Jumping right in with the structural aspect of this challenge, you have done a great job when it comes to building out the wheels' core cylindrical structure, especially when it comes to using many ellipses to build out the subtle curve of its profile - that is, the bump through its midsection that helps to make it appear inflated, as though it would land with a bounce rather than a heavy thunk. I did notice a few cases, like numbers 5, 7, and 21, where the far end definitely should have gotten a touch smaller to finish off that arc, but you did very well with this in most cases.

When it comes to drawing the spokes of your rims however, here you did generally focus on drawing the outward face of the structure, but did not establish their side planes, causing them to appear more paper thin. Always remember to consider every structure you draw as it exists in three dimensions, being sure to gauge how thick it is, and conveying that in how you draw it. Here's an example of what I mean.

Contining onto the textural aspect of the challenge, this is an area where the challenge really is designed at least in part as a trap. Students tend to be so far removed from Lesson 2, where we discuss all of the concepts relating to how we capture texture in this course that they are very prone to simply forgetting it altogether, and skipping over applying it. So, this challenge serves as a sharp reminder that they may want to review those topics before finishing up the course (and as a general reminder that the course covers a great deal, and that we are do not have perfect memories).

It does seem that you've very much fallen into that trap, using purely explicit markmaking to define your textural forms, rather than the implicit markmaking techniques discussed in Lesson 2(which you can read about further here).

As discussed there, when capturing any sort of texture - that is, circumstances where we have forms layered across the surface of a larger object - implicit markmaking where we focus on drawing the shadows those forms cast, rather than outlining the forms themselves, is an extremely useful tool. While floating in the void, each of these wheels look largely fine, but if we were to use them as part of a larger vehicle construction, all of the tightly packed detail would create focal points, drawing the viewer's eye to them whether you intended them to or not, and impeding your ability to control how the viewer's eye moves around the piece, and how they experience it.

Using implicit markmaking allows us to decide to use more or less ink without actually changing the nature of what's being conveyed to the viewer. As we can see here on this example of snake scales, whether we cast larger, deeper shadows, or shallower, thinner ones, the same textural forms are implied as being present.

Now, tackling texture is itself quite challenging because it requires us to actively think about each and every form in question, without drawing them. Immediately this can be immensely overwhelming, but functionally we only really need to think about one form and the surfaces around it at a time - although this in itself is still quite difficult. I find approaching the shadow shapes in a two step process to be quite helpful - first outlining the intended shadow shape can allow us to think through the actual spatial relationship it's meant to establish, between the form casting it and the surface receiving it. The act of outlining it allows us to focus on designing that shape to those specifications, and once finished, we can simply fill it in.

The last thing I wanted to mention is that there is a tendency for students to look at textures with grooves or holes, and end up treating the things they can name - those grooves/holes - as though they're the textural forms in question. They are not, of course, as they are an absence of form, but this confusion can cause us to simply fill those areas in with solid black. Worse still, in some cases (like those wheels with really shallow grooves), approaching it in that manner can get us pretty close to a correct result - but by thinking about the problem incorrectly. For our purposes here, acheiving the correct result is less important than understanding how to think through the problem itself.

This diagram explains how to think about the way in which such textures behave, and how we can tackle them.

So! I am going to go ahead and mark this challenge - being that the "trap" was designed as part of the challenge, it's not something I hold against students. But, you will certainly want to review the texture material, and keep it in mind going forwards.