Originally Posted by
cdmurphy
Regarding the recommendations of Sutherlands, Park Tools, Shimano, et al, where we differ is that I place less stock in their "institutional wisdom." Sadly, most people, including engineers, really don't critically analyze things they "know to be true". (I'm as guilty of this as anyone.)
This is a matter of worldview, I suppose. If you're more comfortable taking the word of one admittedly bright person over the broad consensus of thousands of other bright and professional persons, I'm not sure our discussion will be very fruitful. But let me reiterate that I am not at all interested in being right or wrong about this, because that's a false choice. I'm interested in a general rule, and the reasons for that general rule. Such rules allow for exceptions, since they prove (meaning test) the rule. And out of that testing comes improvement. Exceptions do not destroy the rule until they become more numerous.
In my view, it is important to be skeptical of received knowledge. It is also important to try to determine why that knowledge exists, and also how it is transmitted. I apologize in advance for this, but let's spend some more time critically analyzing the stories we have.
Originally Posted by
cdmurphy
While modern day manufacturers are quite concerned with fatigue testing (Thank you, product liability lawyers), most of the lore regarding square tapers was firmly in place by the 1970's. Back then, bicycle and component manufacturing and design wasn't heavily engineered and tested the way cars, or particularly airplanes were. Most bicycle and component design was an iterative process. Build something that looked "good", then either beef it up if you had failures, or whittle stuff away until you did. (This is a perfectly valid, if slow way to achieve an optimal solution. Fancier, modern methods just get you close, faster.)
I'm not sure where you get your story from, but I think your description of the development process is largely inaccurate. I don't buy it. Bicycles were perhaps the single-most engineered consumer product of the 1890s and early 1900s in Britain, France, and the USA, and continued to be developed creatively for many decades to follow as new metallurgy and technology became available. It was very high-tech for its time; the bike industry wasn't just blacksmiths banging away in forges and chatting about it later in the village pub, this was huge companies with armies of engineers, and cities whose major industry was bicycles and parts. Many of those engineers migrated to the automotive and airplane industries, and brought what they learned about bikes into those industries. Others migrated the other direction, into the bike industry. There was always a lot of cross-breeding between bicycles and other engineering fields.
By the time square-taper cranks were invented in the 1930s, tapered press fits in machining and engineering had been around for almost 100 years. (To my knowledge, machinists recommend a dry taper for their tooling. But I'd suppose there's as much debate about this on their forums as we have about BB tapers.) Is it reasonable to assume that in its eighty-odd year history, not one of literally thousands of qualified engineers has questioned the "institutional wisdom" of dry tapers?
And is that, in fact, the institutional wisdom at all? That Brandt implied it was does not make it so. (He seemed to require something to topple in order to justify his attention to a subject.) It might be that it only appeared to become institutional wisdom because the reasons were not transmitted outside the community of engineers who design and work on them. All that came out was an instruction. For most people, that is enough. Most don't care why. They just want things to work.
Originally Posted by
cdmurphy
Jobst's conversations with engineers at crank manufacturers back in the day revealed their concern that initial over-tightening would lead to cracking of crank tapers. As far as I can tell, this was the original rationale for the "no grease" position. Without testing it myself, ideally over hundreds of cranks, I can't say if this is unfounded or not. Jan Heine's testing of one forged crank seems to point towards it not being a concern, at least on his high quality ones. Perhaps on earlier cranks, with lower quality alloys, this was a legitimate concern. I haven't cracked any tapers either, but that doesn't really prove anything.
Again, I would be very careful not to extrapolate Jan's experience very far. The most you can say at this point is that it applies only to his cranks. Period.
Regarding skepticism: In order for Brandt's account to be valid, We must take him at his word when he offers his account of why dry tapers were used. It seems to exist mainly as a rhetorical device to bolster his own argument. That's fine, but it's not engineering, and honestly, who knows whom he spoke to, or how many actual engineers he spoke to, or what their roles in the companies were? We just don't know that much. He could have embellished it to prove a point. I'm extremely skeptical of that sort of purely anecdotal account, especially from Brandt. He's at his best when providing empirical data and analysis and letting it stand on its own. Not so much when trying to prove a point in an internet argument.
Secondly, even if his story was true, what difference does it make? Maybe the people he talked to didn't know because they didn't need to know. There may be reasons he was not privy to, beyond either warranty concerns or mechanical perfection, that drive a decision.
And let me point out that that's what this is: a decision. There are consequences either way, greasing or not. That's the way it is with mechanical things. At some decision point, you weigh the consequences and make the call. Brandt's win-lose rhetoric notwithstanding, it's about understanding what happens either way
Originally Posted by
cdmurphy
The other issue the gets brought up frequently, is that pushing the tapers too far on to the crank will cause them to grow over time, eventually ruining the crank. In order for this to be a practical mechanism, the crank taper would have to deform in a plastic manner. For quality alloys, this doesn't seem to be the case. (Low quality, cast cranks may well be soft enough, but I suspect they might crack first, as most cast aluminum isn't very ductile.). In contrast to this concern, un-greased tapers commonly cause galling, and scoring of the taper surface during either installation or removal. I've seen several tapers that have been chewed up in this manner, resulting in large pockmarks and low spots that reduce the overall contact, and could well result in taper growth if installed too many times.
Lastly, and to add to what I said earlier in response to randyjawa's question: I am just about positive that Jobst was correct about the cranks squirming further up the spindle after some initial use. I have verified, as I'm sure you have too that a properly torqued crankarm, if checked again after a dozen miles will take another bit of rotation of the crank bolt to get it back to the earlier torque level. The only plausible explanations for this are either: a) the crank arm has moved slightly further up the spindle, reducing the tension on the crankbolt, or b) the crankbolt has loosened slightly. What makes b) seem unlikely, is that this mysterious loosening only happens in the first few miles, then doesn't proceed any further. I can't think of any mechanism that would let the bolt rotate several degrees initially, then stay put for any additional thousands of miles. This suggests very strongly that a) is indeed correct.
I don't doubt this phenomenon. I don't know if greased or dry tapers have anything to do with it directly. More on that later, I hope.
Again I apologize that my arguments so far have been light on engineering. One reason for that is that I am not a trained engineer (but my relatives were train engineers.) Another reason is that most of the people I discuss this topic with are not engineers either. So it usually boils down to "whom do you trust?" as I implied earlier. There are critical methods that can help us analyze that, and I think they can be brought to bear on this topic, as well as others.
So yeah, after having had these same discussions with mechanics and others for over 25 years, I'm weary of the method of argumentation that usually surrounds this. Brandt only fanned the flames of that when he weighed in on the matter. That flame caught on the tinder of the internet and hasn't burned out yet. So here we are.
I will say that I learned very early on in my career the "institutional wisdom" not to grease tapers. I received that and followed it, but not blindly, because there was always some debate about it. I have discussed the matter with knowledgeable people in- and outside the industry, and did my own research into the topic. I encountered exceptions to the rule. I have heard many arguments for and against it, and have tried to understand them on their merits, not having any particular personal stake in it beyond my reputation as a mechanic (which I take seriously.)