Jumping right in with the structural aspect of the challenge, you're doing well when it comes to the body of your wheel structure (in that you're building them out with several concentric ellipses, creating a body that widens through the midsection to help convey that it's not solid all the way through, and instead of landing with a heavy thud, would bounce due to the inflated tire), but I did notice a couple issues regarding how you're handling the spokes/rims.

Most notably, as we see here, you've got the far edge of your spoke's side plane extending all the way to the more forward edge of the inner tube (basically telling the viewer that both edges of the side plane exist at the same level of depth). Instead, the farther of the side plane's edge would be further back, and so in the case of the one I drew over, it would hit the inner tube much sooner. Though I did pick an earlier wheel in the set to depict this issue, it is present throughout the set, and we can see it in cases like 19 as well.

Looking at wheel 20, just a quick reminder that we want to reserve our filled areas of solid black for cast shadows. Filling in the side planes of those spokes is more akin to form shading, where a surface gets lighter/darker based on whether it's facing towards or away from the light source, and as discussed here in Lesson 2, form shading is something we don't worry about in this course.

The last point I wanted to mention on that front is less of a correction, since it's not something you'd have known ahead of time, but more something to keep in mind in the future. For the bicycle wheel you drew for number 3, you had to deal with spokes that were very, very thin - so thin that the reasonable choice would be to, as you did, draw them as singular lines. The issue with this is that a single line or edge doesn't provide the viewer with enough information to understand how that structure exists in three dimensions. At minimum, we would want a 2D shape (so not necessarily something with internal edges, but still with an external outline that encloses a space), as its thickness is generally enough to suggest to the viewer that what they're looking at is 3D. In this case, that means deciding between choosing something that will appear more solid (a shape), and choosing something that is more accurate to the thickness of the spokes in your reference (a line). Because our focus in this course always comes down to spatial reasoning, to making things look convincingly 3D, and to understanding how those structures exist in three dimensions, the better choice in this context would be to draw it as a shape, with at least two edges - meaning it'll be thicker than the reference suggests. That doesn't mean it's always going to be the better choice - the context in which we're operating matters, but since this course focuses on spatial reasoning and 3D structure, that's what is best for the exercises done here.

Anyway, continuing onto the textural aspect of the challenge, this is basically where the challenge becomes a bit of... well, a trap. Given how far removed we are from Lesson 2, it's very common for students - especially considering how difficult texture is as a concept when first introduced - to simply forget to include textural exercises in their regular warmup routine, and to otherwise neglect those concepts going forward. Given how tire treads fit our definition for texture (forms that are arranged along the surface of an existing structure), we've found it's a good opportunity to remind students to consider what concepts they may have allowed to fall through the cracks while progressing through the course, so they can review them before reaching the end.

Students tend to fall into three different categories when it comes to dealing with the textural aspect of this challenge:

• Very rarely, students actually have kept up with the textural concepts, and apply the implicit markmaking (that is, where we draw the shadows the forms we draw cast - not from copying the shadows you see in the reference, but by actually considering the relationship between the form casting it and the surface receiving it, and designing a shadow shape based on that relationship).

• Most often, students remember that there's something about texture involving filled areas of solid black, but they don't necessarily check the Lesson 2 material to confirm that and instead try to work off what they remember - usually resulting in them applying form shading rather than focusing on cast shadows.

• Somewhere in the middle, are the students who pretty much forget that implicit markmaking and texture is something that we've explored, and so they largely rely on explicit markmaking, outlines, and generally constructional techniques (as we apply them to non-textural problems).

In your case, you're a mix of the latter two - I can see places where you're trying to consider how to use filled areas of solid black, as we can see in cases like 5, 10, 15, and 17, but rather than focusing on understanding the relationship in 3D space between the forms casting shadows, and the surfaces receiving them, you tend to use the filled areas more arbitrarily. In many other cases however, you skip over the filled areas entirely, and opt for working with construction to build out the major textural forms.

Of course one might wonder, well why do we need to worry about implicit markmaking and cast shadows and all this? Well, it's true that some of your wheels/tires did come out looking quite nice - but the thing to keep in mind here is that each of our wheels floats in a void, and so we can cram as much visual detail as we want. But when we use them as part of a larger illustration, this presents a problem - the more detailed the wheel is, the more it's going to draw the viewer's attention and create a visual focal point, whether you want to or not. When limited to explicit markmaking (where we draw the forms, rather than the impact they have on their surroundings), we have the choice between adding enough detail to convey the texture that is present there to the viewer, or making the surface appear smoother and less textured in favour of avoiding having the viewer's eye jump straight to the wheel.

Implicit markmaking works differently. 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 we can ostensibly have the same kind of textural detail, but we can choose to convey it with lots of detail (for those cases where the textural forms are a middle distance away from the light source), or with almost no detail (the cases where the textural forms are very close to the light source, and so the shadows get blasted away, or where they're very far away so the shadows become large and expansive, merging into singular shadow shapes whose complexity is limited only to their outer edges. More importantly, these can coexist together, allowing us to transition from areas of high detail to those of low detail density, as the texture analysis exercise from Lesson 2 focuses upon.

To be clear, we're not actually particularly interested in where the light source actually is, and what the perfect, most correct cast shadows would be - rather, it's just the fact that these situations exist that give us permission to be in greater control of where we want to add detail, and where we allow the viewer's brain to fill in the blanks.

One last thing - 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, as I noted before, this "trap" is intentional, and so we don't punish students for it. I'll still be marking this challenge as complete, but it would be best for you to make sure you review the texture material from Lesson 2 (this section of reminders is a good start), and to also consider what other concepts you may have allowed to slip through the cracks, so you can review them as well before continuing on.