Forces on Cranks

On Apr 29, 11:25*am, thirty-six wrote:
On 28 Apr, 19:25, Jobst Brandt wrote:

Beyond that, the torsion, radial (torque) loading and lateral bending
from the center of pressure on the pedal are consistently ignored.

Go and weigh your leg in the cycling position and see how many
pounds. *This is all the weight a crank needs to carry for a
proficient cyclist who rides flattish roads. *The tangential force
applied to a crank from a pedal may be much higher but rarely exceeds
twice this value for a proficient cyclist not racing, hill climbing or
training for such.

What is the source of these rather particular estimates, Trevor?

And what would the higher-end values (racing, hill climbing, an
awkward cyclist) amount to in terms of so many leg weights or whatever
other measure you prefer.

What was that movie called with the cute robot whose tagline was,
"Must have more data. Data! Data! Must have more data!" So far this
thread is thick on evasion, handwaving and abuse and thin on useful
data.

Andre Jute
"The brain of an engineer is a delicate instrument which must be
protected against the unevenness of the ground." -- Wifredo-Pelayo
Ricart Medina

Schwalbe BIg Apples on one's bike also help to stabilize the brain. --
Andre Jute

On Apr 29, 12:31*pm, thirty-six wrote:
On 28 Apr, 22:16, Andre Jute wrote:

Congratulations. Okay, now that you have that off your chest, dear
Jobst, do you agree with me

Huh, why do you want agreement from oddball?

Because Jobst is running around waffling to avoid admitting that I got
it right. He's taken in Krygowski, who in consequence gave the wrong
answer to a simple engineering question! Lovel-ly!

that if fluting on a crank turns it into
some kind of an H or U sectional shape, the longer sides should be
vertical and the web horizontal (when the pedal is at the quarter to
three position)? In short, do you agree with me that lightening/
decoration is best applied to the top and bottom of the arm rather
than the vertical faces to the outside and the inside of the crank?

Make em round and stick polyethylene edges on the forward rotating
face for aerodynamics when you use that monster gear chasing trucks at
60mph plus.

You flatter me, Trevor. Where shall I send the brown envelope?

All the different shapes you may have seen in alloy cranks are not
only due to conflicting hypothesis about which loading is most
important to any one particular rider, but also the hollow you see in
H-section cranks is historically due to the forging process in
improvement of grain in the forging. *This same technique is used in
strong and light fighting knives and swords. *Remember that bicycle
manufacturers rose from the ashes of redundant cutlers in some cases
and the use of such a technique of forging for crank manufacture is
hardly surprising to have arisen. *

Okay, crank manufacturers get it wrong now because they always got it
wrong, because they are the descendents of horse fettlers. Makes
sense.

Whether they managed to get the
orientation theoretically correct is irrelevant, the best alloy
cranks, being this type, would only be of real interest to racers.

Well, Jobst for one is convinced that it matters, though I must say
that anyone who expresses surprise that a crank which had ascended 600
vertical miles of Alps and then broke, as is true of one of the
examples in that oft-quoted site of broken cranks Jobst drools over,
is a cheapskate and an idiot who should have replaced his cranks long,
long since as routine maintenance.

Aerodynamic constraints as well as the dimensional space available
would mean that making the impression facing fore/aft a bit silly.

Imagination, dear boy, imagination. That imagination is in short
supply among the roadies (the pressure of all that excess of blood to
the brain because they have their arses in the air for hours every
day!) is no reason for me to emulate them; on the contrary.

Andre Jute
Visit Jute on Thisthatandtheother
http://www.audio-talk.co.uk/fiultra/...20ARISING.html

On Apr 29, 12:09*am, wrote:


Dear Frank,

Just to make sure that I'm following you, the square cross-section
covering a circle like this . . .
*http://i43.tinypic.com/6r7zog.jpg

. . . is stiffer in torsion because of the extra material at the
corners.

Yes.


But if you melt the square bar and recast it as a circle...

IOW, keep the same cross sectional area (and weight)...

... it becomes
even stiffer than the original bar because the extra material is
evenly distributed?

Yes, the round bar of equivalent weight will be a little stiffer in
torsion than the square bar. About 13% stiffer.




Maybe a dumb question, but would a triangle encompassing a circle...

OK, so now we're not talking about equivalent area (or weight) any
more.

be
even stiffer in torsion than a square encompassing the same circle,...


The triangle encompassing the circle has about twice the torsional
stiffness of the circle. It has less than twice the weight.

while a pentagram would be less stiff?

Less than the triangle.

The general idea is this: For a member in torsion, the material at
the centroid of the cross section isn't working for you. It's just
adding weight. Material further out along a radius does more to
resist torsion. But it's not a simple relationship. The material at
the very tip of that triangular cross section is essentially
unstressed, i.e. not working for you.

But we've been talking stiffness here, which probably isn't the most
important issue. I think it's more accurate to say you want maximum
strength per unit weight, with still adequate stiffness. And you want
that strength for withstanding both torsion and bending. That won't
come from a triangle cross section. A hollow shape would be the
ultimate, I think. (I know that years ago, there were some hollow
steel cranks, brazed or welded up as assemblies.)

It's a complicated problem because the forces and the orientation of
the crank varies so much around the circle, plus you get various
unusual loading conditions (like, say, hitting a pothole while
standing on the lower pedal while coasting). You want a shape that's
optimum for the full range of loading.

While I understand Jobst's frustration at some of the designers and
their designs (including the pedal eye problem), I imagine that most
large firms are using design tools (software, etc.) that are more
capable than - say - the imaginings of pseudonymous Irish fantasy
writers!

- Frank Krygowski

On Apr 29, 10:43*am, Andre Jute wrote:
On Apr 29, 11:08*am, thirty-six wrote:





On 28 Apr, 22:03, Andre Jute wrote:

* Still Just Me * wrote:

On Wed, 28 Apr 2010 07:54:33 -0700 (PDT), Andre Jute
wrote:


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.

Maybe The key question would be whether or not the crank actually
flexes significantly in the direction you suggest. If not, then the
vanity flutes are irrelevant.

IMHE, the (my vintage steel) frame flexes by large, visible amounts.. I
think the stiffness of the crank is far greater than the frame, based
on observation with the bike in a trainer.

At the same time, I do see some flex apparently introduced in the
chainwheels from the cranks when on the road if I start to pedal in a
poor way, pushing out towards the right when pushing hard. I think
that's more of a technique issue than an engineering issue.

So, my rough field observation tells me it's not an issue. But, there
may be laboratory results that further detail. I can say that without
pushing hard, it's all immaterial. It's only when you really "get on
it" that it's noticeable.

I'm not viewing this as problem or a concern for my current cranks. I
have steel cranks and they don't appear to be stressed in the least.
But I'm thinking of designing cranks of my own and having them
machined, and then the question of the forces on the cranks comes up.
Not much point in having plain steel cranks cut just to have your own
design of plain steel crank -- I have plain steel cranks already! So
the question of decor/lightening arises, and with the question of
where it will do the least harm, and we're back at forces and vectors..

Andre Jute
*The rest is magic hidden in the hub.
For rare hub gear bikes, visit Jute on Bicycles at
*http://www.audio-talk.co.uk/fiultra/...20CYCLING.html

Make the cross section of the crank circular. *You are limited
anatomically how thick you can make the crankss, that's all. *Cost may
mean you use less material, this can be put forward as being
'lightweight' and has been a good sales point for 'racer types' for
over a century.

This is the best point made in the responses in this thread so far. I
in fact thought of a hollow section but I don't fancy welded-on ends
for the BB and pedal mountings, and to stop the tube after drilling or
drawing, so the thing will have to be split lengthwise and then glued
(Tune round section hollow alloy cranks are glued lengthwise) or
welded together again. I'm very keen to have it made as one piece.

But how about this for a production process for a round almost-one-
piece crank: Take a bloc of steel, forge or machine a crankshaped
blank. Drill through end of BB barbell lengthwise to almost at pedal
end. Drilling a straight passage will leave thicker walls nearer BB
end. Stop hole at BB end with fine-threaded bolt just long enough to
go from outside to a little way into the now hollow arm. Machine now
hollow-shafted crank blank further, finishing up with a barbell shape,
small bulb at pedal end, bigger bulb at BB end, circular shaft tapered
from thick at BB end to thinner at pedal end. Now tap one end for
pedal and machine other end (right through center of stopping bolt)
for square taper.

A blacksmith way of making this crank would be to start with thickwall
hollow tube, fold over the ends repeatedly until he arrives at a
suitable block of solid metal at each end. Then beat the ends round,
machine the pedal threads and square taper, and polish with fine grit
and elbow grease, then apply black chrome. Ettore Bugatti made bent
hollow-centre solid-ended axles like that, with the added twist that
he started with solid metal and did the gun drilling in his own works.


To be consistent with your subject line, the whole thing would need to
be machine-turned after all other machining steps were finished.
NTTAWTT.

nate

(former owner of a '56 Studebaker Golden Hawk, complete with faux-
engine turned dashboard, which was one of the nicest features of the
whole car...)

On 29 Apr, 15:43, Andre Jute wrote:
On Apr 29, 11:08*am, thirty-six wrote:

Make the cross section of the crank circular. *You are limited
anatomically how thick you can make the crankss, that's all. *Cost may
mean you use less material, this can be put forward as being
'lightweight' and has been a good sales point for 'racer types' for
over a century.


This is the best point made in the responses in this thread so far.

It took me best part of a minute to think and compose. I have
considered the problem before. You may be able to use the steel
chainstays from a roadster or mountain bike. Get your smith to forge
weld the pedal axles to the ends and to pierce out (in) a taper at the
crank bearing to fit the regular ISO taper for alu cranks. The smith
will be able to correct alignment following welding much easier and
harden and temper the finished assembly after making everything smooth
by hammer or file. Give a specification for acceptable surface finish
irregularities, remember a finish can be obtained by filling and
painting.


in fact thought of a hollow section but I don't fancy welded-on ends
for the BB and pedal mountings, and to stop the tube after drilling or
drawing, so the thing will have to be split lengthwise and then glued
(Tune round section hollow alloy cranks are glued lengthwise) or
welded together again. I'm very keen to have it made as one piece.

Flippin 'eck!

But how about this for a production process for a round almost-one-
piece crank: Take a bloc of steel, forge or machine a crankshaped
blank.

FAIL , you're already worse off than a Chinese steel crank if you
machine it. Get this machining crap outa your head, its for
decoration only, not structural.

Drill through end of BB barbell lengthwise to almost at pedal
end. Drilling a straight passage will leave thicker walls nearer BB
end. Stop hole at BB end with fine-threaded bolt just long enough to
go from outside to a little way into the now hollow arm. Machine now
hollow-shafted crank blank further, finishing up with a barbell shape,
small bulb at pedal end, bigger bulb at BB end, circular shaft tapered
from thick at BB end to thinner at pedal end. Now tap one end for
pedal and machine other end (right through center of stopping bolt)
for square taper.

A blacksmith way of making this crank would be to start with thickwall
hollow tube,

No, he starts with a billet and pierces it. He then hammers it out,
slowly drawing the length of the tube around a former. Unless your
smith has an auto hammer the costs arn't feasible even as a hobby. Or
are they?

fold over the ends repeatedly until he arrives at a
suitable block of solid metal at each end. Then beat the ends round,
machine the pedal threads and square taper, and polish with fine grit
and elbow grease, then apply black chrome. Ettore Bugatti made bent
hollow-centre solid-ended axles like that, with the added twist that
he started with solid metal and did the gun drilling in his own works.

He'd not seen a smith pierce a hole then.

In article
,
Frank Krygowski wrote:

But we've been talking stiffness here, which probably isn't the most
important issue. I think it's more accurate to say you want maximum
strength per unit weight, with still adequate stiffness. And you
want that strength for withstanding both torsion and bending. That
won't come from a triangle cross section. A hollow shape would be
the ultimate, I think. (I know that years ago, there were some
hollow steel cranks, brazed or welded up as assemblies.)

Those broke, too.

This conversation has focused mainly on shape, it seems, but material
choice must also play a significant role.

How about an "isotruss"?

http://www.isotruss.org/

On 29 Apr, 16:15, Andre Jute wrote:
On Apr 29, 11:25*am, thirty-six wrote:

On 28 Apr, 19:25, Jobst Brandt wrote:
Beyond that, the torsion, radial (torque) loading and lateral bending
from the center of pressure on the pedal are consistently ignored.

Go and weigh your leg in the cycling position and see how many
pounds. *This is all the weight a crank needs to carry for a
proficient cyclist who rides flattish roads. *The tangential force
applied to a crank from a pedal may be much higher but rarely exceeds
twice this value for a proficient cyclist not racing, hill climbing or
training for such.

What is the source of these rather particular estimates, Trevor?


Totally unbiased and accurate guesstimation based on own experience.
It requires the particular application of an easy and light, round
pedalling technique.


And what would the higher-end values (racing, hill climbing, an
awkward cyclist) amount to in terms of so many leg weights or whatever
other measure you prefer.

That's going back some. For hill climbing urang-utans and sprinters
who overgear then the forces may be double the body weight on the end
of the crank tangentially to it and probably up to 1.1/2 times bodt
weight as a bending load with the pedal at the bottom. On top of this
force you also have to factor in shock loading originating at the
wheel contact, although I think of this will be irrelevant if the
rider does not lock out his knees.


What was that movie called with the cute robot whose tagline was,
"Must have more data. Data! Data! Must have more data!" So far this
thread is thick on evasion, handwaving and abuse and thin on useful
data.

That's because few have a clue and those that do, realise that the
economics of creating the perfect crank far outweigh any benefits the
crank may bring over whatever basic racing crank is on offer, which is
probably in the region of sixty pounds a pair now. Your smith'd
cranks are gonna cost ya 800 spondoolies I reckon, all finished.

On 29 Apr, 19:04, Tim McNamara wrote:
In article
,
*Frank Krygowski wrote:

But we've been talking stiffness here, which probably isn't the most
important issue. *I think it's more accurate to say you want maximum
strength per unit weight, with still adequate stiffness. *And you
want that strength for withstanding both torsion and bending. *That
won't come from a triangle cross section. *A hollow shape would be
the ultimate, I think. *(I know that years ago, there were some
hollow steel cranks, brazed or welded up as assemblies.)

Those broke, too.

Probably due to "computer controlled heat treatment", rrelevant for a
complex for such as a pedal crank. A smith can watch the colour of
oxide form and draw the temper in different amounts for different
areas of the crank. The automated heat treatment relies upon the
design to correctly aid in the treatment. A smith uses his eyes and
experience.


This conversation has focused mainly on shape, it seems, but material
choice must also play a significant role.

So who has used wood and what where the problems?

How about an "isotruss"?

How about chain oil and road dirt?

http://www.isotruss.org/

On Apr 29, 5:29*pm, N8N wrote:
On Apr 29, 10:43*am, Andre Jute wrote:

Ettore Bugatti made bent
hollow-centre solid-ended axles like that, with the added twist that
he started with solid metal and did the gun drilling in his own works.

To be consistent with your subject line, the whole thing would need to
be machine-turned after all other machining steps were finished.
NTTAWTT.

The Ettore ones were plain steel in a very high polish. They were on
cars intended for people who kept a few Krygowskis or whatever local
peasants they had chained in a(n inspection pit) in the garage to
polish plain polished steel daily. Modern owners have mostly had them
nickel-plated, I imagine.

But a machine-turned finish isn't a bad idea, if you can find a
craftsman who understands that if he makes it too regular, it will
look fake, like the "machine-turned" finish in some mid-century
American cars which was both too fine and too regular to be the real
thing.

nate

(former owner of a '56 Studebaker Golden Hawk, complete with faux-
engine turned dashboard, which was one of the nicest features of the
whole car...)

Sorry about that! But it's true: if it were more amateurish, it woulda
looked more real.

Andre Jute
Reformed petrol head
Car-free since 1992
Greener than thou!

On Apr 29, 7:02*pm, thirty-six wrote:
On 29 Apr, 15:43, Andre Jute wrote:
I'm very keen to have it made as one piece.

Flippin 'eck!

But how about this for a production process for a round almost-one-
piece crank: Take a bloc of steel, forge or machine a crankshaped
blank.

FAIL *, you're already worse off than a Chinese steel crank if you
machine it. *Get this machining crap outa your head, its for
decoration only, not structural.

Ever hear of grain-oriented billet stock, Trevor? From everything I
can gather here, the forces on bicycle cranks are nowhere near the
explosive (heh-heh) level of forces on automobile cranks. Hell, you
can buy machined billet alloy round cranks off the shelf, made as
halves and glued together by Tune in Germany; those Tunes are very
highly reputed.

Andre Jute
Feed a tree today, produce more CO2!