Starting with the structural aspect of the challenge, I'm definitely glad that you still opted to go with the ellipse guide - it can be a little intimidating, and when the master ellipse template only offers us fairly small sizes, that can definitely be daunting, but it's still well worth it (as you found out yourself). I am pleased to see that you were aware of the usefulness of making the wheel widen through the midsection (the bubbliness you spoke of), which really helps to convey the sense that the wheel is inflated and would land with a bounce, rather than a heavy thunk. Of course, the degree to which we want this varies between different wheel designs, so I'm glad to see that you chose to apply that differential more or less depending on the wheel in question.

One small thing I would change - and the reason for this will become clear as we get to the textural aspect of the challenge - is that I can see when handling the spokes of your wheels' rims, you tended to fill in the side plane of those spokes with solid black. While I am glad that you paid attention to these side planes at all (one of the issues I look out for is whether students actually define them so as to convey their thickness and make them appear more solid, as well as whether they ensure they connect with the inner tube of the rims further back so as to convey their depth), in this course we want to try and reserve our filled areas of solid black for cast shadows only. What you're doing here is more akin to form shading (making a surface light or dark based on its orientation in space relative to something else), which as discussed here is not something we want to worry about in this course.

Continuing onto the textural aspect of the challenge, you are already a fair bit ahead of most students. This challenge serves as something of a trap - it's pretty normal for students to be so far removed from Lesson 2 that they forget the textural concepts exist at all, and so instead of going back and refreshing their memory on how it all works, they either dive in with purely explicit markmaking (basically constructing/outlining each and every textural form in its entirety), or try and work with what they remember from Lesson 2, rather than necessarily going back and checking it.

While I am very pleased that you're consciously aware of the importance of using implicit markmaking techniques, where you're falling short here is that you're not working with cast shadows much of the time. Rather, it seems that you may be trying to work more based on what you see - that is, drawing the shadows you observe in your reference, rather than using the reference to identify the presence of textural forms, and then designing shadows based on your understanding of how they relate in 3D space to the surfaces around them. Be sure to review these reminders from Lesson 2, as they elaborate on this process and how it differs from simply drawing what you see.

In all fairness to you, 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, which can steer us away from this use of cast shadow shapes - 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 noted above, this is very much an intentional trap - a way to remind students that things do get forgotten as we move through the course, and that as we approach the last lesson, it is a good idea to reflect on what we may have left behind, and what may merit some review. So, I'll still be marking this challenge as complete.