Could This Be Loose Headset?

After a little happy riding in November and then a winter lull, I have
taken to cycling the 9 km to my workplace on my newest bike, Flyzipper,
a Dahon Impulse P21.

The route is hilly, but there is a good section of it about 3km long
which is all downhill. I am learning to appreciate the range of 21
usable gears and when I get on my old 7 speed bikes, they feel really
stiff in comparison.

However, here's the problem. I cannot go really fast on that downhill
stretch because Flyzipper develops a strange wobble in the front wheel
at higher speeds. It is as if he is hyper-reactive to the weight of a
foot on a pedal and starts to slip and swerve in the direction of the
downward foot. So I have to brake and coast until he straightens up. I
have yet to learn what it feels like to spin out on his 21st gear.

There is also a dirt road stretch with some muddy sections. Yesterday in
one of those muddy patches I nearly went down. Fly swerved suddenly to
the left, then nearly flattened me to the right.

What's going on? For months, I had a problem with wobbly _startup_, and
difficulty at low speed, on my Dutch bike, Behemoth, which eventually
turned out to be because her headset was too tight. The ball bearings
were being compressed and could not roll freely. This problem was
resolved with a quarter turn on the headset screw when I took her back
to the shop for her first revision. The pity is it took 6 months to
diagnose.

But Flyzipper was perfect when I bought him, and is OK at startup. His
problem appears when going fast. A screw can loosen itself without any
human intervention, but I have never known one to tighten itself without
intervention.

So could this wobble be due to a _loose_ headset? How does a loose
headset feel?

Also, what screw to I have to tighten, ever so slightly, to eliminate
the problem? I don't want to get it wrong, and I have no LBS help with
Fly, since I quarreled with the guy at the LBS who only wants to
service the bikes he sells himself.

Many thanks,

Elisa Francesca Roselli
Ile de France

Elisa Francesca Roselli wrote:

However, here's the problem. I cannot go really fast on that downhill
stretch because Flyzipper develops a strange wobble in the front wheel
at higher speeds. It is as if he is hyper-reactive to the weight of a
foot on a pedal and starts to slip and swerve in the direction of the
downward foot. So I have to brake and coast until he straightens up. I
have yet to learn what it feels like to spin out on his 21st gear.

Sounds like shimmy - a common but not well understood problem.

Have a read of Jobst Brant's article on Shimmy and see if there is
anything there that helps

http://www.sheldonbrown.com/brandt/shimmy.html

Tony

Tony Raven wrote:

Sounds like shimmy - a common but not well understood problem.


It does sound like shimmy, but then so did Behemoth's problem. Also
Brandt says this happens when one is coasting without hands, something I
have never done in my life, being still incapable of removing even one
hand from the bars for a fraction of a second.

My problem is aggravated, not relieved, by pedaling. Also, it seems to
help to lean forward somewhat.

And I don't remember Flyzipper shimmying in November ...


EFR
Ile de France

Elisa Francesca Roselli wrote:
Tony Raven wrote:

Sounds like shimmy - a common but not well understood problem.


It does sound like shimmy, but then so did Behemoth's problem. Also
Brandt says this happens when one is coasting without hands, something I
have never done in my life, being still incapable of removing even one
hand from the bars for a fraction of a second.

My problem is aggravated, not relieved, by pedaling. Also, it seems to
help to lean forward somewhat.

And I don't remember Flyzipper shimmying in November ...


Riding without hands isn't a requirement for shimmy - it just
exacerbates the problem. Holding the bars too tight can also encourage
it as can pedalling. Have you changed anything since November? Have the
frame pivots loosened? New tyres? Going faster as you get more
confident with it?

When it starts next time while you are not pedalling, with one pedal
down, move your leg to rest on the frame on one side and see if that
damps it out as a starter.

Shimmy can be very difficult to track down and solve.

Tony

Elisa Francesca Roselli wrote:

It does sound like shimmy

It does to me too. A loose headset won't cause any kind of shimmy. In
fact I'm deliberately using a slightly loose headset (because it or frame
has something wrong with that I can't be bothered to fix) and it doesn't
cause any problems besides the odd clunk over a bump, and possibly
increased bearing wear. In any case, you can tell if a headset is loose
(on a bike without front suspension, and with good brake properly tight)
by feeling for play (movement) when rocking the bike back and forth hard
with the front brake on hard.

If I couldn't live with it*, I would try altering weight distribution by
altering the riding position or rearranging any luggage. I used to get
shimmy with my tourer but it became less frequent after /slightly/
changing the handlebar stem and saddle positions, and completely
dissappeared after I completely rebuilt the bike with new components. Not
that I'm suggesting that you resort to the latter! Different size/weight
tyres may also cure it.

* When it happens, for a moment, try slowing down or weaving left to right
(used to work for me); it won't then necessarily come back when you speed
up again. Pressing a thigh/knee against the top tube is also supposed to
help.

~PB

Elisa Francesca Roselli wrote:
It does sound like shimmy, but then so did Behemoth's problem. Also
Brandt says this happens when one is coasting without hands,

"Usually", he says. It certainly can happen with hands on as well.

~PB

"Usually", he says. It certainly can happen with hands on as well.

"Shimmy that concerns riders the most occurs with hands firmly on the bars
and it is rider generated by muscular effect whose natural response is the
same as the shimmy frequency, about that of Human shivering. Descending in
cold weather can be difficult for this reason. The rider's "death grip"
only enhances the incidence of shimmy in this situation. Loosely holding
the bars between thumb and forefinger is a way of avoiding shimmy when
cold."

So to avoid it on a descent when you're shivering with fear, just relax and
loosely hold the bars between thumb and forefinger. Hmmmm. :-)

At various times and on various bikes I have resolved/minimized a front end
shimmy by:

1. Reversing the front wheel in the dropouts

2. Shifting the saddle back on the seat post

3. Correctly lubricating and adjusting the headset

4. Relaxing my grip on the handlebars

5. Pressing my knee against the seat tube on long fast descents

6. Rearranging the gear on the front rack of a fully loaded touring bike.

In other words, shimmy is caused or aggravated by the interaction of a bunch
of different factors, and yes, headset adjustment can be one of them. How
you adjust your headset will depend on the headset design. Older threaded
headsets have two relatively large ( usually 32mm) nuts stacked one on top
of the other, with a washer and/or brake cable hanger stacked between them.
Set the bearing pre-load by adjusting the lower nut, then lock the lower nut
in place by tightening the upper nut with one wrench while holding the lower
nut in place with another wrench. Newer threadless headsets are held in
place by the handlebar stem, loosen the Allen bolts that hold the stem onto
the steerer tube and slide the stem up and down to change the pressure on
the headset bearings. The optimum set up for both types of headset is the
point where the handlebars turn freely, with little or no resistance, but
there is no up and down play in the headset. This may not be possible with a
worn headset, an incorrectly installed headset, or a cheap, low quality
headset.

An incorrectly installed headset may cause the bike to pull to the left or
right, which may or may not be aggravating the problem. I spent almost 3
years wondering why I couldn't ride my Peugeot PX-10 in a straight line
hands free, until the nice folks at Missing Link (Berkeley's leading bike
shop at the time) explained that the headset was incorrectly installed.
HTH,
--
mark

"Elisa Francesca Roselli" wrote:
So could this wobble be due to a _loose_ headset? How does a loose
headset feel?

It's probably NOT a loose headset. To check for a loose headset: Stand next
to the bike. Grip the front brake lever tightly, and rock the bike forward
and back. If you hear or feel a "clunking," the headset is loose.

Art Harris

"Elisa Francesca Roselli" wrote: (clip) I have taken to cycling the 9 km to
my workplace on my newest bike, Flyzipper, a Dahon Impulse P21. (clip)
^^^^^^^^^^^^^^
It may be inherent in the design of the bike. I am not familiar with that
particular Dahon model, but I had a scary experience on my Dahon folder
while going fast downhill. That particular bike had a fairly vertical fork,
resulting in very rapid steering. At high speed it was difficult to
maintain course without overcorrecting. And, of course, each overcorrection
results in a corresponding even larger counter correction, which is the
essence of instability.

If that is what's going on, you may have to limit your speed, and tighten up
your "internal feedback."