HockeyTemper
03-01-02, 01:02 PM
Ok, as you can tell by my other post, Im in the market for a new bike. Im trying to determine how you can tell the difference in a bike by their geometry. I mean whats good for what. I see they have head angle, seat angle, top tube & chain stay....they seem to be the most important measurements in the geometry.
In the bikes Im looking at they all have a 71* head angle Im sure this doesnt mean theyll all ride the same. What other geometry info. would determine what the bikes more suited for?
The most critical frame dimensions are the length of the top tube and, to a lesser extent, the length of the seat tube, as these determine whether the frame can be adapted satisfactorily to your own leg length, torso length, arm length, etc. In general, a frame with a long wheelbase, a long fork rake, and slack angles will provide a stable, comfortable ride over long distances, but may feel soft or whippy during climbs and sprints. It will also provide generous clearances for mudguards and touring tyres. A frame with a very short wheelbase, steep angles, and a short fork offset will sprint and climb crisply, corner sharply, and respond quickly, but many riders may find its ride harsh or its steering "twitchy" and unforgiving. Braking efficiency also tends to be a little better on a close-coupled frame, because of enhanced leverage at the calipers. Exception: I cannot recommend my first-generation single-pivot Campy calipers to anyone.
The specified seat tube angle is the angle between the top tube and the seat tube. The specified head tube angle is 180 degrees minus the obtuse angle between the head tube and the top tube. They are typically within a couple of degrees of each other.
My 1959 Capo has a 104cm wheelbase, 72-degree angles, a long fork rake, and pencil-thin stays, because it was engineered for long races over cobblestones or for long unloaded tours. In contrast, my 1982 Bianchi, engineered for efficiency over much smoother road surfaces, has a 99cm wheelbase, 73/74-degree angles, a much straighter fork, and thicker stays. The contrast is dramatic and has almost nothing to do with the difference between manganese moly (Reynolds 531) and chrome moly (Columbus) steel.
Because the need to handle bumps and fat tyres, most mountain bikes tend to have even longer wheelbases and slacker frame angles than road touring bikes.
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