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Old October 15th 04, 02:41 PM
Gregor Veble
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Hello!

Ruben wrote:
>
> Unless the table is on carpet! Mine is on carpet, which makes it look
> as though all legs are carrying equal weight. But, I suppose this is a
> bit like having suspension.



Yup, as others have said, only with a suspension which has much more
give than the irregularities in the ground can you expect something
close to a uniform weight distribution.

>
> Yes, I see now - a proper model of suspension is what is needed to
> model weight transfer - only, this is a fair bit more complicated than
> I anticipated. It seems to me that the chassis must actually be able
> to pitch and roll which means that the suspension is not fixed at 90
> degrees to the chassis. Is this right?
>


Yeah, pitch and roll is needed in order to compress the springs/dampers,
which in turn provide the load on the wheels (and vice versa, as Sir
Isaac tells you). But, with stronger springs the pitching and rolling is
vastly reduced. It is the relative spring stifnesses that matter in
individual load transfer, not their actual magnitude as long as the road
is sufficiently flat.

The way the suspension works is then really dependant on what sort of
suspension you're dealing with. You can simplify the system a lot,
though, by thinking about the contact point of the tyre moving up and
down relative to the chassis, and from the position and velocity of that
contact point relative to the chassis you get the reaction force for
both the spring and the damper, and that will already give really nice,
plausible results.

> In any case, I am not going to try to model this for now. It's for a
> 2d game - so all that detail would be wasted. But isn't there an
> aproximate way of working out weight transfer from the tyre forces? On
> the other hand, perhaps it's not worth doing at all if not done
> properly...
>


Since you mention you will be dealing with a 2D game, what one can say
something about is the transfer between left and right side of the car
and the same for front and back transfer, but only for the total
transfer in each direction, not the individual wheel loads.

For the forward/backward total transfer, you have to assume that the car
is quasi stationary with respect to pitching (no rotating
accelerations). Then the sum of the torques originating from the loads
on the front and the rear wheels as well as the torques originating from
the longitudinal forces must be 0 with respect to the c.o.g. One must
also make the wheel loads match the force of gravity and the
mass*vertical acceleration, while the longitudinal forces should provide
longitudinal acceleration. From these equations you can determine the
total front and rear loads.

The same can be done for the left/right transfer, but again only for the
total one, not load on individual wheels. But still, IMHO the code that
calculates things from first principle (which is rigid body rotations
and springs/dampers) will be much simpler and will look much more
natural, too.

Cheers,

-Gregor
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