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Speed Wobble - Front End Shimmy


Posted to [T@H] email list on 09-20-04 as part of a discussion concerning the unfortunate death of a 60 year old woman on Cycle Oregon. 

I  have personally fought speed wobbles for 38 years and agree with
Scott that it is a terrifying situation
.  And it seems the harder you
try to stop it or brake the worse it gets.  The oscillations certainly increase
from tensing and trying to control it which is the natural reaction.

When I was 15 years old and purchased by first 531 Reynolds built main
triangle bike, I had my first speed wobble occurrence at about 35-40
mph.  It scared the ..... out of me.  At 15 I was a light rider and I
learned that too much air pressure in the front tire was a factor.

In my early years and during my short stint of racing, I weighed
all of 147-158 pounds at 6'.  The problem would surface during certain
situations that included speeds over 40 mph on long descents
combined with cross winds.  Being 'tense' in these situations proved 
to cause a muscle reflex or shivering that would trigger it.  I learned to 
pedal and pedal hard to bring it to a stop.  Putting my leg against the top 
tube never worked for me.  To pedal hard at high speeds, I ordered a 
Campy 55T ring straight from a company in England back in 1972... 
Holdsworth for those riding at the time.  I still have their orange and blue 
wheel covers on my Campy Record, chromed spoke, wood filled 
Weinmann sew-up rims that were standard issue on a Schwinn 
Paramounts of the day complimented with red cap Clement Criterium 
Seta silk tires on them.

I have put every single owned since 1967 into a speed wobble, nearly
every tandem ridden (from every major manufacturer) and a triplet at
59 mph!

Too much air in a front tire is probably hard to do for most on a
tandem or triplet and has not seemed to be a factor with me during the
past decade and more.  That said, steering geometry, possibly fork
flex, frame flex, being overly tired, nervousness of the captain from
lack of sleep or too much caffeine, and body temperature (shivering)
of the captain and stoker are ALL related and have been positively
identified by me for MY situation.  Weather conditions are a
significant factor that make one tense and react or over react is
probably more accurate.  Another triggering factor is wheel balance.
And yes, bike wheels can be out of balance, more now than ever with
deep section rims as compared to years gone by.

Typical 'good' mechanics seem to put computer magnets opposite the
valve stem to offset the weight of the valve and for cosmetics...
shows their attention to detail.  Usually with deep section rims, this
will actually cause or increase an already existing balance problem
since the heavy spot is usually at the seam area whether welded or
pinned.  The valve during a static balance test will usually go up
rather than down!

I balanced 34mm deep section Campy Atlanta rims some years ago by
placing three computer magnets at the valve area which helped greatly.
The rims were so out of balance that the bike would start to jump out
of the work stand when you would spin the pedals in high gear...
literally.  I learned this 2 months after a 53 mph bad speed wobble
experience coming off of a pass in Colorado with a cross wind.  Others
had the problem on the same pass as a young woman did a face plant at
the same speed.

I have solved my speed wobble problem by using a shorter rake fork
such as a 44 or 45 (with the typical 73 degree head tube angle) which
I understand increases trail and high speed stability.  Also balancing
wheels helps greatly as I am able to relax.  Unloading my rear end
from the saddle will bring the bike out of it should I feel one start.
Touching the top tube with my leg has not worked for me.  Taking my
rear off of the saddle prevents 'most' of the energy from being stored
in the frame that is associated with frame flex.  This is not to say
that no energy is stored in the frame after unloading weight from the
saddle because the shimmy tendency is certainly still there when
riding a flexible flyer single bike in my experience.  All frames are
going to flex it is just that some flex more than others and seem to be
more susceptible to this problem IMO and experience.

I would strongly suggest folks going to the mountains or on any rides
like Cycle Oregon, practice unloading their rear end from the saddle
and learn it to where it is somewhat of a reflexive reaction should
the event occur.  Reading it and knowing it rarely comes to mind when
such a thing happens as it is really all consuming.

Mark Johnson
Precision Tandems, Inc.
Shawnee, KS
On Line Parts & Accessory Catalog

Here is a link to a post made by Jobst Brandt on the subject

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