Cycling and bicycle discussion forums. 
   Click here to join our community Log in to access your Control Panel  


Go Back   > >

"The 33"-Road Bike Racing We set this forum up for our members to discuss their experiences in either pro or amateur racing, whether they are the big races, or even the small backyard races. Don't forget to update all the members with your own race results.

User Tag List

Reply
 
Thread Tools Search this Thread
Old 09-12-07, 09:49 PM   #1
hiromian
Gios my baby
Thread Starter
 
hiromian's Avatar
 
Join Date: Dec 2005
Bikes: Gios 96, Mercier 72, Peugeot 74 X 2, Sears full suspension High rise banana seat, Kona 94, CCM Rambler 70s.
Posts: 1,135
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
high speed wobble

I experienced this for the first time last week decending a mountan at 70K/h. Ahh that was scary. I got out of trouble by not panicing, siting up a bit, bleading off speed with the rear brake which did not do anything to stop the wobble so I pressed my knees into the top tube. Bingo, presto, wobble gone. I have done these high speed decents plenty and since and it has only happened the once.

What is the cause?
hiromian is offline   Reply With Quote
Old 09-12-07, 09:54 PM   #2
DrWJODonnell
Slow'n'Aero
 
DrWJODonnell's Avatar
 
Join Date: Feb 2006
Location: Driving the pace in the crosswind
Bikes:
Posts: 2,599
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
sometimes a harmonic feedback frequency can be set up in your fork, frame, or handlebars. Other times it can be equipment failure (out of true wheel, broken skewer, cracked frame or fork). Finally it can be due to extreme changes in gravitational center. If you were in a normal position, that is not it. I would inspect everything VERY carefully.
DrWJODonnell is offline   Reply With Quote
Old 09-13-07, 05:19 AM   #3
recneps
Senior Member
 
Join Date: Jul 2006
Bikes:
Posts: 1,085
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Putting more weight on the front wheel cures this for me, ive had it happen quite a bit decending at 45k/hr with my hands off the bars.
recneps is offline   Reply With Quote
Old 09-13-07, 05:27 AM   #4
botto 
.
 
botto's Avatar
 
Join Date: Jan 2006
Bikes:
Posts: 40,364
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
also throw in the possibility of a loose headset into the mix.
botto is offline   Reply With Quote
Old 09-13-07, 05:32 AM   #5
NoRacer
Isaias
 
NoRacer's Avatar
 
Join Date: Sep 2005
Location: Essex, MD
Bikes: Ridley X-Fire (carbon, white)
Posts: 5,182
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 5 Post(s)
Quote:
Originally Posted by DrWJODonnell View Post
sometimes a harmonic feedback frequency can be set up in your fork, frame, or handlebars.
I used to have this occur when I first got my Giant. It stopped when I changed my seat height.
NoRacer is offline   Reply With Quote
Old 09-13-07, 06:04 AM   #6
waterrockets 
Making a kilometer blurry
 
waterrockets's Avatar
 
Join Date: May 2006
Location: Austin (near TX)
Bikes: rkwaki's porn collection
Posts: 26,130
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 1 Post(s)
Yeah, I'd check everything over really carefully. I've never had this happen with my hands on the bars at speeds as high as 60mph.

I'll add to the mix to check that your front spoke tension is still high enough, in addition to DRW's trueness check.

My current bike will occasionally shimmy over 30mph with hands off the bars, but what can you expect from a 62cm frame?
waterrockets is offline   Reply With Quote
Old 09-13-07, 06:06 AM   #7
botto 
.
 
botto's Avatar
 
Join Date: Jan 2006
Bikes:
Posts: 40,364
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Quote:
Originally Posted by waterrockets View Post
Yeah, I'd check everything over really carefully. I've never had this happen with my hands on the bars at speeds as high as 60mph.

I'll add to the mix to check that your front spoke tension is still high enough, in addition to DRW's trueness check.

My current bike will occasionally shimmy over 30mph with hands off the bars,
but what can you expect from a 62cm frame?
a jungle gym on wheels?
botto is offline   Reply With Quote
Old 09-13-07, 06:08 AM   #8
jfmckenna
Tiocfáidh ár Lá
 
jfmckenna's Avatar
 
Join Date: Dec 2003
Location: The edge of b#
Bikes: A whole bunch-a bikes.
Posts: 5,440
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 65 Post(s)
Quote:
Originally Posted by hiromian View Post
I experienced this for the first time last week decending a mountan at 70K/h. Ahh that was scary. I got out of trouble by not panicing, siting up a bit, bleading off speed with the rear brake which did not do anything to stop the wobble so I pressed my knees into the top tube. Bingo, presto, wobble gone. I have done these high speed decents plenty and since and it has only happened the once.

What is the cause?
3 of my bikes have high speed wobble. They are all big frames and I believe that has something to do with it. Two are lugged steel and the other is AL. My TI compact frame rides straight and true at any speed. Knee on TT is all I need to do so when ever I descend I clench the TT (or just one knee touching) which is a good aero position anyway.
jfmckenna is offline   Reply With Quote
Old 09-13-07, 06:22 AM   #9
waterrockets 
Making a kilometer blurry
 
waterrockets's Avatar
 
Join Date: May 2006
Location: Austin (near TX)
Bikes: rkwaki's porn collection
Posts: 26,130
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 1 Post(s)
Quote:
Originally Posted by botto View Post
a jungle gym on wheels?
That makes the price justification that much easier
waterrockets is offline   Reply With Quote
Old 09-13-07, 06:34 AM   #10
UT_Dude
Texas Fight!
 
UT_Dude's Avatar
 
Join Date: Oct 2006
Location: Austin, TX
Bikes:
Posts: 2,023
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Quote:
Originally Posted by waterrockets View Post
My current bike will occasionally shimmy over 30mph with hands off the bars, but what can you expect from a 62cm frame?
For it's owner to not be so frigging tall, duh.
__________________
T E X A S F I G H T !
http://www.chriscollins.org
UT_Dude is offline   Reply With Quote
Old 09-13-07, 06:51 AM   #11
Blue Jays
Banned
 
Join Date: Jun 2006
Location: United States
Bikes: roadbikes and full-suspension mountainbikes
Posts: 1,926
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Tire pressure correct front and rear? Headset? Too many peripherals/electronics attached to handlebars?
Blue Jays is offline   Reply With Quote
Old 09-13-07, 07:53 AM   #12
NoRacer
Isaias
 
NoRacer's Avatar
 
Join Date: Sep 2005
Location: Essex, MD
Bikes: Ridley X-Fire (carbon, white)
Posts: 5,182
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 5 Post(s)
Sheldon Brown has this article from Jobst Brandt regarding Shimmy or Speed Wobble:

http://www.sheldonbrown.com/brandt/shimmy.html
NoRacer is offline   Reply With Quote
Old 09-13-07, 10:31 PM   #13
hiromian
Gios my baby
Thread Starter
 
hiromian's Avatar
 
Join Date: Dec 2005
Bikes: Gios 96, Mercier 72, Peugeot 74 X 2, Sears full suspension High rise banana seat, Kona 94, CCM Rambler 70s.
Posts: 1,135
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Quote:
Originally Posted by NoRacer View Post
Sheldon Brown has this article from Jobst Brandt regarding Shimmy or Speed Wobble:

http://www.sheldonbrown.com/brandt/shimmy.html
Thanks for the link. Thanks all for the responces.

I did a thorough check on the bike and found the rear wheel to be a little more out of tru than I like. The wobble initiated in a corner and I think the freqency of the rear tire wobble (more pronounced in a corner yet undetectable by me), + feedback with the frame built up the resonance. My knee pressing the frame broke the cycle.
hiromian is offline   Reply With Quote
Old 09-14-07, 10:54 AM   #14
DrWJODonnell
Slow'n'Aero
 
DrWJODonnell's Avatar
 
Join Date: Feb 2006
Location: Driving the pace in the crosswind
Bikes:
Posts: 2,599
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Another satisfied customer.
DrWJODonnell is offline   Reply With Quote
Old 09-14-07, 12:36 PM   #15
DannoXYZ 
Senior Member
 
DannoXYZ's Avatar
 
Join Date: Jul 2005
Location: Saratoga, CA
Bikes:
Posts: 11,606
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 3 Post(s)
Glad you found the problem. Yeah, I've found that slightly out-of-true wheels, either at the front or rear and start a sympathetic vibration in the frame. Clamping your knees on the frame will dampen the vibrations.

A long-term fix is truing the wheels and stiffening up some part of the frame/fork will raise the oscillation-frequency to higher speeds than you'd ever be able to accomplish. On my bike, going from an alloy fork to a stiffer carbon one moved the speed where the wobble can barely be felt from 55mph to 72mph (which I'd hit maybe once a year if I can find just the right mini-van coming down San Marcos Pass ).
DannoXYZ is offline   Reply With Quote
Old 09-14-07, 03:16 PM   #16
jrennie
Race to train
 
jrennie's Avatar
 
Join Date: May 2005
Location: suffering on the back
Bikes:
Posts: 3,115
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Quote:
Originally Posted by recneps View Post
Putting more weight on the front wheel cures this for me, ive had it happen quite a bit decending at 45k/hr with my hands off the bars.
Why do you decend with your hands off the bars

if you see recneps on a downhill
jrennie is offline   Reply With Quote
Old 06-18-08, 10:29 PM   #17
gr8rcake
Junior Member
 
gr8rcake's Avatar
 
Join Date: Feb 2008
Bikes:
Posts: 10
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
It could also be caused by a wheel that appears true but is supported by a loose hub which aggravates the problem.
gr8rcake is offline   Reply With Quote
Old 06-19-08, 12:24 AM   #18
ridethecliche
Village Idiot
 
Join Date: Jan 2007
Location: Bahstaaaaan
Bikes:
Posts: 20,241
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 2 Post(s)
Quote:
Originally Posted by jrennie View Post
Why do you decend with your hands off the bars

if you see recneps on a downhill
I've ridden downhill on short descents no handed, I'd just got my CAAD9 and I was just blown away by how straight it tracked. I don't make a habit out of it, especially now...
ridethecliche is offline   Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

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

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off



All times are GMT -6. The time now is 03:53 AM.