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Forces on spokes
Hi all.
Just wanted to clear a little question. Thinking of a wheel spoke as a prismatic member, what is the nature of normal forces acting on it. Is it all in tension, all in compression or a mix of both? Thanks. Ron |
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#2
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Forces on spokes
On 28 Aug 2006 05:23:47 -0700, "bicycle_disciple"
wrote: Hi all. Just wanted to clear a little question. Thinking of a wheel spoke as a prismatic member, what is the nature of normal forces acting on it. Is it all in tension, all in compression or a mix of both? Thanks. Ron Dear Ron, Forces on a pre-tensioned wheel loaded at the axle: http://www.astounding.org.uk/ian/wheel/index.html Cheers, Carl Fogel |
#4
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Forces on spokes
On 28 Aug 2006 10:22:51 -0700, "Jeff"
wrote: wrote: On 28 Aug 2006 05:23:47 -0700, "bicycle_disciple" wrote: Hi all. Just wanted to clear a little question. Thinking of a wheel spoke as a prismatic member, what is the nature of normal forces acting on it. Is it all in tension, all in compression or a mix of both? Thanks. Ron Dear Ron, Forces on a pre-tensioned wheel loaded at the axle: http://www.astounding.org.uk/ian/wheel/index.html Cheers, Carl Fogel An interesting and thorough analysis at that link, and yet I am not sure that I believe the final result. He concludes that the load is supported almost exclusively by the bottom few spokes (the ones pointing down toward the road) which are strongly in compression. However, long slender members such as spokes cannot support large compressive loads because of their tendency to buckle (bend). Also, much of the strength of a wheel comes from the fact that all the spokes contribute to the load at all times. I suspect that he has not accounted fully for the pretensioning of the spokes. Jeff Dear Jeff, Actually, Ian's whole article is about accounting fully for the pre-tensioning of the spokes. It's a subject that's been covered repeatedly. That's the nicest online, detailed explanation that I know of. You can find pretty much the same engineering analysis and conclusions in "The Bicycle Wheel" by Jobst Brandt, any edition. And you can see experimental strain gauge confirmation in figures 10 and 11 Professor Gavin's paper he http://www.duke.edu/~hpgavin/papers/...heel-Paper.pdf The icicle-shapes on the graphs show the pre-tensioned spoke losing and then regaining a large amount of tension as it rolls under the loaded axle. Until all the pre-tension is used up, even a string will "support" a compressive load, which is why emergency repair spokes can be made of kevlar string and why whole wheels can and have been made of them. Cheers, Carl Fogel |
#5
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Forces on spokes
Jeff Thomas writes:
Just wanted to clear a little question. Thinking of a wheel spoke as a prismatic member, what is the nature of normal forces acting on it. Is it all in tension, all in compression or a mix of both? Forces on a pre-tensioned wheel loaded at the axle: http://www.astounding.org.uk/ian/wheel/index.html An interesting and thorough analysis at that link, and yet I am not sure that I believe the final result. He concludes that the load is supported almost exclusively by the bottom few spokes (the ones pointing down toward the road) which are strongly in compression. I didn't see that there was any compression in the analysis but rather a reduction in tension. The only spokes that experience a significant change in length are the ones at the bottom. This analysis was done after the publication of "the Bicycle Wheel" in which this matter is discussed at length to avoid any misunderstanding. Because this is appears to be such an unusual perspective, no analysis of the wheel was published before "the Bicycle Wheel". However, long slender members such as spokes cannot support large compressive loads because of their tendency to buckle (bend). Also, much of the strength of a wheel comes from the fact that all the spokes contribute to the load at all times. I suspect that he has not accounted fully for the pretensioning of the spokes. I suggest you review what was written and possibly look at the full analysis in the book. http://sheldonbrown.com/harris/books.html#brandt Jobst Brandt |
#6
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Forces on spokes
Jeff wrote: An interesting and thorough analysis at that link, and yet I am not sure that I believe the final result. He concludes that the load is supported almost exclusively by the bottom few spokes (the ones pointing down toward the road) which are strongly in compression. However, long slender members such as spokes cannot support large compressive loads because of their tendency to buckle (bend). Also, much of the strength of a wheel comes from the fact that all the spokes contribute to the load at all times. I suspect that he has not accounted fully for the pretensioning of the spokes. Someone did a test of a bicycle with a tensiometer providing constant telemetry of spoke tension and found that the spokes under the axle lost tension, the spokes above the axle stayed relatively close, and the spokes at +-90o from those under the axle increased. My ignorant conclusion based on this data was that all the spokes except those directly under the axle contributed to sharing the load, and that the load was shared (this part is even more controversial) by the tendencey of the rim to distort ovally. Others on this ng will now proceed to dismiss this data as insignificant and insist that because the spokes under the axle are tensioned, they are able to support the weight of the bike until the load becomes great enough that they go slack, and they don't really bother to explain convincingly (for me) why the greatest tension rise is seen in the spokes that are _parallel_ to the road surface. For me personally, the tensioned spoke theory would be plausible if the spoke nipple were somehow fixed in the rim, but because the nipple is not fixed, there is no way for the spoke (tensioned or not) to significantly act acgainst the rim to provide support of the weight of the bicycle when the spoke is directly under the axle/hub. |
#7
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Forces on spokes
On 28 Aug 2006 11:41:45 -0700, wrote:
Jeff wrote: An interesting and thorough analysis at that link, and yet I am not sure that I believe the final result. He concludes that the load is supported almost exclusively by the bottom few spokes (the ones pointing down toward the road) which are strongly in compression. However, long slender members such as spokes cannot support large compressive loads because of their tendency to buckle (bend). Also, much of the strength of a wheel comes from the fact that all the spokes contribute to the load at all times. I suspect that he has not accounted fully for the pretensioning of the spokes. Someone did a test of a bicycle with a tensiometer providing constant telemetry of spoke tension and found that the spokes under the axle lost tension, the spokes above the axle stayed relatively close, and the spokes at +-90o from those under the axle increased. My ignorant conclusion based on this data was that all the spokes except those directly under the axle contributed to sharing the load, and that the load was shared (this part is even more controversial) by the tendencey of the rim to distort ovally. Others on this ng will now proceed to dismiss this data as insignificant and insist that because the spokes under the axle are tensioned, they are able to support the weight of the bike until the load becomes great enough that they go slack, and they don't really bother to explain convincingly (for me) why the greatest tension rise is seen in the spokes that are _parallel_ to the road surface. For me personally, the tensioned spoke theory would be plausible if the spoke nipple were somehow fixed in the rim, but because the nipple is not fixed, there is no way for the spoke (tensioned or not) to significantly act acgainst the rim to provide support of the weight of the bicycle when the spoke is directly under the axle/hub. Dear SSTW, Figure 10 in Professor Gavin's paper shows strain gauge results for a spoke on an actual wheel as it was being ridden, with 8 revolutions in 2.0 seconds: http://www.duke.edu/~hpgavin/papers/...heel-Paper.pdf The huge icicle spikes show the loss of tension as the spoke rolls under the loaded axle. They're about ten times the size of the other spikes. Like the huge spikes, the tiny spikes vary, reflecting real world conditions. Figure 11 shows the averaged spoke strain profile from three road tests. The huge icicle spike is about ten times the size of the other variation. It would be very difficult to argue that any significant changes are seen in the experimental data except when the spoke rolls under the axle and its tension drops like a rock. Experiment seems to confirm theory. Cheers, Carl Fogel |
#8
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Forces on spokes
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#9
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Forces on spokes
wrote: Experiment seems to confirm theory. The experiment confirms that the spokes do indeed go slack as they pass under the hub. It doesn't in anyway prove that they are supporting the wheel through compressive loading before they go slack. |
#10
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Forces on spokes
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