'68 R69s base gasket ??

[sherman980]

schrader,
The crankshaft "flex" you refer to is real but not the cause of the torsional "vibration" that a harmonic balancer is meant to address. Two very different issues. That said, when the rubber damper ring in the /2 era BMW harmonic balancer wears, the outer metal ring begins to rotate off-center which does create significant vibration and can result in a catastrophic crank failure (or wipe out your generator, mag, etc - basically everything under the front cover - if the bolt attaching it to the front of the crank were to snap from the force of the now "off-balance" balancer) if not addressed. That is the condition that I was referring to in my comment above that sometimes "the damper assembly did more harm than good".

[niall4473]

" For BMW to have gone to the trouble of adding a crankshaft vibration dampener along with the front engine cover mod to accommodate it, etc., after nearly 40 years of motorcycle engine experience, they must have had their reasons. But I just don't see the actual experience that says it is necessary. And interesting that the follow-on /5, /6, and /7 engines don't utilize a separate dampener either..."

Sherman, you may be right, perhaps it is a complete coincidence that BMW fitted a device to this engine which is designed to prevent this exact result, just a few months after this one was manufactured, and around the time that a few such failures had occurred in long-distance racing, and that the previous and subsequent models never required such a device, it did not feel very anecdotal when it swallowed up my entire motorcycling budget for a year, and when I resolved to do whatever I could to avoid a repeat of it, I am probably just being over-cautious, and the small amount of work and cost required to maintain the device in good condition is probably un-necessary.
Likewise I am most likely being alarmist posting my photograph and recounting my own experiences, which I thought was the point of these forums, in order to save other people having the same trouble, in fact, when it needs a new bush in it, I might just take it out and throw it in a box, after all, it isn't needed and I probably won't have another failure, but if I do, may I send you the bill?

[schrader7032]

Chuck -

Seems to me that with the flex combined with the offset conrod power pulses, the flex becomes a torsional type of motion back at the front. At least it seems to me.

[niall4473]

Shrader,
"Seems to me that with the flex combined with the offset conrod power pulses, the flex becomes a torsional type of motion back at the front. At least it seems to me."

That is it, more or less, the crankshaft acts as a torsion bar, the flywheel is a big mass which likes to stay still, or to keep moving once it acquires some momentum, so the torsional energy is absorbed by the actual twisting of the crankshaft, at a particular rpm, if maintained, that energy will accumulate so that the crank will be twisting the flywheel backwards and forwards until it becomes too much for it to stand, fatigue will play a part in that, too. Watch the famous film of the Tacoma Narrows Bridge for a similar effect, poor dog.
The harmonic damper is, in effect, another flywheel with a resilient bush in it which absorbs those twisting moments so that the crankshaft itself does not have to.
Sherman980 is correct that all engines do this twisting to some extent, but something about the R69S allows this Harmonic to develop within the normal RPM range, none of the other boxer engines do, the R69, particularly, did not. I have been playing with old BMWs for quite a while myself, and one thing that I have never seen is BMW admit that they had made a mistake, they will say that they have improved something, but it was already perfect, a sort of doublethink.
The bang when that crankshaft failed was very loud and I have no wish to hear it again, so I will stick with the damper and put up with the dirt and extra maintenance, if others don't want to, that is their call, but my conscience is clear.

[sherman980]

niall4473/schrader7032,
Appreciate the replies. Couple of thoughts... From an engineering perspective, the torsional stress issue and the axial flex issue are very different and have different engineering "solutions". That said, they both stress a crankshaft and their combination certainly will make the potential for a stress fracture worse. And the harmonics involved are a complex result of materials, mass, rotational speed, and a whole host of other variables. But without getting into a whole discussion on crankshaft harmonics, guessing BMW thought there was reason to add the harmonic damper or they wouldn't have done it. Whether it actually "fixed" the issue they were concerned about is also a question. One which we will likely never know the answer to.

My point was simply that in 50 years of wrenching on, riding/touring on, abusing, hot rodding and occasionally racing /2 era motorcycles, I have never experienced or seen a crankshaft failure like the one that naill4473 posted. Clearly based on the picture, it happened. Anybody else out there have another example of a sheared crankshaft failure like that? If so, please post. And certainly not my place to tell anyone to NOT run a harmonic balancer on their /2 (or earlier) era motorcycle. Just trying to put nail4473's catastrophic failure into perspective...

Hope that helps.
Chuck S.