Lightened Flywheels
lightened-flywheel.htm
Section 60, sub-part 2A
© Copyright, 2012, R. Fleischer
(except the photo)
Below is a photo of a properly lightened
flywheel. Lightened Airhead flywheels are sometimes
installed on the early heavy
flywheel models (pre-1981). After that date, BMW
changed to a totally different type of
'flywheel', which is called a Clutch Carrier and is very much
lighter, and the clutch
parts were totally different, and the transmission input shaft
splined area was also shortened to match. The entire assembly was
much lighter. This accounted for
some of the lack of smoothness of the later models, as the engine
had less stabilizing/smoothing mass at the crankshaft output.
This was particularly noticeable at idle rpm, but also noticeable
at higher rpm,
while actually riding. The later models generally
required quicker shifting, as the rpm drop was faster when the
throttle was chopped, not to mention the often sudden effect on
the rear wheel upon a throttle chop.
Lightening the flywheels on the early BMW's will
enhance shifting, by enabling faster shifting,
because the flywheel has inertia, and with less weight, less
inertia, so the flywheel...with engine crank, etc., attached,
will SLOW DOWN at a faster rate during shifting. You will have
smoother shifting IF you shift FASTER, before the rpm drops too
much (assuming downshifting). The acceleration will be somewhat
faster, as less mass is
being speeded up. If you twist the throttle in neutral, the
engine VERY noticeably speeds up faster. I don't find THAT
important. ...vrooom vrooom...?? I can say this
another way....there is improved engine braking.
There ARE downsides to lightening a flywheel. Engine vibration will increase, but
not too excessively, as the weight of the assembly is still not
as light as the 1981+ models which do vibrate more.
In my
opinion the sensitivity to the preciseness of the carburetor
synchronization will increase. Some of the smoothness, ...some of
this is almost intangible...will be lost, but, not too
excessively.
Depending on the year and the transmission, shifting can be LESS
positive in actually finding the gear/neutral. The so-called
'shift kit' incorporated a number of changes. One of the changes
was
how the detent that identified being fully in a gear (or, in
neutral) works. That change in the cam plate is why there are
two types of neutral switches...one actuates in a valley, one on
the peak.
You will need to shift faster, and perhaps spend a bit more
effort on matching rpm when shifting. One other downside is
that the engine may be more prone to stalling if not fully
warmed-up; and you will have to
slip the clutch more when starting out....this is a reason I do
NOT recommend lightened flywheels on sidecar rigs.
One final thing needs to be said. It is my belief that a
lightened flywheel will promote FASTER WEAR on the transmission
input shafts.
In case any engineer-types read this, NO, I have never heard of
crankshaft cracking/breaking, from lightening the old
flywheels....to explain this, flywheels act as dampeners for
crankshaft torsional vibration modes.
The old BMW's are never going to shift like a Japanese crotch
rocket, no matter what you do to the clutch or transmission. You
can improve things by fine-tuning the transmission
innards....but,
just how far DO you want to go? Are you REALLY taking this
bike onto a racetrack....or do you like its smooth gentlemanly
manners when out for a ride?
You PROBABLY would like the lightened flywheel. I
know I always did....and I think most riders of 1970-1980 BMW
Airheads WILL like the lightened flywheel effect. Do
I recommend it?....YES....but ONLY if you understand and are
willing to accept the drawbacks, see above!!!
The photo below is of a properly lightened Airhead flywheel.
This photo was furnished by Beemershop at my request, as I did
not have any photos from when I lightened flywheels.
The Beemershop (Ted Porter's) is an independent
BMW motorcycle repair center, located in Scotts
Valley, California (831) 438-1100.
www.beemershop.com
They have the ability to
furnish these flywheels, or machine yours, by numerically
controlled, and thus precise
machining methods. It is VERY important that
flywheel
lightening be done properly. There is NO NEED to machine
off the flywheel outer edge marks (S, OT, F, Z...).
NOTE CAREFULLY the needed outer meat
that was left to support the starter ring, threaded clutch
attaching boss places, etc.
HINTs: When installing any Airhead
flywheel, have the pistons at TDC, and OT showing in the timing
window, otherwise you may install the flywheel in the wrong
position onto the crankshaft.
Besides this, do NOT tighten the bolts right away, but have them
SLIGHTLY loose, and CENTER the flywheel on the bolt shoulders, by
trying to move the flywheel circularly clockwise
and counterclockwise...they sometimes show a slight amount of
play. Progressively, somewhat criss-crossedly, tighten the
bolts in stages. Refer to other pages for the
tightening torques,
which vary considerably between years and models. Be
very careful not to overtorque the early (/5 and earliest /6)
bolts, which are 10 mm. The last of the airhead
models used, per
BMW, oiled threads, and a very high torque. I prefer a
somewhat lower torque, although I have seen no reported problems
with the high torque settings. There are two other articles
on the flywheel,
one being a WARNING article, on this website.
The main article that has all the information on the flywheel
torque values, and associated information... is the
CLUTCH.htm article. It is Article
60, subsection 9, but you can click that hyperlink.
Rev:
NEW PAGE, and released 01/28/2011
01/29/2011: go over, fix minor typos, and clarify a few
things.
02/08/2011: Minor clarifications
© Copyright, 2012,
R. Fleischer (except photo)