Downhill Shimmy

Quoted message said:

A few weeks ago, I experienced, for the first time, that I am getting
front wheel wobbling on descents. It was on a steep hill that I have
been down time after time, but this day I was scared half to death
that I was going to lose control of the bike and eat the pavement.
I look my bicycle over, but I can't see any mechanical problems that
might cause that to happen. I seem to be tensing up on descents when
I didn't used to have that problem. Since those few weeks ago, I am
still experiencing the same effect and it is very scary. The people
behind me immediately try to pass me for fear they may end up going
down if I go down.
It's become very annoying. It feels more psychological than anything
else.
I have lost a bit of weight lately from all the riding I have been
doing, so I am flying up hills with relative ease where I used to get
dropped before, but now I am losing a grip on descents when I used to
fly down hills ahead of everyone else, even when I wasn't aero,
because my weight giving me advantage. However, now that I am losing
the weight, I seem to be shimmying like crazy.
It's become like a roller coaster feeling.

Have you read the FAQ on bicycle shimmy, found here-
http://draco.acs.uci.edu/rbfaq/FAQ/8h.5.html? It covers the subject very
well.

But reading your post, it really appears that you've already figured out
what's caused it- your loss of weight. You've changed how you interact with
your bike. Remember that a bike isn't stable without the help of someone on
it, and, well, you changed. That's not to say there might be something going
on with the bike, but we do know, for a fact, that the most-significant
variable, you, are different now than when the bike didn't shimmy.

The question is, what can you do about it? That's a really tough one,
because once you've lost confidence in a bike, you approach it in a way that
makes it more likely to shimmy (by being, as you said, tense). You might
need to make some sort of major change in your position, for no good reason
other than to force you to do things differently than before, and make it
feel as though something has really changed. I've seen cases where it just
doesn't matter; once someone's lost faith that their bike will be stable,
there's nothing you can do to change it. It becomes a self-fulfilling
prophecy.

But first, if you haven't read the FAQ, do so. It often helps to read an
authoritative, unattached (to your situation) view of the problem, and
potential solutions.

--Mike-- Chain Reaction Bicycles
www.ChainReactionBicycles.com

"Michie" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

A few weeks ago, I experienced, for the first time, that I am getting
front wheel wobbling on descents. It was on a steep hill that I have
been down time after time, but this day I was scared half to death
that I was going to lose control of the bike and eat the pavement.
I look my bicycle over, but I can't see any mechanical problems that
might cause that to happen. I seem to be tensing up on descents when
I didn't used to have that problem. Since those few weeks ago, I am
still experiencing the same effect and it is very scary. The people
behind me immediately try to pass me for fear they may end up going
down if I go down.
It's become very annoying. It feels more psychological than anything
else.
I have lost a bit of weight lately from all the riding I have been
doing, so I am flying up hills with relative ease where I used to get
dropped before, but now I am losing a grip on descents when I used to
fly down hills ahead of everyone else, even when I wasn't aero,
because my weight giving me advantage. However, now that I am losing
the weight, I seem to be shimmying like crazy.
It's become like a roller coaster feeling.
One rider, who is a fellow team mate, and is a cat 3 racer, told me to
try pulling my knees in using my them to pull up under the top tube
and sit back farther on the seat and get more aero, but I still feel
like I am shimmying, and he noticed that I was to, even going down a
hill at a mere 25 mph.
Finally, I tried turning the biggest gear on the bike while going down
hill, or at least spin the pedals even if it wasn't giving me
momentum, and it seemed to help. I felt more stable, like my center
of gravity was under better control.

Does anyone have any advice to help me conquer this newly chronic
problem?

Michie, you say you have lost "a bit" of weight. Sounds like it may be
"quite a bit," since you are now flying up hills. I think Frank is
right--you and the bicycle together are a dynamic system, and sometimes a
seemingly small change can move things to an unstable combination. Why
don't you try adding back the weight by filling your pockets and/or a
backpack with heavy stuff, and see what effect it has?

Have you tried another bike just to rule out a mechanical problem?
The only time it ever happened to me was when my headset was loose (even
though the locknut was tight) so make sure it is adjusted correctly. Scared
the [censored] out of me too.
Make sure the front wheel is evenly mounted too.

Apologies if this is all obvious to you,
Z

"Michie" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

A few weeks ago, I experienced, for the first time, that I am getting
front wheel wobbling on descents. It was on a steep hill that I have
been down time after time, but this day I was scared half to death
that I was going to lose control of the bike and eat the pavement.
I look my bicycle over, but I can't see any mechanical problems that
might cause that to happen. I seem to be tensing up on descents when
I didn't used to have that problem. Since those few weeks ago, I am
still experiencing the same effect and it is very scary. The people
behind me immediately try to pass me for fear they may end up going
down if I go down.
It's become very annoying. It feels more psychological than anything
else.
I have lost a bit of weight lately from all the riding I have been
doing, so I am flying up hills with relative ease where I used to get
dropped before, but now I am losing a grip on descents when I used to
fly down hills ahead of everyone else, even when I wasn't aero,
because my weight giving me advantage. However, now that I am losing
the weight, I seem to be shimmying like crazy.
It's become like a roller coaster feeling.
One rider, who is a fellow team mate, and is a cat 3 racer, told me to
try pulling my knees in using my them to pull up under the top tube
and sit back farther on the seat and get more aero, but I still feel
like I am shimmying, and he noticed that I was to, even going down a
hill at a mere 25 mph.
Finally, I tried turning the biggest gear on the bike while going down
hill, or at least spin the pedals even if it wasn't giving me
momentum, and it seemed to help. I felt more stable, like my center
of gravity was under better control.

Does anyone have any advice to help me conquer this newly chronic
problem?

Quoted message said:

Have you tried another bike just to rule out a mechanical problem?

Riding another bike does nothing to rule out a mechanical issue, unless the
other bike were in every way identical. Even then, the psychological
component of shimmy... just knowing that there *might* be something wrong
with your old bike that isn't with the new one... that alone might create
the confidence or lack of being tense that could have caused the issue on
the first bike.

It's not often as simple a problem as it appears.

--Mike-- Chain Reaction Bicycles
www.ChainReactionBicycles.com

"Zimmy" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Have you tried another bike just to rule out a mechanical problem?
The only time it ever happened to me was when my headset was loose (even
though the locknut was tight) so make sure it is adjusted correctly.
Scared the [censored] out of me too.
Make sure the front wheel is evenly mounted too.

Apologies if this is all obvious to you,
Z

"Michie" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

A few weeks ago, I experienced, for the first time, that I am getting
front wheel wobbling on descents. It was on a steep hill that I have
been down time after time, but this day I was scared half to death
that I was going to lose control of the bike and eat the pavement.
I look my bicycle over, but I can't see any mechanical problems that
might cause that to happen. I seem to be tensing up on descents when
I didn't used to have that problem. Since those few weeks ago, I am
still experiencing the same effect and it is very scary. The people
behind me immediately try to pass me for fear they may end up going
down if I go down.
It's become very annoying. It feels more psychological than anything
else.
I have lost a bit of weight lately from all the riding I have been
doing, so I am flying up hills with relative ease where I used to get
dropped before, but now I am losing a grip on descents when I used to
fly down hills ahead of everyone else, even when I wasn't aero,
because my weight giving me advantage. However, now that I am losing
the weight, I seem to be shimmying like crazy.
It's become like a roller coaster feeling.
One rider, who is a fellow team mate, and is a cat 3 racer, told me to
try pulling my knees in using my them to pull up under the top tube
and sit back farther on the seat and get more aero, but I still feel
like I am shimmying, and he noticed that I was to, even going down a
hill at a mere 25 mph.
Finally, I tried turning the biggest gear on the bike while going down
hill, or at least spin the pedals even if it wasn't giving me
momentum, and it seemed to help. I felt more stable, like my center
of gravity was under better control.

Does anyone have any advice to help me conquer this newly chronic
problem?

Zimmy said:

Have you tried another bike just to rule out a mechanical problem?

Even if a different bike did not shimmy, it wouldn't prove there's a
mechanical problem on the first bike. Shimmy is all about resonance,
and the interaction of bike and rider. A different rider on the bike
probably wouldn't get the shimmy.

Art Harris

"Art Harris" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Zimmy said:

Have you tried another bike just to rule out a mechanical problem?

Even if a different bike did not shimmy, it wouldn't prove there's a
mechanical problem on the first bike. Shimmy is all about resonance,
and the interaction of bike and rider. A different rider on the bike
probably wouldn't get the shimmy.

OK, perhaps "rule out" was the wrong term, however if a few different riders
all got a shimmy on the OP's bike then it could indicate a mechanical
problem, right?

Z

Michie said:

A few weeks ago, I experienced, for the first time, that I am getting
front wheel wobbling on descents. It was on a steep hill that I have
been down time after time, but this day I was scared half to death
that I was going to lose control of the bike and eat the pavement.
I look my bicycle over, but I can't see any mechanical problems that
might cause that to happen. I seem to be tensing up on descents when
I didn't used to have that problem. Since those few weeks ago, I am
still experiencing the same effect and it is very scary. The people
behind me immediately try to pass me for fear they may end up going
down if I go down.
It's become very annoying. It feels more psychological than anything
else.
I have lost a bit of weight lately from all the riding I have been
doing, so I am flying up hills with relative ease where I used to get
dropped before, but now I am losing a grip on descents when I used to
fly down hills ahead of everyone else, even when I wasn't aero,
because my weight giving me advantage. However, now that I am losing
the weight, I seem to be shimmying like crazy.
It's become like a roller coaster feeling.
One rider, who is a fellow team mate, and is a cat 3 racer, told me to
try pulling my knees in using my them to pull up under the top tube
and sit back farther on the seat and get more aero, but I still feel
like I am shimmying, and he noticed that I was to, even going down a
hill at a mere 25 mph.
Finally, I tried turning the biggest gear on the bike while going down
hill, or at least spin the pedals even if it wasn't giving me
momentum, and it seemed to help. I felt more stable, like my center
of gravity was under better control.

Does anyone have any advice to help me conquer this newly chronic
problem?

Aside from other responses, I'd first check the bike over carefully to rule
out anything mechanical or structural (as best as I could). Headset tight?
No loose bolts? Fork and frame all sound? Spokes in good shape? Tires on
right?

Put some stress on things and listen for pops or creaks. Once you're
confident that the bike is good, try a different front wheel and see if the
shimmy goes away.

Does it only do it on this one hill? Sounds almost like there's a "shimmy
point" where at a certain speed or place you get the shake? Maybe once you
know the bike's OK, you can sort of blast through it by /increasing/ your
speed?

IOW, I got nuthin'...

Good luck!

Michie said:

A few weeks ago, I experienced, for the first time, that I am getting
front wheel wobbling on descents. It was on a steep hill that I have
been down time after time, but this day I was scared half to death
that I was going to lose control of the bike and eat the pavement.
I look my bicycle over, but I can't see any mechanical problems that
might cause that to happen.

There are no "problems" specifically. The difficulty is that a
bike/rider system has modes of oscillation that are almost undamped.
Under the right conditions, almost any bike/rider will experience
shimmy. What those conditions are varies with the specifics. Each
bike/rider is different. Someone else can get on the same bike, go down
the same hill, and not experience any shimmy. You may experience it
every time you go down that hill.

The conditions needed to produce the shimmy are almost chaotic, that is,
a very small change in the way you sit, or the wind, or the tires,
whatever, can change a no-shimmy descent into a scary one.

But that is also the solution. Change anything, and the shimmy will
probably stop.

I had a bike that would shimmy on downhills. I learned to raise my butt
up off the saddle, and it disappears, immediately. Try that. It's the
cheapest solution. Others will tell you that you need new wheels, new
tires, or a new stem. Or perhaps the headset is too tight, too loose,
or the wrong brand. The point is, if you change anything, you change
the system, and so you change the shimmy conditions. All the advice is
right, in that whatever you change will fix the problem. Mine is the
cheapest advice. Get off the saddle.

Getting your butt off the saddle makes a huge difference in the system,
since you deny the pivot point for the twisting back and forth that is
what shimmy is. Without the pivot, the motion dies immediately.

On the other hand, gripping the bars harder, which is the natural
reaction, reinforces the motion.

I seem to be tensing up on descents when

Quoted message said:

I didn't used to have that problem. Since those few weeks ago, I am
still experiencing the same effect

Because you are tensing up.

Quoted message said:

It's become very annoying. It feels more psychological than anything
else.

No, it is not psychological. But it seems that way because it is a
positive feedback loop when you tense up.

Quoted message said:

Does anyone have any advice to help me conquer this newly chronic
problem?

Lots of advice coming, I am sure.

--

David L. Johnson

Arguing with an engineer is like mud wrestling with a pig...
You soon find out the pig likes it!

Zimmy said:

OK, perhaps "rule out" was the wrong term, however if a few different riders
all got a shimmy on the OP's bike then it could indicate a mechanical
problem, right?

There is no "mechanical problem", even though changing damn near any
part of the bike will fix the problem.

--

David L. Johnson

Arguing with an engineer is like mud wrestling with a pig...
You soon find out the pig likes it!

Michie? said:

Does anyone have any advice to help me conquer this newly chronic
problem?

Did you read the item to which you were given a URL? Try it:

http://www.sheldonbrown.com/brandt/shimmy.html

Jobst Brandt