Starting with the structural aspect of the challenge, by and large you've done a great job. I'm glad to see that you leveraged your ellipse guide well (and I can see that you gave freehanding it a shot, and probably realized just why we recommend using an ellipse guide so strongly - it just lets you get a lot of the other stuff out of the way so you can focus on the meat of the exercise).

I'm pleased to see that you built out your wheels with a number of ellipses, allowing the midsection to widen by varying degrees based on how "inflated" you needed that wheel to appear. I'm also glad to see that when constructing the spokes of your wheels' rims, you were mindful not only of the outward face of the structure, but the side planes as well, which goes a long way to making those structures feel very solid.

Continuing onto the textural aspect of the challenge, this is where the challenge turns into a bit of a trap. Since we're so far removed from Lesson 2 where we talk about how to tackle textural problems and how we can use cast shadow shapes via implicit markmaking to solve them, it's very common for students to either forget about those concepts entirely, or to half-remember them and try to apply them based on what they recall. What is uniquely rare is having students actually go back and review that material, and so I find using this challenge to remind them that reviewing past material is a very good idea, and not something to shy away from, as an effective teaching tool.

By and large, your wheels look lovely - specifically, when floating isolated in the void. When you want to use them as part of a larger illustration however is where things get tricky. Because you've focused largely on conveying the texture through explicit markmaking (drawing all of the forms that are present by outlining them directly or otherwise applying constructional techniques as you would for larger more free-floating structures), you're essentially locked into a specific level of detail density. This creates a visual focal point that draws the viewer's eye to that area of the drawing, whether you want it to or not - and in turn, if you aren't able to control how much detail goes where, you'll have a tougher time controlling how the viewer experiences a piece, which is a big focus of composition.

The obvious answer to this problem is to draw it with less detail - but the thing about explicit markmaking is that between the things that have been drawn and therefore are conveyed as being present in the scene to the viewer, and the things that have not been drawn and therefore are not present in the scene (as far as the viewer is aware), there's not a lot of leeway. It's either one, or the other. Either you've drawn it so it's there, or you haven't drawn it so it's not there.

Implicit markmaking is not so limited - because it does not involve drawing the textural forms directly, it can create the impression that they are present, even where they have not been drawn at all. 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 mechanism on which implicit markmaking works - it relies on the fact that the shadows being cast by the forms, even if the forms are identical, aren't inherently going to be the same. There are other factors at play that legitimize different shadows being cast by the same forms, whether so small that they're imperceptible, or so large that they start merging into other neighbouring shadows.

So, by using implicit markmaking, we can control just how much detail is used in any one spot, and better control how the viewer experiences a composition, without having to change what is physically being represented.

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.

Now, as I stated before, this is very much an intentional trap, and so I don't assign revisions for this. Instead, just be sure to review the textural concepts from Lesson 2 (these reminders are a good place to start) and take some time to reflect on whether there are any other areas that may have fallen through the cracks and not been included in your warmups, so you can review them and sure up those areas before continuing to the end of the course.