Jumping right in with the structural aspect of the challenge, to start you're doing a good job of constructing the core body of each wheel quite well, taking care to use your ellipse guides to build the structure such that each one widens appropriately through the midsection, and includes an appropriate amount of degree shift for the size of the object.

When it comes to the spokes of the wheels' rims, while this didn't necessarily occur all the time, I did notice a number of cases where it seemed the side planes of those structures were extended farther beyond the inner tube of those rims. That explanation is no doubt confusing, so here's what I mean on one of your wheels. As shown in blue at the top, you handled that one correctly (with the back edge of the side plane stopping earlier than the front edge of the same plane). Further down in red, you had the back edge extend as far forward as the front edge, causing it to stop at the same ellipse. I marked out in blue on top of it what would have been more correct.

I also noticed other cases where the side plane wasn't included - I imagine this may be cases where the spokes were very thin in that dimension, though I'd still recommend including as little of a side plane as you can manage to fit in there. It may cause it to appear thicker than your reference, but it'll add solidity that gets lost when we essentially make those structures paper-thin.

Continuing onto the textural aspect of this challenge, this is where things turn into a bit of a trap. Given that we're as far removed from Lesson 2 and its concepts of texture, it's very common for students to either forget that we've addressed these topics altogether (and they end up drawing all of the textural forms just as they would the structural ones - building them up through construction, with outlines and explicit markmaking throughout), or they remember that we leverage filled areas of solid black, and attempt to work that into their approach (but without necessarily going back and reviewing that material), so this challenge serves as a great reminder that sometimes you just gotta go back and check. It's quite rare for students to actually approach this through the implicit markmaking that Lesson 2 actually introduces for conveying texture.

In your case, you fall into the second group - you're not trying to construct everything, and I can see you attempting to leverage filled areas of black, but you tend to fill in the side planes of your textural forms, which is more similar to form shading, where a surface gets darker as it points away from a light source, and lighter as it points towards it. This unfortunately is still a form of explicit markmaking however, meaning we're drawing the forms themselves directly.

The problem with this is that while it allows us to draw very nicely detailed wheels when they're floating in a void as they are in this challenge, once they become a part of a larger illustration, all of that densely packed detail will draw the viewer's eye to it whether you want it to or not, limiting your ability to control how the viewer experiences the piece. This pertains to the topic of composition, which falls outside of the scope of this course, but we do try to give you some tools that'll help you with it in the future.

Instead, we want to use implicit markmaking, which means that what we're drawing is not the textural form itself, but rather the shadow it casts. How that shadow is designed (including how it projects onto and wraps around the surfaces it falls onto) is enough to provide the viewer with information on how the form casting the shadow relates in 3D space to the surfaces it is cast upon.

As shown in this diagram, depending on how far the form is from the light source, the angle of the light rays will hit the object at shallower angles the farther away they are, resulting in the shadow itself being projected farther. While we don't have to worry too much here about the specifics of getting all the light sources right and fully consistent, the fact that this truth exists frees us up to draw the shadows cast by the same kinds of textural forms differently - we can make some smaller (even so small as to not be visible) and others larger, getting up to the point that they start merging into complex shapes, despite the textural forms being the same. This gives us one important thing: control over where we want to include lots of detail density (where the shadow shapes are at a sort of medium size, allowing a lot of light and dark together) as well as areas of low detail density (where the shadows are too small or too big, resulting in larger expanses of flat black or white.

All of which is to say that, given how far removed we are from Lesson 2, it's a good idea to go back and review that material (starting from but not limited to these reminders), as well as to consider any other areas of the course you may have allowed to slip through the cracks, so you can review them as well before moving onto the last lesson and finishing up the course.

One last thing I wanted to share is that when it comes to those tires with shallow grooves, or really any texture consisting of holes, cracks, etc. it's very common for us to view these named things (the grooves, the cracks, etc.) as being the textural forms in question - but of course they're not forms at all. They're empty, negative space, and it's the structures that surround these empty spaces that are the actual forms for us to consider when designing the shadows they'll cast. This is demonstrated in this diagram. This doesn't always actually result in a different result at the end of the day, but as these are all exercises, how we think about them and how we come to that result is just as important - if not moreso.

Anyway, as this is very much an intentional trap, I'll still be marking this challenge as complete. Just be sure to review the texture material, and anything else that may require it.