Re: [Slipstream-devel] Rider upper body roll and translation

[Dimitris Papavasiliou]

Hi,

> WRT traction control etc;
> while I'm sure a preemptive traction control system taking advantage of
> both acceleration and suspension loadings would be beneficial, we've
> been riding bikes for years without, so I don't think we should be
> looking at this as a solution.

Well yes, I did not mean it as a solution just that it might be more
appropriate than a conventional TC but you're right that it should be
drivable without it.  Nevertheless I should point out that we probably
don't ride bikes like this in the real world.  I mean I don't ever
remember doing clutch-less shifts under full throttle while at a 20
deg. lean at corner exit :).  It doesn't seem entirely implausible
that the machine should object by wobbling.  Shakes like these are
common in race footage.  Still it probably is a simulation issue
because it's not the initial wobble that's the problem it's the fact
that it gets worse as if the rider and machine resonate.  I don't
think that should happen but I'm hoping it's a matter of a
misconception on my part with regards to the rider model as I explain
below.

> Thanks for the in-depth discussion of the rider model. From what I
> understood the rotational stiffness of the upper body is a pure moment
> connection between the steering head and the body; ignoring any
> stiffness provided by the torso and lateral force on the bars. If I've
> got that right, it seems fairly likely that there would be instability.
> The rider model is quite the complex challenge compared to the rest of
> the model.

I reread the paper in question and see now that I didn't understand it
correctly before.  A passage follows:

Thus, in the revised motorcycle–rider model, the
rider’s upper body has both yaw and roll freedoms
relative to the main frame. The rider’s arms contribute
modestly to the steering inertia of the front frame and a
parallel rotational stiffness and damping element acts
between the handlebars and the rider’s upper body.
Following reference [16], the steering inertia contribu-
tion is fixed at 0.103 kg m2, while the steering stiffness
and damping parameters are 3.2 N m/rad and
0.72 N m s/rad respectively for relaxed riding and
60 N m/rad and 1.8 N m s/rad respectively for tense
riding. From these values, the rider’s upper body
restraint parameters are estimated as 60 N m/rad and
13.5 N m s/rad in yaw and 380 N m/rad and 34 N m s/rad
in roll for relaxed riding and as 120 N m/rad and 13.5
N m s/rad in yaw and 760 N m/rad and 34 N m s/rad in
roll for tense riding.

When I read this initially I was just looking for numbers for
stiffness and damping for the yawing freedom of the rider's waist so I
didn't read it carefully.  The way I understand this now is that the
upper body has yawing freedom at the waist (which is assumed to be
fixed to the saddle) as if joined with a hinge with a vertical axis.
It's completely free to yaw.  I does however have some sort of
coupling with the steering but I'm not sure how to read that "parallel
rotational stiffness and damping element".  Is it supposed to be a
virtual torsional spring/damper that doesn't constrain motion at all
(like a hinge would)  but only applies torque based on the orientation
difference (and rate of change of it) between the upper body and the
steering stem?  That seems most likely to me, but then what is the
axis of this torsional spring?  The vertical or the steering axis?  In
any case this scenario seems more reasonable as the incorporation of
yawing in the rider model is introduced to account for feedback at the
steering so it's reasonable that the stiffness and damping be
introduced at the torso-steering interface.  I'll play around with it
once I'm done with the transmission (which is almost complete I think,
although I didn't notice much difference with respect to the old
setup).  Let me know if you have a different understanding of the
passage above.

D.