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Shucking the Shakes
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Assumptions can be dangerous. We assume for example that because no one
has ever darted across five lanes of traffic in front of us that no one
ever will. Until it happens. We presume on the future. We engage in
blase acceptance of the status quo. But as the saying goes, we live and
learn. Assuming that the tired old method of checking steering bearings
-- you know, by jacking up the front end and grabbing the fork to feel
for play -- is valid for use on modern motorcycles, is also dangerous.
Because it isn't. Sooner or later, all the larger
late model Hondas and Yamahas develop a problem with the steering
bearings that results in a head shake when decelerating from 40 mph and
less. For want of a better term, we'll call it a "deceleration wobble."
The cause
There is a rule in motorcycle suspension technology which says that
problems in handling that occur under 40 mph are due to defects in front of the steering head, while those occuring at higher speeds are found in causes aft
of the steering. It's a tried and true rule of thumb, and a decel
wobble obeys the pattern. Many things can cause weaves and wobbles,
whether on acceleration or deceleration -- tire wear is especially
critical. But decel wobbles have their own special causes, and if the
front tire isn't excessively worn or the wheel badly out of balance,
the cause is almost always the steering bearings. But we're not talking
looseness. We're talking about something that is not addressed in any
service manuals, factory or aftermarket. Whether because the frame is
made of softer material or what, the steering bearing races "walk" in
the frame. That is, they shift in their recesses and become out of
square with the steering stem, and out of parallel with each other.
Modern motorcycle steering bearings tend
to "worry" -- shift back and forth -- in their frames, especially on
the heavier bikes. The resulting non-parallelness sets up torque forces
in the steering which manifest themselves as attempts by the fork to
correct itself, with the result: shimmy, shimmy. Again, the problem
isn't looseness. Mere tightening fails to correct the problem.
The following procedure is
one circulated by American Honda's District Service Reps. It is based
on the above premise, as well as a procedure found in Honda Service
Letter #126. Three tools are needed: a torque wrench, the special
factory steering bearing nut socket (Honda's is part # 07916-3710100),
and a good quality tubular 0-10 lb. spring scale. A floor jack or
something similar to jack the front of the motorcycle off the floor
will be handy, too.
Follow your manual's instructions for
removing the top triple clamp (Honda calls this a "bridge"), so that
the pair of special castlelated nuts becomes accessible. The upper one
is just a locknut. Remove it and set it aside, along with the special
washer. Jack the front end up off the floor, and feel the bearings as
you turn the bars each side from center. If the bearings are notchy or
the front end has a self-centering action, the bearings need to be
replaced, no second-guessing here. After replacing them if necessary,
continue. Get the front end off the floor. Turn the fork to full right
lock, and with the torque wrench and special socket, tighten the
bearings to 40-50 ft-lbs. The fork will be very stiff. Don't panic.
It's only temporary. Now turn the fork lock-to-lock, repeatedly, at
least twenty times. You will probably notice something interesting:
that ridiculously high tension will loosen up; the bearings will get
looser, indicating that they have squared up and settled into the
frame. In some cases, you won't be able to tell, but even if you don't notice the bearings loosening up, proceed. Turn the fork to full left
lock now and loosen the nut until it's just finger tight, then turn the
fork to the right lock again and tighten it to 7-10 ft/lbs.
Attach your spring scale
onto one fork tube, using a piece of shoestring or something similarly
soft so as not to scratch the tube. With the fork assembly pointed
straight ahead and the tire off the floor, slowly pull the spring scale
straight ahead until the tip of the fender arcs about one inch. Note
the poundage. You're looking for a 5-7 lb. pull. Five for motorcycles
under 600 lbs., more for heavier machines and those with fairings.
Tighten the tensioning nut as needed, a little at a time, and check
with the spring scale.
After adjusting, drop the special washer
back into place, and screw on the locknut, but don't tighten it. Though
you probably found the locknut jammed against the tensioning nut,
that's not the correct way to install it. It should be close to
the tensioning nut, but not jammed against it. Leave a little space --
about 0.020". Then bend the locktabs into the locknut to keep
the two interlocked. The locknut's job is to isolate the torque of the
bridge nut from the steering bearings. Reassemble the rest of the fork
per the manual. If a test ride reveals that there is still a decel
wobble, or the bike sways side to side like a rowboat (the bearings are
too tight), readjust to higher or lower spec as needed. Ride safely.
Mike Nixon
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