#21
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Forces on Cranks
On 28 Apr, 21:46, Andre Jute wrote:
Peter Cole wrote: Andre Jute 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. *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 Jobst has frequently posted on crank failures and causes. Several pictures he http://www.pardo.net/bike/pic/fail-001/000.html I've seen those, thanks. I didn't mention Jobst for fear that he would go into a masochistic ecstacy about the bee in his bonnet about left- hand threads, and never get around to what I want to discuss, which is exactly what happened. -- AJ MMMMmmm. Ha ha ahh ahaha. |
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#22
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Forces on Cranks
On 29 Apr, 02:58, Frank Krygowski wrote:
On Apr 28, 10:54*am, Andre Jute 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. At first glance, I assumed this post was intended as a parody of 17th century technical writing - the sort produced before current engineering vocabulary terms like vertical (vs. "up-down") tangential (vs. "circular motion") torque (vs. "twisting force") were well known. *Based on that, I skipped the rest, as usual. Now that I see that others are taking the question somewhat seriously, the short answer to the question is: No. - Frank Krygowski Too seriously. I see that if decoration is wanted it can be applied to any part of the crank as long as it's big enough to make that decoration viable. Paint is a good choice. |
#23
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Forces on Cranks
On Apr 28, 2:30*pm, Jobst Brandt wrote:
wrote: 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, Where do all the paragraph breaks and empty lines go on your web browser? *This text gets difficult to follow in the form you post. You might also delete items that do not affect your posting. No, the crank is not suitable, as should be apparent by the plurality of three dimensional bending and torsion it supports as has been discussed in this thread. *Even the BB spindle has complex stresses that require careful separation to do any power measurements. *That could be done by a leaf lever that transmits only rotational torque, its flexibility in its thin direction could be ignored. As you see, no one has made such a device, that would be a boon to tandem riders if practical because it could show power of each rider on identical displays on each rider's handlebars to make clear who is doing what part of propulsion work. Jobst Brandt In my puter, the text was very easy to follow. The question appeared by itself as a completely separated post. |
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Forces on Cranks
On Apr 29, 3:20*am, Frank Krygowski wrote:
On Apr 28, 5:34*pm, Jobst Brandt wrote: **The torsional stiffness of an element with other than round cross section is like that of the largest inscribed solid circle. Well, for some value of "like." *A solid square bar is about 1.4 times as stiff in torsion as the solid round bar whose diameter equals the side of the square. *The square is less efficient on a weight basis, though. To visualize torque capacity, relative stress levels and stress directions of a non-circular torsion member, google "membrane analogy torsion" or "soap bubble analogy torsion." - Frank Krygowski |
#25
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Kreepy Krygo caught lying on professional engineering matters, was
On Apr 29, 2:58*am, Frank Krygowski wrote:
On Apr 28, 10:54*am, Andre Jute 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. At first glance, I assumed this post was intended as a parody of 17th century technical writing - the sort produced before current engineering vocabulary terms like vertical (vs. "up-down") tangential (vs. "circular motion") torque (vs. "twisting force") were well known. * That's okay, Franki Shavelegs. I'm not writing for the more arid- minded fascists like you. Far be it from me to support by my language your attempts to regiment everyone and every creative instinct. Based on that, I skipped the rest, as usual. Par for the course, Kreepy Krygo offering opinions without reading the question. Below we'll see with what dire result. Now that I see that others are taking the question somewhat seriously, Poor old Kreey is so lacking in initiative, he has to wait for the street corner gang to run past before he runs after them, shouting, "Wait for me, I'm your leader!" And then he heads in the wrong direction! the short answer to the question is: No. Wrong answer, Franki-boy, especially from a "professor" of engineering, even if at some jumped-up tech. You should have checked what everyone else said: they said "yes". In fact, I already said "yes". That's what a "professor" of engineering gets when he's a) bog- ignorant and b) lets his personal dislikes of people influence his "engineering" judgements and c) snaps out his "answer" without reading the question and d) puts personalities before the facts. - Frank Krygowski You're a dead loss to engineering -- and to cycling -- poor old Franki. And now you've been caught out lying on professional engineering matters for personal reasons. The alternative is that you're an ignorant peasant. Choose only one. Andre Jute Krygo, he say, "Any old number is good number." |
#26
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Forces on Cranks
On Apr 29, 5:09*am, wrote:
On Wed, 28 Apr 2010 19:20:29 -0700 (PDT), Frank Krygowski wrote: On Apr 28, 5:34*pm, Jobst Brandt wrote: **The torsional stiffness of an element with other than round cross section is like that of the largest inscribed solid circle. Well, for some value of "like." *A solid square bar is about 1.4 times as stiff in torsion as the solid round bar whose diameter equals the side of the square. *The square is less efficient on a weight basis, though. To visualize torque capacity, relative stress levels and stress directions of a non-circular torsion member, google "membrane analogy torsion" or "soap bubble analogy torsion." - Frank Krygowski 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. But if you melt the square bar and recast it as a circle, it becomes even stiffer than the original bar because the extra material is evenly distributed? Maybe a dumb question, but would a triangle encompassing a circle be even stiffer in torsion than a square encompassing the same circle, while a pentagram would be less stiff? That is, I'm wondering if the triangle is the stiffest and things gradually decline with more sides until a circle is approximated, or if something about the triangle makes it less stiff in torsion than the square. * triangle * stiffer than squware in torsion as inset circle * square * * 1.4 times as stiff in torsion as inset circle * pentagram *between 1.4 and 1.0 times as stiff in torsion * hexagram * less stiff than pentagram, stiffer than circle . . . and so on, adding more and more sides to reach a circle * circle * * 1.0 stiff in torsion Cheers, Carl Fogel "Inscribed", gee. You might make your trick question more interesting still, dear Carl, by noting that there are two ways to inscribe a triangle on a square, by diagonally halving the square or by inscribing lines from two corners to the middle of the opposite face and cutting away the smaller triangles to the sides. Now, which of the three sizes of triangles so formed will be the stiffest, which will be stiffest in relation to material used, and which will be most efficient on other parameters, say aerodynamically? Andre Jute Wide awake this time of the afternoon |
#27
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Forces on Cranks
On Apr 29, 11:03*am, thirty-six wrote:
This is old hat. Depends whether the brim of the hat section is welded to something how stiff it is. If open, the brim could gain considerable stiffness from a small return of its own. -- AJ |
#28
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Forces on Cranks
On 29 Apr, 15:22, Andre Jute wrote:
On Apr 29, 11:03*am, thirty-six wrote: This is old hat. Depends whether the brim of the hat section is welded to something how stiff it is. If open, the brim could gain considerable stiffness from a small return of its own. -- AJ As to pedal cranks: Unless the grooves have been formed because of the forging process, they will have an overall negative effect on the crank function. The aerodynamics of the con-rods are also in consideration for fast engines. The typical forging pattern is so superior for its cost that it is rarely altered. Welding fabricated cranks is a viable option. Cook Bros, I think is one brand. Can be done in aluminium alloy as well as steel. |
#29
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Making bicycle cranks the Ettore Bugatti way, was Forces onCranks
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. Andre Jute Visit Jute on Amps at http://www.audio-talk.co.uk/fiultra/ "wonderfully well written and reasoned information for the tube audio constructor" John Broskie TubeCAD & GlassWare "an unbelievably comprehensive web site containing vital gems of wisdom" Stuart Perry Hi-Fi News & Record Review |
#30
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Forces on Cranks
On Apr 29, 6:51*am, Sergio Moretti wrote:
On Apr 28, 11:09*pm, wrote: On Wed, 28 Apr 2010 19:20:29 -0700 (PDT), Frank Krygowski wrote: On Apr 28, 5:34*pm, Jobst Brandt wrote: **The torsional stiffness of an element with other than round cross section is like that of the largest inscribed solid circle. Well, for some value of "like." *A solid square bar is about 1.4 times as stiff in torsion as the solid round bar whose diameter equals the side of the square. *The square is less efficient on a weight basis, though. To visualize torque capacity, relative stress levels and stress directions of a non-circular torsion member, google "membrane analogy torsion" or "soap bubble analogy torsion." - Frank Krygowski 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. But if you melt the square bar and recast it as a circle, it becomes even stiffer than the original bar because the extra material is evenly distributed? Maybe a dumb question, but would a triangle encompassing a circle be even stiffer in torsion than a square encompassing the same circle, while a pentagram would be less stiff? That is, I'm wondering if the triangle is the stiffest and things gradually decline with more sides until a circle is approximated, or if something about the triangle makes it less stiff in torsion than the square. * triangle * stiffer than squware in torsion as inset circle * square * * 1.4 times as stiff in torsion as inset circle * pentagram *between 1.4 and 1.0 times as stiff in torsion * hexagram * less stiff than pentagram, stiffer than circle . . . and so on, adding more and more sides to reach a circle * circle * * 1.0 stiff in torsion Cheers, Carl Fogel Yes, the triangle in the problem you state is stiffest. *It also has the largest cross-sectional area and is the least efficient shape. - Sergio Moretti I think you will find the controlling element in Carl's riddle is the word 'inset' (now wait for Krygo to complain that it should be "inscribed" or even "included"). The triangle you posit is one made with all the material of the square rod, not inscribed on it. Still, Carl has confused his own riddle by talking about melting down and recasting the square rod in the same post as he defines the problem as one of insets. Anyhow, all this Rod Science is for Railroad Minds and Other Librarians. Those of us who live in the 21st century rather than the 19th, do Tube Science, with less material disposed edgewise (just for you, Franki Shavelegs, "peripherally"). Some of us have even passed through the stage of triangular small tubes which we called Spaceframes, and to the ultimate largest size of tube for the job, which we call Monocoque, because the cock crowed only once, that being more efficient in an Engineering Religion. Andre Jute Who says the cargo cult is dead? |
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