High speed descent stability ?

I believe that 'shimmy', its effects, causes and the cure is addressed in the rec.bicycling FAQ.
IIRC, in a word, all bikes shimmy at some speed and it may be temporarily stopped by resting your
knee on the top bar.

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

Quoted message said:

The other day while descending one of the steeper hills around here, I hit a rough patch of road,
which caused the front wheel to move rapidly back and forth , I don't know if shimmy is the right
word to use or not. While I managed to bring things back under control (scared the hell out of
me...) I was wondering what frame and or fork factors contribute to this and if I were looking for
a more stable descending bike, what should I be looking for. I am willing to accept that my poor
technique was a contributing factor, but wonder what the equipment options are to help mitigate
the effects of this situation?

Thanks in advance.

Guess I should add that the bike is a 62cm schwinn 754 with the stock fork.

cheers, Paul - email: [email hidden]

(Paul Erickson) said:

I were looking for a more stable descending bike, what should I be looking for. I am willing to
accept that my poor technique was a contributing factor, but wonder what the equipment options are
to help mitigate the effects of this situation?

I've found that relaxing my grip and slightly getting my butt out of the saddle stops and prevents
high speed shimmy. I've also found that frame and fork materials have nothing to do with it.

Barry

B a r r y B u r k e J r . said:

(Paul Erickson) said:

I were looking for a more stable descending bike, what should I be looking for. I am willing to
accept that my poor technique was a contributing factor, but wonder what the equipment options are
to help mitigate the effects of this situation?

I've found that relaxing my grip and slightly getting my butt out of the saddle stops and prevents
high speed shimmy. I've also found that frame and fork materials have nothing to do with it.

Though it's not really "frame and material", there are differences in frames and forks that produce
bikes more or less prone to shimmy. I've found that stiffer frames seem to have much less tendency
to develop a high speed shimmy. For example, I've never had anyone tell me about experiencing shimmy
on a Habanero (probably due to more lateral stiffness than most similarly light frames).

Mark Hickey Habanero Cycles habcycles.comhabcycles.comOpen ↗ Home of the $695 ti frame

Mark Hickey said:

B a r r y B u r k e J r . said:

(Paul Erickson) said:

I were looking for a more stable descending bike, what should I be looking for. I am willing to
accept that my poor technique was a contributing factor, but wonder what the equipment options
are to help mitigate the effects of this situation?

I've found that relaxing my grip and slightly getting my butt out of the saddle stops and
prevents high speed shimmy. I've also found that frame and fork materials have nothing to do
with it.

Though it's not really "frame and material", there are differences in frames and forks that
produce bikes more or less prone to shimmy. I've found that stiffer frames seem to have much less
tendency to develop a high speed shimmy. For example, I've never had anyone tell me about
experiencing shimmy on a Habanero (probably due to more lateral stiffness than most similarly
light frames).

I love my Habanero, but it shimmies at around 20mph if I'm riding no-hands. My last bike, an
Aluminum Cannondale, never shimmied. Frame is 64cm, if that makes any difference.

It doesn't bother me at all, as it's pretty trivial to fix by applying light pressure to the bars.

Sam

Paul Erickson said:

The other day while descending one of the steeper hills around here, I hit a rough patch of road,
which caused the front wheel to move rapidly back and forth , I don't know if shimmy is the right
word to use or not.

It is

Quoted message said:

While I managed to bring things back under control (scared the hell out of
me...) I was wondering what frame and or fork factors contribute to this and if I were looking for
a more stable descending bike, what should I be looking for. I am willing to accept that my
poor technique was a contributing factor, but wonder what the equipment options are to help
mitigate the effects of this situation?

It's a system response, so any change to the system might prevent it -- or make it worse. But there
is a technique that has always worked for me (my old bike shimmied badly). Get out of the saddle --
or at least transfer your load-bearing to your feet. Since you are the most massive part of the
system, changing your intereaction with it will stop the resonance.

Quoted message said:

Guess I should add that the bike is a 62cm schwinn 754 with the stock fork.

Big frames do seem to be more susceptible to shimmy.

--

David L. Johnson

__o | There is always an easy solution to every human problem - neat, _`\(,_ | plausible, and
wrong. --H.L. Mencken (_)/ (_) |

Sam Huffman said:

I love my Habanero, but it shimmies at around 20mph if I'm riding no-hands. My last bike, an
Aluminum Cannondale, never shimmied. Frame is 64cm, if that makes any difference.

It doesn't bother me at all, as it's pretty trivial to fix by applying light pressure to the bars.

In fairness to Mark, this is a really big frame, which is more likely to shimmy. My Habanero has
never shown any tendency to shimmy.

--

David L. Johnson

__o | If all economists were laid end to end, they would not reach a _`\(,_ | conclusion. --
George Bernard Shaw (_)/ (_) |

David L. Johnson said:

Sam Huffman said:

I love my Habanero, but it shimmies at around 20mph if I'm riding no-hands. My last bike, an
Aluminum Cannondale, never shimmied. Frame is 64cm, if that makes any difference.

It doesn't bother me at all, as it's pretty trivial to fix by applying light pressure to
the bars.

In fairness to Mark, this is a really big frame, which is more likely to shimmy. My Habanero has
never shown any tendency to shimmy.

Ditto. My 58cm Habby has never shimmied. It's been up to 60mph descending of the front side of
Moniter pass on The Death Ride. Hands on.

Jim Wilson

Sam Huffman said:

Mark Hickey <[email hidden]> writes:

Quoted message said:

Quoted message said:

Though it's not really "frame and material", there are differences in frames and forks that
produce bikes more or less prone to shimmy. I've found that stiffer frames seem to have much less
tendency to develop a high speed shimmy. For example, I've never had anyone tell me about
experiencing shimmy on a Habanero (probably due to more lateral stiffness than most similarly
light frames).

I love my Habanero, but it shimmies at around 20mph if I'm riding no-hands. My last bike, an
Aluminum Cannondale, never shimmied. Frame is 64cm, if that makes any difference.

It doesn't bother me at all, as it's pretty trivial to fix by applying light pressure to the bars.

Dang, now I have to say "I've never had anyone tell me about experiencing hands-on shimmy...". ;-)

Mark Hickey Habanero Cycles habcycles.comhabcycles.comOpen ↗ Home of the $695 ti frame

On Mon, 28 Jul 2003 19:59:29 +0000, Jim Wilson wrote: e

Quoted message said:

Quoted message said:

Quoted message said:

is 64cm, if that makes any difference.

It doesn't bother me at all, as it's pretty trivial to fix by applying light pressure to
the bars.

In fairness to Mark, this is a really big frame, which is more likely to shimmy. My Habanero has
never shown any tendency to shimmy.

Ditto. My 58cm Habby has never shimmied. It's been up to 60mph descending of the front side of
Moniter pass on The Death Ride. Hands

on.

And mine is a 56. A 64 is a whole different animal.

--

David L. Johnson

__o | It is a scientifically proven fact that a mid life crisis can _`\(,_ | only be cured by
something racy and Italian. Bianchis and (_)/ (_) | Colnagos are a lot cheaper than Maserattis
and Ferraris. -- Glenn Davies

"David L. Johnson" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

Paul Erickson said:

The other day while descending one of the steeper hills around here, I

hit

Quoted message said:

Quoted message said:

a rough patch of road, which caused the front wheel to move rapidly back and forth , I don't
know if shimmy is the right word to use or not.

It is

Quoted message said:

While I managed to bring things back under control (scared the hell out of
me...) I was wondering what frame and or fork factors contribute to this and if I were looking
for a more stable descending bike, what should I

be

Quoted message said:

Quoted message said:

looking for. I am willing to accept that my poor technique was a contributing factor, but wonder
what the equipment options are to help mitigate the effects of this situation?

It's a system response, so any change to the system might prevent it -- or make it worse. But
there is a technique that has always worked for me (my old bike shimmied badly). Get out of the
saddle -- or at least transfer your load-bearing to your feet. Since you are the most massive part
of the system, changing your intereaction with it will stop the resonance.

Quoted message said:

Guess I should add that the bike is a 62cm schwinn 754 with the stock fork.

Big frames do seem to be more susceptible to shimmy.

--

David L. Johnson

__o | There is always an easy solution to every human problem - neat, _`\(,_ | plausible, and
wrong. --H.L. Mencken (_)/ (_) |

It's funny, after reading all the posts regarding shimmy, it seems everyone blames it on the
frame. I always thought it a function of castor and WHEEL BALANCE. By balance I mean off of the
central plane. My wheel didn't seem to shimmy nearly as bad as when I put my magnet for my
cyclocomputer on it. I moved it to the other side of the wheel and it changed quite a bit. I
wanted to experiement with moving it, but figured like the other poster that I should just keep my
damned hands on the wheel...

David L. Johnson said:

Quoted message said:

Ditto. My 58cm Habby has never shimmied. It's been up to 60mph descending of the front side of
Moniter pass on The Death Ride. Hands

on.

And mine is a 56. A 64 is a whole different animal.

Silly question to those who understand frame design:

Why don't larger frames use larger diameter and thicker tubes?

If you just make the tubes longer, the frame becomes more flexible, you then add to that the fact
that you would expect a rider on a 64cm frame to weigh more than the rider on a 50cm frame you would
want to make the larger frame stiffer, rather than more flexy - the logical way to do this would be
to scale the diameter and thickness of the tubes along with the length.

Mark in Maine said:

"David L. Johnson"><[email hidden]> wrote:

Quoted message said:

Quoted message said:

Quoted message said:

Ditto. My 58cm Habby has never shimmied. It's been up to 60mph descending of the front side of
Moniter pass on The Death Ride. Hands

on.

And mine is a 56. A 64 is a whole different animal.

Silly question to those who understand frame design:

Why don't larger frames use larger diameter and thicker tubes?

If you just make the tubes longer, the frame becomes more flexible, you then add to that the fact
that you would expect a rider on a 64cm frame to weigh more than the rider on a 50cm frame you
would want to make the larger frame stiffer, rather than more flexy - the logical way to do this
would be to scale the diameter and thickness of the tubes along with the length.

That's very much necessary if the bikes are being built to the "bleeding edge", but in my case the
"normal" tubes are more than up to the task of handling a strong, heavy rider on a 64cm. Reducing
the diameter of the tubes for the 50cm frame would at best shave a few ounces from the frame - but
would also be the wrong thing to do for that short, heavy rider. Plus, since ANY lateral flex is bad
my philosophy is to design around eliminating it in the worst case scenario, and accepting some
"overkill" for lighter or smaller riders.

And FWIW, there is an optional big down tube for those who think they really need it (round or
ovalized) - 40mm / 32x48mm vs 32.

Mark Hickey Habanero Cycles habcycles.comhabcycles.comOpen ↗ Home of the $695 ti frame

F1 said:

It's funny, after reading all the posts regarding shimmy, it seems everyone blames it on the
frame. I always thought it a function of castor and WHEEL BALANCE. By balance I mean off of the
central plane. My wheel didn't seem to shimmy nearly as bad as when I put my magnet for my
cyclocomputer on it. I moved it to the other side of the wheel and it changed quite a bit. I
wanted to experiement with moving it, but figured like the other poster that I should just keep my
damned hands on the wheel...

See, the problem is that just about anything you might do might fix the problem. The problem is that
the "system" of the bike and rider, as a damped spring-mass system, hits a (close to) resonant
frequency when ridden at certain speeds and certain conditions, and the frequencies of the input
forces (steering, wind, etc.) are reinforced rather than cancelled out. Result is that some
back-and-forth motion is magnified by a feedback loop in the system.

Change the tire, the fork, the frame, and likely the resonant frequencies will change enough to not
be a problem. This is also true when you simply get out of the saddle, or when you lay your knee on
the top tube.

What I was referring to in my reply, though, was the coincidence that many people with shimmy
problems are tall, and so have big frames. It probably concentrates the resonant frequencies lower
in the range, due to the length of the levers (the tubes) in the system.

--

David L. Johnson

__o | Arguing with an engineer is like mud wrestling with a pig... You _`\(,_ | soon find out the
pig likes it! (_)/ (_) |

I agree. I have a big frame and it does it bigtime around 17-19mph. It used to be MUCH worse and at
lower speeds when I had my whatchamacallit (triathalon style bar) on the handlebar. More mass, lower
resonance freq/speed?

"David L. Johnson" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

F1 said:

It's funny, after reading all the posts regarding shimmy, it seems everyone blames it on the
frame. I always thought it a function of

castor

Quoted message said:

Quoted message said:

and WHEEL BALANCE. By balance I mean off of the central plane. My

wheel

Quoted message said:

Quoted message said:

didn't seem to shimmy nearly as bad as when I put my magnet for my cyclocomputer on it. I moved
it to the other side of the wheel and it changed quite a bit. I wanted to experiement with
moving it, but

figured

Quoted message said:

Quoted message said:

like the other poster that I should just keep my damned hands on the wheel...

See, the problem is that just about anything you might do might fix the problem. The problem is
that the "system" of the bike and rider, as a damped spring-mass system, hits a (close to)
resonant frequency when ridden at certain speeds and certain conditions, and the frequencies of
the input forces (steering, wind, etc.) are reinforced rather than cancelled out. Result is that
some back-and-forth motion is magnified by a feedback loop in the system.

Change the tire, the fork, the frame, and likely the resonant frequencies will change enough to
not be a problem. This is also true when you simply get out of the saddle, or when you lay your
knee on the top tube.

What I was referring to in my reply, though, was the coincidence that many people with shimmy
problems are tall, and so have big frames. It probably concentrates the resonant frequencies lower
in the range, due to the length of the levers (the tubes) in the system.

--

David L. Johnson

__o | Arguing with an engineer is like mud wrestling with a pig...

You

Quoted message said:

_`\(,_ | soon find out the pig likes it! (_)/ (_) |

F1 said:

It's funny, after reading all the posts regarding shimmy, it seems everyone blames it on the frame.

Not me, I blamed it on the rider. <G>

Barry

Quoted message said:

It's funny, after reading all the posts regarding shimmy, it seems

everyone

Quoted message said:

blames it on the frame. I always thought it a function of castor and

WHEEL

Quoted message said:

BALANCE.

You're half right. It is the wheel, not the frame. Try using a Mavic MA3 wheel, on the front and
put a 25 tire on it, and I doubt if you get a shimmy. It's those damn light aero wheels that are
giving everyone the shimmys.

Mark Hickey said:

Dang, now I have to say "I've never had anyone tell me about experiencing hands-on shimmy...".

That's not true. You've been around this newsgroup for a while and ALL those who complain about
shimmy were "hands-on" riders. That's the point. If they were no-hands riders they wouldn't complain
because they would know that their bicycles shimmy at a known speed.

Read about shimmy in the FAQ for details.

Jobst Brandt [email hidden] Palo Alto CA

Quoted message said:

Mark Hickey said:

Dang, now I have to say "I've never had anyone tell me about experiencing hands-on shimmy...".

That's not true. You've been around this newsgroup for a while and ALL those who complain about
shimmy were "hands-on" riders. That's the point. If they were no-hands riders they wouldn't
complain because they would know that their bicycles shimmy at a known speed.

Read about shimmy in the FAQ for details.

You're preaching to the choir... my statement was in regards to the lack of reports of shimmy
(of any type) related to me by Habanero customers, until the one I replied to (which was
hands-off shimmy).

I agree that a hands-off shimmy isn't really anything to get excited about, one way or another
(unless of course it happens during the feed zone at the base of an Alpine climb...).

Mark Hickey Habanero Cycles habcycles.comhabcycles.comOpen ↗ Home of the $695 ti frame

I just checked in here, and don't have any earlier messages in this thread. Is there a source for
archived posts other than the FAQ?

I've had a lot of trouble finding a large (~62cm) road frame that descends well. I have long legs
and short torso, so need a top tube of 60cm or less. Hands-off shimmy doesn't bother me, but I've
had a lot of trouble with shimmy on bumpy descents, especially when braking late, or encountering
unexpected bumps or sand/gravel. Most shops don't carry a lot of larger frames, and there are only
two small shops in my town. Buying and trying different frames has gotten too expensive. Even very
stiff oversized aluminum frames (Klein, Cannondale) haven't worked well for me. I haven't found
anyone who could tell me that a certain head angle or fork rake would help. I'm now considering
trying to rig a steering damper of some sort (I've seen both rotary and linear types on
motorcycles), but I don't think that should be necessary. I don't think my technique is the problem,
and I also have experience with fairly aggressive cornering on motorcycles. I typically corner with
cranks horizontal and weight off the saddle. Any ideas, or anyone had a similar experience and found
a solution?

David Mackintosh [email hidden]

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

Quoted message said:

Mark Hickey said:

Dang, now I have to say "I've never had anyone tell me about experiencing hands-on shimmy...".

That's not true. You've been around this newsgroup for a while and ALL those who complain about
shimmy were "hands-on" riders. That's the point. If they were no-hands riders they wouldn't
complain because they would know that their bicycles shimmy at a known speed.

Read about shimmy in the FAQ for details.

Jobst Brandt [email hidden] Palo Alto CA