Originally Posted by GlowBoy
Depending on the bike and the rider, the more-responsive, "twitchier" steering (as pointed out above, they are mostly the same) could result from a combination of one or more of the following:
1. The reduced rotational momentum of a smaller wheel at speed, providing less resistance to changes in direction.
2. Applying conventional bike head tube angle and fork offset to a smaller wheel, resulting in less trail.
3. For some folders, a shortened wheelbase compared to bigger-wheeled bikes.
4. A larger rider on a one-size-fits-all bike that would be considered "too small" for that rider if it weren't a folder.
C'mon, don't overdo it: of the explanations above, twitchiness in small bikes is nearly entirely attributable to trail.
pengyou, trail is the degree to which the front wheel acts like a caster and so resists steering. You can measure trail pretty easily: draw a line straight down the steering axis and mark where the line intersects with the ground. The measure the distance between that spot and the spot where the wheel touches the ground. If the distance is large, the wheel will resist steering and want to go straight: it has a high degree of trail. If the distance is small, the wheel will feel "twitchy" -- you can easily steer it. It has a low degree of trail.
Trail may be useful or not. High-trail bikes are more stable at higher speeds, so racing bikes tend to have a lot of trail. But low-trail bikes are more maneuverable (so mountain bikes often have lower trail) and are much easier to carry loads on the front wheel (so porteur and other work bikes often have very low trail).
On a large-wheeled bike it's easy to make the trail large. In fact the trail can be so large that it's difficult to steer the bike at all. As a result, bike manufacturers may add a fork rake (a forward bend in the fork) to push the wheel axis forward and thus lower the trail.
It's hard to get lot of trail on a small-wheeled bike simply because of the wheel size. To get a lot of trail you'd have to move the wheel rotational axis backwards (a negative rake) or change the steering angle quite a lot (like a chopper motorcycle). Both of these might be weird to steer, and possibly not easy to incorporate into a folding scheme.
A zero rake is easy to achieve though. Oddly, manufacturers still add a positive rake on their small-wheeled bikes for what seems to be no valid reason at all. Dahon's 16-inch bikes (including the Jifo), the Mezzo, and even the Bike Friday Tikit have a small positive fork offset, thus slightly *reducing* an already dismally twitchy trail. The worst offender by far is Brompton, which adds a large fork rake. Brompton seems determined to make their bikes the twitchiest in the industry (a trail of just 24mm).
Last edited by feijai; 08-22-12 at 08:07 PM.