Subject: Re: Clutch weirdness
Author: Patrick Hayes
Date: Jun 29, 2005, 9:24 AM
Post ID: 1719093139
Kevin Graf wrote:
| tension all the time and at the rear of the pack, so, they are already compressed, as you pull the clutch in, the clutch arm pushes on the rod/pressure plate, this compressed the springs to their limit. The longer springs I got from mg, well were longer which gave the springs more travel to be able to disengage the plates all the way.. right? |
Careful with the semantics here. The springs sit in "front" of the
clutch stack. Always refer to Guzzi parts from the perspective of a
rider astride a complete bike. I realize when you are working on the
clutch they appear to be at the "rear" from your mechanic perspective
working on the back side of the motor but, they are in fact at the front
of the clutch.
Longer springs don't inherently give longer travel. The necessary
minimum dimension of the Guzzi clutch starts with dimension ZERO at the
inner surface of the rear or ring gear plate. In front of that surface
is a new 8mm friction disk. In front of that is the floater plate. In
front of that is an identical second 8mm friction plate. In front of
that is the movable pressure plate and then the springs. Stop here.
This sandwich stack has a fixed solid dimension when under spring
pressure. Guzzi only allows for a ONE MM variation in this stack
dimension (0.5mm wear for each of the two friction disks). When the
clutch stack is compressed under friction, the length of the spring has
no part of the computation, only its force. Doesn't matter if the
springs are three feet long (hyperbole). The clutch stack sandwich is a
fixed dimension stack under pressure. The springs under installed
pressure can be too strong or too weak, but they can not be too long or
too short.
Pushing forward on the throwout rod, we move the pressure plate forward
slightly which compresses springs and releases pressure on the sandwich
stack. Allowing for some flex and inconsistencies in part fabrication,
we only need to move the pressure plate forward about 1mm or so to get
full release.
The problem with inadvertent use of a longer spring is NOT its standing
length, but its fully compressed length. The relaxed standing spring
has space between its coils. You can only compress this spring until
the coils bind against each other and it effectively then becomes a
rigid cylinder. Can't compress it any further (unless you add literal
tons of hydraulic force). Presumably, a spring which is longer than
specified to begin with will also be longer than specified when it is
fully compressed to the coil stacking point. The only way it can end up
being shorter under compression and have "...more travel..." is if it is
BOTH longer AND it is made of thinner coil stock so that the resultant,
fully compressed, stacked coil cylinder is now shorter. Unfortunately,
the thinner coil stock would also result in less rebound force and less
friction force to the clutch stack.
So, it is entirely possible that springs which are too long, will bind
down to or get very near to the coil stacking minimum point during the
installation of the ring gear plate of the flywheel. In this
circumstance, nothing you do with the throwout rod will ever release the
clutch. Although I have never seen such, I suspect it would be easy to
notice. The hand effect of the clutch lever would start easily as it
compressed the single spring at the outside back of the transmission and
then it would stop rock hard as the throwout rod reached the clutch. The
clutch won't move at all or it may move only a very little before
getting rock hard stop as the springs reach their coils stack minimum
length dimension. Greg, is there ANY action or feel to the clutch by hand?
Now, if the springs were too short, then the clutch would activate just
fine, but they wouldn't be applying adequate internal pressure to the
pressure plate and the clutch would slip like crazy under throttle. Not
the circumstance we are discussing here.
Greg, I know you've done many clutches. The only caution I have is the
ease at which one can get the floater plate out of alignment and then
bend it during clutch installation and compression. If you bent it, the
clutch will always bind, regardless of the nice feel at the hand lever.
Just a thought. No answers here, just forum discussion.
Patrick Hayes
Fremont CA