[python] Re: Position of Steering Pivot Axis/TT-bike
- From: "Christoph Falkner" <Bernard_Falkner@xxxxxxxxxxx>
- To: <python@xxxxxxxxxxxxx>
- Date: 07 Jan 2006 12:52 GMT
Hi,
here are my five cents about PSI and specially about driveability of TT-bikes
(may be a bit off-topic, I know, but I think it is a REALLY interesting concept
because of its simplicity and the possibility to put rider position and bottom
bracket where you really want them).
First of all my thoughts about physics. On all bikes with moving bottom bracket
there must be the following effect because pedalling force is not exactly in
the middle axis of the frame: if you put your pedalling force to the crank,
you can divide it into a force-vector rectangular to the steering axis and one
in direction of the steering axis. The force rectangular to the steering axis
causes a momentum around the steering axis (because of the pedal-offset from
the frame´s middle axle) - independent from the riders hip-position. This
momentum is zero, if steering is turned so far, that the pedal is in the middle
axis of the frame. The needed steering angle for bringing the momentum to zero
becomes smaller as the distance between steering-axle and BB increases.
My first “knowledge” is: Increased BB to steering-axis distance should reduce
PSI-effects (Python?).
My second “knowledge” is: Momentum around steering-axis is reduced if main
pedalling force is more along the steering-axis instead of rectangular to it.
We can reach this with elevated seat high (no one of us will like this much, am
I right?) or lowered BB (look at Laurent´s low-racer, better solution in my
mind, limited by heel clearance). We can reach the same with shallower
steering-head-angles but there are limitations in driveability again!
By the way: If pedal offset is the cause of all problems, I will open a new
Q-factor-discussion again ;-)
What is not considered in this thoughts: Dampening-effects, gyroscopic effects
and head-angle/trail-effects. For the Pythons the whole body works like a
steering dampener – quite effective, I think, if there is no pedalling induced
steering-effect. For the TT-bikes arms and upper body may have a comparable
function.
May be there are the gyroscopic-effects helpful in reducing the tendency to
oscillate: It could be interesting to test several wheel-sizes/tire-weights in
the same geometric constellation …
Stabilizing effect of TRAIL will play another role in the game. Tom Traylor
used a quite large amount of trail (100mm/ 4 inches) for his bikes and claimed,
that there is NO unintended pedal-induced steering.
Now back to the TT-concept. I googled a while an found a few home-built bikes
and a conversion-kit (http://www.cruzbike.com/kit/) for conventional bikes.
First Tom Traylors homepage (http://traylorfwd.home.mindspring.com). He even
raced with his bikes and seems to be completely satisfied with this concept. A
few others with home build conversions were also very happy with their results.
On the other hand I found some more neutral
(http://www.greenspeed.com.au/NewGSweb/web-content/australia/paul/build.html)
or critical opinions
(http://www.wisil.recumbents.com/wisil/costin/seancostin.htm). Famous racer
Sean Costin wrote about one of his creations: “ … One could call this an early
Low racer. It was hell to get started, but once it was going, it was only a
nightmare to handle …”. Problem of the low seat high? On the other hand
Laurent Dechenne is not only fast, he seems to be happy with his low-racer (or
am I wrong?).
What´s your opinion about it?
My opinion is (only having tested a flevo a few years ago and being not very
satisfied), that getting handled and being happy even when using such an
unconventional concept every day, is more a question of ACCEPTANCE for the
things/feelings that are different from a “conventional” recumbent.
Or are there things, that can make you really hate the TT-concept and make this
concept something NOT worth to spend (building- und learning-) time and money
for?
Cheers,
Christoph
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