Elisa Francesca Roselli said: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.
Quoted message said: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.
Quoted message said: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.
Quoted message said: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
Quoted message said: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
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