Honestly, your work here is by and large very well done. You made excellent use of the ellipse guide to approach the construction of your wheel structures in most cases, although I'd argue that there were some cases where perhaps you relied too much on the basic cylinder structure (like wheel 19) that resulted in tires that felt less inflated. Perhaps that's what the reference image displayed - I can't really determine that for myself - but it does impede the realism not to have at least a bit of a curve/bevel as we reach either end of the wheel. You handled this better in other cases, where the middle ellipse tended to be a touch larger, creating more of a rounded, "fuller" impression. There are however a few cases like #12 where perhaps that middle ellipse gets too much bigger, resulting in a weird dissonance. In these cases, adding additional ellipses between the outer ends and the middle can help bridge across them. These intermediary ellipses would be closer to the size of the middle one, if possible, creating a more graduated curvature.

I think you handled the rims very well. These are admittedly not easy at all, but you did a great job of drawing through them where necessary, and making them feel symmetrical and structurally sound. Some of these definitely get quite complex, but you demonstrated a strong grasp of 3D space and the relationships between those forms within it.

As far as the textural techniques being used on the tire treads goes, this is generally what I call out for students most often in this exercise, as they get more caught up in constructing the chunkier tread forms. I suppose you did do this in #15, but for the most part you actually showed a pretty good grasp of how to think in terms of cast shadows instead of explicitly defining every form present in your construction.

I could definitely see a fair bit of experimentation in how you tackled some of the subtler tire tread textures, but as far as I'm concerned, that's a good thing to see. Trying to figure out how the marks you draw can create the impression of different arrangements of forms, without drawing those forms explicitly, will help develop a stronger repertoire of strategies when it comes to tackling this kind of a challenge.

One small suggestion I do have as far as drawing textures go is that there is a common tendency amongst students to try, when they can, to draw textural marks as individual strokes. This can cause us to get trapped into using individual lines to capture our textures, which in turn can lean more into outlining those forms without realizing it. The thing about lines is that as shown here they're not all that dynamic. We can change their weight here and there, but they've got nothing on the dynamism and liveliness we get out of our shadow shapes. For this reason, drawing those marks with this two step process can help a lot in forcing us to always think in terms of drawing actual shadow shapes, not just lines. Furthermore, this can help us to think about the nature of the form that actually casts the given shadow.

Anyway! All in all your work here is coming along great. I'll go ahead and mark this challenge as complete.