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Сhassis mechanisms

Chassis Mechanisms in GEM RTS

In GEM RTS, chassis mechanisms are used to control the movement and physical behavior of vehicle-type units.

Chassis Mechanisms for Ground Vehicles

These mechanisms enable movement and control for wheeled or tracked units:

  • roller

  • roller_joint

  • align

  • wheel

  • track

  • steer

  • trailer

  • steam piston

Chassis Mechanisms for Aircraft

These mechanisms control the aerodynamic elements of an aircraft:

  • flap

  • rudder

  • aileron

  • elevator

Preliminary Notes

All bone names and associated parameters must be checked for compliance with internal standards. This ensures the correct operation of the mechanism in the game engine.

In GEM RTS, different chassis mechanisms are used depending on the suspension type. Tracked vehicles use either the Roller-Lever (roller + roller_joint) or Roller-Rocker (roller + align) mechanism.

Roller-Lever

Functionality During unit movement, each roller is placed on the ground by rotating the IK bone so that the roller (wheel) maintains contact with the ground surface.

Bone naming rules All roller bones must start with the prefix roller.

Bone Name
Description
Internal Parameters

roller<N>_joint

Bone containing the suspension mesh. Works under Inverse Kinematics (IK) rules.

poly, IKType=revolute, IKMin=<value>, IKMax=<value>

roller<N>

Support bone for the roller/wheel.

radius=<value>

The index N of a roller bone must match the index N of the roller_joint bone to which the roller is linked. Example:

  • rollerL1 must be linked to rollerL1_joint.

  • rollerR1 must be linked to rollerR1_joint.

Bone features

  • The roller<N>_joint bone is positioned at the pivot axis of the lever, to which the roller<N> bone is attached. The Z-axis of roller<N>_joint must align with the lever’s rotation axis.

  • Roller radius: The radius parameter is defined in meters.

The roller’s radius should be slightly larger than the visible roller radius in the model to account for the track thickness. Otherwise, visually, the roller will sink into the track.

Roller-Rocker

Functionality This mechanism functions like a seesaw. The chassis tries to align two bones with defined radii to the ground by rotating the main bone under Inverse Kinematics (IK) rules.

Bone naming rules All alignment bones must start with the prefix align.

Bone Name
Description
Internal Parameters

align

Bone containing the suspension mesh. Works under IK rules.

poly, IKType=revolute, IKSpeed=<value>, IKMin=<value>, IKMax=<value>

roller

Support bone for the roller/wheel.

radius=<value>

Bone features

  • Two child bones act as support points, which must ultimately align with the ground.

Wheel

Functionality Rotates bones based on the wheels’ movement relative to the ground.

Bone naming rules All wheel bones must start with the prefix wheel.

Internal Parameters

  • The radius parameter defines the wheel radius in meters.

If all wheels are identical, the default wheel radius can be set in the basis bone using the wheelradius parameter.

Track

Functionality Track movement is simulated by shifting the V texture coordinate of the track according to the unit's movement.

Internal Parameters Track settings are defined in the .def model file:

  • bone – name of the track bone.

  • length – length of the track in meters.

Steer

Functionality Steering is controlled by setting the rotation angle on the IK bone.

Bone Naming Rules All steering bones must start with the prefix steer.

Internal Parameters Inverse Kinematics parameters:

IKType=revolute, 
IKSpeed=<value>, 
IKMin=<value>, 
IKMax=<value>

Trailer

Functionality The trailer mechanism works similarly to artillery pieces attached to a tractor. It rotates the pivot bone to ensure the support bone remains stationary, ensuring stability and correct movement behavior.

Bone Naming Rules

Bone Name

Description

Internal Parameters

trailer

Support bone. In the bone hierarchy, it is linked to the pivot bone.

poly

<trailername>_joint

Pivot bone defining the rotation point of the hitch. Positioned at the hitch’s rotation axis in the bone hierarchy.

poly, IKType=revolute, IKMin=<value>, IKMax=<value>

Bone features

  • Pivot Bone Positioning The <trailername>_joint bone must be placed at the hitch’s rotation axis, where the trailer connects in the bone hierarchy.

  • Pivot Bone Orientation:

    • The Z-axis must align with the hitch’s rotation axis.

    • The X-axis must be directed towards the support bone to prevent lateral displacement in the rotation plane. Other minor displacements are allowed.

Steam Piston

Functionality Designed specifically for steam locomotives, aligning the piston’s movement with the wheels’ rotation.

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