Steering oscillation ... what to do?

Colin Nelson

<carlfogel@comcast.net> wrote in message
> news:c9d7726e-5d98-4e09-9313-3da9667a6ad3@g16g2000pri.googlegroups.com...
> On Jun 13, 5:28 pm, "Mike Jacoubowsky" <Mi...@ChainReaction.com>
> wrote:
>
> [snip]
>
> > A bicycle isn't inherently stable; if it were, you
> > could give it a push and get it to head down the road a ways. In all cases
> > (with normal bikes) the bike is dependent upon rider input for stability.
> > Which is a problem, since a nervous rider does things that reinforce a
> > tendancy to shimmy. It's all in the FAQ, but I don't think the FAQ hits
> > hard
> > enough on the fact that a bicycle is inherently unstable until you add a
> > person to it. That should be enough to get people thinking in the right
>
> [snip]
>
> Dear Mike,
>
> Like you, I thought that a riderless bicycle was inherently unstable
> and would topple over shortly after being released.
>
> Several posters enjoyed setting me straight with movies like this:
>
> http://ruina.tam.cornell.edu/resear...e_stability.mov
>
> Cheers,
>
> Carl Fogel
>
>

Unable to see that site for some reason ... Is it the one where the forks were reversed? (but bikes need to negotiate bends as well as straights <wink>




--
Colin N.

Lincolnshire is mostly flat ... But the wind is mostly in your face

dustoyevsky@mac.com

On Jun 13, 2:10*pm, "Doug McLaren" <dougmc
+usenet-20080...@frenzied.us> wrote:

> It's a Giant OCR2 2003 or so -- aluminum frame, fork. *It's pretty
> much stock, except that it has 700x28c tires, and a luggage rack.

What's on the rack, if anything? I've had floppy panniers cause a
seemingly intermittent wobble on a touring bike, at the beginning of a
tour in the Rockies. Cured 100% by wrapping spare bungee cords tight
around the rack and front packs, whose "shock cords" had rotted
internally, not obvious until used some and then they could be seen to
be "lumpy" and had suddenly lost all ability to hold tension. I
thought for sure it was the bike as this was the first trip on this
one, while the packs had been used in previous years, same general
riding area and conditions, but on a different frame, with no
problems. Spooky because there didn't seem to be a threshold speed
where the wobble would start, and no other obvious cause. --D-y

jobst.brandt@stanfordalumni.org

someone wrote:

>> It's a Giant OCR2 2003 or so -- aluminum frame, fork. Â*It's pretty
>> much stock, except that it has 700x28c tires, and a luggage rack.

I think there is still a misunderstanding about shimmy from this.
Shimmy is generated by gyroscopic steering effects of the rotating
front wheel and torsional compliance of the top- and downtube. The
rider mass firmly seated on the saddle and the front tire contact
patch is the axis about which the shimmy oscillates at a fairly fixed
rhythmic frequency. It is not an intermittent wobble or divergence.
Beyond that, when riding from a wet area onto dry pavement, the tire
track shows that its path is practically a straight line with
negligible lateral excursions.

> What's on the rack, if anything? I've had floppy panniers cause a
> seemingly intermittent wobble on a touring bike, at the beginning of
> a tour in the Rockies. Cured 100% by wrapping spare bungee cords
> tight around the rack and front packs, whose "shock cords" had
> rotted internally, not obvious until used some and then they could
> be seen to be "lumpy" and had suddenly lost all ability to hold
> tension. I thought for sure it was the bike as this was the first
> trip on this one, while the packs had been used in previous years,
> same general riding area and conditions, but on a different frame,
> with no problems. Spooky because there didn't seem to be a
> threshold speed where the wobble would start, and no other obvious
> cause.

Baggage, unless tight and solidly part of the bicycle frame, acts as a
damper. Beside that, it is not part of shimmy but can cause unsure
steering by shifting. Shimmy occurs at about 4Hz, something baggage
will not follow and therefore acts as a damper.

Jobst Brandt

carlfogel@comcast.net

On Sat, 14 Jun 2008 11:45:53 +0100, "Colin Nelson"
<colin.nelson2@ntlworld.com> wrote:

> <carlfogel@comcast.net> wrote in message
>> news:c9d7726e-5d98-4e09-9313-3da9667a6ad3@g16g2000pri.googlegroups.com...
>> On Jun 13, 5:28 pm, "Mike Jacoubowsky" <Mi...@ChainReaction.com>
>> wrote:
>>
>> [snip]
>>
>> > A bicycle isn't inherently stable; if it were, you
>> > could give it a push and get it to head down the road a ways. In all cases
>> > (with normal bikes) the bike is dependent upon rider input for stability.
>> > Which is a problem, since a nervous rider does things that reinforce a
>> > tendancy to shimmy. It's all in the FAQ, but I don't think the FAQ hits
>> > hard
>> > enough on the fact that a bicycle is inherently unstable until you add a
>> > person to it. That should be enough to get people thinking in the right
>>
>> [snip]
>>
>> Dear Mike,
>>
>> Like you, I thought that a riderless bicycle was inherently unstable
>> and would topple over shortly after being released.
>>
>> Several posters enjoyed setting me straight with movies like this:
>>
>> http://ruina.tam.cornell.edu/resear...e_stability.mov
>>
>> Cheers,
>>
>> Carl Fogel
>>
>
>Unable to see that site for some reason ... Is it the one where the forks were reversed? (but bikes need to negotiate bends as well as straights <wink>

Dear Colin,

See if you can view a copy here:
http://home.comcast.net/~carlfogel/...e_stability.mov

It's a 2.5 mb QuickTime movie showing two runs of a riderless typical
bicycle, showing how stable it is

In the second run, the stupid bicycle recovers after a fellow running
next to bangs the seat to knock it sideways.

Cheers,

Carl Fogel

dustoyevsky@mac.com

On Jun 14, 2:13*pm, jobst.bra...@stanfordalumni.org wrote:

> Baggage, unless tight and solidly part of the bicycle frame, acts as a
> damper. *Beside that, it is not part of shimmy but can cause unsure
> steering by shifting. *Shimmy occurs at about 4Hz, something baggage
> will not follow and therefore acts as a damper.

And that, ladies and gentlemen, is why my bike stopped wobbling when I
put the bags on securely. --D-y

Tom Kunich

<thefronny@gmail.com> wrote in message
news:85156034-d712-4cc2-aacf-fbff587e7425@a9g2000prl.googlegroups.com...
> On Jun 13, 6:30 pm, jim beam <spamvor...@bad.example.net> wrote:
>> jobst.bra...@stanfordalumni.org wrote:
>> > Doug McLaren wrote:
>>
>> >> So, I'm riding to work today, on the most scary part of my ride --
>> >> 0.6 miles over a bridge on a pretty busy highway with 65 mph
>> >> traffic. 12" of unridable curb. (Alas, any other options are miles
>> >> out of my way.) It's slightly downhill, so I usually get a pretty
>> >> good speed going, perhaps 35 mph if I pedal 25-30 mph if I don't.
>> >> It's perfectly straight, and perfectly flat -- no turning, no bumps.
>> >> It scares me, but it's usually over in about a minute, so I do it.
>>
>> >> Today I'm not pedaling, right in the middle of the lane, and I'm
>> >> crouched down on the lower handlebars (no aero bars or anything like
>> >> that) and my odometer says my top speed was 28 mph, so that's
>> >> probably what my speed was. And then the steering starts
>> >> oscillating back and forth, at about 2 Hz. It starts weak, but
>> >> turns into a pretty strong oscillation in about two seconds, and I'd
>> >> say I was a second or two away from a serious crash. I hit the
>> >> brakes, and as I slow, the oscillation goes away.
>>
>> >> I stop and walk the bike somewhere safe, and check it out. Wheels
>> >> are well secured, steering column is tight, etc. I can't find
>> >> anything wrong. I ride it slow at first, no problems, and the later
>> >> I'm going down another hill (in a much safer place) and hit 30 mph
>> >> and no problems (but so ready to hit those brakes!)
>>
>> >> It's a Giant OCR2 2003 or so -- aluminum frame, fork. It's pretty
>> >> much stock, except that it has 700x28c tires, and a luggage rack.
>> >> And interrupter brake handles, but I wasn't on the top part of the
>> >> bars this time. (This might be relevant -- I do most of my riding
>> >> with my hands on the top part of the bars. I don't race, I just
>> >> ride for exercise, fun and to get from point A to B.)
>>
>> >> The bike is pretty new to me, but I'm not the first owner. I've
>> >> personally only got about 70 miles on it now. No problems like this
>> >> before, but this was probably the fastest I'd been on it until today
>> >> too.
>>
>> >> This part of my ride makes me nervous, and so I might have been
>> >> holding on tighter than usual, but I doubt I was actually shaking
>> >> (well, I was after ...)
>>
>> > You don't need to shake, although that can help initiate shimmy.
>> > Unless you hold onto the bars differently (loose grip) or can calm you
>> > response, you are reinforcing the natural frequency of bicycle shimmy.
>> > Generally the tighter you grip the bars the tenser your arm muscles
>> > are and worry about this occurring, the more certainly it will be
>> > initiated,
>>
>> >> I've found a number of pages that talk about this --
>>
>> > http://www.sheldonbrown.com/brandt/shimmy.html
>> > http://en.wikipedia.org/wiki/Speed_wobble
>>
>> >> ... but I don't really know what to do about it. I was pretty
>> >> close to losing it, and if I had, it would have been very bad.
>>
>> > First you must recognize that your natural response frequency is the
>> > same as the shimmy so even if you are not initiating it, you are
>> > reinforcing it.
>>
>> jobst, he's a piece of floppy harmonic meat. he can't help reinforce it
>> if his dynamics coincide with that of the bike.
>>
>>
>>
>> >> One of the articles suggests putting a leg against my top tube,
>> >> which I'll try, but is there more that can be done? Something that
>> >> doesn't rely on me remembering to ride the right way?
>>
>> > This should become natural to you. I think experimenting would help.
>> > Coast no-hands down a grade where this shimmy speed can be achieved
>> > and see whether it occurs. Then you will know what the natural
>> > critical speed is and know when to be prepared to suppress it.
>>
>> that is a band-aid and does not fix the problem. worse, it doesn't
>> prevent it from starting, something a proper cure does.
>>
>>
>>
>> > That it occurs with hands on the bars makes me suspect you are not at
>> > ease at higher speeds. That is another solution.
>>
>> typical - blame the rider.
>
> An age ago I was in a group coming down Gates Pass by Tucson, AZ, and
> a guy on a tall Colnago Super developed a shimmy. It was crazy; the
> rest of us were having a good time and this poor slob was almost
> wetting himself, thinking he was going to die. He blamed the bike.
> Said the headtube angle was too steep. Of course, this was the early
> 70s and there was a lot of voodoo in cycling. Now that we have Science
> all the voodoo is gone. Right?

And my Super would get some real top tube slappers as well. Scared hell out
of me. Turned out it was because my 61 cm Super was made out of SL and the
fork was WAY too stiff for the bike and would put everything into the frame
instead of the fork. When you hit the right bump WOW would it scare living
crap out of you.

I replaced the fork with a carbon fork that had much more "give" and the
bike was transformed.

Tom Kunich

<dustoyevsky@mac.com> wrote in message
news:eb1f085e-17b9-4403-9b9f-6b86378b9770@m45g2000hsb.googlegroups.com...
On Jun 14, 2:13 pm, jobst.bra...@stanfordalumni.org wrote:
>
> > Baggage, unless tight and solidly part of the bicycle frame, acts as a
> > damper. Beside that, it is not part of shimmy but can cause unsure
> > steering by shifting. Shimmy occurs at about 4Hz, something baggage
> > will not follow and therefore acts as a damper.
>
> And that, ladies and gentlemen, is why my bike stopped wobbling when I
> put the bags on securely.

My Atala cyclocross converted to Touring Bike had a heck of a shimmy at
anything above 15 mph. I had to do something to the front wheel and noted
that it was slightly out of the drops - only about 1/16th of an inch -
seating it all the way in completely cured the problem.

dustoyevsky@mac.com

On Jun 14, 6:21*pm, "Tom Kunich" <cyclintom@yahoo. com> wrote:
> <dustoyev...@mac.com> wrote in message
>
> news:eb1f085e-17b9-4403-9b9f-6b86378b9770@m45g2000hsb.googlegroups.com...
> On Jun 14, 2:13 pm, jobst.bra...@stanfordalumni.org wrote:
>
>
>
> > > Baggage, unless tight and solidly part of the bicycle frame, acts as a
> > > damper. Beside that, it is not part of shimmy but can cause unsure
> > > steering by shifting. Shimmy occurs at about 4Hz, something baggage
> > > will not follow and therefore acts as a damper.
>
> > And that, ladies and gentlemen, is why my bike stopped wobbling when I
> > put the bags on securely.
>
> My Atala cyclocross converted to Touring Bike had a heck of a shimmy at
> anything above 15 mph. I had to do something to the front wheel and noted
> that it was slightly out of the drops - only about 1/16th of an inch -
> seating it all the way in completely cured the problem.

You can believe me that, as we were in the Rockies, and hitting speeds/
going around corners that were a whole different ball game from what I
was used to in the billiard-table Midwest, I stopped and looked for a
cause. Wheels in, straight, bearings OK, tight QR, tires OK, headset
OK, etc., thought it was in the (new) frame, which had checked out
fine in earlier rides at home. Finally saw the "knotted" shock cords,
that had no or just about no "snap" left at all. The packs had gone on
nice and tight, as in the past but the cords effectively broke soon
after.

Takes some time to get the confidence back even when the wobble is
gone. Not a nice feeling and no, it wasn't the same "feel" as another
bike I've had that is a voluntary head-shaker.

Long top tubes seem to help make headshake liklihood worse, perhaps
along with a stiff (heavy?) fork as I have a custom-made (for someone
else) Merckx Ti that has an extra-long top tube and fairly massive,
heavy steel fork which is a marvel headshaker. Can start at maybe
12mph, just to guess. Great bike, just doesn't like to be ridden no-
hands <g>. If it does wiggle hands-on, it's hard to say that's what's
happening, or if it's just "road surface" wiggles that stop
immediately or nearly so. I guess that's my reward for being brave?
<g>
--D-y

jim beam

jobst.brandt@stanfordalumni.org wrote:
> someone wrote:
>
>>> It's a Giant OCR2 2003 or so -- aluminum frame, fork. It's pretty
>>> much stock, except that it has 700x28c tires, and a luggage rack.
>
> I think there is still a misunderstanding about shimmy from this.
> Shimmy is generated by gyroscopic steering effects

no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no,
no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no,
no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no,
no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no,
no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no,
no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no,
no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no,
no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no. how
many more times? NO!

this is NOTHING to do with gyros. NOTHING!!! this happens with snow
ski bikes, water ski bikes, basically any system where steering trail
self-centers - no rotating mass required.


> of the rotating
> front wheel and torsional compliance of the top- and downtube. The
> rider mass firmly seated on the saddle and the front tire contact
> patch is the axis about which the shimmy oscillates at a fairly fixed
> rhythmic frequency. It is not an intermittent wobble or divergence.
> Beyond that, when riding from a wet area onto dry pavement, the tire
> track shows that its path is practically a straight line with
> negligible lateral excursions.
>
>> What's on the rack, if anything? I've had floppy panniers cause a
>> seemingly intermittent wobble on a touring bike, at the beginning of
>> a tour in the Rockies. Cured 100% by wrapping spare bungee cords
>> tight around the rack and front packs, whose "shock cords" had
>> rotted internally, not obvious until used some and then they could
>> be seen to be "lumpy" and had suddenly lost all ability to hold
>> tension. I thought for sure it was the bike as this was the first
>> trip on this one, while the packs had been used in previous years,
>> same general riding area and conditions, but on a different frame,
>> with no problems. Spooky because there didn't seem to be a
>> threshold speed where the wobble would start, and no other obvious
>> cause.
>
> Baggage, unless tight and solidly part of the bicycle frame, acts as a
> damper.

rubbish - it becomes part of the reciprocating mass. what did they
teach you in engineering school jobst? because it sure wasn't anything
to do with either materials /or/ dynamics.


> Beside that, it is not part of shimmy but can cause unsure
> steering by shifting. Shimmy occurs at about 4Hz, something baggage
> will not follow and therefore acts as a damper.

rubbish.

jim beam

dustoyevsky@mac.com wrote:
> On Jun 14, 2:13�pm, jobst.bra...@stanfordalumni.org wrote:
>
>> Baggage, unless tight and solidly part of the bicycle frame, acts as a
>> damper. �Beside that, it is not part of shimmy but can cause unsure
>> steering by shifting. �Shimmy occurs at about 4Hz, something baggage
>> will not follow and therefore acts as a damper.
>
> And that, ladies and gentlemen, is why my bike stopped wobbling when I
> put the bags on securely. --D-y

because you change the spring constant for the reciprocating mass.
that's not a damper.

jim beam

dustoyevsky@mac.com wrote:
> On Jun 14, 6:21�pm, "Tom Kunich" <cyclintom@yahoo. com> wrote:
>> <dustoyev...@mac.com> wrote in message
>>
>> news:eb1f085e-17b9-4403-9b9f-6b86378b9770@m45g2000hsb.googlegroups.com...
>> On Jun 14, 2:13 pm, jobst.bra...@stanfordalumni.org wrote:
>>
>>
>>
>>>> Baggage, unless tight and solidly part of the bicycle frame, acts as a
>>>> damper. Beside that, it is not part of shimmy but can cause unsure
>>>> steering by shifting. Shimmy occurs at about 4Hz, something baggage
>>>> will not follow and therefore acts as a damper.
>>> And that, ladies and gentlemen, is why my bike stopped wobbling when I
>>> put the bags on securely.
>> My Atala cyclocross converted to Touring Bike had a heck of a shimmy at
>> anything above 15 mph. I had to do something to the front wheel and noted
>> that it was slightly out of the drops - only about 1/16th of an inch -
>> seating it all the way in completely cured the problem.
>
> You can believe me that, as we were in the Rockies, and hitting speeds/
> going around corners that were a whole different ball game from what I
> was used to in the billiard-table Midwest, I stopped and looked for a
> cause. Wheels in, straight, bearings OK, tight QR, tires OK, headset
> OK, etc., thought it was in the (new) frame, which had checked out
> fine in earlier rides at home. Finally saw the "knotted" shock cords,
> that had no or just about no "snap" left at all. The packs had gone on
> nice and tight, as in the past but the cords effectively broke soon
> after.
>
> Takes some time to get the confidence back even when the wobble is
> gone. Not a nice feeling and no, it wasn't the same "feel" as another
> bike I've had that is a voluntary head-shaker.
>
> Long top tubes seem to help make headshake liklihood worse, perhaps
> along with a stiff (heavy?) fork as I have a custom-made (for someone
> else) Merckx Ti that has an extra-long top tube and fairly massive,
> heavy steel fork which is a marvel headshaker. Can start at maybe
> 12mph, just to guess. Great bike, just doesn't like to be ridden no-
> hands <g>. If it does wiggle hands-on, it's hard to say that's what's
> happening, or if it's just "road surface" wiggles that stop
> immediately or nearly so. I guess that's my reward for being brave?
> <g>
> --D-y

longer tubes will indeed have less torsional stiffness.

thefronny@gmail.com

> > <dustoyev...@mac.com> wrote in message
>
>billiard-table Midwest

Try touring Northeast Iowa.



tf

carlfogel@comcast.net

On Fri, 13 Jun 2008 19:53:20 -0700 (PDT), carlfogel@comcast.net wrote:

>On Jun 13, 5:28*pm, "Mike Jacoubowsky" <Mi...@ChainReaction.com>
>wrote:
>
>[snip]
>
>> A bicycle isn't inherently stable; if it were, you
>> could give it a push and get it to head down the road a ways. In all cases
>> (with normal bikes) the bike is dependent upon rider input for stability.
>> Which is a problem, since a nervous rider does things that reinforce a
>> tendancy to shimmy. It's all in the FAQ, but I don't think the FAQ hits hard
>> enough on the fact that a bicycle is inherently unstable until you add a
>> person to it. That should be enough to get people thinking in the right
>
>[snip]
>
>Dear Mike,
>
>Like you, I thought that a riderless bicycle was inherently unstable
>and would topple over shortly after being released.
>
>Several posters enjoyed setting me straight with movies like this:
>
>http://ruina.tam.cornell.edu/resear...e_stability.mov
>
>Cheers,
>
>Carl Fogel

Aha! An email suggests that the front fork may have been reversed to
improve stability.

Sure enough, if you stop the movie at the start of the second run, you
can see that the fork is backward:

http://ruina.tam.cornell.edu/resear...e_stability.mov

That may account for the recovery when the bicycle is slapped
sideways.

On the other hand, numerous posters have mentioned pushing bicycles as
kids and watching them stay upright for long distances.

Alas, most of us aren't willing to experiment with our own bikes,
since the results is usually a crash.

Cheers,

Carl Fogel

jobst.brandt@stanfordalumni.org

Carl Fogel wrote:

>>> A bicycle isn't inherently stable; if it were, you could give it a
>>> push and get it to head down the road a ways. In all cases (with
>>> normal bikes) the bike is dependent upon rider input for
>>> stability. Which is a problem, since a nervous rider does things
>>> that reinforce a tendency to shimmy. It's all in the FAQ, but I
>>> don't think the FAQ hits hard enough on the fact that a bicycle is
>>> inherently unstable until you add a person to it. That should be
>>> enough to get people thinking in the right

>> Like you, I thought that a riderless bicycle was inherently
>> unstable and would topple over shortly after being released.

>> Several posters enjoyed setting me straight with movies like this:

http://tinyurl.com/262tel

> Aha! An email suggests that the front fork may have been reversed to
> improve stability.

> Sure enough, if you stop the movie at the start of the second run,
> you can see that the fork is backward:

> That may account for the recovery when the bicycle is slapped
> sideways.

> On the other hand, numerous posters have mentioned pushing bicycles
> as kids and watching them stay upright for long distances.

> Alas, most of us aren't willing to experiment with our own bikes,
> since the results is usually a crash.

They undermined their experiment with so little faith that they
doubled the trail to make sure it worked. Well, it would have worked
just as well with the fork forward.

For those of us who ride no-hands any distance, especially down ling
descents, this response is evident and is the mechanism by which one
rides no-hands, using gyroscopic reaction of the front wheel to change
in lean angle.

This steering effect is also what produces shimmy on a bicycle, rather
than front wheel trail as with a caster on a shopping cart. Coasting
downhill, shaking ones knee from side to side produces rapid steering
motions, the kind that correct for change in lean in the riderless
bicycle demonstration... or for that matter, for a shimmying bicycle.

A great example of this was demonstrated at last year's InterBike show
by a rider who had put a forward rotating flywheel between the spokes
of his conventional 36-spoke wheel on an otherwise unmodified bicycle.
This enabled him to ride no-hands reliably at less than 2MPH up and
down the aisles of the show.
--
Jobst Brandt