Starting with the structural aspect of the challenge, I'm glad to see that though you started out attempting to freehand your ellipses, that you ultimately ended up going with the recommendation in the lesson material to utilize a potentially more size-limited master ellipse template. While it certainly results in smaller wheels, it's still massively more beneficial, as it keeps our focus on the tasks and concepts that are central to the challenge itself, rather than pulling cognitive resources away to deal with things like freehanding ellipses accurately and reliably, which other exercises tackle more directly.

When it comes to the body of the wheel, I'm pleased with the arcing profile you've captured for each of these - that is, the widening of the wheel through the midsection to help create the impression that the tire itself is inflated, landing with a bounce rather than a heavy thunk. I did notice that in some cases, like number 19, you didn't draw a central ellipse - if this was because of the limitations of your ellipse guide, then that's fine, but otherwise it is best if possible to include that central cross-section to help build out the structure without trying to do too many things in a given step. The central ellipse gives us an additional "point" through which to pass our curve, helping us maintain a more consistent structure.

Looking at the spokes of your wheels, I have a few points to note:

• Firstly, for really thin spokes like those in number 20's bike-style wheel, if at all possible try to represent these with two lines each (representing the edges on either side of the form, rather than the whole form as a single line). A single line isn't enough to convey it as a solid, three dimensional structure, so while this consideration may make the spoke a little thicker than it realistically would be, it's preferable.

• Secondly, on 18 I can see a case where you drew the outer faces of your spokes, but not their side planes. This is not a consistent issue, but it is something I saw sporadically throughout. In the case of the bike wheel spokes from the previous point, we skip this for two reasons - firstly, we're already increasing the thickness more than is realistic to avoid having the forms represented with a single line each, so we're kind of pushing our boundaries already, so we settle on a compromise. The other significant point is that those spokes are more tube-like, so they don't actually have clear and distinct front/side planes. In the case of number 18 however, those spokes would definitely have an outward face, and a side plane, and capturing both will help the structure feel more solidly three dimensional.

• In cases like number 10, you did draw the side planes of those spoke structures, although you filled them in with solid black. In general we want to reserve our filled areas of solid black for cast shadows only, where we design an entirely new and separate shape based on the relationship between the form casting the shadow and the surface receiving it. When we fill in the side planes of a structure, it's more akin to form shading, which as discussed here in Lesson 2 is something we're generally avoiding throughout this course. Any situation where you catch yourself filling in a shape that already exists as part of the drawing, take a step back and ask yourself whether you're really filling in a purpose-designed cast shadow, or slipping back into applying form shading to your structures. This ban on form shading is of course just a restriction for the work we do in this course specifically - and it's a policy that is somewhat newer than some of our demonstration content, so you may see some contradictions of it (that will be corrected as the overhaul progresses). So, keep that point in mind as you continue forwards.

Continuing onto the textural aspect of the challenge, this part serves as something of a trap for many students, as it's quite common for them to forget all the principles regarding texture from Lesson 2 and neglect to review them, so they just end up tackling texture in whatever way feels comfortable to them. Your case however is one of the few that didn't stumble straight into the trap. While there are certainly cases where you filled in the side planes of textural forms instead of drawing the shadows they'd cast, there are many cases where you've definitely and very specifically worked with cast shadows, and I'm pleased to see it. I noticed this in cases like 16 and 17.

The only issue with these examples (16 and 17) that I would call out is that it appears you may have been "painting" the shadows on stroke by stroke. Instead, approach them in a two-step process, first outlining and designing the shadow shape you wish to draw, while considering the 3D relationship between the form casting the shadow and the surface receiving it, then filling it in.

I do want to note that when you were drawing the protruding structures on number 17, your linework was visibly sketchy, suggesting that instead of applying the ghosting method to your linework to keep it clean and intentional, you allowed yourself to fall back to older habits, approaching them more hesitantly and not committing to full lines, but rather chicken-scratching a little. This suggests that you may not be as in control of the choices you're making as you should be. This often happens when students try to do too much on a small amount of time- the conscious act of choosing what to do next often takes a back seat. So, the solution is to give yourself more time, and perhaps to spread the work out more across separate sessions.

The last thing I wanted to mention is that when it comes to textures that are made up of shallower grooves instead of big chunky protrusions, it can be easier to fall into the trap of drawing those grooves as lines or filled areas based on what we're observing from the reference image, rather than based on what we understand of how those forms relate to one another in 3D space. In other words, skipping the "understanding" step discussed here. It can also be easier to get away with it as well, as the distinctions between the results of one actually considering the information in 3D vs. one simply copying what they see from the reference directly, become far harder to pick up on. While one might think that if the result is basically the same in certain cases, then it's effectively equal - but our purpose here is not to create pretty drawings, or to be concerned with the end results. It's to rewire the way in which we think about what we're drawing.

In this diagram I explain how those grooves - or holes, or cracks - in textures function in 3D space. We often try to draw them directly as lines, or to fill them in completely, because the name we'd ascribe to the texture refers not to 3D forms, but rather empty spaces where no form exists. A hole, a crack, a groove - these are all negative spaces. It is very easy however to hyperfocus on the named element, even if the actual forms are the walls that surround those negative spaces, which cast shadows upon one another. While the difference in the result may be minimal at best, what matters is the difference in how you're thinking about the forms at play.

Anyway, I'll go ahead and mark this challenge as complete. Hopefully the information I've offered will prove useful as you continue forwards.