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#42
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Threadless BB Cartridges: Why/Why Not?
Where's the hatrack writes:
[suggesting a current published non-proprietary standard that includes a larger-diameter BB shell as part of the spec] ISIS Overdrive? http://www.isisdrive.com/isisoverdrive/ This is hopelessly lost. This does not address the failures of BB threads the under size of the shell to house both a sturdy spindle AND bearings The 12mm increase in BB diameter over the current size isn't enough, then? Granted, I'd have plumped for a bit more OD so as to allow the use of robust and inexpensive standard ball bearing assemblies, but at least this seems to be a potentially firmer ground for improvement than the existing "standard" BB. and above all, I find the Isis spline not ideal for the task. Nothing on that assembly strikes me as a significant advance. As was demonstrated, spindles have broken because the transition from spline to shaft was too abrupt, in spite of it's larger diameter than the old square taper. No argument there; if they were going to go with a shoulder at all, they needed a lot more radius on the transition. Perhaps it's time for a Rev B in the ISIS standard. I think that the real problem is Shimano, however. As long as that monster in the marketplace categorically refuses to come to the table for any standards discussions, it is going to remain difficult to achieve a useful consensus on what the successor BB shell design should be. OTOH, I think that an independent agreement implemented by Trek, Bianchi, Cannondale and the other major players would ultimately force Shimano to produce cranks and BBs that acknowledged the change, as long as the successor design was contrived to allow the continued use of the old tech via adapter bushings for at least a reasonable period of time. Backward compatibility could be waived if the design has enough positive features and is an obviously reliable assembly that has been analyzed for peak stresses. I'm thinking of the threadless steer tube for example. In BB's the square taper, for instance, looks good at a glance but has excessively high stress in the taper that is no larger than the last joint of your little finger. These have failed in torsion. Its bearings are overloaded because adjustment is so stiff so that in use the entire load bears on one ball regardless of care. That is why these spindles spall so readily. Beyond that, the need for a left hand threaded right cup proves that forces are so high that this cup moves in use. Of course there is other long term evidence for this as well, such as the destruction of threads in the BB shell. A new BB design should readily pass this sort of scrutiny. In fact its design advances should be obvious to mechanical engineers who see it. I have seen no designs that approach this level. I believe it requires starting from scratch with a series of critical design reviews by people who understand the problems. This is difficult because there are few people who are both capable and have the experience in the field. There are such people, and Tom Ritchey and Damon Rinard come to mind. It is a difficult assignment because there are no obvious parallels in other machinery where such high forces at such low rotational speeds require lightweight precision mechanisms. Shimano is not where to look for new solutions because they have brought too many half baked ones to market to be trusted. Although there are some good and novel designs among their products, there have also been major blunders. I don't think the trend has changed. Jobst Brandt |
#43
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Threadless BB Cartridges: Why/Why Not?
wrote: http://www.isisdrive.com/isisoverdrive/ This is hopelessly lost. This does not address the failures of BB threads the under size of the shell to house both a sturdy spindle AND bearings and above all, I find the Isis spline not ideal for the task. Nothing on that assembly strikes me as a significant advance. As was demonstrated, spindles have broken because the transition from spline to shaft was too abrupt, in spite of it's larger diameter than the old square taper. though this is called "isis overdrive" this is only a proposal for a new bottom bracket and cup threading standard. the specifics of the spindle and bearings is not touched upon here. -Amit |
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