Physics constraint LinearZ problem

Hello eveyrone,

Attached is a sample project which demonstrates the problem. The scene 2 StaticMeshActor actors: a cube and a sphere. The cube has an additional physics constraint component so that the sphere is constrained.

Input setup:
Virtual joystick X-axis: move the cube along the world Y-axis (AMyPawn::MoveX)
Virtual joystick Y-axis: change the linear Z limit of the physics constraint (distance between cube and sphere) (AMyPawn::MoveY)

The problem:
If the cube is moved first when the level starts, everything works properly. However, if only the joystick Y-axis is moved when the level starts, the sphere does move at all. Then if the cube is moved, the sphere moves suddenly according to the z-limit set previously. Can anyone tell if there is anything wrong with my setup/code? Thank you.


It’s been a while and I still have no idea what is causing the problem. For anyone who wishes to see the related code without opening the project, I am posting the content of of MyPawn.cpp here.

	PrimaryActorTick.bCanEverTick = true;

void AMyPawn::BeginPlay()
	for (TActorIterator<AStaticMeshActor> i = TActorIterator<AStaticMeshActor>(GetWorld()); i; ++i)
	{ // find cube actor with physics constraint component
		UActorComponent *c = i->GetComponentByClass(UPhysicsConstraintComponent::StaticClass());
		if (!c) continue;
		cube = *i;
		pcc = Cast<UPhysicsConstraintComponent>(c);
	if (pcc) pcc->SetLinearZLimit(LCM_Limited, zLimit = 125); // set initial z-limit

void AMyPawn::Tick(float DeltaTime)
	if (pcc) DrawDebugLine(GetWorld(), pcc->GetComponentLocation(), pcc->ConstraintActor2->GetActorLocation(), FColor::Red, false); // draw red line

void AMyPawn::MoveX(float v)
//	GEngine->AddOnScreenDebugMessage(-1, 5.f, FColor::Red, FString::Printf(TEXT("%f"), v));
	if (cube && v) cube->SetActorLocation(cube->GetActorLocation() + FVector(0, v, 0)); // move the cube actor along y-axis

void AMyPawn::MoveY(float v)
//	GEngine->AddOnScreenDebugMessage(-1, 5.f, FColor::Green, FString::Printf(TEXT("%f"), v));
	if (pcc && v) pcc->SetLinearZLimit(LCM_Limited, zLimit = FMath::Clamp(zLimit - v, 0.f, 250.f)); // change z-limit: distance between the cube and the sphere

void AMyPawn::SetupPlayerInputComponent(UInputComponent* PlayerInputComponent)
	PlayerInputComponent->BindAxis("X", this, &AMyPawn::MoveX);
	PlayerInputComponent->BindAxis("Y", this, &AMyPawn::MoveY);

Converted the project to 4.17 and the problem still exists. Please, can anyone help? Anyone one from Epic reading this?

Maybe it helps to use ‘wake rigid body’? Physics simulating bodies can fall in sleep mode even on a constraint :wink:

BOBtheRoss, that is exactly what I need! Thanks a lot! :cool: