Headshake causes and cures?

My elderly (small frame tube) Cannondale (bought used in the
late '80's) has always seemed a bit squirrely, and with narrow
(1.125") tires pumped up to 100 psi it will headshake gently
with no hands at 12-15 mph. The frequncy seems rather high,
somewhere between three and five hertz.

There are no obvious problems: The forks look straight, the steering
head bearings are neither loose nor binding, wheelbearings seem ok,
etc, etc.

I'd really like to be able to ride "hands off", at least for short
intervals, and wonder if this is a design characteristic or a hint
that something is amiss with the bicycle. In the past I blamed my
difficulties riding without hands on age and an impaired sense of
balance, but the headshake is clearly not related to either.

Thanks for reading, any counsel appreciated!

bob prohaska

My elderly (small frame tube) Cannondale (bought used in the
late '80's) has always seemed a bit squirrely, and with narrow
(1.125") tires pumped up to 100 psi it will headshake gently
with no hands at 12-15 mph. The frequncy seems rather high,
somewhere between three and five hertz.

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

Just about everything you ever wanted to know about shimmy is in the faq.
Many people have tried to debunk it, but to the best of my knowledge, it
remains the definitive piece on what causes a bicycle to develop the shakes.

In general, shimmy is an inconvenience, not something that causes people to
actually crash. The fact that it occurs for your system (because it *is* a
system, comprised of both you and your bike... a different bike or a
different you changes things and may eliminate it) at a relatively-low speed
is probably less unusual than you think. People don't talk about shimmy much
in that speed range, but I think that's because they don't feel that it's as
scary at such speed, and certainly not as much fun to talk about. As in "Oh
yeah, you should have seen what it was like, flying down that hill at 47.632
mph when I sat up to put on my wind jacket and my bike went into this death
wobble..."

--Mike-- Chain Reaction Bicycles
www.ChainReactionBicycles.com


"bob prohaska's usenet account" wrote in message
t...
My elderly (small frame tube) Cannondale (bought used in the
late '80's) has always seemed a bit squirrely, and with narrow
(1.125") tires pumped up to 100 psi it will headshake gently
with no hands at 12-15 mph. The frequncy seems rather high,
somewhere between three and five hertz.

There are no obvious problems: The forks look straight, the steering
head bearings are neither loose nor binding, wheelbearings seem ok,
etc, etc.

I'd really like to be able to ride "hands off", at least for short
intervals, and wonder if this is a design characteristic or a hint
that something is amiss with the bicycle. In the past I blamed my
difficulties riding without hands on age and an impaired sense of
balance, but the headshake is clearly not related to either.

Thanks for reading, any counsel appreciated!

bob prohaska

Mike Jacoubowsky wrote:

Just about everything you ever wanted to know about shimmy is in the faq.

Well, not quite. I want to make it go away, and was looking for guidance
on things (hopefully specific to old Cannondales) that are plausible culprits.


In general, shimmy is an inconvenience, not something that causes people to
actually crash. The fact that it occurs for your system (because it *is* a

True, to a point. I've had the bike for close to twenty years, and it always
seemed the least competent component was the rider. Seeing it headshake no
hands at low speed made me realize there might be something wrong with the
bike. I'd gladly replace the fork if there was plausible reason to think
it would help.

bob prohaska

Dans le message de t,
bob prohaska's usenet account a réfléchi, et puis a
déclaré :
My elderly (small frame tube) Cannondale (bought used in the
late '80's) has always seemed a bit squirrely, and with narrow
(1.125") tires pumped up to 100 psi it will headshake gently
with no hands at 12-15 mph. The frequncy seems rather high,
somewhere between three and five hertz.

There are no obvious problems: The forks look straight, the steering
head bearings are neither loose nor binding, wheelbearings seem ok,
etc, etc.

I'd really like to be able to ride "hands off", at least for short
intervals, and wonder if this is a design characteristic or a hint
that something is amiss with the bicycle. In the past I blamed my
difficulties riding without hands on age and an impaired sense of
balance, but the headshake is clearly not related to either.

Thanks for reading, any counsel appreciated!

bob prohaska

I had a C'dale of that vintage. There was a recall on forks, although it
didn't cover all models. I am sure C'dale could let you know if yours is
one of those affected. I am sorry I can't recall the details. Mine was
exchanged, and one of the problems solved was an ever-loosening headset.
Ask them, or maybe a long-time C'dale dealer.
--
Bonne route !

Sandy
Verneuil-sur-Seine FR

bob prohaska's usenet account wrote:
My elderly (small frame tube) Cannondale (bought used in the late '80's)

How our perceptions have changed! :-) At the time we thought Cannondale
tubes were immensely oversized...

has always seemed a bit squirrely, and with narrow
(1.125") tires pumped up to 100 psi

Mountain bike?

The head tube angle on my mid-eighties Cannondale MTB measured out to
73 degrees at the local frame builder's shop. Catalog said 71 degrees,
and the stock Tange fork had rake similar to other brands also designed
with 71 degree head tube angles. Everyone who had ridden it said it
felt much quicker handling than other MTBs.

Not that steeper than designed head tube angles are a specific cause of
head shake, but it does change the steering response enough that if
other aspects of the system make it susceptible it might tip it in that
direction.

it will headshake gently
with no hands at 12-15 mph. The frequncy seems rather high,
somewhere between three and five hertz.

Hm, that's lower than the 9 or 10 hertz I've experienced on flexible
road bikes. Maybe speed and frequency are related? Those were around
20+ mph.

There are no obvious problems: The forks look straight, the steering
head bearings are neither loose nor binding, wheelbearings seem ok,
etc, etc.

Right, as the FAQ says these are usually not the problem.

I'd really like to be able to ride "hands off", at least for short
intervals, and wonder if this is a design characteristic or a hint
that something is amiss with the bicycle.

Maybe both? I'd wonder about the head tube angle, for example, but
that's just based on my own experience. Normally Cannondales are quite
stiff, which reduces the tendency to wobble, and theoretically raises
the frequency to very high speeds. Maybe your frame's gotten more
fpexible due to a crack? But then you said it's always done it...

In the past I blamed my
difficulties riding without hands on age and an impaired sense of
balance, but the headshake is clearly not related to either.

Thanks for reading, any counsel appreciated!

The FAQ suggests two solutions: rest a leg on the top tube, or unweight
the saddle. Experiment to see if either of those help in your case?

bob prohaska's usenet account wrote:
My elderly (small frame tube) Cannondale (bought used in the
late '80's) has always seemed a bit squirrely, and with narrow
(1.125") tires pumped up to 100 psi it will headshake gently
with no hands at 12-15 mph. The frequncy seems rather high,
somewhere between three and five hertz.

There are no obvious problems: The forks look straight, the steering
head bearings are neither loose nor binding, wheelbearings seem ok,
etc, etc.

I'd really like to be able to ride "hands off", at least for short
intervals, and wonder if this is a design characteristic or a hint
that something is amiss with the bicycle. In the past I blamed my
difficulties riding without hands on age and an impaired sense of
balance, but the headshake is clearly not related to either.

Thanks for reading, any counsel appreciated!

bob prohaska

what rear wheel do you have?

from what i can see, shimmy is the result of two factors: torsional flex
in the frame and lateral flex in the wheels, which really means the rear
wheel since it's dished, and /much/ more flexible on the drive side due
to the reduced bracing angle. together, if they happen to have
resonance in the same frequency range, you get shimmy. if the resonance
of the two can be "tuned" away from each other, the shimmy goes away.
[assuming you don't have something a loose headset or a broken axle of
course.]

using that info, i substantially mitigated a similar problem on a
shimmy-prone frame with a stiffer rear wheel. i used a dura-ace hub
[greatest flange offset] and plain gauge spokes [the most inelastic],
and it worked well. the original wheels worked fine in other frames.
this frame worked well with the stiffer rear. ultimately though,
replacing that frame with a [torsionally stiffer] big tube model has
given me an unconditionally stable, much more enjoyable ride.

riding with one leg on the top tube is definitely not a "cure". as you
seem to realize, it's a design characteristic, which thankfully, seems
to be recognized my many [but still not all] modern manufacturers.

wrote:
bob prohaska's usenet account wrote:
My elderly (small frame tube) Cannondale (bought used in the late '80's)

How our perceptions have changed! :-) At the time we thought Cannondale
tubes were immensely oversized...

Actually, this one looks fairly normal; maybe a little fatter than old
steel-framed bikes, but nothing like the later incarnations.


Mountain bike?

Quite the opposite; a roadbike with drop bars and downtube shifters.
Not being quite so athletic & coordinated I added thumb shifters and
Scott MTB handlebars along with (until now) fatter tires, usually
1.375" wide.

The head tube angle on my mid-eighties Cannondale MTB measured out to
73 degrees at the local frame builder's shop. Catalog said 71 degrees,
and the stock Tange fork had rake similar to other brands also designed
with 71 degree head tube angles. Everyone who had ridden it said it
felt much quicker handling than other MTBs.


Not that steeper than designed head tube angles are a specific cause of
head shake, but it does change the steering response enough that if
other aspects of the system make it susceptible it might tip it in that
direction.


Steeper head angles make for less stable steering, certainly.
Sounds like maybe this is just an inherently lively frame. That's
the sort of clue I was looking for; nothing to fix, just live with
it or change it. For now I'll live with it.

Thanks!

bob prohaska

jim beam wrote:

what rear wheel do you have?

It's the OEM Wolber 27", original hub & spokes.

bob prohaska

bob prohaska's usenet account wrote:
jim beam wrote:

what rear wheel do you have?


It's the OEM Wolber 27", original hub & spokes.

bob prohaska

so it's been like this since new?

bob prohaska's usenet account wrote:
wrote:
bob prohaska's usenet account wrote:
My elderly (small frame tube) Cannondale (bought used in the late '80's)
How our perceptions have changed! :-) At the time we thought Cannondale
tubes were immensely oversized...

Actually, this one looks fairly normal; maybe a little fatter than old
steel-framed bikes, but nothing like the later incarnations.

That may be the problem. The frame may have poor torsional stiffness. Is
it a large frame?