Starting with the structural aspect of the challenge, I'm pleased to see that when tackling the spokes of your wheels' rims, you're mindful of not only the outward face of the structures but also the side planes. There is definitely a notable tendency amongst students to neglect these - understandably to a point, given the size limitations with those master ellipse templates - but it is certainly important to include the side planes in order for the structures to feel solid. It's good to see that you haven't neglected them here.

Looking at the body of the wheels/tires as a whole though, one thing I'd recommend to consider a little more is making the middle portion of the wheel structure a little larger, so instead of having a straight profile along its length (basically being a basic cylinder), the profile of the wheel would arc more. We can see this in this wheel - although it should be present in many of your others, although to a lesser degree. It helps the tire to feel inflated, as though it would land with a bounce instead of a heavy thud.

Continuing onto the textural aspect of the challenge - that is, how we approach conveying the tire tread, which being made up of forms arranged along the cylindrical surface very much fall into the category of texture as explained back in Lesson 2 - this part serves as something of a trap. It's very common for students to allow those concepts from Lesson 2 to go unpracticed as they push forward through the course, and this is an opportunity for that issue to be highlighted. You certainly have fallen into this trap, as have many others.

Throughout your work here, it appears that you've largely focused on using explicit markmaking - that is, using outlines to construct each of the textural forms. This is unfortunately quite limiting - while your wheels may look fine floating in isolation, when we do this in a larger illustration, all of that packed detail in each wheel becomes a focal point, drawing the viewer's attention whether you want it to or not, and limiting your ability to guide the viewer's eye around the illustration.

Implicit markmaking - which as explained in Lesson 2 has us drawing the shadows our textural forms cast, rather than the forms themselves - allows us to control the manner in which the information is conveyed. We can decide to have greater detail density in certain areas, and less in others, avoiding unintended focal points. Cast shadows - that is, what we get when the light source is blocked by a given form, stopping those rays from reaching part of the surface of another form - are new, distinct, filled shapes. It is the design of that shape which conveys the 3D spatial relationship between the form casting the shadow, and the surface receiving it. So, a lot of the focus falls on how those shapes are designed, and the understanding of the spatial relationships that help us make those decisions.

This diagram looks at what we tackled in Lesson 2's texture analysis exercise, but instead of viewing it from a top-down perspective, we're looking at it from the side in profile. On the far right we have our light source. Because the angle between that light source and the textural forms closest to it is very steep, the shadows that are cast are very small. The further away from the light source we move, the shallower the angle becomes, resulting in a larger/longer shadow. Basically this means that the size of the shadows don't have to be consistent across the entire object, or over the entire scene. We can decide where we want our shadows to be larger and more prominent, and where we want our shadows to only barely imply the presence of detail.

Compare this to your experience drawing the tire treads in this challenge - you're basically locked into the lines you use to construct those textural forms. You can't have lines in one spot, but not another - everything has to be conveyed in the same manner all over the given tire. This takes control out of your hands. This is why implicit markmaking is a useful concept. It focuses on spatial reasoning, like the rest of the course, but it also gives us greater tools to use when illustrating. It is of course considerably more time consuming, and generally trickier, since it forces us to hold the understanding of a given form in our minds while drawing its shadow - that is, without drawing that form directly on the page.

Now, it's clear that this is something you'll need to review - but I'll still be marking the challenge as complete. Before moving onto Lesson 7, start by reviewing these reminders, then go back over the texture material as a whole. I'd also recommend that you reflect on any other concepts that you may have allowed to slip through the cracks.