Sometimes Collision.impulse returns 0 during a hard collision, what could cause this?

I'm really stuck here.

I have a sphere that will sometimes collide with a mesh collider wall. When that happens, I need to determine if I should play a bounce sound (i.e. did the sphere hit the wall hard enough to bounce). To do this, I check the collision impulse during OnCollisionEnter().

Normally everything works fine. However, when the speed of the sphere is high enough, the impulse will return 0 on all three axises. Meanwhile, the game itself behaves normally, and the sphere bounces as expected -- clearly more than zero impulse is at work here.... but that's not what's being reported by Unity. This makes it impossible for me to determine if I should play a bounce sound.

What would be causing impulse of a collision to be 0 on a hard collision?

I'm using Unity 5.3.4

UPDATE: I wrote some pretty ugly code to get around the issue. It turns out that whenever the absolute velocity of my sphere exceeded about 17.2 in any direction, collisions above that speed would return with a 0 impulse. Soooo I made a small array to hold the current and previous frame and during a collision I check the previous frame for sphere velocity and performed collision operations with that.

Not sure if it's a Unity bug, a PhysX bug, or a DivideByNil quirk

 

I faced a similar problem.

Impulse turns to zero on x86 build, whilst impulse on a x86_64 build is ok.

My experiments show a dependance between size of colliders, speed of a bullet and zeroed impulse on x86 build.

 

I have similar thing with 2D physics in 2018.2.1f1. Sometimes normalImpulse just returns 0, randomly.

 

This seems to still be an issue in 2018.3.5f1. The

OnCollisionEnter( Collision collision )

is being called correctly but

collision.impulse

is sometimes zero (including

collision.impulse.magnitude

, a flat zero.) Checking the number of collision contacts this happens with any number of contacts it would seem, including 1 - 6.

 

The same bug in Unity 2019.1.6f1.

 

I'm using 2019.3.f4 and I encountered the same issue!
So it has never been fixed since 2016

 

I am using 2019.3.7f1 and stumbled over the same issue.
It turns out that using "Continuous Dynamic" as Collision Detection Mode on the Rigidbody fixed this issue for me.

Also see the API Reference for Rigidbody.collisionDetectionMode: https://docs.unity3d.com/ScriptReference/Rigidbody-collisionDetectionMode.html

 

Also encountering this issue in 2019.3.15f1. The impulse is usually zero, but sometimes not during OnCollisionEnter. Changing the physics settings and collision modes haven't helped. I did discover that OnCollisionStay seems to always have a valid value for the impulse, which is working for my needs even though I was afraid it would miss some collisions- if it is, it isn't noticeable.

 

Running into this problem in 2019.4.10f1, x86_64 build, using discrete collisions. Played with all the various physics settings available, didn't find a solution. All the other data in the Collision seem to be in order, but impulse for some reason is just zero. This is pretty critical to our game, so I'll defs keep investigating.

EDIT: A solution that worked for our situation was to increase the Default Contact Offset in the physics settings from 0.01 to 0.1. This ensured that smaller collisions correctly passed their impulse values.