Phil, Non-Squid wrote:
> jim beam wrote:
>> Qui si parla Campagnolo wrote:
>>> jim beam wrote:
>>>> bfd wrote:
>>>>> RS wrote:
>>>>>> I want to try a compact on a bike with an Ultegra 6500 crank on
>>>>>> it now and see the Richey WCS compact crank uses the same
>>>>>> octalink BB. Anyone have any comments on this Ritchey crank?
>>>>>> Shift well? thanks
>>>>> Do a search, you may want to check this out before getting one:
>>>>>
>>>>> http://www.flickr.com/photos/swoo/sets/533276/
>>>>>
>>>>> Caveat Emptor!
>>>>>
>>>> entirely predictable - i said as much last year. "i-beam" profiles
>>>> like that are great for static loads but highly susceptible to
>>>> fatigue.
>>> BUT we sold more than a few of these w/o any issue or problem. I
>>> can't argue that the one in the pic broke but we saw no problem.
>>>
>> give it time. believe me, campy and shimano use their rounded
>> profiles for a reason.
>
> Err, shoe rubs on the neutral axis as opposed to the site of highest stress?
shoe rub is not really a feature of this fracture. and the neutral axis
is inside the crank arm.
>
> This flawed design would encompass nearly the entire RaceFace line,
> something they've been doing for a long time.
> http://www.raceface.com/components/component-pop-ups/Deus-crank.jpg
>
> Remember the Turbine LP, which seem to break a lot. Ask alt.mountain-bike:
>
if you have an i-beam, any load resolves so that maximum skin stress is
at the sharpest edges. this is unfortunate because those regions are
also the most easily damaged. even a tiny stone chip or scratch can be
sufficient riser to initiate fatigue. the component is also
increasingly sensitive to surface finish quality in this situation too.
if you therefore want to make the most of your fatigue life, you use a
rounded profile, no sharp edges.
iirc, race face, while it does have the basic i-beam profile, is not as
extreme as the ritchey. it's also machined out of very high quality
material. from a production standpoint, this is a bizarre setup. faced
with small production runs, cnc production of complex profiles from
solid block is the way to go - no question. and use of expensive
material will help fatigue performance, but they're stuck between a rock
and a hard place because fully shaped [rounded] profiles are
prohibitively expensive to make this way - better to stick to profiles
that take single passes using cheaper tool bits, hence i-beam.
big picture, better mechanical properties are achievable with more
economic material when forged to shape, then using more limited
finishing operations. this is what most other people do. initial
tooling costs are higher, but material costs are lower and performance
can be better. race face are in quantity production mode using possibly
the most expensive materials and production process possible. it's far
from the best solution either for them or the consumer.
