It may have been wheely hard, but damnit you've done a fantastic job - so alas, I don't expect this critique to be all that long.

Starting with the structural aspect of the challenge, you've made excellent use of your ellipse guide to create a gentle arc for the wheel's profile, making it appear more inflated, as though it would land with a bounce rather than a heavy solid thunk. You're aslo taking good care with the spokes of your rims in most cases, defining not just the outward face but also the side planes, to help make them feel more solid. For the bike wheel in 15, you did end up using lines for the spokes - I wouldn't recommend this, as lines don't have any dimension. The approach used in 21 is far better, even if it means making them a little thicker than you intend.

Carrying onto the textural aspect of the challenge, where we look at the actual tire treads, this challenge reveals itself as something of a trap, intending to catch all of those who, being as far removed as they are from Lesson 2, forget about all of the concepts of implicit markmaking. You, miraculously, jumped over my little trap, and showed clearly that you were indeed thinking about those textural principles, and applying them quite well in many cases.

The one spot where you do still run into some trouble are the textures that involve very shallow grooves - but these are definitely a lot trickier to deal with, due to the tendency for students to look at the grooves as being the textural forms they're meant to focus on. That's of course incorrect - the actual forms in question are the walls surrounding the grooves, which cast shadows upon one another, and onto the floor of the groove as well. The difference is subtle - in a lot of cases the difference is unnoticeable in the end product, but we are concerned with how you think about 3D space. So cases like 13 and 20 do suggest this kind of issue is present, at least to an extent.

This diagram breaks down the difference between simply filling in the grooves with black, and actually considering the forms present and how light interacts with them to create cast shadow shapes, so give it a look.

The only other thing I wanted to give a quick reminder for is that in wheel 19, I noticed you filled in the side planes of your spokes. In this course, it's important to distinguish between whether we're actually drawing shadow shapes (which are usually going to be their own new shapes that are designed, conveying the relationship between the form casting it and the surface receiving it), or if we're just shading in the side plane of a form, filling an existing shape in, which is more akin to form shading (which we try to avoid applying to our constructions in this course). So, just be sure to keep that in mind.

And with that, I'll go ahead and mark this challenge as complete. Keep up the great work!