Top tube length and the meaning of life- Mtbr.com
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  1. #1
    Not Smart Enough to Quit
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    Oct 2004
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    Top tube length and the meaning of life

    I'm looking to replace my old Trek convert with a dedicated SS rig and I'm a bit unsure on what I want. I was looking at the IRO but in checking the geometry, I noticed that the TT is about 1" shorter than my old faithful. My trek is a medium (18"?) with a 22 1/2" TT while the 18" IRO is 21 1/2". I've also found that many other frames are short like this.

    So, how will this effect handling and feel? I love the way my old Trek handles, but issues like chain length and tire clearance are making me look for a replacement. Any input on other handling factors such as chainstay lengh, wheel base and head/seat tube angles are more than welcome.

  2. #2
    all hail der Fuhrer Bush
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    things to consider

    Quote Originally Posted by xray_ed
    I'm looking to replace my old Trek convert with a dedicated SS rig and I'm a bit unsure on what I want. I was looking at the IRO but in checking the geometry, I noticed that the TT is about 1" shorter than my old faithful. My trek is a medium (18"?) with a 22 1/2" TT while the 18" IRO is 21 1/2". I've also found that many other frames are short like this.

    So, how will this effect handling and feel? I love the way my old Trek handles, but issues like chain length and tire clearance are making me look for a replacement. Any input on other handling factors such as chainstay lengh, wheel base and head/seat tube angles are more than welcome.
    What are the two bikes' seat tube angles? A steeper seat tube angle puts more of the top tube in front of you, because your butt will always find its preferred position over the pedals, and with a steeper seat tube angle, you'll adjust the saddle further back. Opposite is true with a slacker seat tube angle, you tend to sit further forward to compensate, thus shortening the top tube and reach.
    Shorter top tube usually equals shorter wheelbase given the same head tube angles. the shorter the wheelbase, the easier it is to ride tight switchbacks, and the more agile the bike's handling on twisty stuff.
    Short top tube puts more weight onto the front wheel so it's less likely to wash out, without having to consciously lean onto the front end to prevent washout.
    I like the handling of short top tube +longer stem, the bike "steers from the hips" like a good road bike. The opposite, long top tube and a short stem -- genesis geometry etc -- tends to steer more from the shoulders and arms, which always bugs me when I ride bike like that.
    If you never learned to get your weight back on steep descents, a long top tube and short stem can help prevent going over the bar, but that's really a rider skill issue, and I wouldn't want the above compromises just to compensate for not being to shift body weight for descents.
    Finally, with sloping top tubes, there's actual top tube length, measured along the actual tube, and effective top tube length, measured horizontal from center of seat tube at the top tube junction, to the center of head tube. These measurements differ, the more sloping the top tube, the more they differ. Be aware of that when looking at published specs.
    I prefer a shorter top tube and a nice long stem, and I'm man enough to handle the resulting greater agility. Genesis geometry is good for beginners who haven't learned the subtlties of bike handling.

  3. #3
    Out spokin'
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    ... the other side of the coin...

    Conversely, there are those of us with plenty of bike experience who prefer longer top tubes and shorter stems. Personally I ride bikes with ~25" TTs and ~110mm stems.

    It's mostly the taller, stronger, better looking riders who prefer this setup.

    --Sparty
    disciplesofdirt.org

    We don't quit riding because we get old.
    We get old because we quit riding.

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