On Apr 28, 11:25*am, Jobst Brandt wrote:
Peter Cole wrote:
EXECUTIVE SUMMARY
Wouldn't it be more in keeping with the actual forces a crank has to
resolve to lighten/decorate it on its top and bottom surface rather
than at the front and the back?
****
Something about what has been called the "vanity" machining/forging on
bicycle cranks bothers me.
Consider a crank in action. *At the pedal end there are two
directions of force on the crank, a circular motion roughly in the
plane of the crank (if we ignore the angling on the crank to clear
the gubbins), plus an offset twisting moment to the outside on the
pedal, which is at right angles to the crank. *The offset force is
stayed by the bottom bracket end of the crank, and the observed
twist will therefore be larger at the pedal end. *From the point of
highest twist there is then an unwinding action as the crank
rotates.
It seems to me likely that over a full rotation the force in the
up-down plane will be larger than the twisting force on the crank.
Whether at the point in the rotation where the twisting force is
the largest, it is fact larger than the vertical force in the
crank's plane of rotation would depend on the design of the crank,
the force of the pedalist, and the exact offset of the pedals from
the plane of the crank's rotation; we can abstract from these
details because my problem concerns the principle of force in the
crank, not an exact measurement.
Given this description of the forces on a crank, surely it follows
that any lightening (or vanity machining/forging) should be done on
the crank's top and bottom surfaces, not its outside and inside
permanently vertical faces. *The tendency for vanity fluting by
machining or stamping on the classic model is towards creating an
H-beam or U-beam crank. *In practice, as commonly seen on bicycle
cranks, the beam lies on its side with the connecting web vertical.
That's what bothers me. *Shouldn't the two deepest faces be applied
in the vertical plane where they will be able to resolve the most
torque, with the web perpendicular to them? *That is exactly the
opposite of the arrangement we invariably see now.
It seems to me that, because of the engineering considerations I
have laid out above, such "vanity" flutes on the vertical face of
the crank can have no structural justification, indeed the opposite
applies: their engineering effect is negative and destructive.
Such fluting merely creates undercuts which won't survive years of
flexing without becoming the locus of a fracture. *Lightening
machining/forging if considered necessary should, if I am right, be
carried out on the top and/or bottom face of the crank.
Jobst has frequently posted on crank failures and causes. *Several
pictures he

*http://www.pardo.net/bike/pic/fail-001/000.html

The whole crank problem falls apart when the effective forces are
analyzed. *Above all, a left hand thread and significant fretting
damage to both cranks at the pedal shaft shoulder indicate why many
cranks break across the "pedal eye" where the pedal is attached.

Beyond that, the torsion, radial (torque) loading and lateral bending
from the center of pressure on the pedal are consistently ignored.
The fretting of the pedal shaft face is the most important one to me
because I broke at least one crank per 10,000 miles for 30 years,
until I modified the interface to emulate the conical face on an
automobile lug nut. *I have not had a crank failure in the last 20
years as a result.

Talking to crank manufacturers at InterBike trade show, I am convinced
that few if any have an idea where the forces are and have made no
stress concentration tests. *That was brought out by the recent
failure of a Shimano Hollowtech crank right where one would expect it,
there where the crank diverges from the disk of the chainwheel
"spider" that in this design is extremely rigid.

I am amazed when one of these component manufacturers introduces a
reliable design, such as Shimano free-hubs that do not use screw-on
sprockets that warp and become extremely hard to remove... and of
course no screw-on freewheel.

Jobst Brandt

Would the crank arm be a suitable candidate for power (Watts)
measurement through the use of strain gauges and other circuitry?

-Tony