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BMW Transmissions
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 — translator's note). 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 — translator's 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 — translator's 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.
SIDEBARS
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 through the 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 — translator's 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 — translator's 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.
