Jumping right in with the structural aspect of the challenge, you're doing a pretty great job. I'm glad to see that you appear to have picked up a master ellipse template - despite its limitations in terms of the sizes of ellipses it'll allow one to draw, it's still worth it to help avoid distracting us from the focus of this challenge.

I'm noticing that you built up your wheels with successive ellipses of different sizes, helping to create an arcing profile (the widening through the middle), which in turn conveys the sense that the tire is more inflated, rather than just a solid, unyielding mass, and that it would land with a bounce rather than a heavy thunk. I'm also pleased to see that you are also considering not only the outward faces of your rims' spokes, but also their side planes to help make that structure appear more three dimensional and solid. As a whole, very well done.

Carrying over onto the textural aspect of the challenge, this is an area that serves as something of a trap for our students. Being as far removed from Lesson 2 as we are, it's very common for students to forget a lot of the concepts, like what exactly implicit markmaking is, and that it focuses on cast shadows rather than form shading or anything else. Generally those students will do their best with what they recall, but not necessarily go back and review that material, and so it allows us to give them a bit of a rude reminder that yeah, maybe you may want to consider what might have fallen through the cracks, and actually review it more meaningfully, before moving onto the final lesson.

While you did fall into this trap to some degree, it's not as much as most, who will usually just fall straight back to drawing all of their textural forms using explicit markmaking, by outlining each and every one. While you did that in some cases, it's very clear in your work that you understood this approach not necessarily to be ideal, and so you tried a lot of different things. While we see that kind of outlining in your earlier wheels, you do play with other approaches, often shifting more into focusing on making certain planes of those forms solid black.

You're heading in the right direction there, shifting from working with line to working with filled areas of solid black, but looking at cases like 14, it appears that you're tending towards applying form shading (where we make the surface of a given form darker or lighter based on whether it's facing towards or away from the light source), rather than cast shadows. Where a form shadow helps to allude to the relationship between a singular form, or rather a single surface, and the light source, cast shadows establish the relationship between the form casting the shadow, and the surface receiving it. This is very relevant to what we're doing in this course, which focuses as a whole on developing our understanding of spatial relationships - this is just a spatial relationship on a smaller scale, and conveyed through a different kind of tool.

Along with cast shadows being relevant to the larger focus of this course, implicit markmaking is also quite handy when it comes to conveying detail in a way that allows you to control just how densely packed that detail will be. After all, the wheels you've drawn here look great in isolation, floating in the void, but when we place something like the bottom-right wheel on this page as part of a larger vehicle illustration, it's going to create a focal point, drawing the viewer's eye to it whether you want it to or not. That's not a concern for what we're doing in this course, but from a more general illustrative and compositional standpoint, that can be a problem.

Implicit markmaking leaves that control in our hands. 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 means that if what we're drawing are just the cast shadows, we have enough grounds to decide whether we want those shadows to be large and expansive, such that they all merge together into larger, more complex shapes, whether we want those shadows to be so small as to not be visible (in which case once again we end up with a single complex shape of white, with similarly low contrast), or whether we want to be somewhere in the middle with each textural form generating enough of a distinct shadow to create a focal point and draw the viewer's attention with the density of the detail.

Looking again at your work - especially the later wheels - I'm seeing a shift in your approach that suggests that you are in fact moving from filling in those side planes (which again is more akin to form shading), to actually thinking about cast shadows instead - although whether or not that is actually the case is hard to say for sure. When it comes to things like this, the differences can be quite subtle.

To that point, 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, as a whole you're still doing fine - just be sure to review those concepts from Lesson 2, and consider if there's anything else that you might need to review before continuing onto the last lesson. I'll go ahead and mark this challenge as complete.