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[Slipstream-devel] Tire parameter scaling
[Slipstream-devel] Tire parameter scaling
Thu, 24 Nov 2011 02:31:28 +0200
The latest CVS snapshots introduce tire model scaling. You can fiddle
with these at two separate places. One is the tire model
configuration pages which have an extra section entitled "Scaling
factors". These parameters scale combinations of the parameters
specified in the previous sections in a way that could be actually
useful to someone who is not a research scientist. That is to say
they have a physical and practical meaning. More specifically:
Peak longitudinal friction coefficient (λμ,x): This determines the
peak force the tire can generate longitudinally or whether you'll
wheelie or burn-out.
Peak lateral friction coefficient (λμ,y): The same as before only in
the lateral direction so this mostly determines whether you'll make
the turn or drift and low-side.
Brake slip stiffness (λKxκ): This is the slope of the slip to force
curve (before the curve peaks out) so if this is higher the slope is
stiffer and you don't need so much slip to generate a given amount of
force. I'm not sure how exactly this affects handling.
Cornering stiffness (λKyα): The same only for the lateral direction.
Lower values mean you'll need more sideslip to produce a given
sideforce so the steering will be more "sluggish". Whether this will
be better or worse is another matter.
Camber force stiffness (λKyγ): This basically determines how much
"camber thrust" you get per degree of lean (again before the
side-force peaks out. This doesn't determine the maximum value, just
how soon you get there). Camber thrust is the sideforce generated by
camber, or leaning of the tire with respect to the road. It's the
main source of side-force in a motorcycle. This affects handling in
subtle and mysterious ways, especially since it can be different in
front and rear tires. This then would mean that you'd get a different
amount of sideforce due to camber from rear and front tire so in order
to balance the bike you'd need to compensate at the front with more or
less sideslip, that is by steering into or out of the turn (or perhaps
even by counter-steering). Of course since the camber stiffness is a
slope the actual amount of camber thrust difference between front and
rear would continuously change as your lean angle changes so you'd
need to constantly adjust your steering as well. This all happens
anyway of course. This parameter simply allows one to change the
Pneumatic trail (λt): This affects aligning moment which is a torque
that is generated on the tire in the vertical direction as soon as
there's sideslip (or camber) tending to align the tire with it's
direction of travel and therefore diminish sideslip. In effect this
is the lever arm of the sideforce so that the aligning moment due to
sideforce is equal to the trail times the sideforce (plus some
residual moment which has nothing to do with trail).
Camber torque stiffness (λKzγ): This is the aligning moment induced
per degree of lean (or rather a scaling factor of it).
Rolling resistance (λMy): This scales the coefficient of rolling
Radial stiffness, Radial damping: These can be used to scale the
elastic properties of the tire. Presumably this can also be set to a
lower value to simulate the case where the tire is the same but the
ground surface has changed (because you're of the road for instance.)
Apart from the tire these can also be set per ground type. The final
scaling factors applied to the model are a product of the factors of
the tire and the ground. This is useful in two respects.
One is that the off-road areas now feature different scaling factors
than the road to reflect the totally different interaction of a tire
with dirt. The exact values used right now are more or less guesses.
Research on the net has not yielded much on how to scale the road tire
model to get a plausible dirt tire model. This isn't so important at
this point of course as there's no dirt track right now. The general
idea is that if you exit the track at a lean you mostly fall,
otherwise you can re-enter it with caution.
Another use is that the road scaling can be varied too and in order to
make use of that, track variations have been introduced. For now the
track simply has a low-grip and high-grip variation. The former
simulates a very worn or wet road and the latter an ultra-grippy road
useful as an easy mode mostly. Since the new shading model easily
supports variations each track has a different look too.
Feedback is always welcome,
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