Funnily enough, while this challenge serves a number of useful purposes (especially with the specific handling of each wheel's design, down to its tire tread and rims, working as microcosm of the problems we'll face when drawing cars), one of the biggest reasons I assign it here is because we're very far removed from Lesson 2 and its principles relating to texture. So, it serves as a bit of a trap and a reminder of those principles, and the importance of going back and reviewing them.

You, however, seem to have sidestepped that trap altogether. By your comments, and more importantly, your work, I can see clearly that you went in with those textural concepts tucked under your arm, and while it was definitely still a struggle towards the start, throughout the set you showed a continuous effort to consciously think about the specific textural forms present on each surface.

Now there are some earlier on - especially cases like number 7 - where you do fall into explicit markmaking in a couple ways. First off, you outlined each textural form in its entirety (a common mistake), and secondly, rather than drawing the shadows each textural form would cast, you focused instead on filling in the side planes of those textural forms, effectively applying form shading rather than cast shadows. We can see the difference depicted in this diagram. As an extension of this, if we look at the little experiments you did around number 7, drawing chunks floating in space, and filling in those side planes, the fact that they actually exist in this "floating", isolated state, is inherent to the problem. Implicit markmaking is all about establishing relationships, specifically between the textural form and the surfaces around it. Thus, these things cannot exisit (when done correctly) in isolation - they need the surface to which they attach. If they're simply forms floating in space, then those are things we'd definitely approach explicitly, like most of our constructions.

As you progress, I definitely do see improvement. 15 and 21 for example break away from the outlining of forms (although 21 does become less structured as a result, losing the more predictable pattern of the tire tread - but like I said, keeping everything straight in your head is hard when you're not allowed to outline). Shading the side planes instead of drawing cast shadows is still a present concern, albeit a fairly normal one, so that is something you'll want to address in your own practice.

When it comes to the core construction of your wheels, you're doing a great job. More and more as you push through the set, your wheels feel solid, and you do a great job of including that subtle curve to its profile (where the midsection is larger than the side edges), creating the impression that the tires are "inflated" rather than stiff and striaght. 22, 24, and 25, I felt, were especially well done, particularly whe nlooking at the care with which the rims were constructed, with an awareness of their various planes rather than just the face pointing outwards from the wheel.

All in all, I am quite pleased with your work here. You do certainly have room for continued growth, but you're headed in the right direction so keep it up! I'll go ahead and mark this challenge as complete.