Starting with your constructions, you've by and large made pretty good use of your ellipse guides here, incorporating plenty of different ellipses to flesh out the overall structure of each wheel, with plenty of them including a nice profile that includes smaller ellipses along the outside, and a bit more volume through the center to give it that nice "inflated" look that tires really require. You're also doing a good job in building out your rims - there are some where the spacing on the spokes is perhaps a little off (like 23) although it's slight and all things considered, it is a rather difficult thing to nail that really just comes with experience and practice. I mean, there probably are very useful tools/techniques for figuring out how to space a bunch of elements out along a radial but that is certainly outside of the scope of this course.

When it comes to the textural aspects of your tire treads, I did indeed give you a bit of a hint when it comes to the importance of cast shadows over form shading, and you put it to good use. Fortunately, I am not without issues to call out, but I think 25 really does a great job of demonstrating an understanding of how these different forms sit together and create a whole. You did technically use explicit markmaking here to flesh out the different textural forms, but the result still isn't terribly noisy, and more importantly I can see that you were thinking through each separate cast shadow, designing them individually based on your understanding of the existing structures.

Furthermore, there are plenty of other examples where you're employing more strictly implicit markmaking, and while those shadows aren't designed quite as well, it is completely understandable that this is still an area that you will continue developing with practice. What matters to me most is that you're putting a ton of effort into thinking about the nature of each shadow, and how it relates back to the form casting it.

There is really just one thing that I want to suggest to adjust your approach for some of these. I did notice a handful of cases where you ended up working somewhat subtractively - that is, you'd lay down your cylinder/wheel structure, and then when adding the chunky tire treads, you would actually cut back into the existing structure, rather than treating each new tread chunk as its own separate addition to yet further expand the footprint/volume of the wheel as a whole. For example, we can see this in 18, 22, and to a lesser extent in 20.

In truth, the mindset you employed in 25 is exactly right - just like with the organic constructions in lessons 4 and 5, we're always building up, as it is simply very easy to accidentally cut into the silhouette of the existing structure, rather than cutting into it in three dimensions. We see this a great deal in 22, where the result just kind of zigzags back and forth across the existing silhouette. Along the face of the wheel it's okay, but it kinda loses its structure on the opposite side.

In this regard, number 6 was definitely better at its base (in terms of building up solid, 3D structure within the structure that already existed), but adding line weight was a mistake. In tracing along the existing silhouette and jumping from form to form, you were reinforcing the silhouette as a flat shape. Try to keep your use of line weight more limited to establishing how different forms overlap one another, keeping it limited to the specific areas where those overlaps occur, and avoid having line weight jump from one form to another as much as possible. I like to think of the application of line weight that jumps from one form to another like pulling a sock over a complex object - it tends to smooth everything out way too much.

So! I'll go ahead and mark this challenge as complete. Keep up the good work, but take note of the points I've raised as you move forwards.