AWD has too much understeer?

[joevenzon]

The particle numbers at the end of the *.car file - is that a "fudge" to get the polar moment of inertia close to reality or is it based on something real world? Is there a car "generator" somewhere that I should be using to make these numbers? If I have real assembly masses and locations (engine, trans, diff, wheels, etc., etc.) is this what these values and locations are supposed to be?[\quote]

Those particle numbers are used to generate the rotational moment of inertia, center of gravity, and mass. There's no "generator" yet, but that would be nice. Yes, if you have real assembly masses, use those, by all means! :-)

The hinge points so far are all just rough guesses by me based on what I thought felt reasonable, so don't assume they're correct. It'd be nice to define some hinges that we could use generally for macpherson struts or other common suspension geometries, but this hasn't been figured out yet. Here's all of the info I have relating to hinges, based on e-mail conversations with Sam Varner (who wrote the VDrift physics engine -- Vamos):

[quote]
> Sam,
>
> I've been playing with suspension hinges versus the suspension position. I've
> noticed that, on the rear wheels, if I put the suspension hinge fore of the
> suspension position, I get a lot of good oversteer, but if I put the
> suspension hinge aft of the suspension position, I get a ton of understeer.
> Any idea why this happens?

Hi Joe. The suspension hinge allows an approximation of suspension
geometries that reduce squat (under acceleration) and dive (under
braking). The usual configuration puts the rear hinge in front of and
higher than the rear axle. The driving force then tends to push the
rear of the car up. Imagine the wheels trying to drive under the
chassis. Braking pulls the rear down. Also, braking torque pulls the
rear down in this configuration. I can try to make some pictures if
that would help.

If you put the hinge behind the rear axle, you're enhancing squat and
dive, presumably amplifying the weight shifts. So I would expect
oversteer entering a turn under brking and understeer powering through
the turn. Can you see if that's what's going on?

> Also, do you have any recommendations on how to simulate a macpherson strut,
> multilink suspensions, etc, based on placement of the hinge?

As I understand it (which is not very well) a strut allows only linear
suspension travel, i.e. a hinge at infinity. So plug in a big x-value.
The slope (dz/dx) to the hinge will still affect squat and dive, but
braking torque won't.

I avoided trying to simulate multilink suspensions directly. I figured
that the hinge (and roll height) approximated most of the useful
multilink geometries. One effect that's missing is the change in camber
with suspension travel.

> One other thing... how would I go about simulating a dependent rear suspension
> (just one big axle)?

I think that would be difficult with the current code. The left and
right sides would have to communicate. There is some communication
already for anti-roll, you might be able to use the same mechanism but
it sounds hairy.

Hope that helps. Just reply if you think that one of us is confused.

A later e-mail:

> Any chance you could, if you have some time, list some examples for hinge
> offsets from the suspension position for various common suspension geometries
> (MacPherson, wishbone, multi-link, etc)? I'm having a hard time figuring out
> what makes sense, because sometimes even large changes in the hinge position
> end up creating fairly subtle effects....

The problem is that just knowing it's a strut or a wishbone doesn't tell
you enough about the geometry. What matters is how the wheel is
constrained to move as it's displaced. You'd need to know the positions
of the joints and how those joints constrain the motion of the wheel in
order to determine how the car will behave. Unfortunately, as it
stands, the code could not use that information even if you had it. I
know that's bad news if you're trying to simulate a particular car.

The hinged suspension in the code doesn't directly correspond to a real
suspension type, but it does (as far as I know) reproduce the important
performance characteristics of real suspensions--anti-squat and
anti-dive. Depending on their precise geometries, two different
multi-link (for instance) suspensions may have different
characteristics, and some multi-link suspension may have the same
characteristics as some double wishbone. There's not a direct
correspondence between suspension type and hinge position.

Does that make sense?