Whew, you certainly put a lot into these wheels! I'm very pleased with all the experimentation you did here. It's clear that the constructions performed with an ellipse guide (the smaller ones) came out great - they felt solid and the ellipses were arranged well to achieve a strong illusion of solidity, and to that point I won't spend time on pointing out the fact that your larger, freehanded ellipses aren't as good - as they're still coming along nicely however, all things considered. I'm also pleased to see the attention placed on the rims themselves, building them out as solid forms. There's a lot of room there for oversimplification, but you continued to focus on how everything relates to one another in 3D space.

One of the biggest challenges, as you certainly noticed, comes down to capturing the texture of these tire treads. The shallower ones with less grip are considerably easier, and you handled those well, but where most students will just (incorrectly) construct the big chunky treads and move on with their lives, I can see that you tackled a number of different strategies for capturing these larger textural forms while avoiding the urge to just make them visually noisy and distracting, and in this you had various degrees of success.

12 was definitely one of those where you just eschewed textural, implicit drawing techniques, and just constructed the tread completely, and you were clearly aware of the fact that it was not an ideal result. A similar approach was employed with #15, with each bit of tread being outlined entirely, with full interior edges.

9 and #18 however were quite different. I can see you there trying to leave more open gaps, allowing the viewer's eye to push through the texture without as many barriers, and putting much more focus on cast shadows. This is absolutely the correct direction, and I'm pleased to see this. It isn't perfect, however, but only in that you didn't push it far enough in that direction, as you still clung to those internal edges to make them feel 3D.

Usually the diagram I provide in this case is this one, but unfortunately in your case, the left side doesn't apply so much, because you didn't fall into the trap of just filling the side planes with solid black. The right side is still relevant, however. Notice how we can still make the form feel like a three dimensional box, even without a single line existing within the silhouette? As long as the silhouette itself is defined (which we can achieve through other cast shadows that might exist around the form, cast by other similar pieces of tread), we can still show that the form is 3D even without having to put a single bit of information along its inside. And even if you don't have deep enough shadows to define the silhouette all the way around each individual piece of tread, that's totally fine. As long as you have some that are more fully formed in this way, the viewer's eye will fill in the rest. That's the trick with texture - you by no means have to draw each textural form. You just have to give the viewer enough information, and they'll fill in everything else.

So! Hopefully that helps make sense of how to tackle situations like this. Other than that, I'm very pleased with your results, and am happy to mark this challenge as complete.