As a general rule when dealing with 700-C wheels, a trail of about
56mm will give a frame set "neutral" handling. My use
of the term "neutral" here refers to two things. First,
neutral handling means that a frame set will respond to steering
input in the same manner no matter what speed the bicycle is traveling.
Second, while cornering, a neutral handling bike will have neither
a tendency to climb out of a turn nor have a tendency to dive
into the turn, it will simply hold the line that the rider sets
up unless further rider input is applied.
Decreasing trail below the neutral range has a couple of effects
as you might expect. The first thing a rider will notice about
a low trial bike is that it appears to resist attitude changes
(or lean angles). It requires more physical effort to get the
bike to lean into a corner and more effort to get it to straighten
up. The second thing that you will notice is that while cornering
at higher speeds, the bike will have a tendency to climb out of
the turn on its own. Finally, you will find that the way the bike
responds to rider input is effected by the speed of the bike.
As you might have guessed by now, at lower speeds, a low trail
bike will have a tendency to want to go straight and do so pretty
much on its own. What you will find at higher speeds (like over
30mph) is that a low trail bike will become quite vague in the
front end. The front wheel will feel as though it is wandering
a bit and the contact patch feel will go away.
Increasing trial above the neutral range will cause opposite effects
for the most part. At lower speeds, handling response will be
light and consequently, attitude changes will be much easier.
During cornering, the bike will have a tendency to drop into a
tighter arc than the rider might have intended. Finally, speed's
effect on handling is reversed. While low speeds give a light
feel during handling maneuvers, high speed sets up a very solid
front end feel.
Although high trail frame sets give safer (more inherently stable)
handling than low trail frame sets do, high trail frame sets are
still inconsistent in the way they respond to rider input. Interpreting
from the basics above you can see why we usually aim for neutral
trail. It does not require the rider to consciously hold a bike
down during hard cornering, nor does it require different rider
input depending on changing speeds.
For some frame designers though, it is not always that simple.
For example, look at the way Eddy Merckx designs most of his frames.
He usually uses less trail than the "ideal" as he did
much of his racing on the pave and likes the way a low trail frame
tracks under really horrible conditions. Granted, they do not
act as consistently under a variety of speeds on good roads, but
they really work on northern Europe's country tracks.
Track frames are a whole different thing though. If you would
like me to, I can get into that. Let me know.
1190
Dorney Road, Breinigsville, PA 18031-1123 phone:1-610-398-1986
