Starting with the structural aspect of the challenge, you've handled this quite well. I'm pleased to see your use of the ellipse guide to help you sort through the cross-sectional slices, and the fact that you've been mindful of considering how much to widen the midsection to create the appropriate degree of "inflation", and help convey that many of these would land with more of a bounce rather than a heavy thunk.

While I am glad to see that you defined both the outward surface of your rims/spokes as well as their side planes in many cases, I do want to recommend that given the limitations of the tools we're working with (specifically the fact that we're working with stark black and white), that you avoid filled in the side planes with solid black. This relates more to the tools we leverage in the textural side of things, so I'll bring things back around to this point shortly.

Moving onto the textural aspect of the challenge, it's intended to be something more of a trap. Being as far removed from Lesson 2 as we are this far into the course, it's very common for students to forget the heavy focus on implicit markmaking and how we leverage cast shadows to achieve that, and so more often than not they end up either falling back to using completely explicit markmaking techniques (so for example, outlining the textural forms in their entirety as we can see in cases like 20 and 21 where each of the big chunks in the tire tread were constructed individually) or trying to be lighter on the detail without necessarily approaching every textural mark as an individual cast shadow shape.

While approaching our wheels with explicit markmaking can come out looking pretty good (for example, number 9), the thing to remember is that we aren't usually going to be drawing singular structures floating in a void. As soon as these wheels end up being used as part of a larger illustration, the approach you've used will create a densely packed area of detail that will draw the viewer's eye to it whether you want it to or not. This can interfere with compositional control (guiding the viewer's eye around your illustration), which while outside of the scope of this course, will still rely on the tools we learn here.

Implicit markmaking has the benefit of not tying the way in which the texture is actually drawn (in terms of whether we're using an area of densely packed detail, an area of sparse detail, or whether we need to transition between these extremes in different parts of the drawing) to what is actually being represented (the specific forms that are being conveyed to the viewer as being present). When working explicitly, if you remove some of the lines to make the detail more sparse and less eye-grabbing, you're going to be removing the forms that go with them. Not so with implicit markmaking, because what we're drawing are cast shadows, not outlines or the textural forms themselves.

As shown in this diagram, depending on how far the form is from the light source, the angle of the light rays will hit the object at shallower angles the farther away they are, resulting in the shadow itself being projected farther. This is what we take advantage of as a tool - even without worrying about the light source itself, we can take from this the freedom to control how densely we want to pack detail into specific areas, by simply opting to make the shadows larger/deeper, or to make them smaller to the point of not being visible. As long as there is enough shadows to imply the presence of the textural forms in some areas, the viewer's brain will fill in the rest without having their experience of the image changed from what the artist intended.

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.

Now, this was an intentional trap so we don't assign revisions based on that. Rather, I want you to take this as a sign to review the textural material from Lesson 2 (starting with but not limited to these reminders), and then consider whether there might be any other concepts that may have slipped through the cracks that you would want to review as well. Once you've done that, you can move onto the next lesson.