Starting with the structural aspect of your wheels, you've generally done a good job in terms of building out your wheels with successive ellipses. You've correctly included a gentle arcing to the profile of the wheel (with smaller ellipses on the outer sides, helping to convey that the tire itself is inflated, and would land with a bounce rather than a solid thunk), although I do think this can at times be emphasized more. Cases like number 19 and 20 end up feeling a little too rigid, without a visible enough arc. If this was intentional (some wheels/tires can be very rigid, in which case this would be appropriate), then that's fine - but if not, that's something to keep an eye on.

Another point that stood out to me is that you have a tendency to use your filled areas of solid black incorrectly - at least, in the context of how we're meant to use them in this course. Right now you tend to fill in the side planes of forms, like the spokes of the rims, where you've drawn the structure with both an outward face and side planes to give the structure thickness, but then filled those side planes in. This is more akin to form shading, where the orientation of a given surface determines whether it should be lighter or darker. As discussed here in Lesson 2, form shading should not be employed for our drawings in this course. Instead, we want to focus our filled areas of solid black primarily on capturing the shadows forms cast upon one another.

When it comes to filling in the side planes of our structures, it actually can flatten the drawing out by taking what was a three dimensional structure with distinct planes, and causing them to read as though they are no longer surfaces of the same structure. They get separated out into black areas (which the viewer's brain wants to interpret as cast shadows, given the fact that we're working with full black and white with nothing in between), and leave us with the outward face existing as a singular plane with no thickness.

Continuing onto the textural aspect of this challenge, it appears that all of your tires here are generally of the same type, each one featuring very shallow grooves. Admittedly it is especially useful when students include some tires with chunkier tread textures, as these can make it far more clear whether the student is working with implicit markmaking (drawing the shadows those textural forms would cast, and thinking about how those textural forms sit in space in general, as explained here). In general, this challenge serves as a bit of a trap for those who may have forgotten the textural concepts from Lesson 2, given how far removed we are from them.

Fortunately I can see signs that you likely did fall into this trap, and would want to review that material. While the shallower grooved tire treads are much more forgiving when we deal with explicit markmaking, to the point that we can in some cases get the correct result without necessarily going about it as instructed in Lesson 2, there are signs that stand out when a student is not necessarily thinking about those textures as a series of forms arranged along a surface, and that they're instead focusing more on drawing what they see without thinking about what it represents in 3D space.

This is an issue we can see with textures involving holes, grooves, cracks - anything narrow that is likely to just be represented as being filled in completely with black. Filling them in does not accurately represent the relationships between the forms at play, however, as shown here in this diagram.

We end up thinking of the holes/grooves/etc as being the textural forms we're meant to be focusing on - but of course, they're not forms at all, they represent an absence of form. An empty space. The actual forms in question are the walls surrounding the grooves, which cast shadows upon one another and onto the floor at the bottom of the hole. Understanding how these structures exist in three dimensions allows us to determine when we can get away with filling a given shape in completely, and when we need to consider which areas won't be covered in the shadow being cast by a given form. At its core, it comes down to the fact that just drawing what we see without thinking it through takes a lot less time - but does not convey the spatial relationships accurately.

So! I'll leave you to ponder that, but I will be marking this challenge as complete. Just be sure to revisit the textural concepts in Lesson 2 prior to finishing up the course, so that you're aware of what you need to keep in mind as you continue on your own.