THE KEY STARTING POINT
A necessary foundation for good handling is proper frame and chassis
alignment. Identifying the cause of poor handling conditions can be
tricky. This section covers
HOW to recognize a misaligned frame and chassis component
WHERE the problems are
WHEN to have the chassis components checked for misalignment
WHAT is the probability of a chassis component being out of alignment?
WHY check frame and chassis?
Some key factors for determining if it's frame or chassis damage, and
a definition of some terms:
1. Pull Bike
tends drift in one direction or the other
A common complaint is that the bike turns great in one direction but
wont turn worth a damn in the other. This forces the rider to
compensate by counter steering applying more steering input on
one bar than the other. In some cases, the rider then has to shift riding
position off the bikes center of gravity to maintain a straight
line. If you removed your hands from the bars (not recommended) the
bike would dive/pull in one direction.
So what causes
this condition? This can be caused by one or more chassis components
being misaligned. On a list of things to check, the top item would be
a twisted/bent steering head. (The center line through the frames
steering head). A bent or misaligned swing arm would be next on the
list. A key indicator for a misaligned swing arm would be excessive
chain and sprocket wear. Also a misaligned subframe could be a factor
causing the riders weight to shift off the center of gravity.
Other things to look for would be bent fork legs in the front suspension,
and bent or twisted triple clamps. Another indicator of misalignment
could be irregular tire wear.
When a bike understeers through a corner
The front wheel loses gripping contact with the road surface causing
the bike to run wide. This can happen entering, during, and exiting
a corner. One of the causes for this condition may be excessive rake
or caster angle. Example: if your frames spec rake angle
calls for 25 degrees of caster but your frames steering head angle
is 30 degrees, this will cause a bike to push. This condition will also
cause the bikes steering to be heavy and slow. This may or may
not occur in both directions. If it happens in only one direction, there
is likely to be a twisted centerline in the frames steering head.
If it occurswhen turning both directions, it is probably not twisted,
but is as described above.
3. Wobble and Weave
Instability of the steering head causing either high or low oscillating
frequencies ( gyro effect)
Weave is a low frequency large gyro condition. Wobble is a high frequency
small gyro condition of the steering head. Example: a good comparison
for wobble is a shopping carts front caster that wobbles. This
condition may be associated with a misaligned steering head, in which
the rake/caster angle is too steep. Example: Your bike frames
rake/caster angle spec is 25 degrees, but your angle is now 20 degrees.
This condition also causes steering to be light and excessively quick,
making the bike unstable in a straight line.
These are just some
of the main causes of the above described conditions. There can also
be many other factors that are not described in this section. They may
not necessarily be related to a misaligned frame or chassis component.
But remember, to get the best handling characteristics from your bike,
first eliminate any possibility of a bent or misaligned frame/chassis