calbmwtriumph
com (CALIFORNIA BMW TRIUMPH)Subject: MO Transmission Translation
Mo-Magazin 2/96 PP. 34-39, Text and photos by Jochen Soppa
Translated by Kari Prager
From: karicalbmwtriumphcom (CALIFORNIA BMW TRIUMPH)
Subject: MO Transmission Translation
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.
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)
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?
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 — translator's 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.
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.
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.
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.
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. |
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.