Rigidbody.mass and large values: what is the problem?

Unity Engine
1

Hi

At the moment I am trying to resolve some weird issues I am having with my vehicles. I use the integrated PhysX Physics engine, and I am expieriencing a lot of weird issues with collisions lately.

My vehicles physic models is consisting of four wheel colliders, and some convex mesh colliders for the body and turret. now I developed and tested my project with nothing but terrain and some road geometry (generated from easyroads 3D) and the collisions where fine.

But now as I started to insert level props, I get a lot weird collisions with them. Some things I have seen and seem weird:

  • I built a very crude looking ramp. Gave it a mesh collider and "smooth sphere collisions. but when I drove my vehicle on it, the wheel collider went mad and the vehicle started dancing on the collider mesh, even when at rest.

  • I also have a big issue with collisions between my vehicle body collider and level geometry other than the road or the terrain. I use a physic material with very low friction, but even if my convex body mesh collider hits a level geometry collider at a very low angle and should just scrape alongside it because of the low friction, it seems to stick to the other collider and turns the vehicle around instead.

After trying almost everything, I stumbled upon this sentence:

holy moly! I haven’t read that, and set my vehicles Rigidbody to 6000 ( = 6 tons, right?)… now, apart from the weird collision, I have not seen too much weirdness in the physics system (apart from the usual fight with PhysX to get it to behave like it should… it is, as far as I understand, nothing but a low quality simplified physics engine in the end).
Though I had to heavely modify its behaviour until everything feeled and played right (like setting the bounce threshold to 200000 to prevent all the stupid bounces because PhysX does not know of any Material Deformation :slight_smile: )…

Now, I gave setting the mass to a lower value a spin… but boy, is this a mess! Not only do I need to tweak all the wheelcollider settings and my other vehicle drive parameters, it seems the rest of the physics engine now really treats the vehicle as being 1 kg heavy…

Does anyone know what kind of instability is caused by large rigidbody masses? Should I care? Could my Collider woes be coming from the rigidbodies large mass (hard to test with my whole wheel colliders and drive script going haywire because of the low mass, so I’d like to know in advance before I spent hours on it)?
If I have to set my vehicles mass to tiny value… how should I tell the physics system to treat the rigidbody like something thats 1000 times heavier?

Thanks for any help

Gian-Reto

2

Rigidbody mass is unitless, so 6000 doesn’t mean 6 tons. I remember the warning used to be something like “always keep the mass between 0.1 and 5” so I guess something changed :). I never experienced any problems with it, but i’ts better to be on the safe side though, so what you could do is divide all the masses you have by 1000 so your 6 tons becomes 6 and you still maintain weight proportions. :slight_smile:

3

well, that is what I tried any way… but it seemed that that this does not help with my issues, and as I am not seeing any other real issues I was interested as to what the symptoms of this “large mass instability” was. As I already spent hours tweaking all the settings so the vehicle would not roll over, and just downsizing sadly will make all this work invalid. with all the Wheelcollider settings, this is kind of a bitch to finetune.

I agree, thats maybe just me trying to avoid some extra work because of no direct problem. But still, if it does not help with my current problem, I will push it to later.

Anyway, thanks for the answer, its appreciated.

EDIT: And it seems, with zero Friction the colliders now behave like I wanted… hm, strange that it needed such a low value to work… well, still I would be interested what is meant with “instability”