A quick tutorial about rigging shock absorber in Modo 401:

1. Divide geometry into mesh items: Spring, Casing, Rod
2. You'll need a morph map to animate Spring.
3. Align pivots to proper places using Align to Workplane button.
4. Select Rod + Casing: Apply Directional Constraint.
5. Select Casing + Rod: Apply Directional Constraint.
6. Make sure Axis and Up Vector are OK.
7. Parent Spring to Casing.
8. Select Rod + Casing: Apply Measure Distance
9. Open Channel Links: Load Driver - Rod, Load Driven - Measure Distance
10. Direct Link World Posiotion to Position From
11. In Channel Links: Load Driver - Casing
12. Direct Link World Posiotion to Position To (it can move exactly 8mm)
13. Add Morph Deformer to Spring
14. In Channel Links: Load Driver - Measure Distance, Load Driven - Morph Deformer
15. Create Relationship between Distance Output and Opacity
16. Use Graph Editor to create linear relationship between Distance and Spring compression (I used exactly the same distance *minus* 8mm)