Starting right in with the construction you have by and large done really well throughout these, and have made excellent use of your ellipse guide. I'm very pleased to see the use of a variety of ellipses to build out the profile of each wheel, although there are a few that I'm... let's just say, a little suspicious of. Specifically, with wheels like 13 and 21, these end up looking really straight acrooss their widths, with no bulging through the midsection that typically gives the impression of an inflated tire.

Given that you are doing this correctly for all your other wheels, I can only assume that your reference wheels were indeed this stiff. It's definitely not wrong, but there is value in looking to see if there's any bulging, even slightly, in your reference image that can be exaggerated. Sometimes taking what is present in our reference image and pushing it further than it is there can allow us to successfully convey what it is we see, more than what is strictly present. It's a bit of a weird situation where earlier on we stress the importance of drawing what is actually there, but in truth our brains and eyes do a lot of processing of the things in front of us, and what people really look to us to capture is the human experience of an object (through all of its filtering), even if that isn't strictly accurate. But of course that probably goes well beyond the scope of what we're doing in this course.

Getting back to that construction, I'm pleased with how you've handled the rims/hub caps as well, with a lot of attention to not only the front face that points outwards from the wheel, but also the side planes that give the structure its thickness and solidity. You've also done an excellent job of spacing out your spokes, even in some especially complicated ones like 22.

The other part of this challenge comes down to the tire tread and how we can convey its texture. It's pretty common for students to forget about the textural principles from Lesson 2, being as separated from it as we are here towards the end of the course, and this challenge serves as a bit of a reminder, like sitting on a thumbtack, that such things do not cease to be important. Fortunately, I'm pleased to see that you did not fall into my trap, and you've been doing a good job of focusing on the use of implicit markmaking rather than using explicit outlines for your textural forms - though there are some points I can draw your attention to.

If we look at number 20, what I'm seeing of those filled black areas is that they appear to be capturing the side planes of the tread forms, rather than the actual separate shadows that those forms cast. The difference is quite subtle, and can be very tricky to pin down, but it really comes down to how we design the silhouette of the textural form.

Here's a quick diagram of the difference. On the top we have the form drawn explicitly, with outlines. Then going down from there, on the left we've just filled in the side planes, whereas on the right I've eliminated all of the internal lines and then drawn a separate filled shape outside of the form's silhouette, as the cast shadow. And below that on the third row, is an analysis of the specific elements of the silhouette being defined of the textural form, which includes more than just the basic straight edges, but also diagonal ones in between, capturing the 3D structure more completely, rather than just its top face.

This is of course quite a difficult point to grasp and it really speaks to the heart of why texture is so damn hard (especially considering that we have to figure this out without ever outlining those forms). As it stands though, you still showed a clear awareness of the fact that these tire treads must be tackled with textural/implicit drawing techniques, which puts you a little ahead of those who'd forgotten.

All in all, you've done very well. I'll go ahead and mark this challenge as complete.