Originally Posted by
jimmuller
I'm not sure this is the complete answer but here is a point not mentioned yet.
Even if rake is "correct" such that trail is the same between one frame and another, the shallower head angle will induce more camber change for the same amount of steering input. In other words, the wheel leans into the turn. This lowers the wheel center.
With a "normal" bike, the non-zero trail means that the contact patch rises when the steering is turned, which really means that the wheel center falls, which means that gravity wants to pull the wheel sideways (i.,e. flop over). But now imagine two bikes, one with a vertical head angle and no rake, and one with a "normal" head angle but enough rake to give it zero trail. The flop-over contribution from trail is zero in both cases, but the camber induced by the non-zero head angle still means the wheel center will fall and thus "want" to flop over.
One final question is where the "extra weight at the front" is located. If it's in a handlebar bag it is in front of the steering axis. If you're talking about rider weight, it is obviously further back. So you'd need to be more specific.
See, that is the thing.. OK, wildwoods question. I came across this when building my tandem frame. Tandems are funny and what everyone seems to be talking about relates to the amount of weight on the front wheel and speed. Tandems and tourers have a great deal of outside forces trying to effect the balance. I don't know the answer.
If you take the average British roadster and try to ride the sidewalk at a snails pace, it's difficult. Take that same roadster down a choppy dirt road at speed and it's pretty darn nice. Also, with tandem it's better with steering geometry that is less effected by subtle lean angles.
The head angle seems to create a leverage ratio. If you had a 90 degree head angle, the bike would ride a circle radius that matched the angle of the handlebars. As the head angle gets slacker the resulting turn radius becomes larger. Lift begins to take place and the contact patch increases. It seems a balance of "wanting to go straight" and the ability to initiate lean lean in a short amount of time is pretty ideal. Weight distribution seems to be what changes day to night with a certain set of numbers.
I think trail provides lean "effort" with more leverage to upset the balance. This allows any outside influence to effect the balance. Though that does not really sound correct. I think it's being done with a steeper head angle and the resulting lost trail is how it's described.