Motion-Ratio

Motion-ratio, particularly Spring-motion-ratio in the context of suspensions is the ratio of the spring displacement to that of the wheel displacement. Motion ratio is an important parameter that is used in evaluating the wheel rate of a suspension.

Calculating Motion-Ratio

Motion-ratio calculations are available for ride springs in the models that are assembled using the Assembly Wizard in MotionView's Vehicle Tools extension. These output requests are available for models under the Car/Small truck and Two Wheeler libraries.

To calculate motion ratio, the instantaneous displacement between I and J markers of the spring is divided by the displacement of the wheel part along the Z axis of the vehicle body reference frame.


Figure 1.
For instance, at time = t:
  • Let the spring length be equal to S.
  • And the wheel center position be C (Z coordinate in the vehicle body’s reference frame).
And at time = t + Δt:
  • Let the spring length be equal to S’.
  • And the wheel center position be C’ (Z coordinate the vehicle body’s reference frame).

The motion ratio is calculated as: (S-S’) ÷ (C-C’).

Motion Ratio Output Requests in Library Models

When you build a model using the Assembly Wizard, the motion ratio outputs are included in a separate system with the label prefixed with Request Motion Ratio. This system is included within the spring or strut systems of suspension in the model.


Figure 2.
This system contains the output requests that you can plot after a simulation. The entities that are necessary to measure the motion ratios are included as attachments to this system.


Figure 3.
A Solver variable is created to calculate the motion ratio.
SolverVariable Purpose Function
Motion ratio Measure the motion ratio of the spring entity USER(100, {m_frame_cm.idstring}, {m_wheel.idstring}, {spring.idstring})

Args:

#1: Branch ID. Default = 100.

#2: Reference frame ID for reporting wheel center displacement along Z.

#3: ID of the marker on spindle body located at wheel center.

#4: Spring entity ID. Motion ratio of this entity will be calculated.

FUNCTION: MOTION_RATIO

The value of this variable is used in the output request with label prefixed with Motion ratio - Request Motion Ratio.