Starting off with your wheel constructions, you've done very well. I can see that you've gone well beyond just building up basic cylinders, but rather incorporated as many ellipses as were needed to achieve the more nuanced, "inflated" form of those wheels and tires. You've also made good use of your ellipse guides throughout the process, and have tackled a wide variety of different styles of wheels and tread patterns.

Now, moving onto the tire tread patterns themselves, we get into the trap I've laid for students. By the point one reaches this stage in the course, we end up being pretty far removed from the textural principles shared back in Lesson 2, so it's pretty common for students to completely forget about things like implicit markmaking techniques and the use of cast shadows.

Tire treads are basically made up of a bunch of small forms that adhere to the surface of a larger structure. Sometimes they're grooves (subtracting from the surface of a form, though that's the same thing as having a bunch of other forms built up around the grooves) and other times they're larger chunks. The grooves are less of an issue, but it definitely stands out more when we use explicit markmaking techniques (outlining the textural forms themselves, or approaching them the same way we'd construct larger structures) for the chunkier treads like those in 13, 15, and 17.

There you've ended up with a lot more visual noise since you opted to outline everything in its entirety. Instead, the marks we put down should consist primarily of the shadows being cast by those textural forms, as this would allow us to more strategically focus that visual information in specific areas, instead of having to cover the whole thing. We can control how directly the light hits the surface to allow for some areas to get blown out, while allowing others to receive just enough light to convey information to the viewer. That's our focus here - it's all about giving the viewer just enough to communicate what's going on. That isn't generally going to require covering the whole structure.

While this isn't a tire tread, I find sharing this example of a viper's scales helps convey the same principles.

So! All in all, you are doing fine, just be sure to review the principles of texture and implicit markmaking from Lesson 2. Most students end up being sent there for a refresher at this stage, so it's nothing abnormal.

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