Starting with the structural aspect of the challenge, I think a lot of the calls you made in terms of where to use that perhaps-too-big ellipse to create the bump in the center, and where to leave the bump out (for your airless tire) were completely spot on, and show a deep understanding of how the way in which you approach drawing a structure impacts the way in which it is interpreted by the viewer. Beyond that, as a whole your wheels came out looking very solid, both in terms of that important bump in the center to create an inflated effect, but also in how you've worked with the limitations of your ellipse guides, and in your attentiveness to establishing the side planes of each of the spokes of your rims, rather than merely focusing on the outward face. I can imagine it may have been tempting only to draw that outward face as well, given how little room you had to work. All in all, structurally speaking you've knocked it out of the park.

I will say that we are working on the ellipse guide matter ourselves - a whole set is of course super expensive (over $100), but while it probably won't be out in time for you to take advantage of it, I've already gotten our pen sales guy a laser cutter, and we've sourced acrylic, and we're working on figuring out how we can reasonably offer a full set of ellipse guides (with decent degree/size increments, up to around 3.5" on the major axis) for something closer to the $30 range.

Fortunately though, that master ellipse template will serve you quite well in Lesson 7 - not so much with the structures themselves (though there will be places where it'll come in handy there as well), but more with the basic way we use ellipses to lay down a basic unit grid in order to nail the vehicles' proportions. This doesn't require large ellipses, and since it's something we do right at the beginning, we have some leeway to choose our degrees, so we can pick the ones that fit before having the rest of the structure abide by it.

Anyway! The second part of this challenge is about how we capture and convey the actual texture of the tire tread, and up until you hit 19, you were definitely relying a lot on explicit markmaking, so we should still talk about that first, before we get into your later strategies.

Constructing all of our textural forms actually works perfectly fine, when we're looking at the wheels floating arbitrarily in the void - but when we actually start diving into wheels that are part of a larger construction, it gets tricky. Reason being, when we have wheels with really complex treads, with each form fully constructed, it results in a lot of lines packed into a compressed space, which will catch and draw the viewer's eye, whether you want it to or not. It becomes an issue of controlling composition, where the viewer's eye falls, and so those explicit marks actually become troublesome, and we need some way to reduce the visual complexity there without changing the nature of the texture we're conveying. That's where implicit markmaking comes in, which, as you can see in this non-tire example of an african bush viper's scales, we can make the shadows deeper or smaller without changing the number and nature of the scales being implied.

While you did catch onto this issue later in your set, the way in which you actually approached drawing those filled black shapes do have some room for improvement however. What I noticed was that you appear to actually use the filled black areas to represent the side planes of those textural forms - not to establish the shadows they cast on their surroundings. This is not an uncommon issue, and I actually already have an explanation using another student's work prewritten, so I'll share that below.

First, take a look at this diagram, then read the quoted explanation:

On the top, we've got the structural outlines for the given form - of course, since we want to work implicitly, we cannot use outlines. In the second row, we've got two options for conveying that textural form through the use of filled black shapes. On the left, they fill in the side planes, placing those shapes on the surface of the form itself, and actually filling in areas that are already enclosed and defined on the form and leaving its "top" face empty. This would be incorrect, more similar to form shading and not a cast shadow. On the right, we have an actual cast shadow - they look similar, but the key point to pay attention to is shown in the third row - it is the actual silhouette of the form itself which is implied. We've removed all of the internal edges of the form, and so while it looks kind of like the top face, but if you look more closely, it has certain subtle elements that are much more nuanced - instead of just using purely horizontal and vertical edges, we have some diagonals that come from the edges of the textural form that exist in the "depth" dimension of space (so if your horizontals were X and your verticals were Y, those diagonals come from that which exists in the Z dimension).

And that about covers it! All in all, I'm very pleased with your results. The textural issues are still pretty expected - we're far enough removed from Lesson 2 that students tend to forget, and so this challenge serves as a reminder before we hit the end of the course. That said, you picked up on the issue yourself and attempted to correct it, which is good to see.

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