Very nicely done! Starting with your construction, you've done a pretty solid job of building out your wheels, both in terms of the cylindrical structure as well as with the assorted style of rims. There are a few things I'll call out when discussing the tire treads, but all in all your work here has come along well, and I'm especially pleased to see that you didn't always just limit yourself to constructing a simple cylinder - by using a series of ellipses along the length of the wheel, you captured the more subtle bulging through the midsection of the wheel, while retaining the form's solidity.

Moving onto the tires, what I like most about this challenge is the fact that it serves as a reminder. By the time a student reaches this stage, they're many lessons removed from Lesson 2's texture section, and so most of them tend to forget some of the nuances of capturing textures.

The key to pinning down these tire treads is the same as handling any texture - it is to focus not on the patterns of lines that you can see in your reference, but to identify the actual individual forms that are present. Every solid piece of rubber, and the spaces that separate them. The marks we draw are not defined by these gaps, but rather by the shadows each form casts into the gaps and onto the forms around them. We imply these textures through shadow shapes alone, and refrain from using line as much as possible.

One common mistake students make is something we can see in your 9th wheel - you constructed each tread "chunk" separately, defining its various faces and separating them with edges. Similarly, looking at wheel 19, you also have some places where you tried to separate these different planes of a given tread form by filling in the sides with solid black. As shown here, it is much more effective to draw the shadow actually being cast by the form, and simply leaving the inside of the form itself empty. The corners of the form's silhouette are enough to imply the shift from one face to another, so they don't need to be established quite so explicitly as you have done.

You've done similar things with the tread in #25, as well as in the rims for 13, 16, 20, 22, etc. The silhouette of a form really is incredibly powerful, but it's easy to undermine its effectiveness by putting too much on the page. So don't be afraid of pulling yourself back, and letting the silhouette do its job. That is definitely at the core of how we approach simplifying the tire treads.

As for students who did a good job capturing the tire treads, because of how I like using this challenge to provide the shock of "you forgot this thing you learned in lesson 2!", most students make similar mistakes. One recently who did a better job than most however, was EvilBob. You can see some of his wheels here.

Another example, though not actually tire related, that can help is this old demo I have of drawing the scales of a viper around a cylinder. Note how every textural mark is a cast shadow shape, rather than a stroke or an outline, and so they're able to expand and shrink in response to the surface's relationship with the light source.

Anyway, all in all your work here is still very well done. I'll go ahead and mark this challenge as complete.