[python] Re: Trail

George Durbridge wrote:

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.
I have often wondered about this, and came to the conclusion in at least my mental theory on python dynamic stability that when the python is turning at speed, and the rider is leaning into the curve then the hanging pendulum effect must in a essence be helping the bike hunt for the correct line in the curve, rather than exhibiting oversteer or understeer. Or does the Python want to steer straight.
There should be other forces as work too. As the Python leans, the point of contact of the tires with the ground would be moving to the side of the tires, but what also tends to happen (on positive trail bikes at least) is that the contact patch starts to move forward around the front rim and backward around the read rim thus extending the wheel base and changing dynamic forces pushing on the tires on the sides of the tires, and at different angles than on an upright bike. On the Python does the point of contact with the front tire on the leaning bike actually move forward or does the negative in some way cause it to move differently? What influence to you think this has on the cornering dynamics. On a positive trail bike, there is a point where an extreme lean will in fact neutralize the effect of positive trail to the point where it actually becomes negative and causes instability, but generally the pivot angle is set at such an angle that this is only a theoretical risk. 



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 rake here are you are talking about the pivot angle or fork offset?


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.
The effect being that you changed the pivot angle with the different tire size and as such the handling characteristics. Did you draw any conclusions as to which angles you felt were better from this? If you posted it here I am afraid I must have missed it. 

-h
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