At 12:43 AM -0700 6/15/07, Donald Gillies wrote:
>Seriously, though, the bike has what looks like to me maybe a 71 or 70
>degree head angle (swag), which means LOTS of trail (see richard jow's
>1970's bicycling magazine article on trail).
>To summarize that article:
>Bikes with more trail tend to go into a straight line, (think shopping
>cart, where the wheel trails its swivel point) but when you finally
>wrestle the handlebars into a turn there is more "wheel flop" (think
>of a chopper motorcycle) as turning the bars doesn't so much rotate
>the wheel around a perpendicular line coming straight up out of the
>ground, rather, it rotates the wheel around a line through the hub
>that is nearly parallel to the ground.
When you go in a straight line on a two-wheeler, you always weave from side to side a bit to balance the bike. Thus, wheel flop is a factor even when going in a straight line, and a bike with a lot of trail and wheel flop doesn't need to be "wrestled into a turn." Instead, it will tend to veer off the course inadvertently.
Wheel flop is caused by the front of the bike lowering as you turn the handlebars. (The combination of fork rake and inclined steerer axis is the reason the bike lowers as you turn the bars.) So if you put weight on the front of the bike, this will have a tendency to turn the handlebars, because the handlebars never are pointing perfectly straight-ahead for more than a split-second. Wheel flop is a bit like power steering, reinforcing every handlebar movement, but only in one direction - away from the straight-ahead.
Wheel flop is a good thing - it helps you overcome other factors that stabilize a bike. A bike with too little wheel flop is very hard to steer at low speeds. However, too much wheel flop will make a bike veer off the course at the slightest provocation. The ideal amount of wheel flop depends on many factors, such as speed, tire size, weight distribution and also rider preference.
Many modern racing bikes have a lot of wheel flop, and you see their riders weave slightly from side to side as they ride along. How much they weave depends on the rider, of course, but there are many who prefer classic 1950s, 1960s or 1970s geometries.
The Spring 07 Bicycle Quarterly had a detailed article on "How to Design a Well-Handling Bicycle" that describes the various factors influencing bicycle handling as we understand them today. -- Jan Heine Editor Bicycle Quarterly 140 Lakeside Ave #C Seattle WA 98122 http://www.bikequarterly.com