Re: 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
.

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