Starting with the structural aspect of the wheels, you've done a great job. You've been mindful of approaching the constructions in stages, and have been quite fastidious in ensuring that you're not skipping any steps. I'm also pleased to see that you've widened the midsection of your basic wheel structures to make the tire feel more inflated, as though it would land with a bounce rather than a heavy thud, and that you also were mindful not only of the outward faces of the spokes/rims, but also their side planes to help ensure each one felt solid and three dimensional.

On that front, you've done an excellent job. In regards to the other aspect of the challenge - that is, where we get into drawing the tire tread patterns, which falls very much into the definition of a "texture" we use in Lesson 2 (because all of these textural forms rest along the surface of a larger form and follow it as it deforms in space rather than existing independently of it), you appear to have forgotten to apply the approaches mentioned in Lesson 2 in regards to conveying texture through implicit markmaking (using cast shadows) rather than explicit markmaking as we would when constructing our major forms.

To be completely honest with you - that's largely expected. This challenge serves as a somewhat rude reminder to students that the course is dense, and most are prone to allowing certain concepts we've discussed and explored to fall through the cracks, especially when it comes to texture due to the fact that it's rather frustrating. So I wouldn't be too broken up about it, but take it for what it is - a sign that you have material to review.

While any of these wheels would work well floating in a void, the reason implicit markmaking is a useful technique to learn is because when these wheels play a role as part of a larger drawing or illustration, all of that detail you've packed in to convey every bit of texture ends up drawing the viewer's eye. It creates a focal point, whether you want it to or not, and that impedes our ability to actually control how a viewer experiences a piece. That pertains more to composition, which is well outside of the scope of this course (although we used to have a lesson on it, which is archived here), but implicit markmaking itself still pertains largely to understanding how your forms sit in 3D space and how they relate to one another within it, and so it falls squarely within the goals of what we're doing here.

Implicit markmaking hinges on cast shadows - rather than drawing a form directly, we draw the shadows it casts upon its surrounding surfaces. That shadow shape, through the design of the shape itself, conveys the relationship between the form casting it and the surface receiving it, and allows us to convey the textural information (at least aside from where those forms would break the silhouette, which as mentioned here still does need to be drawn explicitly) without being locked into a specific level of detail density in that part of our drawing.

Explicit markmaking establishes the expectation in the viewer that everything that has been drawn is present in 3D space, and everything that hasn't been drawn isn't present. It's a very clear contract being made. What I draw is here, what I don't draw is not. Therefore if we wanted to make the detail density sparser, we'd actually be modifying what it is we're conveying to the viewer. Implicit markmaking is not limited in that fashion, for the simple reason that cast shadows aren't as consistent as lines. A form of any size, depending on its relationship with the light source, can cast a large shadow, a medium shadow, a small shadow, or virtually no shadow at all - but even when that form casts no visible shadow, because of the expectation it establishes in the viewer, that isn't immediately interpreted as the form not being there. Rather, the viewer instinctually looks at the other cast shadows around it, and assumes that there's a pattern being followed - and therefore if there are gaps, those gaps get filled in by the viewer's own mind. You can see this variation in shadow shape size 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.

These kinds of things are much more prominent in textures with large, chunky textural forms. 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.

Anyway - as I mentioned before, this is a sort of trap that we expect students to fall into, and our only intent is to provide the reminder to reflect on what may have fallen through the cracks, so you can review that material before continuing onto the last lesson in the course. So, be sure to review the texture material (I'd recommend reviewing all of it, but start with these reminders), and consider anything else you may have allowed to be left behind as you pushed forward.