Get a tape measure, 6 foot straight edge, level, chalk and
a clean, dry,
level floor to work on.
Hold the bike upright and have someone sit on the seat while you take these
measurements. Align your level with the axle centerline and run it
straight down to the floor.
Put a mark on the floor at that point.
Now use your straight edge to follow the angle of the neck
(Not the fork angle, the neck on the frame) all the way to the floor.
Put a mark on the floor at that point.
Measure from the first mark to the second. It should be between 2 and 6 inches.
(This is a commonly held theory, although higher trail numbers have been used on
some
bikes without incident).
What this all means to you:
Too Little Trail:
With too little trail, the bike will handle well at slow
speeds but might develop a
high speed wobble that could be very dangerous and possibly fatal.
***Never set your bike up with negative trail***
(neck mark on floor behind axle mark).
Too Much Trail:
With too much trail, The bike will be hard to steer at high
speeds
(it will want to go straight no matter what you try to do)
and it will be unsteady at low speeds.
Normal Trail:
With 2-6" of trail, the bike will handle well at both high
and low speeds.
If you use a fat rear tire, you should keep the trail as high as possible.
A slightly larger trail is also practical for touring (4-6").
Another problem is too much rake.
With too much rake the tire will flop over rather than steer.
Just something to keep in mind if you prefer function over form.
There are many bikes out there with astronomical rake numbers although
many of them are more show than go.
Of course, there are exceptions to every rule and this is just a basic rule of
thumb
to point you in the right direction.
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