Why do BMW/Bosch Airhead Alternator Rotors Fail?
(and, replacing a rotor)
15-E
rotorsfail.htm
©
Copyright, 2011, R. Fleischer
Section 1...why do they fail?
I have thought, many times, about why rotors fail. I have
some basic ideas. Back in the early days, rotor windings were held in place with
some sort of lacquer or shellac substance. After enough heating and
cooling cycles, it might begin to flake off, letting windings move about
from centripetal force. I don't
think the stuff did nearly as much good as modern epoxies applied
in a vacuum chamber, but it was pretty much standard treatment for motor/generator/alternator windings back then.
I have personally seen rebuilt rotors fail from not being
assembled correctly...one was a brand-new rebuild, right out of the box. I have seen rotors short circuit; but most have OPENED, and
sometimes that is intermittent, and the internal connection can close at some
rpm....or open. It's a mixed bag. If the rotor is properly assembled, and then vacuum
impregnated with epoxy, it should hold up darn near forever.
Rotors run fairly hot in the core, particularly when a high
percentage of alternator maximum output is used. Rotors on the faired RT
models (particularly the early ones with NO louvers in the front
fairing piece) run hotter. I think that the wider the heat-cold range the rotor
is exposed to; and, the more often, the less the reliability.
I believe that higher output usage is part of the problem. With rotors, voltage regulators, and diode boards....the higher the output, the more heat is generated in these parts, and cycling heat-cold tends to bring about failures in things electrical and electronic. A combination of a RT fairing, and high output, likely is a contributing factor.
Another possible factor is that the air that passes over
the stator, rotor, and diode board, is not likely the same quantity,
nor velocity,
between some models. The air flow is not the same, for instance, between the
clamshell air cleaner models and the rectangular air cleaner models. Take a look at
the air flow differences....look at the outer timing chest metal cover
which is NOT the same design on all models. Look at how air
gets inside, passes around and over the parts, and exits....and you will see why I
think this is a factor.
I have NOT run any sort of survey to prove this
one way or the other....by belief that a goodly percentage of rotors fail due to how
the the rider shifts gears, and how often, over the many miles. Those that shift abruptly downward might be particularly prone to
the sudden acceleration forces on the rotor windings. I have not made
up my mind as to whether or not JUST high rpm is a factor, but I
suspect that it IS involved.
The Omega alternator rotor runs very hot in its core. That
alone might detract some from its reliability. But if
the rotor is vacuum chamber impregnated with epoxy, it may be fine. It
remains to be seen how many fail over the coming years, as miles on them
increases and the number sold increases. The EnDuraLast should be immune to rotor failures, but whether
or not the system is more or less reliable than stock...or the
Omega.....it will likely be some years before we know.
I think we just do not have enough data to know if these two
aftermarket alternators will be more or less reliable than the stock
alternators, as the miles are accumulated.
It appears that the last of the airheads rotors are more
reliable than the earlier ones, but it is difficult to make an informed
statistically good statement....after all, they are also YOUNGER, and have had,
in general, fewer heat-cold cycles and miles.
I suspect that the only way to determine what is going on
would be to do a rather detailed survey of miles, number of heat-cool cycles, rpm and
shifting types
(smooth, match rpm?...or tends to shift a bit jerkily....)....
average trips being very short, short, or long....RT? type of front outer
cover? Clamshell model?....etc.
Many things could contribute...making it difficult to say what
is going on....and making it somewhat meaningless (not totally,
perhaps) on how long rotors last, or do not, for any one particular rider.
I have had ONE rotor fail on one of MY
Airheads; it was an improperly made aftermarket rotor. I've also had ONE diode
board fail (standard non-bent leads on as delivered Wehrle board, 1983
R100RT). Since "Airheads" begins with the /5, I suspect that means half a
million miles. I have a LOT of miles on /5 bikes...with NO alternator
problems. I am not so sure ANYthing can be concluded from THAT. Can we conclude anything much from my statement about MY
rotor experiences on RT's? MAYBE...maybe NOT. I have always done FEW short
jaunts; never overtaxed my alternators; don't shift down with snap rpm
rises. On the other hand, much of the mileage has been on RT's with rectangular
airboxes...which are definitely harder on the alternators, due to heat effects
(granted one of my three RT's came with the NON-louvered front
fairing and I DID soon after purchase, modify it).
I think it would take quite an in-depth survey to determine what was causing the various failures. Even if we could determine trends, etc......what would you DO about it? Increase cooling to the parts?...HOW??...yes, there ARE a few changes that help...some I am not sure about.
Section 2: How to remove and replace a rotor
FIRSTLY.....you MUST HAVE THE CORRECT TOOL!
You can buy the tool from BMW; or, make an equivalent. Joe ('cuda)
at one time was making his version of the needed tool, and there
is an article on how to make his type, on the
www.airheads.org
website.....under Technical Tips....dated 5/01.
So, to replace a bad rotor, your FIRST STEP is to
get the factory tool, or an aftermarket one.....or make one.
You can get a factory tool from BMW dealerships under part number 88-88-6-123-600.
You can also get an aftermarket version from many
places, including some dealerships!
Here are two places that have their own tool:
www.northwoodsairheads.com
www.cycleworks.net
NOT .com! Cycle Works LLC, located at
5805 Haskins Street, Shawnee, KS, 66216 (913) 871-6740.
Contact the owner, Dan Neiner, at:
Dan@cycleworks.net ((NOT
.com!!)). Dan bought out the tools+ section of Ed Korn's
well-known Cycleworks business.
WARNING!!! DO NOT FAIL TO USE A
HARDENED TOOL!!!
Second step is to disconnect all battery negative leads.
Third step is to pull off the push-on connections at the stator,
and remove the three long screws holding the stator to the
engine. The stator housing, with the stator attached, may be a
bit stuck to the engine. Pry on it CAREFULLY, using a piece of
hardwood, between the engine case and the stator....you will see
where to do that at three places...do it a bit at a time, evenly all around;...you will see
why...until the stator and housing come out as a unit.
Be
CAREFUL that as you do this, that the brushes do not hang up on
the slip rings, as you can break the brushes...or the plastic
brush holder if not cautious.
DO NOT!! pry with a metal tool
(unless very careful)...you do NOT want to nick nor scratch the
stator laminations nor nick the stator copper wiring. When
replacing the stator/housing after replacing the rotor, you will
want to have the engine case and stator where they fit together,
cleaned up some. I use a rag, then a bit of sandpaper or a wire
brush.
The rotor is threaded, and those threads are used only when
removing it. Otherwise, the rotor is held onto the
crankshaft by the hex headed bolt you see when you look at the
rotor. That hex headed bolt tightens the rotor to the
crankshaft, as the nose of the crankshaft is internally threaded. Once the tapers have been tightened, they are
unlikely to come apart easily, even if the bolt was
missing...which is why only light torque on the bolt is really
needed upon reassembly. I tighten LESS than the book says.
A bit deeper into this:
The crankshaft nose is drilled and threaded to accept the
stock bolt you see. The interface between rotor and crank nose is
a TAPER. The taper must be VERY clean and DRY when replacing the
rotor. In order to remove the rotor, this tool I mentioned
is needed. How the tool works is the same whether a homemade
multi-piece type, or the official tool (one piece). The
tool, one or several pieces, is inserted into and through the
rotor, and the inner end of the tool is not threaded!! The tool
has threads near its hex head!...that screws into the rotor
threads. The far end (tip) of the tool, as noted it is not threaded, but it has
clearance, 'bottoms' in the drilled/threaded hole of the
crankshaft which is not drilled too deep. Thus, as the tool is tightened, it forces the rotor
off the
crankshaft nose.
As you tighten the bolt into the rotor,
and it can take a fair amount of torque, ....suddenly the
rotor will pop off usually with a bang noise, and fall;....so DO
have your hand ON the rotor as you tighten the tool.
Save the bad rotor. Rotors
can be rebuilt and have some value. Have the bike in
gear; rear tire on ground...you might even have to hold the rear
brake....otherwise you MIGHT be rotating the motor as you tighten
the removal tool.
The HOMEMADE removal bolt tool is made from the same grade
8.8 metric steel bolt material (or 10.9). The factory tools
I have seen are grade 8.8.
I prefer to own the factory style removal bolt, which is one
piece. I usually keep one in my on-bike tool tray on my
airheads....and even carry one in my K bike tool tray....never
can tell when I will need to help someone.
WARNING...and heed this
well! Under NO CIRCUMSTANCES should you use a common low
grade bolt when making the
tool. Use 8.8 or higher! There is a great
danger with a softer bolt that as you tighten it, the innards
will BEND, trapping the tool part inside the rotor. THEN you
have a truly nasty job to deal with.
When replacing the rotor...clean and dry (not even fingerprints!)
on the tapers!....and the stock bolt need NOT be overly
tight....it is the TAPER that really holds the rotor in
place...the bolt is a safety in actual effect.
rev:
09/14/2010: add section 2 on how to replace
a rotor
06/24/2011: Clean up and clarify minor points.
© Copyright, 2011, R. Fleischer