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Forces on Cranks
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 |
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#2
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Forces on Cranks
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 |
#3
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Forces on Cranks
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 |
#4
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Forces on Cranks
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 |
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Forces on Cranks
* 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 |
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Forces on Cranks
On Apr 28, 7:25*pm, 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. I just included them all in my analysis above. 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. Congratulations. Okay, now that you have that off your chest, dear Jobst, do you agree with me 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? Snipped, more whining about component manufacturers that would be amusingly scurrilous if I were not so ****ed off at having my thread diverted by the swarm of wasps on Jobst's belfry. Do try to stick to the point, Jobst. I already know what to do about the pedal/crank interface fretting: you've told us all that. Now I want to move on to dealing with the other forces. 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 |
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Forces on Cranks
Jobst, the question we're discussing is:
***Given that someone *will* decorate/lighten a bicycle crank, is the balance of forces on it such that material is less damagingly removed from the top and bottom faces than from the outer and inner faces, as happens now?*** "Given that someone will decorate/lighten a bicycle crank" means we're not interested in the normative case (engineers telling us we can't do what we already decided to do) but in starting at the managerial decision already arrived at and moving forward. If you can't (or more likely won't) help, at least try not to get in the way. Andre Jute Check out Andre's recipes at http://www.audio-talk.co.uk/fiultra/FOOD.html |
#8
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Forces on Cranks
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 |
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Forces on Cranks
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 |
#10
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Forces on Cranks
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 |
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