Starting with the structural aspects of these wheels, overall you're doing decently, but there are a few cases that I want to call out.

• You do a good job with those like this one, where you're building out the entirety of the structure, smaller ellipses along the sides and a larger one in the center to help give it a sort of "bump" in its profile, which in turn makes it feel more inflated, rather than a solid piece of metal.

• As you get into wheels that are turned away from the viewer, some of your wheels do appear more awkward, like this one, simply because we're missing the structure on the opposite side. In such cases, it may be beneficial to, at least for the purposes of these exercises (which always come back to spatial reasoning), establishing the structure that exists on the opposite side, even though we wouldn't generally be able to see that section. Basically like drawing through the boxes from the 250 box challenge.

• For this one, just a minor point about the spokes. Aside from the topmost one, you didn't establish the side plane of those structures, making them appear more like a paper-thin structure. Here's how you would establish the side planes of the spokes.

Continuing onto the other section of the exercise, we get to the tire tread textures themselves. Here you've got some varying success, although I can clearly see that as you stated yourself, you're trying to apply the principles of textural and implicit markmaking. There are definitely cases where I can clearly see that you're trying to think in terms of cast shadows, although more often I do see that you fall into the trap of filling in the side planes of your textural forms. This is more akin to form shading, although when we deal with relatively subtle tire textures, the difference can be unclear.

I've got this diagram I drew for another student who was running into a similar issue, along with the following explanation:

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).

The other side of this however is the fact that a lot of your tire textures are shallower does also introduce its own confusion, mainly because it's easy for students to think that the grooves themselves are the textural form. But they're not - they're an absence of form. The forms themselves are the walls that surround the grooves, and the floor of the groove itself. Here's a diagram that explains how to think about this.

As a rule, try to avoid any situation where you're filling in shapes that already exist. Always try and think about the cast shadow shapes as their own new shapes that you have to add, with the specific shape being what establishes a relationship between the form casting it, and the surface that receives it.

Oh, before I forget, one thing circling back to structure - on this wheel, I noticed you forgot to add some of the additional ellipses/curves that would have helped to flesh out this structure. Without them, you end up missing important information that establishes how each of these protruding rings actually have their own thickness.

Anyway, I've shared some points for you to keep in mind here, but I'll still be marking this challenge as complete.