[python] Re: Trail
- From: George Durbridge <gdurbrid@xxxxxxxxxxxxxx>
- To: python@xxxxxxxxxxxxx
- Date: Thu, 01 Mar 2007 23:28:31 +1100
Henry,
This is an interesting marshalling of the available data, but you don't
explain why you apply trail figures from positive-trail bikes to
negative-trail bikes. If trail is positive, the force due to the slip
angle of the front wheel will tend to keep it straight, overcoming the
tendency to flop. On a Python, which is a negative-trail machine, we
rely on a potential energy effect related to wheel flop (the hanging
pendulum) to overcome the slip angle force. There is no reason why the
compromises which suit positive-trail machines should be the same as
those which best suit negative trail machines.
Since the gravitational effects are not affected by speed, but the slip
angle effect increases with speed, positive-trail bikes get more stable
with speed, but negative-trail bikes eventually become unstable at
speed.
A factor which seems not to be taken into account in any of the Python
discussions is the extent to which the lean of the bike in a turn
increases the hanging pendulum effect.
There are interesting discussions of trail in books on bike design by
Mike Burrows and Tony Foale. Both have conducted experiments (on
positive-trail machines) which satisfied them that steering rake is
important only for ergonomic reasons, and that the only important factor
in the dynamics of the bike is the mechanical trail. This conclusion
was applied by Burrows to the steering of the Windcheetah, which has
negligible rake and about an inch of trail.
By contrast, steering rake is critically important to the dynamics of
Pythons. On Robert's machine, we changed the rake by fitting different
sized rear wheels (pity about the brakes ...) and could feel clear
differences in the stability.
On Thu, 2007-03-01 at 21:48 +1000, Henry Thomas wrote:
> This is a little off topic, more about Speculoos & TT clones than
> Pythons I am afraid, so those who aren't interested I apologize -- and
> please ignore this thread.
>
> I did a bit of research and analysis into Trail to figure where I went
> wrong with my Jetrike front end and as usual, wrote what I have learned
> up on a new page on my web site...
>
> http://jetrike.com/trail.html
>
> If you would cast your eyes over what I have written and let me know if
> I have gotten any aspect of this wrong, I would greatly appreciate it.
>
> Thanks in advance
>
> -h
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George Durbridge
+61 3 9481 3500
+61 409 413 945
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