Jumping right in with the structural aspect of the challenge, one thing that umped out to me was that the ellipses drawn for your wheels frequently appeared to be of the same degree (although not always). They were also of a pretty considerable size, which when using ellipse guides is only something we get when we use the larger full sets of ellipse guides (which tend to be very pricey, as they involve many separate sheets, each dedicated to a particular degree of ellipses, allowing for a wider range of sizes on any one sheet). This made me suspect that if you aren't working with a full set of these larger ellipse templates, you may have accidentally picked up one or two sheets with a more limited range of degrees.

While this isn't that much of an issue in this challenge, it will be a problem in Lesson 7 where we use ellipses of varying degrees to construct bounding boxes to a specific proportional scale. For that, using a "master ellipse template" as recommended in the assignment section of the wheel challenge is better - it does limit the size of the ellipses you can work with, but it is more versatile and gives us what we need to construct those bounding boxes in Lesson 7. You'll find more information about all of this in the ellipse guide section of this video from Lesson 0.

Another point that stood out to me was that at a glance, some of your wheels appeared a little skewed, and upon closer inspection (which you'll find here) I checked the actual minor axes of the ellipses and they were off by a small but notable margin from the minor axis you intended to follow. Now this isn't a big concern if that alignment is estimated by eye, though most ellipse templates will have little markings to denote the minor axis, and sometimes the major axis, of the ellipse. This allows you to align them to an existing line and ensure that they're aligned correctly. Not every ellipse guide has this, but most do in some form. If yours doesn't, not a big deal, but if it does, be sure to make use of it when aligning your ellipses.

Admittedly your alignment does improve later in the set, though there are still some cases like 18 on this page which is more notably misaligned.

The last point I wanted to note in regards to the structural aspect of the challenge are your rims/spokes. You've approached them in a few different ways, and most are generally well done, although there are cases like 21 where your linework is visibly sketchy (in the sense that it doesn't seem you've made these marks as intentionally and with as much forethought/planning as the others), which undermines their solidity. There's also 15 where no rims/spokes were drawn at all - that's a missed opportunity for further practice within the bounds of the assignment. And lastly, as we see in some cases like 9 and for the most part, 14, we see cases where you mainly focused on drawing the outward face of the rims/spokes, but without defining their side planes. You defined those side planes in most other cases, which is good to see as it helps remind us that we are looking at three dimensional structures with thickness to them, although as we'll note in the next section of this critique, filling in their side planes with solid black is something you should avoid in this course. It has a tendency to separate the structure into two distinct graphic elements (which undermines the three dimensional aspect of it, as we can see in 17 where the side planes feel like they're a separate element than the front-face of the rims). This is also more akin to form shading, where we make the face lighter or darker based on its orientation (whether it's facing towards or away from the light source). As you'll note here in Lesson 2, we generally want to avoid including form shading in our drawings for this course.

Carrying onto the textural aspect of the challenge, this is where the wheel challenge serves as something of a trap. Texture - specifically the use of implicit markmaking (through the use of cast shadows) to imply the presence of textural forms rather than drawing them directly - is something students have a tendency to let slip through the cracks, and being as far removed as we are from Lesson 2, it's very common for them to - as you have here - try to tackle texture through more explicit markmaking strategies. By highlighting that here, it reminds the student to go back over previous material that they may have allowed themselves to forget or ignore when choosing exercises for their warmups.

Looking at your work, you certainly have fallen into that trap. I am pleased to see that you've tried working with filled areas of solid black, which is a good step, but the way in which you've done so (by filling in those side planes, effectively applying form shading) still requires you to draw the entirety of the form. That's pretty much why students tend to do it - actual implicit markmaking is very difficult, and it requires them to design a new shadow shape based on what they know of the textural form and the surfaces around it, without first drawing the textural form itself. But it is only by practicing that, by having it come out badly a lot that we're able to develop the capacity to hold more of that spatial information in our heads, so we can design the shadow shapes to reflect those 3D relationships.

The reason that using explicit markmaking for all our textural details doesn't work is that it removes a certain amount of control and decision-making about how much detail we are required to draw in a given space. So for example, wheel 18 looks great in isolation, but if we use it as part of a vehicle, those wheels are going to draw a lot of attention, creating a focal point in the illustration whether we want it to or not. 14 would draw even more attention, because of all the areas of black and white crammed into a small space.

Implicit markmaking on the other hand allows us to leverage the fact that, 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. This means we can choose to use larger or smaller shadows in order to change just how densely packed the area of detail is in our drawing, without altering what is actually being conveyed to the viewer in terms of what forms are present.

It does however take practice - so I'll leave that to you to review the material from Lesson 2 pertaining to textures (I would start with these reminders), and to also consider anything else in the course you may have allowed to slip through the cracks. I will however mark this challenge as complete, as the trap you fell into was by design.