Starting with the structural aspect of your wheels, overall you've done a great job. You're mindful of the importance of having the profile of your wheel/tire widen through its midsection to give it an appearance of being inflated (as though it'll land with a bounce rather than a heavy thud), and I'm pleased to see that you're conscious of the different planes of your rims/spokes, establishing both the outward facing planes as well as the side ones to establish a fairly solid structure. One area where I do feel you have room to get a little more comfortable is in executing linework in cases where it's in a cramped space - for example, if we look at the rims on number 7, we'll see that the lines don't maintain as consistent a flow or trajectory as they could, and tend to be more broken up. This can undermine the solidity a little, so be sure to keep at it. Usually it's just a matter of taking a little more time with the planning and preparation of each individual mark, and it will improve as long as you're conscious of it.

Moving onto the textural aspect of the challenge, this is an area where the challenge serves as a little bit of a trap for those who've forgotten about the principles of textural and implicit markmaking from Lesson 2, as it's very common for students to fall back to exclusively explicit markmaking, where they're outlining all of their textural forms and drawing every little detail, whereas what we really want to be doing is thinking about how those forms sit in space and how they relate to the forms around them, and ultimately using that understanding to help us decide how to design the shapes of the shadows the forms cast upon their surroundings.

While earlier on you do end up leaning more towards explicit markmaking (there are more cases where we're dealing with very shallow grooves, which are much more forgiving in this respect and also make it easier to slip back into explicit markmaking - we'll talk a bit more about this in a moment - but there are some cases with chunkier tread patterns like number 2 where you've focused on drawing each form in its entirety, rather than focusing on the shadows they'd cast), as you progress through the set you clearly show a shift towards being more cognizant of the concepts from Lesson 2.

One thing I am seeing as we move through the set however is that while you do focus more on cast shadows, they do often blend together with the side plane of those textural forms. This is definitely at least in part because it's very difficult at the scale we're working to distinguish between those two elements, but it's more the intent rather than the result that I want to highlight. It's pretty common for students to try to fill in the side planes of their textural forms, when attempting to apply implicit markmaking, but this isn't entirely correct. Because it's filling in an existing shape, it doesn't allow us to actually define the spatial relationship between the form casting the shadow and the surface receiving it, which is achieved entirely through how the shadow shape itself is designed. Since we're filling in an existing shape (which in this case is more akin to applying form shading to that, which as discussed here is something we avoid in this course, the shape never has an opportunity to take that spatial relationship into consideration.

Instead, always ensure that you're thinking about how the shadows you're casting are actually shaped, and how they're designed, and when making those decisions be sure to always consider the relationship in 3D space between the form and the surface upon which the shadow is being cast.

To this point, I want to share with you a diagram I drew on top of another student's work, along with the explanation I provided, as may help to flesh this concept out further:

Here's the diagram: https://i.imgur.com/SEwsEfO.png

In the top, we've got the structural outlines for the given form - of course, since we want to work implicitly, we cannot use outlines. In the second row, we've got two options for conveying that textural form through the use of filled black shapes. On the left, they fill in the side planes, placing those shapes on the surface of the form itself, and actually filling in areas that are already enclosed and defined on the form and leaving its "top" face empty. This would be incorrect, more similar to form shading and not a cast shadow. On the right, we have an actual cast shadow - they look similar, but the key point to pay attention to is shown in the third row - it is the actual silhouette of the form itself which is implied. We've removed all of the internal edges of the form, and so while it looks kind of like the top face, but if you look more closely, it has certain subtle elements that are much more nuanced - instead of just using purely horizontal and vertical edges, we have some diagonals that come from the edges of the textural form that exist in the "depth" dimension of space (so if your horizontals were X and your verticals were Y, those diagonals come from that which exists in the Z dimension).

Now the last bit of advice I wanted to offer pertains to those shallower groove type textures I alluded to earlier. It also applies to textures consisting of cracks or holes. As shown in this diagram, it's very common for students to simply fill those holes, grooves, or cracks in entirely, but this robs us of the nuanced information that helps the viewer to understand how those grooves exist in 3D space. Filling them in completely makes no real difference from having a mark upon the surface of the structure, and provides us with no cues as to the depth. Instead, considering the hole, groove, or crack as being made up three dimensional elements - side planes which cast shadows down onto one another, and a floor below.

The key distinction is that the textural "form" we're focusing upon here is not the crack, the groove, or the hole - although we're often inclined to think of them as such, because it's the "named" entity we're dealing with. Rather, it's the structures around that negative space which constitute the actual forms, which cast the shadows. The named entity is actually an absence of matter, and while that distinction can yield very minimal and sometimes unnoticeable differences in our results, ensuring that we're thinking of those relationships and the role each element plays will help us approach the problem with the correct nuance.

So! With that, I'll go ahead and mark this challenge as complete. All in all you've done quite well, and while there are some points to keep in mind, you're heading in the right direction and hopefully what I've offered here will help you continue along in that manner.