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#21
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wobble on fast curve?
Callistus Valerius wrote:
yes, a rider can "deal" with the problem, but with respect, you're falling for the premise that's been bullied into people here over the years - that the physical dynamics that cause the problem are "acceptible" and therefore there's no point addressing them. jobst doesn't think this is a serious issue because he has an old frame with thicker walled tubes and old wheels with much less dish than modern wheels. modern frames with thin walled tubes [of the old standard diameter] and highly dished wheels are shimmy nightmares. shimmy can be solved with bigger diameter tubes offering better torsonal stiffness and wheels with better lateral stiffness [thicker drive side spokes, less dish, stiffer rims, etc.] the truth is, shimmy is no more an acceptible characteristic for a bike than yaw instability on a plane. shimmy /is/ solvable. the right frame/wheel combo /will/ give a bike on which shimmy does /not/ occur, and the rider does /not/ have to accept this b.s. argument about this very dangerous situation being /their/ lack of skill. That's why the speed wobble is such a paradox. Jobst says the rider is the dunce, you say the bike or wheel is a lemon. That is the paradox. why is it a paradox? i currently have 8 steeds in my stable and many more pairs of wheels. they all behave differently. i've also owned a decent number of other bikes so i've therefore had the opportunity to try a good number of different frame/wheel combos. i, as their rider, am a constant. what i find is that some combos shimmy, some don't. and yes, i do take the trouble to test using same tires/pressures. if the frame with the heaviest gauge, largest diameter tube [i.e. high torsional stiffness] is shimmy resistant with any wheel, but another frame of the same wheelbase, angles, size, but skinny light gauge tube [i.e. low torsional stiffness] shimmys with any wheel, you /have/ to conclude the torsional stiffness of the frame is a major factor. and if another yet another frame shimmys with laterally flexible wheels but not with laterally stiff wheels, that indicates the wheels also have a part to play in the equation. that's not so fantastic when you think about the engineering either. stiffness/spring constants dramatically affect resonance of all other dynamically loaded systems, so to ignore known principles in favor of some ethereal supposition about "shivering" is quite frankly, ridiculous. Solving it, is what is fun. As Hannibal would say "how did you feel............when the wheel wobbled?" Tonight I will dine on lamb chops with some fava beans and a nice Chianti, and think about speed wobbles. |
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#22
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wobble on fast curve?
jim beam wrote: i currently have 8 steeds in my stable So THAT'S where all the muck is coming from!!!!! |
#23
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wobble on fast curve?
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#25
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wobble on fast curve?
why is it a paradox? i currently have 8 steeds in my stable and many more pairs of wheels. they all behave differently. i've also owned a decent number of other bikes so i've therefore had the opportunity to try a good number of different frame/wheel combos. i, as their rider, am a constant. what i find is that some combos shimmy, some don't. and yes, i do take the trouble to test using same tires/pressures. if the frame with the heaviest gauge, largest diameter tube [i.e. high torsional stiffness] is shimmy resistant with any wheel, but another frame of the same wheelbase, angles, size, but skinny light gauge tube [i.e. low torsional stiffness] shimmys with any wheel, you /have/ to conclude the torsional stiffness of the frame is a major factor. and if another yet another frame shimmys with laterally flexible wheels but not with laterally stiff wheels, that indicates the wheels also have a part to play in the equation. that's not so fantastic when you think about the engineering either. stiffness/spring constants dramatically affect resonance of all other dynamically loaded systems, so to ignore known principles in favor of some ethereal supposition about "shivering" is quite frankly, ridiculous. There's another side to that coin. I have 3 bikes, 6 sets of wheels, all are different in every respect. If fear enters my head, all three bikes will shimmy exactly like they were all the same setup. I have a tendency to support that the rider, not the bike is the problem. |
#26
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wobble on fast curve?
Callistus Valerius wrote:
why is it a paradox? i currently have 8 steeds in my stable and many more pairs of wheels. they all behave differently. i've also owned a decent number of other bikes so i've therefore had the opportunity to try a good number of different frame/wheel combos. i, as their rider, am a constant. what i find is that some combos shimmy, some don't. and yes, i do take the trouble to test using same tires/pressures. if the frame with the heaviest gauge, largest diameter tube [i.e. high torsional stiffness] is shimmy resistant with any wheel, but another frame of the same wheelbase, angles, size, but skinny light gauge tube [i.e. low torsional stiffness] shimmys with any wheel, you /have/ to conclude the torsional stiffness of the frame is a major factor. and if another yet another frame shimmys with laterally flexible wheels but not with laterally stiff wheels, that indicates the wheels also have a part to play in the equation. that's not so fantastic when you think about the engineering either. stiffness/spring constants dramatically affect resonance of all other dynamically loaded systems, so to ignore known principles in favor of some ethereal supposition about "shivering" is quite frankly, ridiculous. There's another side to that coin. I have 3 bikes, 6 sets of wheels, all are different in every respect. If fear enters my head, all three bikes will shimmy exactly like they were all the same setup. I have a tendency to support that the rider, not the bike is the problem. but are the bikes different? if they're all the same material and similar tube guages, they're all going to be similar in terms of torsional stiffness. all my frames are different in this respect - that's why i bought them - so i could test them. the big tube models are way more torsionally stiff than the standard tube ones. and the one with the heaviest tube gauge of all simply refuses to shimmy regardless of how i try. that's not /all/ good because it's a brute to ride, but it sure is stable. |
#27
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wobble on fast curve?
wrote:
wrote: This is not chickening out or from braking in a curve. It is a natural resonance of the muscles (that of shivering) which closely matches bicycle frame shimmy frequency. It occurs easily when fatigued, in cold weather chilling the body or excessive muscular stress when grasping the bars. When the condition arises, do not grip the bars more tightly because that can invoke a shivering response. Try to lay the hand loosely with thumb and forefinger draped on the crook of the bars. I have trouble with this explanation, as every time I have felt a violent shimmy of the front end it occurred while I was descending no hands and completely relaxed. http://draco.acs.uci.edu/rbfaq/FAQ/8h.5.html |
#28
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wobble on fast curve?
Robert who? writes:
This is not chickening out or from braking in a curve. It is a natural resonance of the muscles (that of shivering) which closely matches bicycle frame shimmy frequency. It occurs easily when fatigued, in cold weather chilling the body or excessive muscular stress when grasping the bars. When the condition arises, do not grip the bars more tightly because that can invoke a shivering response. Try to lay the hand loosely with thumb and forefinger draped on the crook of the bars. I have trouble with this explanation, as every time I have felt a violent shimmy of the front end it occurred while I was descending no hands and completely relaxed. Your problem is that you didn't follow the thread, that of a rider, with hands on the bars, who experienced substantial shimmy. I do not doubt that a bicycle can shimmy ridden no hands and that pedal position has an influence on that since the pedals are one of the two places where the mass of the rider interacts with the bicycle when riding no hands. I think that is adequately discussed in the FAQ item. http://www.sheldonbrown.com/brandt/shimmy.html There is probably medical research that shows the frequency of muscular response in the arms and that it appears most readily in the auto response we know as shivering. It does not mean the person is freezing cold, although from my experience it is fatigue enhanced. Jobst Brandt |
#29
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wobble on fast curve?
41 wrote:
If you doubt shivering can trigger it: my bicycle has never shimmied on its own but for amusement I have tried to provoke it by shivering and so on and succeeded. Try it and I'm sure you will be able to do the same. I tried it, and couldn't start a shimmy. But I will keep trying. R |
#30
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wobble on fast curve?
wrote:
Your problem is that you didn't follow the thread, that of a rider, with hands on the bars, who experienced substantial shimmy. I do not doubt that a bicycle can shimmy ridden no hands and that pedal position has an influence on that since the pedals are one of the two places where the mass of the rider interacts with the bicycle when riding no hands. I think that is adequately discussed in the FAQ item. http://www.sheldonbrown.com/brandt/shimmy.html There is probably medical research that shows the frequency of muscular response in the arms and that it appears most readily in the auto response we know as shivering. It does not mean the person is freezing cold, although from my experience it is fatigue enhanced. Okay, got it. Still, I find the whole concept of hands-on-the-bars shimmy to be dubious, as I can't remember ever experiencing it myself and I've done plenty of descending while cold and tired. I'll have to take y'alls' word for it that this really happens. Robert |
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