I am very pleased to see that after wheel 6, you ended up leaning into your ellipse guides - while they can be somewhat restrictive, they definitely help us focus on the core of this exercise, without additional distractions.

Starting with the structural aspect of the challenge - that is, constructing the core cylinder of the wheel, as well as how you approach building out structures like the spokes/rims in the center, you have largely done quite well. You've included a subtle bulge through the middle to help convey the impression that the tires themselves are inflated, and would land with more of a bounce rather than a solid and unyielding thunk, and you have been mindful of not just drawing the outer face of your rims, but rather also included the side planes to establish them as solid structures.

One thing I would note however is that when you're working with the kinds of very thin spokes found in bike wheels - or really any kind of a very thin structure - you're still generally going to want to draw it as a shape with two edges, rather than just the single line. A single line tells us of infinitesimally negligible widths - like the thickness of a sheet of printer paper - and in most cases that is not what we're after. So, if you have to choose between too thick (drawing two edges and enclosing a tiny bit of space between them) or too thin, always err towards the side that has you enclosing space - so go for the thicker one instead. So, instead of what you did through much of this page, the manner in which you approached the wagon wheels here was definitely better (though I understand they were different circumstances).

Continuing onto the other section of the challenge, this is where we get into something of a trap. See, being as far removed as we are from Lesson 2, it's very common for students to pretty much forget a lot of the specifics relating to how we tackle texture throughout this course. Tires are, of course, very much textures, a series of rubber chunks arranged along the surface of a much simpler cylinder.

I can see that you have tried to work out how to push the use of filled areas of solid black (rather than relying entirely on the constructional outlining we're more familiar with), but you do still lean into aspects of explicit markmaking, rather than the implicit markmaking we're looking for. In effect, the difference comes down to whether you're drawing the textural form directly (outlining it, filled in its side planes, etc.) versus drawing the impact (the shadow) it has on its surrounding surfaces.

So for example, on number 15, you ended up filling in the side planes of the big tread chunks (which is more akin to form shading, where the whole surface gets lighter/darker based on its orientation), whereas cast shadows would be new shapes we design based on the relationship between each textural form and the surfaces around it. The fact that filling in those side planes doesn't involve designing a new shape is a key to look out for, as it likely means that you're leaning into form shading instead. This is also why filling in the middle of wheels like 14, or filling in the inner rim of 8 wasn't entirely correct either (at least within the restrictions of this course).

The other situation that can be a lot trickier is where we deal with tires that have much shallower grooves. In such situations - similarly to dealing with holes in a texture - it's easy for a student to look at the groove itself as being the textural form. It's not, of course - it's not a form at all, but rather is the absence of form in an empty space. Rather, it's the walls that surround it which are the forms, and so they're what we need to be thinking about, in terms of how they cast shadows upon one another.

This diagram gets into this point further. What makes grooves so tricky is because often times drawing them simply as lines representing the grooves themselves can come off as good enough - but the manner in which we think about it makes for a subtler difference that can't always be picked up on from case to case. Still, because of how it relates to the larger textural considerations, pushing yourself to always think about "what is the textural form i'm dealing with next" and "how does it relate to the surfaces around it" is going to help for those situations that aren't quite as forgiving.

And of course, this is all specific to what we're doing in this course, and comes back to that core goal of developing one's spatial reasoning skills.

Anyway! While you did stumble into that trap somewhat, it's entirely normal, and not something I hold students back over. So, I'll go ahead and mark this challenge as complete.