Rigging Model T Steering System

[smb]

I'd like to rig the front end steering system of a Model T so the arms of the two spindles (the steering axes of the wheels) and steering yoke (which connects the spindles) all move and stay connected as they should.

Attached images show the system from above. The two spindle arms, yoke, and axle form a quadrilateral.

For example, when the right wheel rotates about its spindle's Y axis, the spindle arm should pull the yoke left or right. The yoke in turn should pull the arm of the left spindle to rotate that wheel.

Joints A and B must remain fixed (relative to the car) and the ends of the yoke (joints C and D) must move in the Z direction as well as X. As it moves, the yoke does not remain parallel to the axle.

I'm thinking that there must be a way to do this with constraints, but I can't figure out how they should be set up. If there's a way of rigging it with joints I'd consider that, too.

Any suggestions would be much appreciated, thanks.

Steve

[osidedan · Accepted solution]

With a combination of joints and constraints, you can indeed make this work. I made a mock-up of your scene, and used the following steps to hook it up. (I'm also attached a video of it in action, and my scene file for reference. )

First off, since we only need the movement in X and Z, switch to top down view to make things easier.

Secondly, once you're done with the steps, the locator named "axle_locator_right" is what will control the motion of the entire rig. Rotate it in the Y axis to move everything else.

Creating the rig:
1. Create 4 locators above where you want your rotation points to be.

2. Create a 3 joint chain with the joints snapped to the locators shown. (hold v to snap quickly to locators). You might want to hide the geo for this part to make snapping to the locators easier.

3. delete yoke_locator_right, it was just there to place JointC correctly.
4. create an ik handle from JointA to jointC.
5. select axle_locator_right, then select the ik handle you just created, and do a parent constraint with default values.
6. select jointB, then select axle_locator_left, and do a an aim constraint for all axes with maintain offset checked.
7. select jointB, then select yoke_locator_left (make sure its the yoke locator and NOT the axle locator this time) and do a point constraint for all axes with maintain offset checked.
8. select jointC, then select yoke_locator_left, and do an aim constraint for all axes with maintain offset checked.

Controlling the geo with the rig:
1. select axle_locator_left, select your left spindle arm geo, and do a parent constraint with default values.
2. select yoke_locator_left, select your yoke geo, and do a parent constraint with default values.
3. select axle_locator_right, select your right spindle arm geo, and do a parent constraint with default values.

for the wheels, it's up to you how to rotate them from the spindle arms, but in my example I just made mine children of the spindle geo.

That should be all you need! Let me know if you need any more clarification on any of that, and don't forget that the rotation of "axle_locator_right" controls the entire setup. 

[damaggio]

Very nice Dan, thanks.

[smb]

This works perfectly. Very clear presentation, and I learned a few more things about constraints. Thank you.