Nice work! I'm glad to see that you went ahead with an ellipse guide for this exercise, and judging based on the size of the wheels, you likely went with one of the master ellipse templates. Even though this does add some annoyance and limitations in terms of how big you can work, I think it's definitely the right call to be able to focus on the core of this challenge, in dealing with the character of each wheel and its tire tread, rather than fussing over freehand ellipses (which of course will continue to be practiced with the many exercises we've addressed previously).

As a whole your constructions are looking good. One thing I did notice was that there are definitely a lot of cases where you've mainly focused the wheel construction around a basic cylinder - 8 stands out in this regard, as does 6, 9, 13, etc. This may be due to some of the ellipse guide limitations, depending on the nature of the one you picked up, but I did want to mention that wheels are generally just a little more nuanced than basic cylinders. They tend to be a little smaller along the sides, resulting in a bit of a curve in its profile, rather than a straight edge across from one end of the cylinder to the other. I demonstrate this in this step, where I place a larger ellipse in the middle (signifying a larger circular cross-section), although for some wheels we might opt to split that into two, pushing them closer to either end to maintain a mostly straight profile across, with tapering on the ends.

There are no doubt some wheels where a basic cylinder is going to be the best bet, but the nature of pneumatic tires generally lends to this sort of dispersion of air and pressure, so keep an eye on that.

Moving onto your tire treads, throughout most of these you've made good calls in terms of how you choose which marks to put down, where to focus on certain cast shadow shapes, and where to work more in line. There's definitely been some experimentation, but I can see that you've clearly tried to apply the principles from Lesson 2's texture section - for example with number 4 you did explicitly draw out each 'chunk' tread form, but moving onto number 5, you tried to work more strictly with shadow shapes. This shows me that you were aware of the problem, and were trying to navigate your way around solving it. 11's another example where you handled capturing the cast shadows rather than the outlines quite effectively.

Of course, because of the scale of these, when we deal with smaller, shallower grooves, as is with most of these tires, lines make sense, and work fine. It's the chunkier treads that give us an opportunity to explore this more complex problem.

Lastly, you may not have had much fun with the rims, but I think you pulled them off quite well. You've got a wide variety here, and you've drawn them each to feel solid and cohesive, capturing some remarkably fine detail in a relatively small space. Well done!

All in all your work here is looking solid. I'll go ahead and mark the challenge as complete.