Could This Be Loose Headset?

My gearhead colleague has just had a look at the bike here beside me. He
thinks the front wheel is slightly warped. When I hold up the bike and
spin the wheel, it oscillates slightly on the left side. He says that at
25-30 km an hour, Flyzipper's speed on the downhills, this could expand
and become perceptible as a wobble. He says one good bump on a road
could warp a wheel like that. He also thinks the cheapest and simplest
solution is probably to change the wheel. There is also a subtle and
very expensive art of wheel balancing that consists in delicately
changing the tightening of the spokes, but that is probably not
accessible to me.

What do folks think here about warped wheels and shimmy?

EFR
Ile de France

On Mon, 04 Apr 2005 09:07:41 +0200, Elisa Francesca Roselli
wrote:

My gearhead colleague has just had a look at the bike here beside me. He
thinks the front wheel is slightly warped. When I hold up the bike and
spin the wheel, it oscillates slightly on the left side. He says that at
25-30 km an hour, Flyzipper's speed on the downhills, this could expand
and become perceptible as a wobble. He says one good bump on a road
could warp a wheel like that. He also thinks the cheapest and simplest
solution is probably to change the wheel. There is also a subtle and
very expensive art of wheel balancing that consists in delicately
changing the tightening of the spokes, but that is probably not
accessible to me.

Take the wheel into a bike shop. They'll have the skills and tools
needed to "unwarp" it. Here in the UK such a service costs about 10
pounds, much cheaper than a new wheel.


Tim

Elisa Francesca Roselli wrote:
My gearhead colleague has just had a look at the bike here beside me. He
thinks the front wheel is slightly warped. When I hold up the bike and
spin the wheel, it oscillates slightly on the left side. He says that at
25-30 km an hour, Flyzipper's speed on the downhills, this could expand
and become perceptible as a wobble. He says one good bump on a road
could warp a wheel like that. He also thinks the cheapest and simplest
solution is probably to change the wheel. There is also a subtle and
very expensive art of wheel balancing that consists in delicately
changing the tightening of the spokes, but that is probably not
accessible to me.

What do folks think here about warped wheels and shimmy?


Possible but unlikely unless its gross. When you say it oscillates on
the left hand side, it should be equal on both sides unless its the tyre
that slightly distorted or badly seated.

Truing a wheel is not hard provided you are methodical about it.
Sheldon Brown's website will take you through how to do it and you can
do it with the bike upside down and using the brake pads as indicators.
There's a lot of extra stuff included on the site about building the
wheel and dish and tension that you can ignore and just use the truing
stuff.

http://sheldonbrown.com/wheelbuild.html#tensioning

Tony

Elisa Francesca Roselli writes:

My gearhead colleague has just had a look at the bike here beside
me. He thinks the front wheel is slightly warped. When I hold up the
bike and spin the wheel, it oscillates slightly on the left side. He
says that at 25-30 km an hour, Flyzipper's speed on the downhills,
this could expand and become perceptible as a wobble. He says one
good bump on a road could warp a wheel like that. He also thinks the
cheapest and simplest solution is probably to change the
wheel. There is also a subtle and very expensive art of wheel
balancing that consists in delicately changing the tightening of the
spokes, but that is probably not accessible to me.

What do folks think here about warped wheels and shimmy?

Shimmy occurs at a frequency unrelated to single wheel rotations and
does not arise from wheel unbalance or alignment. This sort of
specious guess is tossed out often here on wreck.bike. Don't be
swayed by such claims, ask for proof. For instance have your expert
install the wheel in his bicycle and demonstrate that it is caused by
that wheel and not by the one he usually rides.

Elisa Francesca Roselli wrote:

What do folks think here about warped wheels and shimmy?

Plenty of cyclists ride around with out-of-true wheels and no shimmy.

~PB

"Elisa Francesca Roselli": (clip) What do folks think here about warped
wheels and shimmy?
^^^^^^^^^^^^^^^
Almost any "gearhead" cyclist with a spoke wrench should be able to reduce
the wobble in about 15 minutes. Even if it's not the source of the problem,
it's worth doing. I certainly wouldn't replace the wheel at this point.

Jobst's comment is pertinent: If the wheel is not true, it is going to make
a wavy path on the road--it is not going to shake the bike.

On Sat, 02 Apr 2005 14:41:15 +0200, Elisa Francesca Roselli wrote:

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.

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.

This sounds like shimmy. Any bike will shimmy at some speed; what we hope
for is that the shimmy-speed is beyond what we could reach for other
reasons. It is usually encountered in downhills because we go faster then.

Shimmy is a complex phenomenon, a resonance response between the several
parts of the "system", which includes the rider as the largest component.
Changing nearly anything can affect shimmy, for better or worse.

The easiest thing to change will be the rider. Next time you feel shimmy,
raise yourself up a bit off the saddle. Sounds scary, but it will work.
This is because shimmy is oscillation, side to side, about the axis of the
largest component... you. Getting off the saddle removes the pivot, and
the shimmy will stop.

Your Dahon is an unusual bike, with very small wheels. That makes it
likely to have unusual shimmy behavior, and shimmy at the speed you
describe is unusual.

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

A loose headset is diagnosed as follows. Straddle the bike with your feet
on the ground. Hold the front brake tight, and push/pull the bike
forward and back. If you feel a clunk each time as you rock back and
forth, the headset is loose. Not likely to cause shimmy, though, except
to the extent that everything is involved.

Also, what screw to I have to tighten, ever so slightly, to eliminate
the problem?

I think your bike has a traditional threaded headset. You adjust this by
first loosening the locknut at the top of the headset, then tightening
the lower nut just a bit. Re-tighten locknut and check again. Repeat
until satisfied.

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

Hmm. You live in a fairly large city... with lots of bike shops.
Certainly you can find another shop willing to help out.


--

David L. Johnson

__o | Let's not escape into mathematics. Let's stay with reality. --
_`\(,_ | Michael Crichton
(_)/ (_) |

On Sat, 02 Apr 2005 15:08:01 +0200, 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.

I know you can get the impression from that article, but lots of people
have shimmy while downhilling at speed, with hands on bars. Often the
shimmy is made worse by the death-grip you apply once shimmy starts.

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

Still consistent with shimmy.

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

What's different now?

--

David L. Johnson

__o | Become MicroSoft-free forever. Ask me how.
_`\(,_ |
(_)/ (_) |

David L. Johnson wrote:

This sounds like shimmy. Any bike will shimmy at some speed; what we hope
for is that the shimmy-speed is beyond what we could reach for other
reasons. It is usually encountered in downhills because we go faster then.

Shimmy is a complex phenomenon, a resonance response between the several
parts of the "system", which includes the rider as the largest component.
Changing nearly anything can affect shimmy, for better or worse.

The easiest thing to change will be the rider. Next time you feel shimmy,
raise yourself up a bit off the saddle. Sounds scary, but it will work.
This is because shimmy is oscillation, side to side, about the axis of the
largest component... you. Getting off the saddle removes the pivot, and
the shimmy will stop.

Your Dahon is an unusual bike, with very small wheels. That makes it
likely to have unusual shimmy behavior, and shimmy at the speed you
describe is unusual.


I'd add to this, since I know Elisa's not all that confident a cyclist -
shimmy can also be stopped (though this means stopping pedalling,
somethign you don't sound keen to do!) by gently resting the inside of a
knee/thigh on whatever passes for a top tube on your Flyzipper. Just
that contact seems to dampen the shimmy for me, and shimmy's always more
prevalent if I am tense rather than relaxed.

--


Velvet

Elisa Francesca Roselli wrote:

Flyzipper,


Why's it called Flyzipper?

Tony