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

[James Bamford]

On Fri, 1 Oct 2004 16:07:42 -0400, Shamyl Zakariya <shamyl at zakariya.net>
wrote:

> 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.

I've yet to model the steering differential or attempted to drive the
wheels at different speeds when cornering.. how much of a difference can
this make.. i'll certainly have a go at the angular differential when i
have change... surely a lot of the problems comes from using spheres and
not cylinders!? I know again when i roll and end up on my side that the
fact that they're spheres means you can actually twizzle the car on its
side quite signifantly in a way that you obviously couldn't with a real
car.. has anyone had any success with cylinders as wheels.. there is a
cylinder library around isn't there?

>
> 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...
>

You're description of a rollbar (to turn it over when its flipped) sounds
interesting, but i guess its not something that helps it before its
flipped? i imagined a rollbar as something to apply some forces to oppose
the flip as it starts to happen.. almost to simulate the braces you can
get to strengthen up the chasis of sportscars

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

Looks neat... like the rendering style.. same as your four legged creature
you were discussing in the laser range finder posts isn't it?

Jim

> 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
>>
>>
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