A Cycling & bikes forum. CycleBanter.com

Go Back   Home » CycleBanter.com forum » rec.bicycles » Techniques
Site Map Home Register Authors List Search Today's Posts Mark Forums Read Web Partners

High-speed shimmy, Speed wobble



 
 
Thread Tools Display Modes
  #1  
Old September 29th 03, 05:23 PM
Shayne Wissler
external usenet poster
 
Posts: n/a
Default High-speed shimmy, Speed wobble

This weekend I was descending a canyon, and at around 42MPH my LeMond Buenos
Aires started to get a bad speed wobble. This is the first time it's ever
happened (and I've been at that speed many times over the summer).

I've read about speed wobbles but have never experienced one. At first I
thought my front tire had gone completely flat and was going out of control,
or as if the front tire had become extremely out of true--I'd guess that the
wobble amplitude was about 2-3", and a frequency of about 5Hz. But the
braking was responsive. As I slowed down it continued to wobble for a while
though lower speeds (I'll guess through 30MPH--but I don't know). I finally
stopped, checked that my front tire was in tact and not out of true.
Everything appeared OK, so I continued down the hill, this time not going
over 30 or so.

My owners manual says that if I experience this, to take it into the shop. I
called the shop and they want to see the bike. But based on my reading of
the shimmy FAQ, I'm half expecting that they won't find anything.

I have made two changes to my bike recently. The most recent was that I
lowered my handlebars by about 2". They used to be almost parallel to my
seat. I did tighten the headset well. The other is that I replaced my front
tire with a lighter, skinnier one.

I have descended with the tire before, but I can't say whether I reached
42MPH or not. I can say that ever since I put that tire on, I have noticed
there being less stability while riding no-hands--it does start to shimmy a
little, but at lower speeds and not so violently. Is it probable that this
low-speed (~20MPH?) no-hands shimmy is from the same source as the violent
shimmy?

Any suggestions would be appreciated.


Shayne Wissler


Ads
  #2  
Old September 29th 03, 06:01 PM
Werehatrack
external usenet poster
 
Posts: n/a
Default High-speed shimmy, Speed wobble

On Mon, 29 Sep 2003 16:23:31 GMT, "Shayne Wissler"
may have said:

Is it probable that this
low-speed (~20MPH?) no-hands shimmy is from the same source as the violent
shimmy?

Any suggestions would be appreciated.


Observing that the problem appeared after the tire change, the
connection to the reduced diameter is possible but not proven.

An observation: Reducing the diameter of the front tire will move the
steerer axis more toward the vertical, which moves the point of
intersection of the steerer axis with the ground toward the tire's
contact patch. The distance between the axis intersection and the
patch is the "trail" of the fork. Insufficient trail will cause
exactly the problem that you describe...but if the amount of trail
before the tire swap was small enough that the change produced this
problem, I would be a bit surprised. If the bike has a suspension
fork, try cranking the preload up to the max, as that will tend to
minimize such a problem.

Somehow, though, I think the lbs is likely to find that there is
something else wrong. What I describe is, in my opinion, not terribly
likely to be the problem.

--
My email address is antispammed;
pull WEEDS if replying via e-mail.
Yes, I have a killfile. If I don't respond to something,
it's also possible that I'm busy.
  #3  
Old September 29th 03, 06:10 PM
Mike Jacoubowsky
external usenet poster
 
Posts: n/a
Default High-speed shimmy, Speed wobble

I've read about speed wobbles but have never experienced one. At first I
thought my front tire had gone completely flat and was going out of

control,
or as if the front tire had become extremely out of true--I'd guess that

the
wobble amplitude was about 2-3", and a frequency of about 5Hz. But the
braking was responsive. As I slowed down it continued to wobble for a

while
though lower speeds (I'll guess through 30MPH--but I don't know). I

finally
stopped, checked that my front tire was in tact and not out of true.
Everything appeared OK, so I continued down the hill, this time not going
over 30 or so.


Once you get a shimmy, my advice is not to slow down, but stop, take a
breather, and then get going again. Lack of confidence doesn't help the
situation any.

I have made two changes to my bike recently. The most recent was that I
lowered my handlebars by about 2". They used to be almost parallel to my
seat. I did tighten the headset well. The other is that I replaced my

front
tire with a lighter, skinnier one.


Simplest experiment would be to raise the bars back up again and see if the
shimmy goes away. As you've read in the FAQ, shimmy isn't a function of the
bicycle alone, but the entire system, which includes you on it. The
combination of your position being altered and the skinnier tire (which
speeds up response on the front end) sounds like a good place to start,
almost a no-brainer.

--Mike-- Chain Reaction Bicycles
http://www.ChainReactionBicycles.com


"Shayne Wissler" wrote in message
news:70Zdb.634208$o%2.291570@sccrnsc02...
This weekend I was descending a canyon, and at around 42MPH my LeMond

Buenos
Aires started to get a bad speed wobble. This is the first time it's ever
happened (and I've been at that speed many times over the summer).

I've read about speed wobbles but have never experienced one. At first I
thought my front tire had gone completely flat and was going out of

control,
or as if the front tire had become extremely out of true--I'd guess that

the
wobble amplitude was about 2-3", and a frequency of about 5Hz. But the
braking was responsive. As I slowed down it continued to wobble for a

while
though lower speeds (I'll guess through 30MPH--but I don't know). I

finally
stopped, checked that my front tire was in tact and not out of true.
Everything appeared OK, so I continued down the hill, this time not going
over 30 or so.

My owners manual says that if I experience this, to take it into the shop.

I
called the shop and they want to see the bike. But based on my reading of
the shimmy FAQ, I'm half expecting that they won't find anything.

I have made two changes to my bike recently. The most recent was that I
lowered my handlebars by about 2". They used to be almost parallel to my
seat. I did tighten the headset well. The other is that I replaced my

front
tire with a lighter, skinnier one.

I have descended with the tire before, but I can't say whether I reached
42MPH or not. I can say that ever since I put that tire on, I have noticed
there being less stability while riding no-hands--it does start to shimmy

a
little, but at lower speeds and not so violently. Is it probable that this
low-speed (~20MPH?) no-hands shimmy is from the same source as the violent
shimmy?

Any suggestions would be appreciated.


Shayne Wissler




  #4  
Old September 29th 03, 07:04 PM
jim beam
external usenet poster
 
Posts: n/a
Default High-speed shimmy, Speed wobble

shayne

i used to have similar experiences on an older road frame and i managed
to "tune" it out.

as you read in the faq, there are many reasons for the shimmy, but
basically, you're looking at a dynamic system which has a resonance,
just like that famous footage of the tacoma narrows bridge.

the initiation of the resonance aside, and tires are one, the frame and
the wheels facilitate the resonance. just like the bridge, the wind
initiated the oscillation, but the resonant frequency of the bridge was
disasterously "in tune".

so, if you are to address this problem with your bike, you have to
address these two main areas - frame & wheels. i was lucky enough to
have several rear wheels and noticed that different wheels had different
effects on the resonance. in my case, the lighter spoked wheels were
much more prone then the heavier spoked ones. following this pattern, i
ended up building a "stiff" rear wheel with straight gauge 2.0mm spokes
on the drive side and butted 2.0/1.8/2.0mm on the non-drive. it's very
rigid and quite a harsh ride, but it killed the shimmy completely.

i have since replaced that frame and can report that while i can induce
some degree of shimmy in any of the three others i now have if i really
try, none of them are susceptible in the same way.

ymmv, but in my opinion, start with wheel variants and if that doesn't
work, move on to a different frame. my aluminum frame is the most stable.

jb




  #5  
Old September 30th 03, 01:25 AM
David L. Johnson
external usenet poster
 
Posts: n/a
Default High-speed shimmy, Speed wobble

On Mon, 29 Sep 2003 16:23:31 +0000, Shayne Wissler wrote:

My owners manual says that if I experience this, to take it into the shop.
I called the shop and they want to see the bike. But based on my reading
of the shimmy FAQ, I'm half expecting that they won't find anything.

Quite lkely.

I have made two changes to my bike recently. The most recent was that I
lowered my handlebars by about 2". They used to be almost parallel to my
seat. I did tighten the headset well. The other is that I replaced my
front tire with a lighter, skinnier one.


The maddening thing about shimmy is that it can be just about anything
that will cause, or cure, it. It's as likely the bar height (and the
change in your position that results) as the tire.

I have descended with the tire before, but I can't say whether I reached
42MPH or not. I can say that ever since I put that tire on, I have noticed
there being less stability while riding no-hands--it does start to shimmy
a little, but at lower speeds and not so violently. Is it probable that
this low-speed (~20MPH?) no-hands shimmy is from the same source as the
violent shimmy?


I'd say it's possible.


Any suggestions would be appreciated.


Try it with the bars a bit higher, or with another front tire. If it
_does_ shimmy, press one knee against the top tube. That should stop it.
Also, unloading your weight from the saddle will stop the shimmy.

--

David L. Johnson

__o | "Business!" cried the Ghost. "Mankind was my business. The
_`\(,_ | common welfare was my business; charity, mercy, forbearance,
(_)/ (_) | and benevolence, were, all, my business. The dealings of my
trade were but a drop of water in the comprehensive ocean of my
business!" --Dickens, "A Christmas Carol"
  #6  
Old September 30th 03, 03:04 PM
Claire Petersky
external usenet poster
 
Posts: n/a
Default High-speed shimmy, Speed wobble

"Shayne Wissler" wrote in message news:70Zdb.634208$o%2.291570@sccrnsc02...

The other is that I replaced my front
tire with a lighter, skinnier one.


I bet this is it. When I developed a "high-speed" no-hands shimmy
earlier this year, I read everything on rb* on shimmies. It seemed
like the fact I had replaced the rear tire with a slightly fatter one
was the likely culprit. I returned to a skinnier rear tire, and the
high speed shimmy disappeared.

Warm Regards,


Claire Petersky )
Home of the meditative cyclist:
http://home.earthlink.net/~cpetersky/Welcome.htm

Books just wanna be FREE! See what I mean at:
http://bookcrossing.com/friend/Cpetersky
  #7  
Old September 30th 03, 03:31 PM
Callistus Valerius
external usenet poster
 
Posts: n/a
Default High-speed shimmy, Speed wobble

Is it probable that this
low-speed (~20MPH?) no-hands shimmy is from the same source as the

violent
shimmy?

Any suggestions would be appreciated.

..
I'll tell you this just once, and I assume you don't want the sh*t
scared out of you again. The skinny tire did it. Put a 25 width tire on
the front and enjoy life.


  #8  
Old September 30th 03, 03:39 PM
Callistus Valerius
external usenet poster
 
Posts: n/a
Default High-speed shimmy, Speed wobble

It seemed
like the fact I had replaced the rear tire with a slightly fatter one
was the likely culprit. I returned to a skinnier rear tire, and the
high speed shimmy disappeared.


It doesn't matter what you have on the back wheel, it's the front wheel
that counts.


  #9  
Old September 30th 03, 07:50 PM
external usenet poster
 
Posts: n/a
Default High-speed shimmy, Speed wobble

I suppose we need another FAQ item on this. What is missing in most
of these testimonials is how and at what speed these shimmy episodes
occurred. In prior airing of this sort of thing, it was claimed that
balancing the front wheel would prevent shimmy or that a different
weight wheel would do it.

I put a large imbalance on a front wheel and made test runs with and
without imbalance (I balanced the wheel as well as greatly imbalanced
it) with no discernable difference in shimmy. I use a 5% grade,
smooth half mile section of road, coasting at between 25 and 30 mph
no-hands and can consistently develop shimmy.

The next test was to completely fill the front tire with water. This
changed the shimmy frequency slightly but did not stop it from
occurring.

That said, I descend mountain roads often at speeds up to 50mph with
no problem while placing my hands on the stem, knees together. Shimmy
is a characteristic of most larger bicycle frames and should not be
allowed to occur by the rider. SInce most people cannot ride
no-hands, they are not aware of the tendency of their bicycle to
shimmy. Then when it occurs, it is a disaster about to happen.

http://draco.acs.uci.edu/rbfaq/FAQ/8h.5.html

Jobst Brandt

  #10  
Old September 30th 03, 09:57 PM
Tom Nakashima
external usenet poster
 
Posts: n/a
Default High-speed shimmy, Speed wobble

I was also experimenting with descending shimmy this past weekend, not by
choice at first.
I just built a bicycle with upright steel handlebars (older schwinn type)
using an older Specialized Expedition touring bike (long wheelbase). The
wheels are Mavic MA-2 with Campagnolo 36h hubs, 126mm spacing in the rear
13-24 6sp. The tire are the older Avocet Duro Plus 32's, tire pressure 85
psi.
On a descent grade of 3-6 % for about 1/3 mile I was able to induce shimmy
with my hands on the handlebars.
My speed was 26-32mph.
I've also noticed that with upright bars the steering is very responsive
from what I was used to, riding drop bars.
I'm not sure if this is because of the handlebars being further behind the
stem. In any-case it was easy to induce shimmy.
I thought this was interesting, so I rode back up the hill and repeated it
again. As I got the bike to shimmy I wanted to see if I could correct this.
I took weight off the handlebars by every so slightly removing both hands
about an inch away from the bar. My speed was about 27 mph, and before I
knew it, the shimmy corrected itself and was completely eliminated.
I repeated this a 3rd time, to make sure it wasn't a fluke and actually
timed how long it took for the shimmy to correct itself. The time was about
5 to 7 seconds for the shimmy to completely vanish.
I'm going to try this again since it's one of my training routes. This time
I changed tires, with Avocet Carbon 12 Road 700 x 25c.100psi.tire pressure.
These are the tires I normally ride with my roadbike. Strange, but my
roadbike doesn't shimmy at all on this stretch of my training ride.
-tom

wrote in message
...
I suppose we need another FAQ item on this. What is missing in most
of these testimonials is how and at what speed these shimmy episodes
occurred. In prior airing of this sort of thing, it was claimed that
balancing the front wheel would prevent shimmy or that a different
weight wheel would do it.

I put a large imbalance on a front wheel and made test runs with and
without imbalance (I balanced the wheel as well as greatly imbalanced
it) with no discernable difference in shimmy. I use a 5% grade,
smooth half mile section of road, coasting at between 25 and 30 mph
no-hands and can consistently develop shimmy.

The next test was to completely fill the front tire with water. This
changed the shimmy frequency slightly but did not stop it from
occurring.

That said, I descend mountain roads often at speeds up to 50mph with
no problem while placing my hands on the stem, knees together. Shimmy
is a characteristic of most larger bicycle frames and should not be
allowed to occur by the rider. SInce most people cannot ride
no-hands, they are not aware of the tendency of their bicycle to
shimmy. Then when it occurs, it is a disaster about to happen.

http://draco.acs.uci.edu/rbfaq/FAQ/8h.5.html

Jobst Brandt



 




Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
High quality Single Speed Bicycle on a budget? Lobo Tommy General 24 April 3rd 04 09:01 AM


All times are GMT +1. The time now is 03:26 PM.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 CycleBanter.com.
The comments are property of their posters.