Jumping right in with the construction of your wheels, you've done a great job with this. Earlier on you did have a tendency to construct wheels/tires that were more straight across in profile, but as you progressed through the set, I could see you experimenting much more with adding a more gradual bump through the middle, which increases the impression of an "inflated" object.

You've also done similarly well in the construction of your wheels' rims/spokes, taking care not only to draw the front face of these structures, but also their side planes to ensure that they feel solid and three dimensional.

The other part of this challenge comes down to texture - and in that regard, it's kind of a trap. A trap you did happen to stumble into, so let's take a moment to talk about that. Basically, it comes down to this challenge being so far removed from Lesson 2 and its specific principles on texture, on implicit vs. explicit markmaking, and so on, that a lot of students otherwise finish the course having forgotten all about it. By putting down this little snag where texture is in fact very much relevant, it serves as a reminder to perhaps review that material.

Tire treads are, by their nature, a texture. They're made up of smaller forms sitting along the surface of a much simpler cylindrical structure. When we capture those textures using strictly 'explicit' markmaking (like we see in cases like 17 for example, where you basically constructed those chunky treads with all their outlines and internal edges, although it is present throughout the majority of these), this can look okay for a wheel that's on its own, floating in the void. But as soon as you get into drawing a car, all of a sudden that densely packed linework becomes a very strong focal point, drawing the viewer's eyes in whether you want it to or not.

The solution to this is to convey the presence of these forms strictly through filled, designed, cast shadow shapes - basically implying that these forms are there without actually drawing them, but drawing the shadows they cast on their surroundings. You can of course read up on this back in Lesson 2 (and certainly should).

This demo, which is decidedly not a tire tread (it's an african bush viper's scales, but the principles are the same) demonstrates how we can use implicit markmaking and control just how dense we want that texture to be, and thus either create or avoid focal points as needed.

One thing that some students do where they only partially fall into the trap is that they'll try to use implicit marks, but only by filling in the side planes of those textural forms. While moving in the right direction, this is also explicit markmaking (because we're still drawing what's on the form itself), and is more in line with applying form shading to those textural forms instead of cast shadows.

I prepared this explanation for that difference for another student, and while I didn't see you doing this here (at least not in the context of texture - you did do it on the rims for number 18), I do think it's still valuable to provide as it's an easy mistake to make. 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).

While you did fall into the trap, it's the nature of a trap to expect as much. This challenge was meant to only be a reminder, not something to hold students back, so I'm still marking this challenge as complete. Just be sure to review that texture section from Lesson 2.