Understanding geometric impact of different frame designs

Discussion in 'Bike buying advice' started by thimble, Sep 9, 2010.

  1. thimble

    thimble New Member

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    Used to be that all cycles had a horizontal top tube. Now there are some with a top tube that goes from the handlebar stem to the rear axle, almost in a straight line. Like this Specialized Crossroads hybrid:
    Specialized Bicycle Components : Crossroads Sport

    What's the practical impact of such a design? I tried one of these in a bike shops parking lot and liked it a lot, but want to understand more about the effect of the frame design and how it works. Can anyone help out? Does this make for a stiffer ride that's responsive, how does it affect nimbility (being nimble?) etc.
     
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  2. tonyzackery

    tonyzackery Well-Known Member

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    Practical impact is manufacturers can produce less sizes of the bike (s, m, l, xl vs. 48cm, 50, 52, 54, 56, 58, 60, 62) with the diagonal top tube (called "compact") and still reasonably fit 95+% of the bike-buying public with simple adjustments (among others) to seatpost length and handlebar height (adjustable stem and/or spacers). Manfacturing less different sizes ultimately translates to higher profit margins...my $0.02CAN worth...
     
  3. thimble

    thimble New Member

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  4. alienator

    alienator Well-Known Member

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    What Tony said is a big factor in the popularity and proliferation of "compact" frames, that is frames whose top tube slopes. What practical difference such designs have for the rider depends and may be none. For riders of atypical proportions, compact frames can offer the benefit of having more stand-over clearance while offering longer reach to the cockpit (a benefit for people with comparatively short legs and long torsos). There may be no real performance benefits for compact frames over traditional frames (with level top tubes), so absolute statements about things like stiffness, "agility", and etc. can't be made. Such things are unique to each design.

    I don't think the article says that. If all other frame geometry metrics are equal, steeper head tube angles (i.e., bigger angles) create quicker steering, more nervous bikes. This is because steeper head tube angles reduce trail (the distance from the center of the tire's contact patch to the point where the axis of the head tube intersects the ground) and because the steeper HT angle causes a larger deflection of the tire at the contact patch than that caused by a more shallow HT angle. Less trail means that there is a lower re-centering force for the wheel which creates the more "nervous" behavior. Alas, all other things aren't equal as steering behavior is also modified by fork offset (the horizontal distance from the axle axis to the head tube axis). All other things being equal, shorter fork offset yields a slower steering and more stable feeling than does a larger fork offset. Wheelbase is another factor. A longer wheelbase will tend to be a bit more stable than a shorter wheelbase and will also tend to steer a bit more slowly (whether the rider can tell the difference or not is an entirely different matter.).
     
  5. thimble

    thimble New Member

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    So what you're saying is that it's all important but in the end it's just up to the user to decide...
     
  6. alienator

    alienator Well-Known Member

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    Pretty much.
     
  7. thimble

    thimble New Member

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    Understood. Thanks for lightening the mental load of what should be sheer enjoyment.
     
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