Summary

This is my steering wheel knob (or brodie knob) project for car (or boat)

I needed this to increase my capacity to maneuver my city car during the parking, as I had a small accident on my left hand. It worked very well, despite its simplicity.
I decided, in fact, to avoid bearings (not really necessary due to the limited number of turns it makes)

Here the list of the metallic parts: (all sizes in millimeters)

1 bolts M8 (80 x 8) (hexagonal head)
3 nuts M8 (6 mm thickness)
4 washers M8 (ext. diam. 15 mm.)

2 bolts M3 (20 x 3) (round head)
2 nuts M3

I added several versions, choose the model which correspond better to the diameter of your steering wheel.

Print Settings

Printer: Ordbot Hadron

Supports: No

Notes:

I made this in ABS, just to avoid possible problems with the sun heat (possible with PLA). Printed with a 0.5 mm nozzle and 0.4mm layer thickness, without supports. The filling was set at 25% for the jaws and 15% for the knob.

Custom Section

Important:

I had a comment concerning the safety and the legality of the knob.
Please note:
About the legality of the object please refer to your local legislation, I cannot be considered responsible if in your country its use is not allowed. (About USA, searching through each state's statutes, there were no laws in the 50 states banning the use of steering wheel knobs on regular cars. Nor in France, where I live. (internet sources))
About the security, the correct use of the safety belts avoids normally any contact with the steering wheel, and the knob position doesn’t prevent the regular operation of the Airbag.
More, as you can see in the project, the axis of the knob is not on the steering wheel but laterally, this mean that any bump on the top of the knob, if mounted in a non-excessively rigid manner, would make it rotate, further reducing the eventuality of a serious blow.
In any case is up to you to evaluate the risks. Drive carefully.
Regards