[Slipstream-devel] Rider upper body roll and translation

[Dimitris Papavasiliou]

The current implementation allows the upper body of the rider to roll
or lean and also to move laterally.  This simulates the actual
behavior of the rider which is important in terms of vehicle stability
as the rider not only dampens the oscillation of the vehicle frame but
also feeds some of them back through her hands to the steering stem.
This latter part has an interesting side-effect.

This is because I though it wise to actually force the rider to lean
and move the upper body laterally into the turn as the motorcycle
itself leans into the turn to mimic this typical behavior of real-life
riders but this seems to have the side-effect of applying steering
torque too.  In fact when setting "roll and lateral translation gain
with respect to motorcycle roll" to 0 the motorcycle seems to be much
more stable at large lean angles (reaching as much as 59-60 degrees of
lean which is a bit of an embarrassment really) and it also needs more
steering torque to do so.  The later is presumably because before some
steering torque was provided by the motion of the upper body.

Another interesting result of setting roll and translation gain to 0
(which means that the rider always tries to stand upright resisting
the tendency of the bike to throw him around) is that now it seems to
have become more unstable in terms of wobble during brake. Changing
the stiffness and damping of the rider body to numbers found in [1]
seems to remedy that though.  The translational stiffness was absurdly
high before because otherwise the bike would be unstable so it's
rather reassuring that now sane values actually lead to stability
(which is what the paper predicts too).

My current rider control values are:

Upper body roll 1760 Nm/rad  stiffness and 220 Nms/rad  damping.
Upper body yaw  120 Nm/rad  stiffness and 13.5 Nms/rad  damping.
Upper body lateral translation 38862 N/m stiffness and 660 Ns/m.

All gains are zero by default and in fact have been removed altogether
as a feature because they were unrealistic:  the stiffness and damping
of the rider-motorcycle coupling is not the same when the rider is
upright as when he is hanging from the bike at full lean.  This is
evident in the current setup as the rider's butt serves as a perfectly
good stand when the bike is dropped.

These settings will probably be made default on the next commit so any
feedback on whether my observations are correct or not is welcome.

Dimitris

[1] The Significance of Frame Compliance and Rider Mobility on the
Motorcycle Stability, Roberto Lot, Vittore Cossalter, Matteo Massaro