Starting with the structural aspect of your wheels here, you've done a great job and have really put a lot of time into establishing some really complex, but still structurally solid rims/spokes throughout your wheels. You definitely didn't shy away from picking more difficult ones, but you certainly met the challenge you set out for yourself.

One thing I would recommend when it comes to the overall wheel's core cylindrical structure is to add a bit of a larger ellipse for the one in the center. Given the limitations of the master ellipse templates most students use, you needn't worry about achieving a degree shift for this, but just getting an ellipse that is a bit larger in the middle will give the wheel an arcing profile, which helps imply that the wheel is more inflated, and would land with a bounce, rather than the sort of heavy thunk we get from a solid mass. This is mentioned in this step from the instructions, and I see a little of it in your earlier wheels (like 11 and 12) but it tends not to be present in many of the later ones. This was likely because the reference itself didn't demonstrate this effect as obviously - it's one of those things where we're considering what it is we want to convey to the viewer about this wheel, and that may at times mean exaggerating things that aren't there in as obvious a fashion.

Continuing onto the textural aspect of the challenge, here you've done a very good job at drawing wheels that look fantastic, but when it comes to the principles of texture from Lesson 2, they were certainly neglected to varying degrees. Ultimately this is expected - I intend this challenge to serve as a trap for those who, being as far removed from Lesson 2 as we are, have forgotten those concepts. It helps to give a bit of a rude reminder that we may want to review some of the concepts we've left behind before finishing up the course.

The main issue comes down to this - your wheels are lovely, but they're very visually dense, and when they become part of a larger vehicle construction, they will draw the viewer's eye whether you want it to or not, becoming focal points in the composition. This is the curse of "explicit" markmaking as discussed in Lesson 2. We draw everything exactly as it is, and so there's little room for flexibility. The marks that exist on the page, and what is actually being conveyed to the viewer (in terms of which forms/structures exist) are inextricably linked. Remove a mark, and you remove a form.

Implicit markmaking is different in that it disconnects what is being conveyed from the specific marks that are drawn. We imply the presence of those forms not by drawing them directly, but by drawing and designing the shadows they cast on their surroundings, based on what we understand of their relationship with those surrounding surfaces in 3D space. You can read more about this in these reminders, and you can see what I mean in practice with this example of african bush viper scales. Because we're drawing cast shadows, and cast shadows can change in their properties based on the behaviour of the light, we can bring that light in close and turn it up to blast away a lot of the shadows, or we can pull it back and allow more of those shadows to be present. Either way there's still enough information in what shadows are present to convey the gist of what textural forms are there on the surface. This is the foundation of "implicit detail" which is used a great deal in all kinds of illustrations where giving all the detail would be more distracting than useful.

Another point I wanted to mention is that students tend to have more difficulty when engaging with textures that consist of grooves, cracks, or holes. As explained in this diagram, we tend to look at those named things - the grooves/cracks/etc. as though they're the textural forms we're meant to focus on, because they're what we can ascribe names to. This is not the case however - those things are an absence of form, they constitute negative space. The actual forms are the walls that surround them, which cast shadows upon one another and onto the floors at their depths.

Because students have a tendency of making this mistake, they'll focus on drawing the grooves, like you have done in cases like 19 and 20. They draw the negative space, and so they don't end up considering the forms at play. When approaching any texture, always ask yourself, "what are the forms that make up this texture, how do they relate to one another, and how does that influence the shadows they'll cast?"

The last thing I wanted to mention is more of a quick reminder. It's not at all uncommon for students to confuse cast shadows with form shading, and end up filling in the side planes of their forms, thinking that they're actually drawing the cast shadows. While I can't speak to your intent, 18 is a good example of where the side planes of textural forms were filled in with black.

Remember - cast shadows generally involve introducing a new shape, which you'll design as you draw its outline, then filling it in. It's this shape design that captures the relationship between the form casting it, and the surface receiving it. If you find yourself tempted to fill in an existing shape in your drawing rather than introducing a new one, take a step back and ask yourself whether it is truly a shadow you're drawing. There are cases where it may will be - even when it comes to the aforementioned cracks/grooves, there are cases where the end result is the same whether you're thinking of it correctly, or if you're just drawing the grooves, because sometimes that groove is deep enough, or the lighting is such that it's going to be filled in anyway. The important part is taking that moment to ask yourself and assess whether or not in this case, it is the appropriate course of action.

So! As this is very much an intentional trap for students, it's not one I generally hold them back over. So I will be marking this challenge as complete - and again, your wheels here are very beautifully drawn. It all comes down to how they relate to the concepts of the course. So be sure to review that material, along with anything else you may have left behind.