Fantastic work! So, I rely pretty heavily on the fact that by the time students reach this far ahead into the course, they kiiinda forget about the principles covered in Lesson 2 regarding capturing texture using implicit drawing techniques and focusing on the use of cast shadow shapes. It allows me to use this challenge as a way to smack their hands a little bit and say "YOU FORGOT - go back and refresh your memory!"

Unfortunately for me, you didn't forget. In fact, you absolutely knocked this challenge out of the park. Both in terms of your overall construction - you've used your ellipses guides to great effect here to help you build solid, tangible structures and you didn't hesitate to create many different ellipses within one construction to capture some of the more nuanced elements.

The only issue I noticed was with this wheel where instead of using your solid black areas to capture the shadows cast by the tire tread chunks, you used it to fill in one of the side planes of said chunk. While it's often a bit tempting to use solid black to distinguish one plane of a form from another, as shown here it's far more effective and communicates far more about the relationship between that form and the surfaces around it, to use filled shapes only as cast shadows.

Aside from that, your work here is fantastic. So, I'll happily go ahead and mark this challenge as complete.