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

[Shamyl Zakariya]

How complex a suspension are you willing to use?

I made a suspension which, while nowhere near complicated ( no 
double-wishbones, for example ) did the job nicely. Also, I modeled the 
behavior of a differential to keep wheel spin in line.

It's easier to describe with a screenshot:

http://home.earthlink.net/~zakariya/files/TruckPlatform.png

While I'm not writing a racing game -- this is actually just a 
quasi-autonomous wheeled platform -- I've never seen it flip for any 
reason other than falling off the ramp in the distance. For that, I 
have ( subsequently to the screenshot ) added an automatic rollbar 
which turns the car back over...

For what it's worth, this bugger takes fast sharp turns all the time -- 
even on bumpy surfaces -- and doesn't flip.

Shamyl

On Sep 30, 2004, at 7:44 PM, James Bamford wrote:

> 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
>
>
> -- 
> Using Opera's revolutionary e-mail client: http://www.opera.com/m2/
> _______________________________________________
> ODE mailing list
> ODE at q12.org
> http://q12.org/mailman/listinfo/ode
>