No Rocket Surgeon
Join Date: Jan 2005
Location: Corona and S. El Monte, CA
Bikes: Cannondale D600, Dahon Speed T7
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For one thing, it helps the bike track straighter.
If you take a bike that has a fork with the dropouts offset ahead of the head tube centerline and turn the handlebars, You will notice that the front of the bike raises slightly. When you have your weight on the bike, the wheel will want to point straight. Much better than that floppy wheel on the shopping cart!
Another reason is this: I helps reduce bending in the frame. Mostly, you will see this design in road bikes.
On a properly constructed frame, the trapazoid formed by the head tube, top tube, down tube and seat tube will behave as a triangle. Find the imaginary point where the top tube and down tube would intersect in front of the bike. The front axle should be directly beneath this point. In a static loading condition, the tubes will be in compression only, with no bending.
Before you flame me on this, lay it out and do the calculations. It works. I'll bet my Masters in Mechanical Engineering on it!
Last edited by eubi; 01-07-05 at 07:54 AM.