Jumping right in with your construction, as a whole you've handled this really quite well. I can see that your wheels tend to be made up of fairly consistent degrees from one end to the other - while this is incorrect, I see it less and less as you progress through the set, and towards the end you definitely end up with a lot more dynamism to the structure of those wheels. It's not just that you shift the degree of the far end more (which is great), but you're also including more of a "bump" through the midsection, creating that impression that the wheels/tires are inflated, rather than just a rigid cylinder.

I'm also pleased to see that you're paying a lot of attention to the construction of your rims, not only establishing the front face, but also the side planes of those structures. 19 is especially complex, but you demonstrated well developing spatial reasoning skills in building it out even without some of the internal edges. Very nice work.

Continuing onto the other half of this exercise, is the texture. You noted it yourself, and you were correct - the tire treads are very much a problem of the sort we'd have explored back in Lesson 2's texture section, given that it's made up of a lot of small forms that sit along the surface of a larger structure. I actually use this challenge as something of a trap for students who, being as far removed from Lesson 2 as we are, tend to forget about those principles altogether. While you do use a lot of explicit marks/outlines towards the beginning, you explain this issue in your question about holes, and I do have some advice to offer on that in a moment.

Further down as you start to deal with more protruding textural forms, you definitely are trying to think about how to approach these implicitly, though it is certainly still something you're struggling with. One piece of advice - something I've actually stressed more recently here may help. The marks we draw when capturing texture are, as you understand, cast shadows. But these are not cast shadows we're meant to copy off our reference image. Rather, the reference image serves as a source of information. It helps us understand which textural forms are present, how they're arranged, and how they relate to the surfaces around them in 3D space. It's that understanding of these textural forms that we want to pull out of the reference, so that we can then combine it with our own understanding of 3D space to determine what kinds of shadows the form should be casting.

The main point there is that we're not copying the shadows we see in our reference. We're creating those shadows absed on our understanding of the forms, and of the lighting situation we need. This allows us to choose if we want heavier shadows, or lighter ones, depending on just how high contrast and eye-catching the wheel should be. Floating in isolation like this, packing in lots of marks may be fine, but once we use it in an actual vehicle construction, a wheel like that would immediately draw the viewer's eye, whether you want it to or not.

It also helps immensely to actually approach creating those cast shadows in two steps - first outlining the shape you intend to create, considering the form that's casting it and designing that shape so it conveys that relationship between the textural form and the surfaces around it. Then, once that outline is done (which you should do with your standard 0.5mm fineliner), you can fill it in (with a thicker pen, a brush pen, or whatever else). Avoid painting shadows on stroke by stroke, as it is a lot harder to intentionally design the shadow shape in that manner.

Another point that may help is some advice I provided to another student who had a tendency to, instead of drawing cast shadows, simply fill in the side planes of the textural form (which is as you noted more similar to form shading). This isn't an issue I'm seeing a ton of in your work, but I still think the explanation I provided to them would be helpful to you.

Take a look at this quick demonstration I provided them. On 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).

Now, before I finish this critique, let's circle back to your questions. First, about holes. This is something we can run into with the grooves on our tires, as well as with other surfaces like sponges, or cracked dirt, etc. The thing is, the student focuses on the hole, or the crack - the empty, void, negative space - as though it's the textural form. But it's not. The relevant forms are actually the walls and floor of the hole. It's the walls that cast the shadows, and the other walls as well as the floor that receive them. Here's a visual breakdown of what I mean.

For your second question, you asked if there is a point at which a form gets so big that it's no longer a texture. The answer is... kind of. It's more that the distinction between things we'd use implicit marks vs. explicit marks aren't quite so cut-and-dry. For example, if you look at a city scape, it'll look like it's a bunch of independent, constructed boxes along the surface of the earth. But if you zoom out and look at it from overhead, especially from a considerable distance, all of a sudden those buildings are just little boxes adhering to the surface of the earth, like a texture.

The "rule" we use in this course is that if it sits along the surface of a larger, defined structure, and follows that surface, then it's a texture. If you stapled a bunch of fish to a wall, that wall would have a texture of... fish. But when you're drawing your own stuff, there's a lot more flexibility. For example, in this drawing of a derelict house I did a while back, you'll notice that I actually used a lot of implicit marks for the windows, and a lot of architectural structures. These aren't strictly forms sitting along the surface of a larger structure, but it still works, because I didn't want to end up with a ton of really explicit, detail-dense linework in the drawing, it would draw too much attention. So, I changed up how I made my marks in order to tone down how much of the focus they grabbed. But of course, it ultimately was still built on top of my understanding of the forms that were present, regardless of whether I drew them explicitly or implicitly.

So, I hope this helps. I'll go ahead and mark this challenge as complete.