Re: Knackered crank?


jobst.brandt@xxxxxxxxxxxxxxxxxx wrote:
Peter Chisholm writes:

Can't argue with that. I mean, after years of square taper, some
bright spark at Shimano decided splined was the way forward.
After not too long, they decided to change it - the main reason
has to be because they're guff. As for outboard bearings, they
may solve the problems of Octalink, but do they do anything
square taper didn't? Other than wear expensively, I mean.

What is less obvious is that the square taper is not as strong as
we would like it to be, the square end being no larger than the
last joint of your little finger. I have broken a couple of these
and seen others.

In the past, speaking of square taper spindles, you yourself said:
"there are not many crank failures". And another very experienced
observer (Andrew Muzi) said, in the same thread: "do you mean a
spindle snapped in half? I've seen 2 or 3 in my lifetime".

http://tinyurl.com/rexel

But now, you make it sound a commonplace hazard in dire need of
fixing. What's up?

Approaching failure cannot be anticipated or inspected for.
Because it is a catastrophic failure, manufacturers have been
looking for a clean solution. As you see, it isn't easy.

The spindle breaks off in torsion, just inside the press fit of
the aluminum crank where stress is greatest. The fractures I had
initiated in the center of the flats leaving a spiral four toothed
face, the corners being the ends of the "teeth".

I've seen it 3 times in 21 years. Not common, certainly not a
reason for Octalink or outboard bearings.

Well, I've experienced it twice on my bicycle.


Twice in how many total miles? And, remind us of your size, please.



Looking at the last
joint of one's little finger, visualize pushing hard on a pedal that
is in effect a 5" lever (pedal load from the spindle) bending it while
a 7" lever (the crank) is twisting it. That isn't reasonable to me.



So, in the end, it seems more of a *theoretical problem* (just look at
that poor 'lil spindle, it looks ready to break at any minute) than a
practical one (the spindle *actually* broke twice over many, many miles
under a large, powerful rider).


Octalink definitely solved the spindle breaking

Which didn't happen very often.....


and assembly problems,
but did so while violating other design criteria, such as bearing
durability, and above all the backlash problem that loosens retaining
bolts causing spline failure.


So, Octalink obviated a very infrequent problem and replaced it with a
much more frequent failure mode. This is progress??

And now, Octalink is another (about to be) abandoned Shimano standard,
joining Dyna-Drive pedals, etc., in the Shimano Hall of Shame.

.

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