Measuring Fork Rotation (was Fretting)

Joe Riel said:

As a first attempt to measure the amount that the front fork rotates
while coasting downhill, I taped an aluminum rod to the stem. The end
of the rod was just short of the front of the seatpost. The idea was
that this would allow me to observe the motion of the handlebars
relative to the frame. The particular implementation could be improved.

First, the readily available rod was thin; it vibrated significantly,
limiting the "precision" of the measurement. Second, putting the
viewing end near the seat post required looking down and back
(i.e. not watching the road ahead). That wasn't a significant problem
with the hill I descended, but it didn't improve the situation.
The rod also limited the range of motion of the bars, and probably
presented a minor risk of injury.

If I can get hold of some thin walled aluminum tubing---that should
reduce the vibration significantly---I'll test again, this time with
the "needle" projecting forward along with a parallel reference that
is mounted to the top tube.

The preliminary observation was that coasting with the hands off the
bars produced little motion; however, there were gusting winds that
required frequent correction, so nothing conclusive. Back to the lab.

Joe

Dear Joe,

Or what about cutting a long rubber band, attaching it to
the handlebars, and running it around a small pulley mounted
on the top tube? Here's what I mean, looking down from the
rider's position:

stem
left bar---O---right bar
\ | /
\ | /
\ | /
\O/ pulley with tell-tale pointer pointing
|\ back toward rider's right
| \
|
seat

A small enough pulley with a tell-tale might magnify very
small steering movements enough to be clear, despite
vibration.

The trouble is that everyone agrees that very small steering
motions (somewhere around 0.2 degrees of steering should be
produce 1 degree of bearing rotation), and the wind and road
vibration at 30 mph make it hard to measure accurately.

If nothing else, the fretting thread has produced an
interesting technical puzzle--how to measure steering motion
accurately and cheaply. Whatever trick is used, it has to be
something that can be applied easily by the rider after he
gets going in what he feels is a good, straight roll down a
hill, and then stopped after a few seconds.

It also would be nice to see how much more steering motion
is actually produced when the rider pedals.

Carl Fogel

On Sat, 28 May 2005 22:04:46 -0600, [email hidden]

Quoted message said:

Joe Riel said:

As a first attempt to measure the amount that the front fork rotates
while coasting downhill, I taped an aluminum rod to the stem. The end
of the rod was just short of the front of the seatpost. The idea was
that this would allow me to observe the motion of the handlebars
relative to the frame. The particular implementation could be improved.

First, the readily available rod was thin; it vibrated significantly,
limiting the "precision" of the measurement. Second, putting the
viewing end near the seat post required looking down and back
(i.e. not watching the road ahead). That wasn't a significant problem
with the hill I descended, but it didn't improve the situation.
The rod also limited the range of motion of the bars, and probably
presented a minor risk of injury.

If I can get hold of some thin walled aluminum tubing---that should
reduce the vibration significantly---I'll test again, this time with
the "needle" projecting forward along with a parallel reference that
is mounted to the top tube.

The preliminary observation was that coasting with the hands off the
bars produced little motion; however, there were gusting winds that
required frequent correction, so nothing conclusive. Back to the lab.

Joe

Dear Joe,

Or what about cutting a long rubber band, attaching it to
the handlebars, and running it around a small pulley mounted
on the top tube? Here's what I mean, looking down from the
rider's position:

stem
left bar---O---right bar
\ | /
\ | /
\ | /
\O/ pulley with tell-tale pointer pointing
|\ back toward rider's right
| \
|
seat

A small enough pulley with a tell-tale might magnify very
small steering movements enough to be clear, despite
vibration.

The trouble is that everyone agrees that very small steering
motions (somewhere around 0.2 degrees of steering should be
produce 1 degree of bearing rotation), and the wind and road
vibration at 30 mph make it hard to measure accurately.

If nothing else, the fretting thread has produced an
interesting technical puzzle--how to measure steering motion
accurately and cheaply. Whatever trick is used, it has to be
something that can be applied easily by the rider after he
gets going in what he feels is a good, straight roll down a
hill, and then stopped after a few seconds.

It also would be nice to see how much more steering motion
is actually produced when the rider pedals.

Carl Fogel

Bah! Instead of a single pulley arrangement connected to the
handlebars by Rube Goldberg, how about a series of pulleys,
like the intermediate crank that lets a 55x11 connected to
another 55x11 produce an overall 25:1 gear ratio for those
drafting speed records?

1 turn
55t 11t
xxxxx===============x
| 11t
xxxxx============x
55t 25 turns

Wrap a belt around the stem, lead it back to a pulley
smaller than the stem.

On the small pulley's shaft, mount a large pulley, and
connect it by belt to another small pulley.

PPPPP===========p magnified
| | steering movement
stem=====p PPPPPP========p

tiny
steering
movement

If I had any metal-working talent, I'd know how much work
this would be.

Carl Fogel

Quoted message said:

Or what about cutting a long rubber band, attaching it to
the handlebars, and running it around a small pulley mounted
on the top tube? Here's what I mean, looking down from the
rider's position:

stem
left bar---O---right bar
\ | /
\ | /
\ | /
\O/ pulley with tell-tale pointer pointing
|\ back toward rider's right
| \
|
seat

A small enough pulley with a tell-tale might magnify very
small steering movements enough to be clear, despite
vibration.

My bike has dimensions

handlebar width = 16"
stem length = 5"
top tube = 22"

Moving the pulley two inches forward of the seat tube,
the gain of the system is then (computation omitted)

Gain ~ K/Dp

where
K = 3.3"
Dp = pulley diameter

To get reasonable gain (say 10x) the pulley is going to be rather
small. I'll try a small diameter tube. I suspect that a rubber band
will vibrate too much. Better might be to use fishing line with a
small rubber band at one end to keep it tensioned (the length changes
as the bar turns).

Joe