Before I get started, I wanted to note that the images you've included are fairly low resolution - each wheel gets about 200x200 pixels or so - so while I will do my best to provide you with useful feedback, this does make things quite a bit more difficult so my apologies if I miss something.

Starting with the structural aspect of the challenge, overall you're doing pretty well. Most notably, the body of the wheel overall shows consideration to the fact that it's not just a straight cylinder each time, but rather that many of them widen to varying degrees through the center to give that impression that they're inflated and would land with a bounce instead of a thud. I did notice a couple little issues however.

Firstly, when drawing your spokes, while I'm glad to see that you're considering not only the outward facing surfaces but also the side planes that give those structures thickness, you tend to extend those side planes well beyond where they clip into the inner tube of the wheel, as shown here. I think you might be lapsing in your attention, and forgetting that you're drawing something that exists in three dimensions, where these different surfaces interact.

Secondly, you tend to fill the void spaces within the wheels with black, in a few different ways. Sometimes like in 14 and 20 this seems to engulf those important side planes, causing the spokes themselves to appear flat and thin, which feels off, since very few things in the world are that thin. While the other cases aren't so bad, do keep in mind that generally in this course, we try to keep our use of certain tools consistent, so as to avoid confusion. So for example, filled areas of solid black are a tool that we can use to convey form - but as discussed back in Lesson 2's texture section, we generally use them to fill cast shadow shapes - that is, shapes that have been added independently of any existing surface, where we actively design them before filling them in to reflect a specific spatial relationship between the form casting it and the surface receiving it. Ensuring that the same tool signifies the same thing helps focus our work in this course in a specific direction without distraction, and so a good rule of thumb is to avoid filling any existing shape in with black. We'll talk a little bit more about this a little further into the critique.

Continuing onto the textural aspect of the challenge, this is something that tends to trip students up, and it's become somewhat intentional over time as it provides us with a really poignant way to remind students that sometimes you leave concepts and material behind as you progress forward. Texture is a big one of these, as not only is Lesson 2 so far behind us at this point, but with all the difficulty students have with it, they tend to prefer not to think about it anymore, and so it doesn't factor into their warmups, or much else for that matter. So, when they hit this lesson with its tire treads, they often forget how texture is meant to be tackled.

In some cases they abandon any semblance of the implicit markmaking we discuss there, although more often they'll remember a bit of it, and try and figure out how it all fits together. It's a little unfortunate though, most don't go back and actually look at the material itself, instead guessing based on what they recall.

You certainly have fallen into this trap, but it's clear that you've tried to figure out how to work with those filled areas of solid black so as to avoid overwhelming each wheel with detail. Your approaches aren't entirely correct, but they're moving in the right direction.

If we look at number 10, this is a good example to discuss because it's got really chunky textural forms along its tire tread, and it's also a very prominent example where you definitely tried to work with filled areas of solid black. The only issue here is where you placed the black, and what you used it for - you filled the side planes of the textural forms, essentially applying form shading to them (in the sense that form shading is all about making surfaces that point away from a light source dark, and surfaces that point towards a light source white).

Texture, and implicit markmaking as we employ it in this course, is all about cast shadows, because cast shadows are like the echo of an object - they tell us it's there, without specifically showing it to us. A cast shadow also defines a 3D spatial relationship between two physical elements - the form casting the shadow, and the surface receiving it, which ties it all back to the core principles of what thise course focuses upon. It's for that reason that we generally avoid form shading altogether as discussed here.

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 basically means that we can have the same textural form at different locations along a surface, and we can draw the textures that imply their presence in different ways. For our purposes here, the specific relationships to an actual light source matters less - what matters is that this gives us the control to choose more generally, "here's where I want to concentrate my detail" and "here's where I want to leave things more empty for the viewer to fill in their own mind". Both extremes - where the shadows are virtually invisible from being so slight, and where the shadows are so large they fuse into a single complex shape, give us a low-contrast, low-detail area in our drawing, whereas the middleground gives us lots of interposed areas of light and dark, creating more contrast and allowing us to convey more detail.

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.

The big takeaway here is just to focus on thinking about the relationships between your forms in 3D space, to focus on reserving your filled areas of solid black for shapes you actively designed before filling. There are some mild exceptions - like where in Lesson 7 you'll see that I tend to fill in the interior surfaces in the cab of a car, but my weak-but-sufficient argument there is that the shadows of the exterior structure is casting shadows inside - but really it's because it helps separate those two levels of depth.

Anyway, I'll still be marking this challenge as complete, so be sure to review the texture material (these reminders are a good start), and consider anything else you may have allowed to slip through the cracks, and then feel free to move onto the last lesson.