Starting with the structural aspect of these wheels, your work is fantastic. You're mindful of maintaining the illusion of solid, 3D form both in your wheel structures (where you're also sure to widen the structure through its midsection to create a sort of "inflated" impression, making it appear as though it would land with a bounce rather than a thud), as well as in the spokes of your rims, where you've ensured not only the solid construction of the outward face of that structure, but also their side planes and how they connect to the inner tube of the wheel.

You've been mindful of these things both in places where they are more easily executed (in terms of the constructiosn being quite simple), but you also did an excellent job with this in cases like number 9, which definitely get considerably more complex, as well as cases like number 9 where those little holes could very easily be overlooked.

Continuing onto the textural aspect of the challenge, this is where the challenge serves as something of a trap for those students who, being as far removed from Lesson 2's textural concepts, have kind of let them untouched for a long time, and as such have forgotten to apply them altogether. It's not uncommon to see students trying to define their textural forms in purely explicit, constructional markmaking, rather than focusing on the use of cast shadow shapes to imply their presence.

You did not fall into this trap - and you're one of the very few who hasn't - but there are some ways in which your approach still isn't entirely correct. Looking at number 21 as an example, I noticed that while you were definitely focusing on the use of shadow shapes, you had a tendency to fill in the side planes of your textural forms - something more akin to form shading where the surface gets darker as it turns away from the viewer, or lighter as it turns towards the viewer - instead of focusing on designing a new shape to represent the shadow being cast, and therefore the relationship between the form casting that shadow and the surface receiving it in 3D space.

I have illustrated this here - in the bottom one, which lays out the structure of the textural form in red, then marks out the area you filled in with orange, illustrates how the section you filled in was the side plane of the form. The upper example does the same, but instead marks the actual cast shadow out in blue - note that it is not on the textural form itself, but rather being cast by it onto the surrounding surface.

This is a very easy mistake to make, especially when we rely too heavily on observing our reference. That's not to say observation is bad - it isn't, by any stretch. Observation is always good, but it serves a specific purpose. Observation is how we derive information, and it is that information that we consider and use to decide on the next action to take, the next mark to draw. If however we focus too much on only observing, we can fall easily into the trap of seeing filled areas of solid black, and carrying them over into our drawing.

While the difference seems small - and in a lot of ways it is - the result is quite significant. As with all explicit markmaking, we're effectively telling the viewer that everything that has been drawn is what exists in the scene, and everything that has not been drawn does not exist. This leaves no room for implied detail - though you certainly made a valiant effort to use it to that end, the result often doesn't come through as clearly to the viewer as it could. Working with implicit markmaking however disconnects what is drawn from what is actually represented in this object. Cast shadows are subject to the light shining on the object - if that light hits the textural form from a lower angle, it results in a much longer, more expansive shadow shape, and if that light hits the textural form at a higher angle, it results in a smaller shadow. This gives us the control to be able to decide where we want our detail to be concentrated, while being able to still tell the viewer that these textural forms exist in the same manner across the surface, even if they've not been drawn.

This dynamism in shadow length can be seen here - that diagram illustrates a texture arranged along a surface, seen from the side. Where it's closer to the light source, the textural forms are hit at a sharper angle, resulting in a smaller shadow shape, and where it's farther away, the shadow shape gets longer due to the shallower angle. This is precisely why in the texture analysis exercise, the shadows get smaller and eventually disappear as we move from left to right, towards the light source.

I do want to reiterate the point that your work here is still considerably further in the right direction than most - so don't take this feedback as declaring that you've done things wrong, just that there's still room for further growth and improvement as you solidify your understanding of these spatial relationships.

The last thing I wanted to quickly call out is that when you're adding texture, it's important that you always push yourself - regardless of how much time it might take - to be as consistent with the layout of those textural forms as you can be. That means ensuring that they protrude cleanly and consistently from the tire's silhouette when that is needed (looking again at what I drew on top of wheel 21, you'll notice that your protrusions are not very clear and consistent, and I pushed them much further in my correction). It also means maintaining regular patterns as they wrap around a non-flat surface, which is a lot harder than it sounds. Plotting down little points to ensure regular spacing can help - I demonstrate this in the demo video for this challenge, towards the second half, so be sure to give that another look.

Anyway, all in all, good work. I'll go ahead and mark this challenge as complete.