Starting with the structural aspect of the challenge, you've done a great job. You're making solid use of your ellipse guides, and I'm pleased to see that those wheels that are more bouncy/inflated have a widening through the middle section to convey that they'd land with a bounce rather than a heavy thud, while those that are indeed more solid reflect that in a much straighter, denser profile.

You have also done a good job of considering not only the outward faces of your structures (I'm mainly looking at the spokes/rims of your wheels for this), but also their side planes to ensure they look solid and three dimensional.

Carrying over into the textural aspect of the challenge, this is something of a trap that we set for students to remind them that they've likely forgotten or neglected certain sections of the course (specifically the texture notes from Lesson 2), so they can review them before finishing up this course. Almost everyone falls into that trap to some degree, but you did not. Throughout your work here you've demonstrated a pretty strong understanding of how to leverage implicit markmaking - that is, using cast shadows to imply the presence of our textural forms - and while there are a couple places I have extra thoughts on, as a whole you've done a fantastic job.

The first one I wanted to note isn't entirely fair, given that it's literally the first wheel and this issue doesn't seem to persist at all after that point, but given that there isn't too much to offer in terms of critique here, I've gotta take what I can.

The main issue here is that the only face that is visible is very obviously the outward/top face of those textural forms - meaning, the side plane has been filled in with solid black. Generally we want to avoid this and leave that side plane blank as well (as you did it in other cases), so that the only section that is actually black is the cast shadow. This of course only applies to how we approach it in this course, because we don't want to mix up cast shadows with form shading, and instead want to focus only on cast shadows which actually have a much stronger role in representing the 3D relationships between the form casting the shadow and the surface receiving it.

The other point I wanted to note real quick is more of a matter of advice, rather than a specific issue I saw signs of in your work. When it comes to those tires with shallow grooves, or really any texture consisting of holes, cracks, etc. it's very common for us to view these named things (the grooves, the cracks, etc.) as being the textural forms in question - but of course they're not forms at all. They're empty, negative space, and it's the structures that surround these empty spaces that are the actual forms for us to consider when designing the shadows they'll cast. This is demonstrated in this diagram. This doesn't always actually result in a different result at the end of the day, but as these are all exercises, how we think about them and how we come to that result is just as important - if not moreso.

Anyway, you've done a great job here so I'll go ahead and mark the challenge as complete.