[ODE] Questions about wheel slip and damping

[Yizhen Zhang]

Dear all,

I am pretty new to the ODE community. I am just trying
to implement some basic vehicle dynamics on a simple
car model built in Webots. The current model only
contains four cylinder wheels and one box body. The
two front wheels are driving wheels which I apply
torque on, and the two rear wheels were motor off.

First I noticed that when the car finished
acceleration and entered steady state (moving forward
at constant speed) stage, the following relationship
seems to hold:
v = (wf + wr) * r / 2                 (1)
where v is linear speed of the car, wf and wr are
angular speeds of the front and rear wheels, and r is
the wheel radius.

The other thing I noticed is that there is always a
difference between front and real wheel angular
speeds. And this difference seems to depend on wheel
radius. Actually when the wheel radius is too small
(say in centimeters), the rear wheels do not rotate at
all, and they look locked and just slide forward, i.e.
wr = 0. But equation (1) still holds, so I get v = wf
* r /2, i.e. the front wheels slip at 50%. When I
increase the wheel radius (say, to > 0.2m), the rear
wheels start to turn, but I always get wf > wr, and
the following seems to hold:
(wf - wr) * r * r = constant        (2)
So the rear wheels still skid as if some brake force
were applied to them. I tried to change friction
coefficients and FDS, but (1) and (2) always hold for
steady state. I wonder why it is implemented like this
in ODE, and I can't believe this is the case in real
cars. I hope there is other ways to control the wheel
slip percentage instead of changing wheel radius. 

On the other hand, I defined a simple slider joint
between the box body and the car body (the part move
with wheels), so that the box could move up and down
relative to the car. I just want to implement a very
simple spring and damping system to simulate the
suspension. So I added opposite spring forces to both
bodies, which are proportionate to the vertical
distance between the two. However I observed damping
effect even I only added spring forces. I can
understand if ODE developers added damping here to
stablize the simulation. But I wonder how does ODE
decide how much damping to add here. It seems to be
small, around 0.01, but changes with the spring
constant and maybe some other factors.

Sorry for the message length and I appreciate your
attention and help.

Best regards,
Yizhen

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