Jumping right in with the structural aspect of the challenge, you've done a fantastic job. Your use of your ellipse guides to control the profile of your wheels (mainly to create that widening through the midsection to provide a strong sense that the tire itself is inflated, and would land with a bounce rather than a heavy immobile thud) is coming along quite well - although I did notice that the stronger degree shift towards the far end of the wheel in conjunction with the amount of shift in its scale did result in a sense that the wheel was sloping unevenly towards the back. While that's not great, I think this is in large part due to the limitations of your ellipse guides, which force you to choose between very set intervals of degrees and sizes, which can be challenging to work around. Ultimately this forces you to make the best decision for the circumstances - and so I don't actually see this as any kind of a significant concern. But for the sake of completeness, here's what I mean, and it can be addressed somewhat by making the profile of the wheel on that side curve a little more.

Your patience and care extends to the spokes/rims of the wheel, where you've created some fairly complex structures that would definitely give me a run for my money. Just make sure that you're closing off the spokes' structures as shown here so you can clearly and specifically define where they connect with the inner tube.

Now, if the entire challenge was one of a structural nature, and the tire treads were things we'd approach in the same manner, you'd have knocked this one out of the park. Unfortunately it's not - there's a lot to do with texture here that you largely missed, but fortunately that is very much expected. Being as far removed from Lesson 2 as we are, it's pretty normal for students to forget about the specifics of how we tackle texture (with some students going as far as to try and approach their tires constructionally, as they would other structures - something you did quite a bit, although with a degree of patience and care that was rather astounding). We use the challenge in this manner because showing students that they've allowed certain concepts to fall through the cracks, instead of simply telling them upfront, creates a much starker lesson that is absorbed more fully. That we should always be mindful of what we may have disregarded, and take time to reflect upon what kinds of things that might be.

When it comes to texture specifically - at least, how we handle it here, which is very specific to this course and what it seeks to develop in our students - we are ultimately looking at the same kind of problem that the course as a whole explores: spatial reasoning. We imply the marks we draw (you can refer to the implicit vs explicit markmaking section for more specific information on this) by drawing the shadows our textural forms cast on their surroundings, not by drawing the forms themselves (in terms of outlining them, or otherwise drawing anything about the form itself). It's the shape of the shadow itself, which is designed based on our understanding of the relationship in 3D space between the form casting it and the surface receiving it. And so, as stressed in these reminders, in this course we're never just drawing what we see. We're looking at our references, and understanding what they tell us about the forms in question, and then deciding on how to convey the relationships between them in space.

The reason we use implicit markmaking instead of explicit is fairly simple, although it's not always obvious. For example, looking at any of your wheels like both of these on this page, it looks excellent floating in the void, all full of detail. But when it becomes part of an existing drawing, all of that packed detail can actually work against you by drawing the viewer's eye to it whether you want it to or not. This interferes with our ability to control composition (which is all about dictating how the viewer experiences a piece, what they look at and in which order), which while outside of the scope of this course, is still something I want to give students the tools to engage with more easily.

Explicit markmaking basically locks us into an agreement with the viewer: whatever is drawn is present, and whatever has not been drawn, is not present. And therefore to convey each textural form, we have to declare its presence explicitly. Implicit markmaking on the other hand gives us more freedom by disconnecting the marks we draw from the specifics of what is present.

Implicit markmaking has no such limitations. 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 is the principle on which implicit markmaking is based. Even if we set aside concerns relating to maintaining a consistent light source (which is important, but not relevant for us right now), this gives us a simple fact to work with. That even if you have the same textural form - completely identical in every way, just positioned in a different place on your object's surface - it can cast an entirely different shadow, ranging from none at all, to a distinct one based entirely on its own structure, or one so large that it merges with the shadows cast by other forms to create something entirely different.

With that fact in our toolbelt, it allows us to have far greater control in increasing or decreasing the amount of visual noise and detail used at different points of an object or an illustration.

The last thing I wanted to mention 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.

So in effect, there are some cases where you've employed more implicit textures - like number 14 on this page - but the approach you're using still isn't rooted in that fundamental concept. Rather, we can see that the darker portions tend to be the side planes of structures (which is more akin to form shading, which as discussed here is not a concern for this course). So it's not just a matter of whether we're working with filled areas of solid black - it's also a matter of what they represent. Only cast shadows provide us with enough information to define the relationship between forms in 3D space. Form shading may tell us about the relationship between a surface and a light source, but this course and everything we do within it comes back to the concept of the relationships between forms.

Anyway! Be sure to review the concepts relating to implicit markmaking and texture (these reminders are a good place to start), but I will go ahead and mark this challenge as complete.