Question Unable to set max velocity for Rigidbody and code examples that claim to work online do nothing.

I am trying to set the max velocity to a tank with the script below. It doesn't matter if I place the velocity clamping code in Update or FixedUpdate, I've tried various code snippets from forums in each, nothing clamps the speed. Here is my code that controls the tank below. The tank is designed to move forward and backward by using forces and rotate left and right by using transform.Rotate. Someone please help me implement this the proper way.

Code (CSharp):

1.  
2. using System.Collections;
3. using System.Collections.Generic;
4. using UnityEngine;
5.  
6. public class PlayerMovement : MonoBehaviour
7. {
8.     public float speed = 85.0f;
9.     public float turnSpeed = 30.0f;
10.     public float maxSpeed = 0.01f;
11.     public float currentVelocityMagnitude;
12.  
13.     private float horizontalInput;
14.     private float verticalInput;
15.     private Rigidbody tankRigidbody;
16.  
17.     // Start is called before the first frame update
18.     void Start()
19.     {
20.         // Lower the player tank's center of mass so that the vehicle won't flip during movement.
21.         GetComponent<Rigidbody>().centerOfMass += new Vector3(0, -1f, 0);
22.  
23.         // Get the tank's rigid body component to avoid unecessarily calling the "GetComponent<Rigidbody>()" function later.
24.         tankRigidbody = GetComponent<Rigidbody>();
25.     }
26.  
27.     // Update is called once per frame
28.     void Update()
29.     {
30.         // Gather input from axes in Input Manager
31.         horizontalInput = Input.GetAxis("Horizontal");
32.         verticalInput = Input.GetAxis("Vertical");
33.  
34.         // Turn the tank left or right based on horizontal input
35.         transform.Rotate(Vector3.up * Time.deltaTime * turnSpeed * horizontalInput);
36.     }
37.  
38.     void FixedUpdate()
39.     {
40.         // Record current velocity for TESTING.
41.         currentVelocityMagnitude = tankRigidbody.velocity.magnitude;
42.  
43.         // Move the tank forward and backward based on vertical input
44.         tankRigidbody.AddRelativeForce(0, 0, speed * verticalInput, ForceMode.VelocityChange);
45.  
46.         // Ensure player tank's max velocity doesn't exceed the defined max velocity.
47.         if (tankRigidbody.velocity.magnitude > maxSpeed)
48.         {
49.             tankRigidbody.velocity = tankRigidbody.velocity.normalized * maxSpeed;
50.         }
51.     }
52. }
53.  

 

You can let the physics engine dictate the max speed by using drag and friction. The laws of physics dictates the maximum speed of things. There is no hidden intelligent force watching over a car to make sure its speed doesn't go over a value in God's database.

But if you really want to dictate the max speed then you can do something like this:

Code (CSharp):

1.     void FixedUpdate()
2.     {
3.         Vector3 moveDir=tankRigidbody.velocity.normalized;
4.         Vector3 pushDir=new Vector3(Input.GetAxisRaw("Horizontal"),0,Input.GetAxisRaw("Vertical")).normalized;
5.         // Push the tank if its moving below max speed or if pushed in the opposite direction of travel (we still want to be able to slow down if moving too quickly (maybe going downhill))
6.         if (tankRigidbody.velocity.magnitude<maxSpeed || (Vector3.Dot(moveDir,pushDir)<0))
7.             tankRigidbody.AddRelativeForce(0, 0, speed * verticalInput, ForceMode.VelocityChange);
8.     }

 

Code (CSharp):

1. Vector3 thrust = Quaternion.Inverse(body.rotation) * -body.localVelocity;
2.  
3. // "m" is how powerful force impulse is
4. // where 4.0 making velocity change sharp and lower values make movement "floaty"
5. float delta = (desiredVelocity + thrust.z) * m;
6.  
7. // set drag to zero or increase this value to overcome it
8. thrust.z = Mathf.Clamp(delta, deceleration, acceleration);
9.  
10. body.AddRelativeForce(thrust, ForceMode.Acceleration);

 

For testing purposes I'm sure I can ignore this part:

Vector3 moveDir=tankRigidbody.velocity.normalized;
Vector3 pushDir=new Vector3(Input.GetAxisRaw("Horizontal"),0,Input.GetAxisRaw("Vertical")).normalized;

and this part:

|| (Vector3.Dot(moveDir,pushDir)<0)

as the tank will never be pushed in the opposite direction of travel since I have no hills or ramps in the scene. With this in mind I tried the following portion of your posted code:

if (tankRigidbody.velocity.magnitude < maxSpeed)
{
// Move the tank forward and backward based on vertical input
tankRigidbody.AddRelativeForce(0, 0, speed * verticalInput, ForceMode.VelocityChange);
}

This still doesn't achieve the desired effect and instead makes the tank rapidly jump to a velocity above the maxSpeed and back to the maxSpeed while increasing the current speed variable in the Inspector window during testing. Did you have this issue when you tested your code?

 

This solution is to complex for me as I am still a beginner at Unity and C#.

 

I mean your add force method is using

ForceMode.VelocityChange

which immediately jumps to a certain velocity, so I'm not sure why you're surprised at your current behaviour.

If you want a gradual chance you want to use the other modes.

 

You could add a threshold. I am no expert on this, but if the speed is below the max speed, the next force could easily put it over that max speed.

 

You may need to lower the speed variable in the inspector if it's making the tank move too quickly. But I suspect the real issue is that you've done a typo and your script is changing the speed variable for some reason. The code I posted doesn't change the speed variable.

 

Even the following code in Update:

Code (CSharp):

1.         // For TESTING
2.         if (speed < 500)
3.         {
4.             speed += 10;
5.         }

and the following code in FixedUpdate:

Code (CSharp):

1. tankRigidbody.AddRelativeForce(0, 0, speed * verticalInput, ForceMode.VelocityChange);
2. tankRigidbody.velocity = Vector3.ClampMagnitude(tankRigidbody.velocity, maxSpeed);

still fail to limit the tank to the maxSpeed value even though each of these code snippets are the last line of code in both Update and FixedUpdate. Can you please test this with the following starting values for speed and maxSpeed?:

Code (CSharp):

1. [SerializeField] float speed = 10.0f;
2. [SerializeField] float maxSpeed = 5.0f;
3.  

I'm trying to understand why the maxSpeed is ignored even though the very last thing done on each FixedUpdate loop is literally supposed to clamp the rigidbody's vector to the maxSpeed value.

 

Strange.. I've not had any problems clamping the velocity of a rigidbody

You may be struggling with a weird quirk of Unity in that if you make an initialized variable visible to the inspector by using public or [SerializeField] then later you decide to change the initialized value in script, Unity won't register the new value until you reset the script in the Inspector. So perhaps you're changing maxSpeed in the script and not in the Inspector?

 

I don't even have to change maxSpeed in the inspector or the script during testing and I still run into this same problem where maxSpeed gets ignored. I only change the current speed during testing and changing it within the inspector during play mode or with code as mentioned above still doesn't do anything different. Did the same above code work for you?

 

Yes.

Try this:

Code (CSharp):

1.         tankRigidbody.AddRelativeForce(0, 0, speed * verticalInput, ForceMode.VelocityChange);
2.         Debug.Log(maxSpeed);
3.         tankRigidbody.velocity = Vector3.ClampMagnitude(tankRigidbody.velocity, maxSpeed);

Now you'll be able to see if maxSpeed is actually set to what you think it should be set to.


.

 

.

After reading MelvMay's messages I did some more tests and he's right. If you add too much force to a rigidbody then clamping its velocity becomes unreliable.

Personally I wouldn't try to restrict a rigidbody's velocity directly but if you really feel its necessary then try being less forceful when moving your objects around. Try using ForceMode.Impulse.

 

Oooh! I didn't even know there was a maxLinearVelocity. Unity 2021 user here!.