Originally Posted by
Carbonfiberboy
That's why I posted my question in post 202. And thanks to all who made a useful contribution to my understanding. I can ride any bike just fine, but I wondered why some rode differently than others and what advantage the designer was after when they designed those bikes.
Here's another observation that someone might understand: A tandem team with whom I've often ridden climb a bit oddly when they stand, The front wheel tracks back and forth with every pedal stroke, describing a series of arcs, whereas the front wheel on our CoMotion tracks completely straight. I don't think it's experience because the wobbler captain has far more tandem time than I do. I think it's something to do with steering geometry.
I think I can answer that question. The wobbler captain you talked about like most of us probably likes the bike he has and is used to it but you've isolated a feature that you don't think you'd like and want to know what's causing it, right?
Sure enough, trail is the key. A bike’s trail is a meaningful way to address this feature primarily because it’s an easy way to easily relate the important factors that go into handling: heat tube angle (HTA), fork offset (rake) and wheel size (as you say, the steering geometry).
When riding a tricycle you manually change direction by using a handlebar to turn the fork (from its straight-ahead neutral position to the direction of travel). Unlike a tricycle, a road bike also leans into turns with the bars turning in the same direction of the lean and vice versa.
For the road bike with more trail, the bike will steer in the direction of the lean. This tendency then is for the fork to return to the neutral position as the rider leans back out of the turn. Accordingly, a bike with more trail is said to be more stable than a bike with less trail because of this self-correcting feature.
Having some amount of trail is an absolute necessity for a road bike. No one argues otherwise. More stability obviously is a good thing. The negative consequence, however, is that a bike with too much trail resists turning—i.e., seems sluggish. And, the wheel may flop to one side or the other at slow speeds.
For any given amount of rake you reduce the trail by increasing the HTA and for that there are desirable and undesirable consequences: a bike with less trail will lose some of its stability so when leaning out of a turn the rider must make up for the loss in steering-correction tendency by manually using the bars to turn the fork back to a neutral position. A bike with less trail, however, is more maneuverable at slow speeds and the fork is less responsive to leaning.
So, how about the wobble?
As discussed above, the bike with more trail has a self-correcting feature which is a tendency to keep the bike righted as the rider leans back and forth. Imagine however that you rise from the saddle and lean the bike from side to side as you pedal: for a bike with more trail the fork will steer into the lean and flop back when the bike is leaned in the opposite direction. Accordingly, the larger the trail the more the wheel will swing back and forth in such circumstances.