3 ms·

This is not correct because gravity only acts downwards. This means a lower center of gravity will always be more stabilizing for a vehicle the right way up. As

This is not correct because gravity only acts downwards. This means a lower center of gravity will always be more stabilizing for a vehicle the right way up. As the car rolls the center of gravity is pushed outward. When it’s too high it quickly passes over the pivot point of the wheels and the car will roll as gravity is applying additional moment about the wheel. When it’s lower than the pivot point which is not usually physically possible it’s always resisting the roll until the vehicle is basically nearly upside down. Draw it on a piece of paper if you need to convince yourself. This is an incredibly common trick to stabilise rigidbody physics simulated vehicles in games. Edit: I took some time to setup OBS on my non-work machine and make a little video explaining what's going on (apologies for kids in the background and that for some reason my voice only comes out of the left channel): https://youtu.be/qvMbdzbaAMQ https://youtu.be/qvMbdzbaAMQ

snovv_crash 5y ago
In your video you show the force in red being applied somewhere up top. What is this force? What applies it? Your force sideways is actually from the ground, on the tyres. This acts on the CoG (or better to call it Center of Mass), so if this CoM is above the tyres you get a torque, because the mass doesn't instantly accelerate, but resists the force. This torque then needs to be countered by having more force on the tires on one side than on the other, which when you take into account things like suspension, causes body roll, or in the extreme case the car lifting wheels on one side completely off the ground. If you put the center of mass below the car then think what happens not during turning, but during braking. The center of mass will swing forward underneath like a pendulum (think of what happens to something hanging from the rearview mirror during braking) and if you took it really far down you could even get the car to do a wheelie while braking, which is of course ridiculous. If you want the car to be completely flat during turns, acceleration and braking then you need to put the center of mass at the ground level. This way when you turn and the ground puts a horizontal force on the car at the contact points with the road (ie. the tyres), it goes through the center of mass and there is no torque generated. No torque = no body roll.
- meheleventyone 5y ago
  The torque to roll the car in my example is provided by an external force, the dark brown/red arrow being applied to the top right of the 'car'. This is why the car would still roll even if you had the CoM at the ground level. I'm simplifying the true situation (suspension, roll due to the forces involved in turning and so on) to isolate just the impact of CoM position on body roll. So you need a force to induce roll otherwise the car would just sit still. If you don't believe me you can download Unity, UE, Godot or the rigidbody simulator of your choice and just setup the experiment. Of the engines mentioned UE is perhaps the easiest to set this up with a straightforward car example.
  - snovv_crash 5y ago
    Well sure, if an external object hits your car above the CoM then it will roll. I suspect we're talking past each other, I was referring to the handling characteristics, not the response to an impact.
    - meheleventyone 5y ago
      I think you misunderstand. My demonstration uses an external force for simplicity. To illustrate the stabilizing effects of moving the CoM up and down. The results are the same for other sources of body roll as well. We're very much not talking past one another. I recommend just doing the experiment to convince yourself because as I and Animats note this is a very common thing to do in games. As I said in my last reply just grab a game engine (they're all free to download) and test it yourself!
      - dTal 5y ago
        I'm sure we all agree that a lower CG is always more "stabilizing" in the sense of righting a car that is not already flat on the ground. But that's not what snovv_crash and I were talking about - we were talking about plausible handling characteristics. A CG below the ground will produce vehicle rolling motions opposite to real life. The outer wheels will lift in a tight turn, and the car will rear up when braking instead of pitching down. There are only two forces involved - inertial forces which act on the CG, and friction forces which act at ground level. Swap the order, and you swap the direction of the torque.

snovv_crash 5y ago

In your video you show the force in red being applied somewhere up top. What is this force? What applies it? Your force sideways is actually from the ground, on the tyres. This acts on the CoG (or better to call it Center of Mass), so if this CoM is above the tyres you get a torque, because the mass doesn't instantly accelerate, but resists the force. This torque then needs to be countered by having more force on the tires on one side than on the other, which when you take into account things like suspension, causes body roll, or in the extreme case the car lifting wheels on one side completely off the ground. If you put the center of mass below the car then think what happens not during turning, but during braking. The center of mass will swing forward underneath like a pendulum (think of what happens to something hanging from the rearview mirror during braking) and if you took it really far down you could even get the car to do a wheelie while braking, which is of course ridiculous. If you want the car to be completely flat during turns, acceleration and braking then you need to put the center of mass at the ground level. This way when you turn and the ground puts a horizontal force on the car at the contact points with the road (ie. the tyres), it goes through the center of mass and there is no torque generated. No torque = no body roll.

meheleventyone 5y ago
The torque to roll the car in my example is provided by an external force, the dark brown/red arrow being applied to the top right of the 'car'. This is why the car would still roll even if you had the CoM at the ground level. I'm simplifying the true situation (suspension, roll due to the forces involved in turning and so on) to isolate just the impact of CoM position on body roll. So you need a force to induce roll otherwise the car would just sit still. If you don't believe me you can download Unity, UE, Godot or the rigidbody simulator of your choice and just setup the experiment. Of the engines mentioned UE is perhaps the easiest to set this up with a straightforward car example.
- snovv_crash 5y ago
  Well sure, if an external object hits your car above the CoM then it will roll. I suspect we're talking past each other, I was referring to the handling characteristics, not the response to an impact.
  - meheleventyone 5y ago
    I think you misunderstand. My demonstration uses an external force for simplicity. To illustrate the stabilizing effects of moving the CoM up and down. The results are the same for other sources of body roll as well. We're very much not talking past one another. I recommend just doing the experiment to convince yourself because as I and Animats note this is a very common thing to do in games. As I said in my last reply just grab a game engine (they're all free to download) and test it yourself!
    - dTal 5y ago
      I'm sure we all agree that a lower CG is always more "stabilizing" in the sense of righting a car that is not already flat on the ground. But that's not what snovv_crash and I were talking about - we were talking about plausible handling characteristics. A CG below the ground will produce vehicle rolling motions opposite to real life. The outer wheels will lift in a tight turn, and the car will rear up when braking instead of pitching down. There are only two forces involved - inertial forces which act on the CG, and friction forces which act at ground level. Swap the order, and you swap the direction of the torque.