Getting a child collider to not affect the weight distribution of a rigidbody

Hi! See attached image. The object "Player1" has a rigidbody. Its child object "Colliders" has some colliders, in the image these are illustrated with pink color. These I want to be used as weight to the character.

There is also another child object "ExplosiveCollider" which has a single collider, it is illustrated with blue color in the image. It is used for encapsulating picked up explosives which can be thrown. The purpose of this collider is so that the character and its picked up explosive will behave as one object, i.e. the character cannot walk into a wall with an explosive.

This works fine, the character walks up to a wall and the picked up explosive's collider makes it stop before going into the wall.

There is a little problem though: because of this collider the character gets unstable and starts to tilt, sometimes even falling.

How can I solve this? I still want the collider to be a part of the character and to be used for collision detection, but I don't want it to make the character becoming unstable.

Have a nice day!

 

Thanks for the reply! But if I set it to be a trigger it will just go through the wall. What I want to do is to still get a physics reaction (i.e. stop at the wall), but I don't want it to affect the center-of-mass. Is it possible to force a collision reaction inside OnTriggerEnter?

 

@UnresolvedExternal You can easily fix the tipping over issue by simply toggling on the "Freeze Rotation" option for the X and Z axis under "Constraints" on the rigidbody of your character, unless you actually need those to remain free for some reason?

 

Hi! I already have "Freeze Rotation" option for X and Z checked. The tilt rotation that occurs is around the local forward axis. Wouldn't this be equal to Z and thus shouldn't rotate since it is freezed?

When I checked Y as well (i.e. freeze rotation all) the character stopped tilting, which I find strange.

Worth to mention is that the rotation of the character is handled with "transform.rotation" in Update, could that have any impact?

Code (CSharp):

1. _newRotation = Quaternion.AngleAxis((-Mathf.Atan2(Axis.z, Axis.x) * Mathf.Rad2Deg) + Camera.main.transform.rotation.eulerAngles.y + 90, Vector3.up);
2.  
3. transform.rotation = Quaternion.Slerp(transform.rotation, _newRotation, Time.deltaTime * RotationSpeed);

The forward/backward movement is handled with AddForce in FixedUpdate.

 

@MelvMay You keep referencing the 2D physics system, but I think @UnresolvedExternal is using the 3D physics system? However, If game play is restricted to 2D then switching to 2D physics might make working with physics a bit simpler here.

Another possible solution to this whole "center of mass" issue could be to simply create a set of predefined child objects on the player. Each representing a different pickup. Then instead of attaching pickups to the player when they collect them, you would simply destroy the pickup and display the corresponding child pickup model on the player that represents it. Then when the player drops the pickup you simply hide the child pickup on the player and Instantiate a new pickup object to drop.

Now the key here would be that all the predefined child pickups on the player would be visual only, with no colliders attached to them. Meaning they wouldn't effect the rigidbody's center of mass at all. Now if you don't want these child objects to clip through walls I would just create a second wider collider on the player that is only activated when a child pickup is displayed. Or maybe just make the player's collider bigger to start with.

 

Hi! Thanks for a thorough explanation @MelvMay!

The reason I have freezed the X and Z constrains is because I don't want the rotation of the character to be affected by physics. So the only way to make it rotate is by changing the rotation of the transform, correct me if I'm wrong.

I tried to move the rotation logic from Update to FixedUpdate, but the results were the same. Is it better to keep it in FixedUpdate even though I'm using transform.rotation?

I also tried to store the initial center-of-mass value in Awake and then when a collider is added or enabled I set rigidBody.centerOfMass back to the initial value. By using a custom editor that draws the center-of-mass I have verified that the initial value is used at all times. But, the problem still persists.

I have probably built up my hierarchy wrongly, there must be a simple solution for this, is there any other way I can build it?

To sum up, what I want is this:

• Character is moved with AddForce
• Character is rotated without physics reaction
• An extra collider is enabled if the character picks up an item:
  • This collider should belong to the character
  • It should not affect the character's stability, i.e. not wobble
  • It should detect collision and stop at walls

Hope we can figure this out!

 

I created a simple script below that should do what you want. Just create a new rigidbody object for the player to test it with, lock all rotation on it, add an empty child named "Hand" and position it somewhere in front of the player (this will be used to hold and position any items picked up), and add a trigger to detect pickups. Then create another rigidbody object and name it "Pickup" and place it in the scene near this player object and run the scene to test it.

Code (CSharp):

1. using UnityEngine;
2.  
3. public class PlayerPickupDemo : MonoBehaviour
4. {
5.     public float speed = 10f;
6.  
7.     Rigidbody playerBody, pickup;
8.     Transform hand;
9.  
10.     void Awake()
11.     {
12.         // Optimization - Cache reference to components.
13.         playerBody = GetComponent<Rigidbody>();
14.         hand = transform.Find("Hand");
15.     }
16.  
17.     // Always handle physics updates here.
18.     void FixedUpdate()
19.     {
20.         // Get input.
21.         float h = Input.GetAxisRaw("Horizontal");
22.         float v = Input.GetAxisRaw("Vertical");
23.         Vector3 movement = new Vector3(h, 0f, v).normalized;
24.  
25.         // Create a quaternion (rotation) based on the movement vector.
26.         Quaternion moveDir = Quaternion.LookRotation(movement);
27.  
28.         if (movement != Vector3.zero)
29.         {
30.             // Add movement force to the player's rigidbody.
31.             playerBody.AddForce(movement * speed, ForceMode.Acceleration);
32.  
33.             // Set player's rigidbody rotation to the new rotation.
34.             playerBody.MoveRotation(moveDir);
35.         }
36.  
37.         // Drop any pickup when you press the Fire1 button.
38.         if (Input.GetButton("Fire1"))
39.             DropPickup();
40.     }
41.  
42.     // We use a trigger attached to the player to detect pickups.
43.     void OnTriggerEnter(Collider other)
44.     {
45.         // It's a much better idea to check tags instead of names.
46.         // Example: if (other.gameObject.CompareTag("Pickup")),
47.         // however this is easier to setup for the demo.
48.         if (other.gameObject.name.StartsWith("Pickup") && !pickup)
49.         {
50.             // Disable physics on the pickup.
51.             other.attachedRigidbody.isKinematic = true;
52.             // Place pickup in player's hand.
53.             other.transform.position = hand.position;
54.             // Attach pickup to player's hand.
55.             other.transform.parent = hand;
56.             // Save reference to pickup.
57.             pickup = other.attachedRigidbody;
58.         }
59.     }
60.  
61.     void DropPickup()
62.     {
63.         if (!pickup)
64.             return;
65.  
66.         // Disconnect pickup from player's hand.
67.         pickup.transform.parent = null;
68.         // Re-enable physics on the pickup.
69.         pickup.isKinematic = false;
70.         // Add a little throwing force for fun.
71.         pickup.AddForce(playerBody.velocity + hand.forward * speed, ForceMode.Impulse);
72.         // Remove reference to pickup.
73.         pickup = null;
74.     }
75. }

Only issue with this is that the pickup wont collided with walls when it's attached to the player?! @MelvMay is this a bug or am I doing something wrong? Is a rigidbody supposed to stop colliding with other objects when it's parented and kinematic?

 

I'm requesting this! Working with procedural animation you often want to figure out the physics of some kind of simple main body first, and then attach limbs and protrusions to it. Another example is an enemy that has some kind of energy shield hovering in front of it - the energy shield obviously needs a collider, but you don't want the enemy to topple forward because of it.

I suppose a workaround would be to cache or serialize the Rigidbody.centerOfMass, but a checkbox on the colliders would be super convenient.

 

Hello there I have similar problem and it is sooo bothersome, I have Bulldozer(having only rigidbody) and its blade with seperate colliders and blade being child of bulldozer in my game which I want bulldozer to rotate when its blade touches something when rotating itself because the object blade is touching is soo heavy results not be able to be pushed so it recoils the force to blade in the end bulldozer must be go up everything is OK up until here, but when I rotate blade while moving bulldozer no matter what I try, blade changes centerOfMass of bulldozer rigidbody resulting bulldozer rotate upwards so we should really be able to define percentage of how much a child collider should affect center of mass I beg you please implement this feature @MelvMay

 

I have the same problem. The collider influences the mass distribution of the rigidbody. I'm using wheels collider for my vehicle, but when i add decorative objects with collision center of mass and weight distribution are affected.
They change the way they turn and the driving sensation. In the worst case the weight forces the vehicle to lean on two wheels. Is not an option add this feature but i need it.

Has any solution been implemented? or has anyone managed to fix it?

 

You can manually set the inertia tensor and CofG... https://docs.unity3d.com/ScriptReference/Rigidbody-inertiaTensor.html https://docs.unity3d.com/ScriptReference/Rigidbody-centerOfMass.html

You can see what the values are in the inspector under the rigidbody - info. I have not checked whether the rb updates these if they have been set by script, but an additional collider is added at run time. However, that is easy to check and easy to reset.

Sorry if I have misunderstood the problem.

 

Thanks a lot! Inertia Tensor solves the problem completly!
A simple example of how to implement it to the thread of the topic

Code (CSharp):

1. using UnityEngine;
2. public class PlayerPickupDemo : MonoBehaviour
3. {
4.     public Vector3 baseInertiaTensor;
5.     public Quaternion baseInertiaTensorRotation;
6.  
7.     Rigidbody playerBody;
8.     GameObject myItem;
9.  
10.     public void TakeItem(GameObject item)
11.     {
12.      myItem = item;
13.      myItem.transform.SetParent(transform);
14.    
15.     playerBody.inertiaTensor = baseInertiaTensor;
16.     playerBody.inertiaTensorRotation =
17.     baseinertiaTensorRotation;
18.     }
19. }

The values of the inertia tensor can be taken at the inspector.
It should be noted that it does not take into account Wheels colliders until they have been initialized.
As a recommendation, take this value in runtime when all objects have been initialized-