The How and Why of BMW R-bike transmissions ....

[Sir Loin of Beef]

This article was forwarded to me by one of the (very helpful) folks at the BMW dealership where I bought my
RSL. The article goes into considerable detail regarding the engineering details and resulting operating
characteristics of the R-bike transmissions. It is, frankly, somewhat disheartening. However, the details
provide the information needed to properly shift the transmission. Enjoy.

-Glenn

======================================================================
======================================================================

Mo-Magazin 2/96 PP. 34-39, Text and photos by Jochen Soppa
Translated by Kari Prager
Not for commercial use. Not professionally translated. May be used by
non-profit club magazines as far as I am concerned but I did not obtain
rights or permission from MO for this.

****************************

BMW Transmissions - The MO factory visit with BMW supplier Getrag sheds
some light upon the darkness of this perpetual irritation.

CHATTERING TEETH

...A splendid new boxer. But still criticized for shifting. How
come? MO has a look at the builder of BMW's gearboxes.
Bitter reproaches for the new BMW R 850 R in MO 11/95. Tester
Guenter Wimme addressed the issue of shifting behavior unequivocally: "In
the old BMW tradition the great weakness remains: the gearbox. Difficult
shifting, long shift lever travel, graunchy shifts between first and
second, and still the old tale of woe in finding first at stoplights."
In that article's conclusion, our (MO's) journalist Wimme
recommended that the BMW factory seek out a new transmission manufacturer.
As we now find out, shortly after the introduction of the R 1100 RS, BMW
was actually in the process of looking... The Bavarians had already
contacted gearbox builders in Japan, Italy and Austria. But after ongoing
testing of the prototype proposals the source will remain where it has been
since 1978: at Getrag in Ludwigsburg, or more precisely, at the Getrag
subsidiary, ZWN in Neuenstein/Hohenloh.
Understandably, the people at Getrag don't like hearing the
reproach that they can't build a decent transmission for the prestigious
BMW bike, all the moreso since the 3,000 workers of the factory supply the
cream of the international automakers with their gearboxes. The list is
exceptional: Porsche, Maserati, Alfa Romeo, Alpina Bi-turbo, Jaguar, Ford
Cosworth, Dodge Stealth...
Of course, their biggest single customer is BMW. The complete automotive
line, from the 3-series on, is equipped by Getrag, and also the motorcycle
line: old Boxers, new Boxers, and K-series.
The exception is the F-650 with the Rotax motor. In round numbers,
ZWN/Getrag has built about half a million motorcycle transmissions for BMW.
Up till 1978, BMW built their own motorcycle transmissions. (...and they
didn't shift any better than these... translator's note)

HIGHER STANDARDS
Nobody needs to explain the secrets of gear-shaping to the people
at Getrag. The know-how of the company, founded in 1935, is incontestable.
The in-house development department with its state-of-the-art test center
is imposing; the production facilities make an exemplary impression.
Getrag is proud, and rightly so, of their high automotive
product-standard, which applies equally to the motorcycles. All the parts
are calculated to take the maximum possible stress from fully-loaded
machines, difficult mountain passes and maximum torque. This is a
reputation to which we can attest, having performed so many long-distance
endurance tests on BMW's.
Transmissions of the Bavarian motorcycles are still in spotless condition
after 100,000 kilometers and can boast tolerances which would be expected
from new parts.
Nevertheless, BMW's new boxers don't come close to delivering the
same standard of gear-changing that has been set by comparable machines
such as Honda's ST 1100. What is the dirty secret here?

O-RINGS AS NOISE-SUPPRESSORS
A clamor of complaints arose together with the introduction of the
new Boxer generation. The clattering noises emanating from the hot
gearboxes drove more than just those with delicate hearing back to the
dealerships in swarms. Gearboxes were replaced in large numbers, and the
public rapidly seized on the image of time-bomb gearboxes. (lit.
-"Kaputt-guaranty").
In fact, Getrag had built a faultless transmission according to
their contract, which, through a chain of circumstances derived from the
unique technical characteristics (of the BMW motorcycle), created an
acoustical nightmare (lit> "defektbombe").
Analyzed in detail, the truth of the story is revealed. A
peculiarity of BMW motorcycle design is that as in an automobile, the
transmission is separate from the engine, isolated in its own housing.
Between the motor and transmission is the single-plate dry clutch. A
characteristic of this design is that transmission noises can clearly be
differentiated. In normal Japanese engine architecture the motor and
transmission share a common housing. The noises emanating from cam drive,
alternator, pistons and transmission blend together, making it difficult to
assign exact sources to to the component sounds. The chorus of transmission
noises become a section of the overall symphony of mechanical noise.
Motor and transmission castings on the new Boxer are manufactured
by a pressure die casting process, which produce a thin-walled housing.
Delicate, thin-walled die castings do not block noise as effectively as the
somewhat coarser, thicker-walled sand castings (...as formerly used by BMW
- translator's note). The actual source of the annoying gearbox clatter is
the gear teeth themselves, as the flanks of the freewheeling gear teeth
click against each other, accelerated by by the uneven rotation of the
crankshaft. Imagine that you are a piston. Unlike an electric motor, which
revolves in a continuous and even rate, the individual rotations of a
piston-driven motor are characterized by a series of accelerations and
decelerations. The sparkplug fires, "paff", and the piston and connecting
rod thrust hard against the crankshaft, which accelerates. Then the piston
reaches bottom dead center, losing energy, as simultaneously the cam chain
encounters the resistance of the valve springs, which induces a small
deceleration in the rate of the crankshaft's rotation.
At steady, small throttle openings (as in neutral/idle - trans.
note) and at minimal combustion pressures (power output) this variation
in rotational speed is especially noticeable, moreso as the new BMW boxer
is programmed to run at very lean, environmentally optimal mixtures. The
resulting surges in rotational speed send a shaking effect through the
input shaft to the gear cluster. This is what generates the gearbox rattle
in neutral, which is all the more apparent when the transmission is hot and
the oil is thin.
Faced with substantial numbers of customer complaints, BMW had to
react quickly. To dampen the clatter the Bavarians resorted to the
introduction of rubber o-rings in the gearbox. These were installed between
the transmission shafts and the inner diameters of the freewheeling gears.
By this simple trick the disengaged gears are always under a slight drag
and cannot spin freely on the transmission shafts, their teeth cannot
chatter so easily against one another and the noise is noticeably reduced.
For a self-respecting transmission manufacturer to need to take
such a step is something of a fiasco. The efficient operation of the
transmission diminished by the o-rings, and the shiftability also suffers.
However, it happens that the noisy rattling in neutral is also an
issue in modern, emissions-optimized automobiles. The manufacturers have
resolved the problem with a so-called double-mass flywheel, which dampens
the torsional fluctuations before they can reach the transmission input
shaft. Such a flywheel cannot be used in the BMW boxer motor, as it would
be too heavy and might have durability problems withstanding the higher
revs of a motorcycle motor.
Thus BMW customers of the future will find o-rings in the gearboxes
of all the new BMWs, in spite of the fact that the noisy transmissions of
the first ('94) series, with the exception of the objectionable neutral
noises, were in fact the better-shifting transmissions. These early
transmissions, in point of fact, had no pattern of mechanical defects, only
an unattractive noise in neutral when the gearbox was hot.

WHEN FIRST GEAR WON'T ENGAGE....
The light turns green, you pull in the clutch and step on the
shift lever of our brand-new Boxer. It doesn't snick into first, it makes
no "clack"...On the dash the big neutral "0" still shows on the digital
gear indicator and first gear just won't go in. The "hard cases" just keep
standing on the pedal and let the clutch out slowly; the more delicate
riders go back to neutral, let the clutch back out and start the process
over.
How do you suppose the BMW automobile developers would react if
they were told that the new top-of-the-line 7-series BMW sedans would only
go into first gear only 50% of the time? Of course, they would say, "Guys,
get your tails right back to the drawing boards!".
So why did things turn out as they did with the new BMW Boxer? A
condition of the contract with Getrag was "No noise when first gear is
engaged!" Thus Getrag allowed a "roll-down" time with the clutch pulled in
of .8 seconds at operating temperature (note: the time necessary for the
free-spinning gears to idle down to a standstill - trans.). After the
introduction of the anti-rattle update, the roll-down time was reduced by
the increased internal friction of the o-rings to only .4 seconds. Thus the
transmission gears spin down to a standstill very quickly. If the opposing
gears have not stopped in just the right position, it is very difficult for
the shift dogs to find engagement in their intended slots. Result: the
gear will not engage.
This BMW characteristic is made yet more noticeable by the use of a
perfectly disengaging dry clutch. Unlike Japanese motorcycles using a
multi-plate oil bath clutch (which always "drags" a little when disengaged
- trans.), the BMW uses an automotive dry clutch (made by Sachs), which
disengages totally, without any clutch drag at all. Thus the BMW clutch
provides optimal declutching.
The Japanese transmission, even with the clutch pulled in, is still
under continuous rotation from the dragging clutch plates. The result:
first gear engages effortlessly. The disadvantage: epecially when it is
cold and the oil is thick, first gear engagement is often accompanied by a
terrible grinding noise in many motorcycles. And that is exactly what BMW
wanted to avoid.

CLANKS AND CRUNCHES
Shifting a BMW transmission requires great concentration. In
practice it is a good habit to maintain preload on the shift lever for a
moment or two after each shift. Thus one is assured that the gears remain
in engagement. This is especially important when starting off and when
making the leap from first to second. In general the BMW transmission, even
in the higher gears, shifts notchily and unwillingly.
The Getrag engineers have an illuminating explanation for this
characteristic as well. The BMW designers required a minimum of freeplay
between rotating parts in the gearbox, so that the "driveline slop",
eagerly criticized by magazine testers, would be reduced as much as
possible. Minimal rotational play, however, results in more difficult
shifting. This is easy to visualize, as the smaller the clearance between
the shift dogs on one gear and their respective openings in the mating
gear, the more difficult it becomes to find the optimal position for
engagement.
To banish this blemish the Getrag designers developed a two-tiered
(stepped) shift dog. The slightly extended, narrower tip of the shift dog
easily engages the (now relatively large) opening in the facing gear. When
the gears have turned just a little more, the entire shift dog now slides
into the engagement slot.
If a neophyte Boxer owner just nudges the shift lever and feels
the tip of the shift dog drop in, it may seem as if the shift has been
completed successfully. However, it may happen that under load the gear
jumps back out of engagement. The seasoned BMW gear-banger, by keeping the
pedal under preload, can feel both the first and second stage of complete
engagement take place. Subjectively, the impression the transmission makes
is "notchiness".
Loud clanks and bangs have been inherent in BMW transmissions since
the beginning of time. These noises have always been especially "robust"
when shifting down from second to first gear. This BMW peculiarity is best
explained by the basic mechanical characteristics of the gearbox. The mass
of the three-shaft transmission (input shaft w/torsion damper & spring,
intermediate cluster & output shafts - trans. note) and large diameter dry
clutch inevitably carries a great deal of rotational energy. There are also
rather large relative differences in the ratios and speeds of the gears in
the lower ranges and at low rpms. Finally, the solid shaft drive system
does nothing to dampen the impact of the gears' engagement.
Thus, the current situation... The question remains: who is guilty
of creating this miserable transmission? On one hand it would seem that the
bulk of the answer lies buried in the conceptual foundations of the
Bavarian Boxer. Tradition is often good for Marketing but not necessarily
best when dealing with matters of functionality. On the other hand, the
example of BMW/ Getrag shows what can happen when two partners are not
sufficiently engaged in communication during the development process... The
alloted time suddenly runs out; the World Introduction inexorably arrives,
and the dealers, gnashing their teeth, are left to whitewash the
shortcomings as best they can. It is lucky that in spite of this the
customers are convinced by BMW's offering and continue to buy Bavarian with
unbridled enthusiasm.
Lessons have been learned from this experience by both parties. In
the future Getrag/ZWN will lead the transmission development process as the
responsible vendor. Hitherto the final development authority rested with
BMW. At Getrag there is now a single, coherent design team in place. These
are not desk-jockeys, but actual motorcyclists in the flesh. You can
already feel the impact! MO has ridden a new prototype transmission in a K
1100 RS. And we can tell you this: total BMW shifting pleasure is coming -
and soon.

THREE SIDEBARS TO THE MAIN ARTICLE FOLLOW:

1. IN THE PLANS: IMPROVEMENTS TO THE BOXER TRANSMISSION
In the planning stages: high load roller bearings with
fine-particle oil filtration (clean bearings) will replace the previously
specified barrel roller bearings on the input shaft. By this means the
previously reduced (through the introduction of the o-rings) roll-down time
will be increased to approximately 1 second. This will make engaging first
gear from a standstill easier.
Simple shift dogs with slightly increased engagement clearance will
replace the stepped shift dogs. The transmissions of the K models have
always been equipped with this shift dog design. This measure will improve
the shifting haracteristics.
BMW has recognized that the rider likes to hear an acoustical
signal that first gear has been engaged. In the future even a BMW will
generate a little noise and produce a perceptible commotion through the
shift lever when first gear is engaged. It will all of course be of the
highest and most satisfying quality.
To this end a new dry clutch is being developed with a carefully
defined amount of clutch drag when disengaged. This measure will bring the
engagement of first gear from a standstill up to the appropriate standard.
Further detail work on the transmission will make the o-rings
superfluous.
The mentioned modifications will be introduced in series production
up throughthe model year 1997.

2. CONVERSATION WITH GETRAG/ZWN
Transmission-Questions: What the builders say.
MO spoke to Getrag development engineers Klaus Sommer and Eberhard
Schaetzle. Both ride their own privately-owned BMWs. Sommer a K 1100 RS
and Schaetzle the new R 1100 GS.

MO: Are the transmissions of the R models similar to those of the K
models, broadly speaking?
SOMMER: There exists similarity in concept only. For example the gear
ratios, shift mechanism and engagement dogs are different on the K model,
and the K model even has a different housing.

MO: Can Getrag-ZWN look back on other motorcycle experience besides BMW?
SOMMER: At Getrag some time ago we manufactured the Yamaha XS 750
shaft drive system, which was designed by Porsche.

MO: Getrag produces these transmissions at considerable expense.
Forexample the highest-loaded 5th gearset is finely honed to produce the
best possible surface on the flanks of the teeth. The rest of the gears are
shaved. Do these transmissions still need to be broken in?
SCHAETZLE: Every fully asssembled transmission goes onto a test stand
and is thoroughly tested with hot test-oil. Afterwards the hot oil is
extracted and along with it the first wearing-in material.The transmission
does not need to be broken in. It can be be fully loaded from the very
beginning.
SOMMER: After the first rides, the o-rings in the boxer gearboxes settle in
a little. Then the transmission may shift a little easier. This has nothing
to do with wearing in. Only the braking effect of the o-rings is slightly
lessened. After sufficient operation and with a hot motor a slight
ticking noise in neutral can be audible even with an o-ring transmission.
This ticking will be louder when the idle synchronization is not
meticulously adjusted.

MO: What kind of oil recommendation for the BMW transmission can you
give us?
SCHAETZLE: Oil should be seen as an integral part of the transmission.
When designing the transmission the load bearing capability of the oil is
part of the calculation. We fill the BMW transmissions with SAE 90 GL 5
gear oil manufactured by Fuchs, a brand mainly found as an OEM supplier.
SOMMER: SAE 90-Oil should be used throughout the whole year. It istrue that
in winter the shifting will suffer at first from the thick oil, but it
should improve during a very short ride. For those to whom this is
disturbing, because they make many short trips, for example, can use 75w90
GL 5 in winter as an alternative. In summer it must be changed back to SAE
90 GL 5.

MO: How do you regard oil additives?
SCHAETZLE: Oil additives are always factors which cannot be taken into
account when designing and building a transmission. In the best case they
don't do any harm. For example, it has not been researched yet how oil
additives might react with the new "clean bearings". We therefore
rigorously recommend against their use, especially in automotive
transmissions. Synchro rings, for example, function only if they can exert
some braking effect on the gear wheels. Special extra-slippery additives
can result in big damages.

3. THE BMW BOXER CONCEPT: ADVANTAGES AND DISADVANTAGES
A QUESTION OF PRINCIPLE
Both Boxer motors and the K series are built on the same fundamental
principles at BMW (automotive design - trans. note). The construction
principle: crankcase with additional cast bell-housing for the dry clutch
and starter drive gear. Bolted up to this is a separate transmission
housing containing the final drive output shaft. All the shafts are
oriented in the same direction. In the world of motorcycling manufacturers
today, only Moto-Guzzi retains an identical construction concept.

ADVANTAGES:
Motor and transmission as separate entities may make later repairs more
economical, since less labor time is required to service separate
components.
The separate gearbox can be lubricated with special transmission oil. Thus
in principle it should be possible to provide a "lifetime" lubricant. The
oil remains clean forever, since there is no wear material from the dry
clutch being deposited in the oil (as would be the case with a wet clutch
- trans. note).
The motor lubricant is not burdened with the wear and contamination of the
transmission and clutch.
Thus longer oil change intervals can be recommended.
All rotating shafts are lying parallel in the same plane, resulting in
better mechanical efficiency.

DISADVANTAGES:
Noise suppression of the separate transmission and crankcase housings and
large dry clutch bell housing
is not very effective.
The primary drive reduction must take place in the transmission after the
clutch. This increases the rotating masses in the transmission. This has an
adverse effect on shifting comfort and noise when changing gears.
Crankshaft rotation perpendicular to the rotation of the wheels may have an
adverse effect on chassis stability.
Crankshaft torque reaction is not cancelled by a counter-rotating clutch or
similar heavy rotating mass.
The motorcycle leans along its long axis under acceleration.
This constructions results in a long engine/transmission package. Thus a
shorter wheelbase, desirable for good handling, is scarcely possible.
Shaft drive vastly increases the unsprung weight of the the rear wheel
assembly. This impairs the suspension behavior of the machine.
Shaft drive mandates a wide separation of the footpegs. This is not ideal
for the seating position.

END

Regards, Kari

***********************************