Jumping right into the structural aspect of the challenge, your work here is well done. You've made good use of your ellipse guide, and have built out your wheels with an appropriate consideration to just how much larger the midsection should be compared to the outer sides, so as to convey as much of a sense of being "inflated", or that the wheel would land with a bounce as opposed to a heavy thud, as required by each individual wheel's tire. Some were more rigid (like number 16, which is appropriate given that it's made of wood and would have no give), and others were nicely inflated, like number 5, where you maintained that larger size through much of the wheel's width, tapering it off nicely at the edges. This shows a great deal of control over that structure.

I'm also pleased to see that your rims' spokes show a similar level of regard for their structure and three dimensional nature, although I did notice some spots where you got a little confused when drawing the back edge of a given spoke's side plane, and where it should stop, as shown here on number 17.

Moving onto the textural aspect of the challenge, admittedly you somewhat dropped the ball, but that's not remotely out of the ordinary, nor outside of what's expected in this challenge. Being as far removed from Lesson 2 where we discuss concepts like texture, explicit markmaking vs implicit markmaking, and how we can leverage cast shadows to imply the presence of forms, it's very common for students to either entirely forget that those points were discussed there, forget that they might be relevant to some of the problems one might face in this challenge, or remember their relevance but not the specifics of how to use them (and not actually go back and review that material). The vast majority of students fall into one of these categories, yourself included.

Ultimately looking at your work, it seems you may have jumped straight to attempting to solve the problem at hand with the same explicit markmaking we employ when constructing the structural elements.

When it comes to texture specifically - at least, how we handle it in this course, which is very specific to this course - 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, nor easy to implement. For example, looking at any of your wheels, for example numbers 18, 19, 20, 23 on this page (honestly there are a ton throughout the set that could be used as examples for this), 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.

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. We don't even have to specifically concern ourselves with the position of the light source or keeping it consistent (which does matter more broadly, just not a concern for what we're doing here right now or in this course) - just the fact that the same form in different locations can result in a different shadow gives us license to decide the nature of the shadows - whether they should be so big that they merge into complex compound shadow shapes (in the case of the light source being farther away), so small that they're virtually invisible or hidden by the textural form itself (in the case of the light source being really close), or in between where we get sizeable but distinct shadows creating a lot of visual detail density.

The last thing I wanted to mention is to do with 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! Take this as a reminder that there may be things introduced throughout the course that may have fallen through the cracks as you progressed forwards, so be sure to reflect on what areas they could be, and take some time to review them (starting with the texture section, for which these reminders are a good starting point). Once you've done that, you can continue forwards.

I'll go ahead and mark this challenge as complete.