Jumping in with the structural aspect of your wheel constructions, overall you've done well, but there's one issue I do see come up enough that it is worth calling out. Before that though, I want to note that you've done a good job of using your ellipse guide despite its limitations, and I think it was still very much the right choice to use it instead of freehanding. You leveraged it well to build out the body of your wheels with varying degrees of widening through the midsection, which helps to give some of your tires a sense of being inflated, as though they'd land with a bounce, rather than a heavy thunk.

Continuing onto that issue I was seeing, I've noted one case of it here in number 23. Basically when drawing the side plane of your spokes (which at times you leave out, causing the spokes to appear very flat, which would also be a mistake unless they were razor-sharp and thin). You tend to extend the back edge as far as you do the front edge, causing it to penetrate through the inner tube of the rims. It's important to consider the relationship between these elements in 3D space. You do it more correctly, allowing the back edge to end where it would intersect with the tube, in other cases, but it's kind of a mixed bag, so be sure to keep that in mind.

Moving onto the textural aspect of the challenge, this is an area where the challenge is honestly a bit of a trap we set for students. Being as far removed from Lesson 2 and its discussion of texture, using implicit markmaking, and so forth, it's pretty common for students to forget about it. Some remember how we try and rely on using filled areas of solid black instead of outlining every textural form or applying texture as a series of individual lines, but they tend to forget that the focus is on cast shadows, and so they more often end up filling in the side planes of forms in a manner more similar to form shading (which as discussed here isn't something we worry about in this course.

As discussed in Lesson 2, texture is basically just forms that run along the surface of an existing structure. How they flow through space is dictated by the structure they follow. As discussed in these reminders which sum this up pretty well, we don't draw what we see - we observe what we're looking at, use that information to think about the textural forms that are present as they sit in 3D space, and then we design cast shadows based on our understanding of those spatial relationships. Each cast shadow shape defines a relationship between the form casting it, and the surface receiving it.

Now many of your wheels do look nice, and are quite detailed - but it's not all about how they look floating in a void. If we take one of these wheels, like 18, and include it in a larger illustration, all of its complex detail will create a focal point, grabbing the viewer's attention whether you want it to or not. This can be tricky, because guiding the viewer's eye around a piece (something more closely related to composition, which we don't get into here) is an important tool for an illustrator, who has to tell a story across multiple moments, in a single frame. We do this by being able to control (to some degree) where the viewer looks first, then second, and so forth.

The problem with using explicit markmaking techniques for textural detail, as you do here, is that you can't really take out any of that detail without telling the viewer that those textural forms aren't present. It's very on/off - if it's drawn, it's there, if it's not drawn, it's not there. Implicit markmaking doesn't suffer from this, entirely because instead of drawing the textural forms themselves, we're drawing their shadows, which aren't the form itself, but they do tell us that it's there.

As shown in this diagram, depending on how far the form is from the light source, the angle of the light rays will hit the object at shallower angles the farther away they are, resulting in the shadow itself being projected farther. This serves as a very useful tool - it means that a form can cast a shadow that is so small that it can't be seen, or that a form can cast a shadow so big that it merges with other shadows to create a larger shadow shape that only directly describes the textural forms casting the shadows that make up its edge. It also allows us to allow those cast shadows to change, to be represented by small shadows or big shadows or something in between, within the exact same surface - depending on how far they are from our "light source". We don't even have to necessarily worry about where the light source is (although it's a good thing to keep in the back of your mind for other reasons - just not for our textural problems here) - the fact that we can have shadows that are big or negligible gives us more freedom in whether we want areas to be detailed (where the shadows are somewhere in between, big enough to be seen but small enough not to all merge together), or whether we want areas to be sparsely detailed (where the size of the shadows are on either extreme). As long as you have some detail, it's enough for the viewer to be able to fill in the gaps in their own mind, because they're give enough information to work with, without being told "this is all there is".

Now, one last thing I wanted to share is that when it comes to those tires with shallow grooves, or really any texture consisting of holes, cracks, etc. it's very common for us to view these named things (the grooves, the cracks, etc.) as being the textural forms in question - but of course they're not forms at all. They're empty, negative space, and it's the structures that surround these empty spaces that are the actual forms for us to consider when designing the shadows they'll cast. This is demonstrated in this diagram. This doesn't always actually result in a different result at the end of the day, but as these are all exercises, how we think about them and how we come to that result is just as important - if not moreso.

As I'd mentioned before, this is very much an intentional trap, so I'll still be marking this challenge as complete. It's just useful to give students a bit of a scare at this point, to remind them that with a course this long it's very easy for some things to fall through the cracks and be forgotten, even though we should be keeping up with those exercises in our warmups. So, take this opportunity to reflect on anything else you may have allowed to slip by, and be sure to review those sections, starting with the textural concepts from Lesson 2.