[ODE] Re: Car stability with lowered centre of mass leads to problems when rolling

[Pete Baron]

Hi Jim,

I've been playing with driving simulations for a while now (though not as
long as some of the folks here!) and one thing that I've always wanted to
add to my car models is 'active suspension'.  Many performance cars now
feature this and given the kind of speed we tend to drive in simulations,
and the limited control inputs we're normally given, it seems like an
essential addition.
To simulate this should be fairly easy - simply measure the displacement of
all four wheels from their neutral position along the vertical axis, average
it, and add a force to push each wheel towards the average offset using the
suspension (with joint motors).  (It seems like you might need to add a
slider joint to each wheel to achieve this, but maybe clever use of the
hinge2 can gain the same effect).
The effect of this change would be that when the car body starts to roll due
to centrifugal (centripetal?) forces, the outside wheels suspension would
push harder and attempt to keep the car level.  Yet when the car lands after
a jump or bump, the suspension is still able to absorb the shock and the car
will ride down reasonably level until it bottoms out and springs back up.
I'm not sure but I vaguely recall reading an article about sports cars years
before the advent of the new hydraulic active suspension systems which
talked about a way of linking diagonally opposite wheels with chains to
achieve a similar result.

Let me know if you use this idea and get it working... like I say, I've been
thinking about it for ages and just haven't managed to find the time to try
it!

cheers,

Pete Baron
sibaroni at hotmail.com
http://home.btconnect.com/pete/homepage.html


> From: "James Bamford" <fromweb at jimtreats.com>
> Subject: [ODE] Car stability with lowered centre of mass leads to
> problems when rolling
> To: "ODE mailing list" <ode at q12.org>
> Message-ID: <opse5yvtg8n75mj0 at jims-win2k>
> Content-Type: text/plain; format=flowed; delsp=yes;
> charset=iso-8859-15
>
> Hi folks..
>
> I've been picking over the ode archives and got some really useful tips on
> basic vehicle simulation with ODE.. I've managed to get a set of different
> types of vehicles working to a reasonable degree and now i want to just
> tidy up the remaining problems I have with the simulation.
>
> At any kind of speed (really quite low in my setup) with a sensible centre
> of mass the cars were always tending to roll over at the slightest hint of
> cornering, after discovering that the built in ode mass translate
> functions weren't suitable i went the geom transform route to offsetting
> the body to be in a lowered state with the geom positioned correctly
> offset via the geom transform... I've not tweaked this that heavily but it
> really seems to require the new centre of mass to be very close to the
> ground to get a nice sense of stability, especially at higher speeds.
>
> This is all great but when your driving skills (or perhaps some niggles in
> the controls/simulation) get you into trouble and you do end up rolling
> the vehicle the rotation is of course about the body pos which in its
> lowered position is around floor height.. you get the real feel of the car
> orbiting around a point with some kind of translation associated with the
> visible body instead of just rotation as is the case with the rotation
> point sensibly placed near the centre of the geometry.
>
> I've seen this on a few of the other demos out there where the centre of
> mass has been similarly lowered and was just wondering what strategies are
> available to reduce/remove this problem.
>
> I had the idea of either snapping or tweening the centre of mass position
> back to the centre of the object when i detected that the car was tilted
> on its side (about to go into a spin) .. this involves setting the
> transform geom offset to 0 and compensating for this change with body
> position changes for the chassis, wheels and their hinge's anchor
> positions from having the wheels vertically level with the old body pos to
> being well below the body pos when its back in the centre of the vehicle.
>
> I've managed to get this working and aside from a slight wobble in the
> wheel hinges its pretty imperceptable snapping the centre of mass rapidly
> between the two positions. I thought i was halfway there to solving this
> problem when i started to notice that my car wouldn't drive in a straight
> line anymore.. further stress testing in stranger orientations allows you
> to get the hinges of the wheels well and truly broken facing in many
> different directions at steering rest. Its as if the hinge takes whatever
> state its in when i alter the anchor point and sets this as its new
> centred orientation.. I can think of no way to undo this as I can't see
> anyway to directly set the hinge's values back to a correct setup.
>
> I know that messing with the vehicle's positions etc out of the simulation
> isn't really a good idea but i couldn't think of an alternative to improve
> this body rolling rotation problem.
>
> Anyone got any ideas why the hinges get corrupted like this, or have any
> other ideas how to fix it or make our lowered centre of mass cars perform
> more like they should when in mid air and rolling down a slope after a
> crash?
>
> Thanks for your time, sorry for the post length
>
> Regards
>
> Jim