On Sunday, May 25, 2003, at 11:46 AM, ADP wrote:
> I'm hoping someone here has some insight for me on geometry and ride
> characteristics.
Ann, Geometry is just one of the many factors in bicycle designs. In a real-world sense I think of geometry working on the way we fit on a bike in three ways. You either sit "in", "on", or "ahead of" the bike. With bikes with "slacker angles", <72.5, degrees have you sit in the bike, basically you feel more inside and between the wheels. You can feel this with classic roadsters and touring bikes. Strong effective braking, comfort, and stability are the characteristics of these bikes. Sitting on the bike is the balanced feel you get with bikes between 72.5-74 degrees. Most standard road bikes fall within this realm and have a good balance between power output, braking, and performance. In bikes with geometry >74 degrees you're ahead of the bike. Crit and time-trial bikes are good examples of this. In this range your bike gets really twitchy because your weight is so far forward. Braking is sacrificed for power output with these steep angles.
Think about everything in the terms of the front-center measurement (distance between vertical lines bisecting the BB and front hub). The majority of the rider's weight is in the back of the front-center area in slack angles and is moved forward with steeper angles. This of course is all based on that both the ST and HT measurements are the same. In general, matching angles only work well in medium sized frames. Let's look at the extremes in design. Most production bikes <50cm use steep seat angles and slack head angles. This produces a frame with short chainstays, toptube, and allows for toe-clip clearance. If you sketch this out on paper you'll see that the rider is quite forward in the front-center. This is what makes the small bikes seem unstable and is the largest complaint in smaller bikes. In frames >60cm you get slacker seat angles and steeper head angles. This is an attempt to make a long TT, keep weight over the rear wheel, and move the rider over the front wheel at the same time. These are compromises the companies have to make to fit the bikes into production parameters.
Ideally you would just scale up or down the bike. This is next to
impossible in a production setting, but is quite simple in the custom
setting. For a company to produce these scaled up or down bikes it
really takes separate production lines set up specifically to do it.
The great thing about custom builders is they're just that, custom.
This is one of those that things had been discussed ad nauseum on the
framebuilders list. If you need more information I'd advise going to
the framebuilders and bicycle-science archives at
http://search.bikelist.org/
--Sarah Vowell--
++++++++++++++++++