Car-Wheel Constraint
 
 
 
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Create panel (Helpers) reactor Carwheel

Animation menu reactor Create Object Car-Wheel Constraint

reactor toolbar (Create Car-Wheel Constraint)

You can use this constraint to attach a wheel to another object; for instance, a car chassis. You can also constrain a wheel to a position in world space. During the simulation, the wheel object is free to rotate around a spin axis defined in each object's space. Linear motion is allowed for the wheel along a suspension axis. You can also add limits to the wheel's movement along this axis. The constraint's child body always acts as the wheel, while the parent acts as the chassis.

The Car-Wheel constraint also has spin parameters. If these are nonzero, the constraint turns the wheel during the simulation. You can specify a target velocity and a target gain: the maximum angular impulse that the motor can apply to the rigid body in order to achieve the target velocity.

A Car-Wheel constraint is simulated if it has the correct number of rigid bodies attached and is included in a valid Constraint Solver. When not selected, an invalid Car-Wheel constraint is red in the viewport.

Procedures

To create a Car-Wheel constraint:

To attach objects to the Car-Wheel constraint:

  1. Create the constraint and the objects to connect using it.
  2. On the constraint’s Properties rollout, click the Child pick button, and then, in one of the viewports, select the object you want to use as the wheel.

    This attaches the constraint to the wheel at the wheel's pivot point. The spin axis is aligned to the X axis of the wheel object and the suspension axis is aligned to the Z axis.

  3. If you do not want to assign a chassis to the Car-Wheel constraint, turn off the Parent check box if necessary.
  4. If the Car-Wheel constraint is to be two-bodied, repeat step 2, using the Parent pick button to specify the chassis object for the constraint.

To create the constraint and attach objects in a single step:

To change the positions and rotations for the bodies:

  1. With the constraint selected, open the Modify panel.
  2. In the modifier stack, open the constraint’s sub-object list.
  3. Access the sub-object level corresponding to the object whose constraint space you want to move: Parent Space for the chassis or Child Space for the wheel. If the constraint is single-bodied, Parent Space represents the world attachment position and rotation of the constraint axes.
  4. The space, represented by just a spin axis for the wheel and by the spin and suspension axes for the chassis, becomes active in the viewport and can be moved or rotated using the Move or Rotate tool, respectively. The space maintains its position and rotation relative to its corresponding object. For example, if you move or rotate the wheel, Child Space moves with it.

  5. To reset the attachment point to the child's pivot, reattach the objects to the car-wheel constraint or choose the Align Spaces To Child Body option.
  6. You can also rotate the suspension axis independent of the spin axis. To do this, highlight the Suspension Rotation sub-object level; this lets you rotate the suspension axis with respect to the chassis’s spin axis.

To change the suspension limits:

  1. With the car-wheel constraint selected, open the Properties rollout.

    The default limit settings in the Suspension Parameters group are Min Limit=0.0 and Max Limit=0.0 . This means that in a situation where the constraint spaces are fully aligned for the objects, the wheel cannot move along the suspension axis.

    The limits define the allowed motion of the wheel along the suspension axis with respect to the chassis, and are displayed as a line with respect to the chassis. This means that from a stable position where the sub-objects are aligned, if the limits are –5 and 10, the wheel will be able to move 10 units away from chassis along the suspension axis in the direction of the icon’s arrow and 5 units in the opposite direction.

  2. Change the limits and observe the difference in the simulation.
  3. You can also change Friction for the Car-Wheel constraint; this inhibits the wheel’s motion along the suspension axis.

To spin the wheel:

These controls are found in the Spin Parameters group.

  1. Set the Velocity value to the required value. This is specified in radians per second.
  2. Set the Gain. This is the maximum angular impulse the constraint can apply to maintain this angular velocity.

Interface

Modifier Stack

Parent Space

At this sub-object level, you can move and rotate the representations of spin and suspension axes for the chassis. This describes where on the chassis the wheel is attached and also the directions of the spin and suspension axes for the chassis. This is defined in chassis space, which means that if you alter the position or rotation of the chassis the axes will move as well and maintain their offset rotation and translation with respect to the chassis. When you assign a chassis to the car-wheel constraint, Chassis Space’s translation is set to the wheel’s pivot point, the spin axis is aligned to the wheel’s X axis, and the suspension axis is aligned to its Z axis. If the car-wheel constraint is single-bodied, then Chassis Space represents the world orientation and position of the constraint.

Child Space

At this sub-object level, you can move and rotate the representation of the constraint for the wheel. This describes both the location on the wheel that the constraint considers as its attachment point and the rotation of the spin axis for the wheel. This is defined in wheel space, which means that if you alter the position or rotation of the wheel the space moves as well and maintains its offset rotation and translation with respect to the wheel. When you assign a wheel body to the Car-Wheel constraint, Wheel Space’s translation is set to the wheel’s pivot point and the wheel’s spin axis is aligned to its X axis.

Suspension Rotation

This level allows independent rotation of the suspension axis with respect to the Chassis Space. This means that your suspension axis doesn’t have to be perpendicular to your spin axis, allowing for a greater range of constraint setups.

Properties rollout

Parent

When on, the constraint has two bodies and you can designate an object to be the optional chassis rigid body attached to the Car-Wheel constraint. When off, the constraint is single-bodied and the point occupied by the Parent Space sub-object is the parent.

You assign the chassis by clicking this button and then selecting a rigid body from any viewport. Thereafter the button displays the name of the chassis object.

Child

Displays the name of the second rigid body attached to the constraint. You assign the child object by clicking this button and then selecting a rigid body from any viewport.

Align Spaces To

Use these options to align the bodies' local constraint spaces. You can find out more about each option in Working With Constraint Spaces.

Lock Relative Transform

When on, the relative transform between the child and parent constraint spaces is locked: If you move either space in the viewport, the other space moves and rotates along with it, and vice-versa.

Strength group

Strength/Tau

Govern the impulses applied to the constraint's bodies in order to maintain the constraint, and so how strongly the constraint works to restrict their movement. You can find out more about these parameters in Strength and Tau.

Suspension Parameters group

Min Limit

The distance the constraint allows the wheel to move along the suspension axis relative to the chassis, in the direction opposite that indicated by the icon’s suspension arrow. For example, if Min Limit=–5, then the wheel cannot move more than five units backwards along the suspension axis with respect to the constraint’s spaces.

Max Limit

The distance the constraint allows the wheel to move along the suspension axis relative to the chassis, in the direction indicated by the icon’s suspension arrow. For example, if Max Limit=10 then with respect to the constraint’s spaces the wheel cannot move more than 10 units forward along the suspension axis.

Friction

The level of friction applied to the objects as they try to move relative to each other along the suspension axis. Available with Havok 1 only.

Strength

The strength value of the suspension spring. Higher values cause less vertical movement. Available with Havok 3 only.

Damping

The damping value of the suspension spring. Higher values suppress vertical oscillation. Available with Havok 3 only.

Spin Parameters group

Velocity

This value indicates the angular velocity in radians per second that the constraint will seek to achieve for the wheel.

Gain

This value indicates the maximum angular impulse the constraint can apply to attain the specified angular velocity.

Breakable group

Breakable

When on, the constraint is breakable. If its breakable limits are exceeded during simulation, it ceases to exert impulses on the attached objects. For more information, see Breakable Constraints.

Display group

Size

Lets you change the size of the constraint space display in the viewport.

Reset Default Values

Returns the constraint's parameters to their default values.