Jumping right in with the structural aspect of the challenge, you've done a good job of building out your wheels' bodies using your ellipse guide. While technically speaking we'd use a wider degree for the farther end of each wheel than the end that's closer, with the more limited ellipse guides most students employ don't have a lot of intermediate options, this essentially forces you into one of two choices - either jump to the next degree available and end up with a somewhat dramatically foreshortened/distorted wheel, or stick with the same degree throughout the body of the wheel. While either is arguably acceptable, I think the choice you made - to use the same degree throughout the entire wheel's body - is the more correct one. Since wheels are generally pretty narrow, there isn't that much foreshortening that'll occur, so the absence of that foreshortening manifesting with a degree shift isn't really noticeable.

In addition to this, I'm pleased to see that you've generally been pretty mindful of building out your wheels' spokes/rims in stages, respecting both the definition of the outward faces as well as their side planes to ensure that they look structurally solid.

Continuing onto the textural aspect of the challenge, this is an area where many students fumble, and by design. Being as far removed from Lesson 2, where we introduce how we engage with texture in this course, specifically through implicit markmaking (that is, drawing the shadows textural forms cast on their surroundings, rather than the forms directly), it's very common for students to forget those concepts and require a refresher, and so by assigning a heavily texture-focused exercise here, we can provide a much more poignant reminder that students should perhaps go back and review that material, as well as anything else they may have allowed to slip through the cracks and that might merit some extra attention.

Implicit markmaking is important both because it hinges on the same spatial reasoning this course focuses on. The shadow shapes we design based on the information we observe from our reference images, rely on us considering how those textural forms exist in 3D space and relate to those around them (as discussed here in these reminders from the texture section), and so just as the intersection lines between forms define the relationships between structures we might construct through explicit markmaking (outlining and other forms of direct representation of those forms), the shapes of our cast shadows allow us to convey relationships between forms that we don't draw as directly. This also provides a foundation upon which we can base our choices when we try to imply details - that is, where we're not necessarily trying to pack a drawing full of detail (which can create unintentional focal points, drawing the viewer's eye whether you mean for them to or not), and are instead relying on the viewer's brain to fill in the gaps.

In order to achieve that, the marks we do put down have to follow a logic that the viewer's brain can latch onto and understand (in order for them to extrapolate from it), so using cast shadows, which adhere to general physical rules/properties that the viewer will already be well accustomed to as an observer is an effective approach.

Needless to say, this is an area where you've obviously fallen into that trap - you don't appear to have considered the use of implicit markmaking here at all, and instead applied explicit markmaking to all of your textural forms, either outlining/constructing the protruding structures or simply drawing the grooves themselves as lines. While this isn't ideal, it's not unexpected. It's simply a sign that you've allowed those textural concepts to slip through the cracks, and will definitely at the very least want to go back and review the notes and videos from the texture section of Lesson 2, in particular the use of implicit markmaking, which is all about drawing the shadows our textural forms cast upon one another, and how the shapes of those cast shadows themselves are what convey to the viewer the relationship in 3D space between the form casting it, and the surface receiving it. While you may wish to consider the texture challenge (which is the texture analysis exercise done 25 times), that is not required.

Similarly, it may well be worth reflecting upon what other concepts from the course thus far may have been allowed to slip through the cracks, what exercises may have been left out of your warmup rotation, and so forth.

Anyway, I'll leave you to that, and will go ahead and mark this challenge as complete.