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.

jim beam wrote:

i currently have 8 steeds in my stable

So THAT'S where all the muck is coming from!!!!!

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.

Robert

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 o ccurs 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.

If you notice, it also refers to the natural bicycle frame shimmy
frequency. The point is that shivering etc can trigger shimmy, as can
any other suitable small disturbance.

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.

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.

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.

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

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

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

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