wobble on fast curve?

Yikes!

Upon leaving a national park of steep hills and starting a nice swift
ride down the road leading to it, I was going maybe 30 mph when the
road curved and I leaned the bike accordingly. ...

.... Suddenly, my wheels started wobbling like crazy (or so it felt).
For maybe 10-15(?) seconds I was barely able to keep control. (I
didn't dare peek at the wheels -- I didn't dare take my eyes off the
road while trying to avoid a very nasty spill!) Once I slowed down
and straightened out, everything returned to normal.

What happened? I've never had that happen before. Is something wrong
with my bike?

As far as I could see, everything on the bike was (& still is) tight.
Tires were inflated to about 105+, wheels were re-trued two before by
a mechanic, & quick releases seemed firmly locked. The tires are
tight enough on the rims to be a little hard to get off when replacing
tubes. This is a 1996-ish Trek 2300 road bike.

FWIW, the road itself was smooth, I'm pretty sure. (It didn't feel
anything like riding over holes or cracks or ripples in the road, and
I would've seen anything like that, I hope.)

Is there any "typical" explanation for something like this? 30mph-ish
is too slow to feel out-of-balance wheels, and I'd have noticed that
on straight 30mph runs if that were the case. Maybe the tire was
starting to be flexed out of the wheel rim? I dunno.

Craig

Craig No Joke writes:

Yikes!

Upon leaving a national park of steep hills and starting a nice
swift ride down the road leading to it, I was going maybe 30 mph
when the road curved and I leaned the bike accordingly.

... Suddenly, my wheels started wobbling like crazy (or so it felt).
For maybe 10-15(?) seconds I was barely able to keep control. (I
didn't dare peek at the wheels -- I didn't dare take my eyes off the
road while trying to avoid a very nasty spill!) Once I slowed down
and straightened out, everything returned to normal.

What happened? I've never had that happen before. Is something
wrong with my bike?

As far as I could see, everything on the bike was (& still is)
tight. Tires were inflated to about 105+, wheels were re-trued two
before by a mechanic, & quick releases seemed firmly locked. The
tires are tight enough on the rims to be a little hard to get off
when replacing tubes. This is a 1996-ish Trek 2300 road bike.

FWIW, the road itself was smooth, I'm pretty sure. (It didn't feel
anything like riding over holes or cracks or ripples in the road,
and I would've seen anything like that, I hope.)

Is there any "typical" explanation for something like this?
30mph-ish is too slow to feel out-of-balance wheels, and I'd have
noticed that on straight 30mph runs if that were the case. Maybe
the tire was starting to be flexed out of the wheel rim? I dunno.

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

Jobst Brandt

The famous speed wobble, I've gone through this myself. Forget all the
technical explanations you will get. I found out it was me. Even though I
wasn't aware of it. You chickened out, and when that happens the bike
wobbles. This isn't criticism, having gone through this myself. Do the
exact descent again, try to figure out what made you nervous. Was it the
car passing you too close, was it the rumble of a big truck behind you. I
found out by doing this technique myself, I found out, it was the nature of
the traffic behind me, would sometimes rattle me on steep descents. Kind of
like battle fatigue. The shelling begins and you start unvoluntary shaking,
when there may be no rational reason to be nervous. Fear is a good thing,
when it's controled, when not, it is very dangerous, as I found out also.
Good luck. BTW, you didn't brake on the curve did you? Brake before the
curve.

On Tue, 02 Aug 2005 02:31:40 GMT,
wrote:

Craig No Joke writes:
... Suddenly, my wheels started wobbling like crazy (or so it felt)
....

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

Jobst Brandt

Thanks, Jabst. Shimmy sounds like a good theory.

That reference describes it affecting front wheels. I was pretty sure
I was feeling the effect from both wheels, but if the seat and pedals
were somehow involved in the effect I might not have been able to tell
the difference without looking. I didn't actually see the front wheel
wobble, but my hands felt it, so that part might be consistent. I
wonder if my partially carbon frame (the middle triangle) could've
made shimmy affect both wheels or just the rear.

The Sheldon Brown article makes me imagine that shimmy might not be as
dangerous as it felt, is that right? If its safe to experiment, I
might try that hill again that way and try to get a better sense of
what which wheel(s) was affected. Hopefully I won't be able to
duplicate it, though.

Thanks again,

Craig

Callistus Valerius writes:

The famous speed wobble, I've gone through this myself. Forget all
the technical explanations you will get. I found out it was me.
Even though I wasn't aware of it. You chickened out, and when that
happens the bike wobbles.

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.

This isn't criticism, having gone through this myself. Do the exact
descent again, try to figure out what made you nervous. Was it the
car passing you too close, was it the rumble of a big truck behind
you.e I found out by doing this technique myself, I found out, it
was the nature of the traffic behind me, would sometimes rattle me
on steep descents. Kind of like battle fatigue.

I propose it's more physical fatigue and at a point where shakes are
natural, easily excited by normal bicycle frame shimmy. If you have
the option, place the hands on the bar tops near the stem where the
arms will have less leverage on steering. If you need to brake, it
will unfortunately require placing your hands at the brake levers
where the arms typically have more muscular influence.

The shelling begins and you start unvoluntary shaking, when there
may be no rational reason to be nervous. Fear is a good thing, when
it's controled, when not, it is very dangerous, as I found out also.
Good luck. BTW, you didn't brake on the curve did you? Brake
before the curve.

I think you should reassess your analysis. Braking has no effect on
this and nervousness may be your interpretation of fatigue and cold.
Don't tell the shivering person in the ski lift line to not be so
nervous.

Jobst Brandt

On Tue, 02 Aug 2005 02:46:21 GMT, "Callistus Valerius"
wrote:

The famous speed wobble, I've gone through this myself. Forget all the
technical explanations you will get. I found out it was me. Even though I
wasn't aware of it. You chickened out, and when that happens the bike
wobbles. This isn't criticism, having gone through this myself. Do the
exact descent again, try to figure out what made you nervous. Was it the
car passing you too close, was it the rumble of a big truck behind you. I
found out by doing this technique myself, I found out, it was the nature of
the traffic behind me, would sometimes rattle me on steep descents. Kind of
like battle fatigue. The shelling begins and you start unvoluntary shaking,
when there may be no rational reason to be nervous. Fear is a good thing,
when it's controled, when not, it is very dangerous, as I found out also.
Good luck. BTW, you didn't brake on the curve did you? Brake before the
curve.

Nice theory, but I have to doubt it. Or else you're just trying to
pull my leg ... oh, gee, there's an explanation: maybe you were
tagging along pulling my leg! ... just kidding ;-)

There was no traffic at the moment -- just trees and smooth road and
sunny warm weather. My speed was not scary, no reason to imagine any
problem. I was exhausted, but I rested a bit before starting down
that hill. Besides, I can't imagine a way to shiver or shake in a way
that would feel like it's coming from both wheels instead of from me.

I don't remember braking on the curve until after the wobbling started
(and then felt I had no choice). I think it's more likely I was
peddling to gain speed, or starting to coast, when it started, but
it's possible I started to brake a little (though I doubt it).

Thanks,

Craig

On Tue, 02 Aug 2005 03:18:08 GMT,
wrote:

Callistus Valerius writes:
....
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.

In my own case, I doubt that muscle shivering was involved. However,
I could imagine that my trying to stabilize (stiffen) my steering
might have worsened the effect by adding counterforces too weak to
stop the wheel movements but strong enough to reinforce them,
prepetuating some kind of resonance flexing.

Being on a downhill curve, I couldn't risk not trying to stiffen the
steering and cautiously braking. If the problem were really something
else more serious, .... well, you get the picture.

Still, in some form, the shimmy theory seems the most likely
explanation.

Thanks again,

Craig

CraigNJ wrote:
On Tue, 02 Aug 2005 03:18:08 GMT,
wrote:


Callistus Valerius writes:

...

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.


In my own case, I doubt that muscle shivering was involved.

i doubt it ever is.

fact: jobst has never tested for the effect lateral wheel stiffness has
on shimmy.

fact: jobst has never tested for the effect frame torsional stiffness
has on shimmy.

so, in the absence of any data on the two most relevant varaibles in the
harmonic response of the bicycle system, he blusters & grasps for an
ethereal straw that can be argued by plain old bullying - shivering!
yeah! right!

jobst has quite astounding chutzpah.

However,
I could imagine that my trying to stabilize (stiffen) my steering
might have worsened the effect by adding counterforces too weak to
stop the wheel movements but strong enough to reinforce them,
prepetuating some kind of resonance flexing.

Being on a downhill curve, I couldn't risk not trying to stiffen the
steering and cautiously braking. If the problem were really something
else more serious, .... well, you get the picture.

Still, in some form, the shimmy theory seems the most likely
explanation.

Thanks again,

Craig

On Mon, 01 Aug 2005 20:51:55 -0700, jim beam
wrote:

CraigNJ wrote:
On Tue, 02 Aug 2005 03:18:08 GMT, wrote:
Callistus Valerius writes:
... natural resonance of the muscles (that of shivering) ...
In my own case, I doubt that muscle shivering was involved. ...
i doubt it ever is.
fact: jobst has never tested ...
fact: jobst has never tested for ...
so, ... bullying - shivering!
....
jobst has quite astounding chutzpah.

I think my trimming of context may have misdirected you, Jim. It was
Callistus who claimed I must have been shaking/shivering like he
himself has experienced, and I think jobst was just trying to explain
what might have caused the shaking Callistus experienced. I see no
bullying ... although bullying might be a way to make a person start
shaking in their boots. ;-)

My apologies for causing the confusion,

Craig

"CraigNJ" wrote in message
...
On Tue, 02 Aug 2005 02:31:40 GMT,
wrote:

Craig No Joke writes:
... Suddenly, my wheels started wobbling like crazy (or so it felt)
...

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

Jobst Brandt

Thanks, Jabst. Shimmy sounds like a good theory.

That reference describes it affecting front wheels. I was pretty sure
I was feeling the effect from both wheels, but if the seat and pedals
were somehow involved in the effect I might not have been able to tell
the difference without looking. I didn't actually see the front wheel
wobble, but my hands felt it, so that part might be consistent. I
wonder if my partially carbon frame (the middle triangle) could've
made shimmy affect both wheels or just the rear.

The Sheldon Brown article makes me imagine that shimmy might not be as
dangerous as it felt, is that right? If its safe to experiment, I
might try that hill again that way and try to get a better sense of
what which wheel(s) was affected. Hopefully I won't be able to
duplicate it, though.


I used to get a shimmy on a particularly fast hill ( 35 mph ) on the way to work
some mornings. To me it felt like the rear wheel was shaking, but of course it
was not a time to carefully peer at it. Try to avoid it, it doesn't do the
frame joints much good.

Did you have anything on a rack behind you?