frames are being built with lots more standover room, so that shouldn't be an issue with a bigger frame.

    The longer wheelbase could provide a lot more stability and keep that twitchy feeling away, while the short stem helps keep quick steering.

  5. #5
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    Thanks, yeah I just googled that one up.

    Good question. I'll let those with more experience answer it.
    I would rather have a bottle in front of me than a frontal lobotomy. W.C. Fields

  6. #6
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    Quote Originally Posted by ashwinearl
    http://frameforum.net/

    Go here.
    or the frame builders mailing listserv at
    http://www.phred.org/

    I'm curious about longer wheelbase / longer toptube frames married to a shorter stem.
    Such as sizing up on a frame say going from a 15" to a 17" and then putting a short stem on like a 60mm or so.

    I've got long legs/short torso and more modern frames are being built with lots more standover room, so that shouldn't be an issue with a bigger frame.

    The longer wheelbase could provide a lot more stability and keep that twitchy feeling away, while the short stem helps keep quick steering.
    What you're describing is essentially what Fisher started with his 'Genesis Geometry'. basically if you take total cockpit length, there's many ways you can arrive at that number, using a combination of stem length, top tube length, and angles. The real issue as far as I'm concerned is weight distribution, and 'total cockpit length' doesn't give you any indication of that.

    Personally, I like everyone on every size bike to use a stem between 100-120mm for XC. That way I can predict the handling a little better than I can if someone orders a frame with a 24" top tube and is unbeknownst to me going to run a 60mm stem with the same 71 degree head angle as someone with a 110mm stem.
    No longer member of the bike industry nor society, so don't hassle me.

  7. #7
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    Quote Originally Posted by Thylacine
    What you're describing is essentially what Fisher started with his 'Genesis Geometry'. basically if you take total cockpit length, there's many ways you can arrive at that number, using a combination of stem length, top tube length, and angles. The real issue as far as I'm concerned is weight distribution, and 'total cockpit length' doesn't give you any indication of that.

    Personally, I like everyone on every size bike to use a stem between 100-120mm for XC. That way I can predict the handling a little better than I can if someone orders a frame with a 24" top tube and is unbeknownst to me going to run a 60mm stem with the same 71 degree head angle as someone with a 110mm stem.
    Thanks for your thoughts. I've talked to two knowledgeable riders. One a pro downhiller who is also a skills coach and one a former downhiller and now pro XC racer. Both favor the longer top tube short stem approach.

    One suggested going with a longer top tube for the longer wheelbase it provides
    He said: "Traditional wisdom is a long wheel base is slower turning, but my experience is that with the stability of the long wheelbase, I can maintain grip at more extreme lean angles, and actually corner faster, although it does require a little more aggressive technique (fall with your hip vs. the more intuitive push with your arms)."

    I recently had a bike fit done on my current frame. And due to reach/hamstring flexibility issues we went with a shorter stem 80mm. I found the handling to improve as well.

    I know that it is a combination of many many factors that go into making the handling right, and not just one dimension like top tube or wheelbase. I'd like to try two identical frames, one in a 15" for example and one 17" with an 1" longer top tube between them to see how to handle compared to each other with the same reaches but with different stems.

  8. #8
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    Quote Originally Posted by ashwinearl
    One suggested going with a longer top tube for the longer wheelbase it provides He said: "Traditional wisdom is a long wheel base is slower turning, but my experience is that with the stability of the long wheelbase, I can maintain grip at more extreme lean angles, and actually corner faster, although it does require a little more aggressive technique (fall with your hip vs. the more intuitive push with your arms)."

    I recently had a bike fit done on my current frame. And due to reach/hamstring flexibility issues we went with a shorter stem 80mm. I found the handling to improve as well.
    I can't say I agree with your DH friends. If you come from a DH background you really don't know what's going to work in all situations, and if you're used to a short top tube, a 68 degree head angle and a 50mm stem, then anything else isn't going to 'feel right' anyway.

    The problem with long wheelbase still persists, and it's not wheelbase that makes your front wheel wash out in a turn - it's weight distribution.

    Not having enough weight on the front wheel is also bad for climbing the steep stuff, too. if you have no weight on the front wheel you can't steer which forces you to creep forward on the saddle and then you loose traction....it's a vicious cycle.

    Of course, how much of a problem all this is depends on where you ride, too. This should not be underestimated. Some XC tracks aren't super technical, and from our collective experience, that's where you can benefit from a more long-legged steed.

    However, when I design a bike, I'm looking for superb balance, regardless of whether you're going up or down, fast or slow, or whether your riding in Fruita, Pisgah, Mt.Beauty or the Karakorum Highway, so you'll never see any bikes from us with a 26" top tube and a 60mm stem.

    Naturally that's not to say there isn't someone out there with the opposite philosophy. That's the beauty of custom bikes!
    No longer member of the bike industry nor society, so don't hassle me.

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