Using a shorter fork, but with the same offset (rake) will DECREASE trail and quicken the steering. This is a common modification used to quicken the steering on triple-clamp type forks. Raising the tubes in the clamps effectively shortens the fork.
I did read about increased wheel flop that results from a shallow head angle and large fork rake. In analysing this, it turns out that the trail decreases more rapidly as the fork turns, when using such touring geometry, but is only relevent at low speeds and relatively sharp steering angles.
David Snyder
Auburn, CA
> The tricky thing is that shorter fork for a given headangle increases
> trail - and many people say more trail means more stability. But - that
> depends on the definition of stability.
>
> High trail bikes also have more wheel flop. So a bike can be very stable
> in the sense that it does not want to turn easily, but when it does start
> to turn the wheel flop causes it to dive fast thereby knocking you you
> off the bike if not careful.
>
> Now I'm not sure anyone has this all figured out, but Jan Heine's Vintage
> Bicycle Quarterly has been trying to quantify this issue in depth.
>
> Mike Kone in Boudler CO
>
> -------------- Original message --------------
>
>> As you may know I am trying to find a replacement fork for my trashed
>> Raliegh fork. It has mid 80's twitchy geometry. The ride I prefer is
>> more stable than was popular than the extremes we saw during the last
>> two decades. I like the short wheelbase it has, but don't necessarily
>> need the cushy ride provided by forks that have lots of rake. I feel
>> that a race bike should have just enough steering stability so that you
>> can take your hands off the bars and sit up for a bit. I could still do
>> that on my Raliegh because the frame was so straight, but the overall
>> effect was that a sudden breeze could send you to the pavement. I want
>> a bit more stability than that. I have a lovely Italian fork that
>> features more rake, but if I am to understand correctly, that a more
>> raked fork will provide more cush but less stability. If that is the
>> case than I should go with a fork that has less rake and therefore more
>> trail. Will the effect than be to provide more stability? That may be
>> the trouble with these specialized type bikes: Any less rake could
>> shorten the wheelbase to the point where it becomes a problem, and any
>> more could make the handling even more twitchy. Is this understanding
>> correct? This is where it pays to choose bikes that were designed with
>> what you had in mind in the first place.
>>
>> Garth Libre in Miami Fl.