Starting with the structural aspect of the challenge, you're doing fairly well. On a number of these, you've mindful of including several ellipses throughout the profile of the structure, allowing for an often subtle but well appreciated "bump" through the midsection that helps it to appear more inflated, as though it would land with a bounce rather than a solid thunk. That said, I do think this is something that could be exaggerated a bit further. I'd even go as far as recommending that you not push the foreshortening too hard (and avoid making that far end so much smaller), as it can actively counteract this subtle curve - and given the size of our wheels, it's pretty normal not to have too much foreshortening on objects of this scale.

Now I can totally understand that given a more limited set of ellipse guides, as most students end up using here, you may simply not have the ellipses to achieve this. That's totally understandable, so temper any advice I offer on that front with your own knowledge of the tools you have at hand. That said, if you have to pick one side to sacrifice, I might go with maintaining a more similar degree on the far end (even though it technically should be wider), if it means having more flexibility when it comes to varying the scale through the midsection to exaggerate the bump.

Moving on, one aspect of the structure that I do want to call out is the importance of establishing not only the outward face of the rims/spokes, but also the side planes of those structures. I do see you generally focusing more on the outward face, aside from a few exceptions like 7 where you filled in the side planes (which I wouldn't recommend, as it both falls under form shading which as explained here shouldn't play a role in our drawings for this course, and because it tends to create the impression of a separation between the front face (which still appears flat) and the black shape behind it (which the viewer will naturally want to interpret as a cast shadow first and foremost, resulting in more visual confusion). There's also 15 where you actually specifically drew the side planes but not the outward face.

13 was one of the cases where this was handled well - you've got both the outward face and the side planes, and as a result it all feels more more solid and three dimensional. 23 is also really well done in this regard.

Continuing onto the textural component of the challenge, this exercise is really meant to be something of a trap. Being as far removed from Lesson 2 as we are, it's entirely normal for students to forget the concepts discussed in Lesson 2 - and having that revealed here can spur them on to go and review that material with a certain amount of.. urgency, and perhaps a touch of embarrassment. It does appear that you've fallen into the trap as well.

In tackling your tire treads, there are two main ways in which you've approached it. The vast majority of these involve pure outline-based construction, where you've built out the textural forms as you would any other textural form - using explicit markmaking, as discussed in Lesson 2. This ends up looking great when dealing with these wheels as they float in isolation, but when they become part of a larger construction or drawing, all of that compressed contrast/detail creates a focal point, drawing the viewer's eye whether you want it to or not.

Working implicitly however - where we draw the shadows cast by those textural forms, and not the forms themselves - allows us far more flexibility, where we can choose to work with more or less contrast without physically changing the nature of the structure being represented, as shown here on this example of bush viper scales.

It can be easy to confuse the use of cast shadows with the use of form shading however, and that brings us to the other approach you've used. In 22, we've got an example where you have attempted to use filled areas of solid black, but where you ended up using it to fill in the side faces of those textural forms. As discussed in regards to the spokes on number 7, this is more akin to form shading - we make the faces darker or lighter based on their orientation in space.

Now, I do have an explanation as to the distinction between this and actually using the cast shadows that these textural forms cast upon their surrounding surfaces that I'd written up for another student that includes a diagram. I'm going to paste them below, but you may find that the explanation is a little hard to make heads or tails of at first. If it's not, that's great - but if it is, that's fine too. Give it a couple read throughs now, but sit on it for a while and come back to it later. You may find it makes a bit more sense once you've had a chance to let it sink in over time.

Here's the diagram and here's the explanation:

On the top, we've got the structural outlines for the given form - of course, since we want to work implicitly, we cannot use outlines. In the second row, we've got two options for conveying that textural form through the use of filled black shapes. On the left, they fill in the side planes, placing those shapes on the surface of the form itself, and actually filling in areas that are already enclosed and defined on the form and leaving its "top" face empty. This would be incorrect, more similar to form shading and not a cast shadow. On the right, we have an actual cast shadow - they look similar, but the key point to pay attention to is shown in the third row - it is the actual silhouette of the form itself which is implied. We've removed all of the internal edges of the form, and so while it looks kind of like the top face, but if you look more closely, it has certain subtle elements that are much more nuanced - instead of just using purely horizontal and vertical edges, we have some diagonals that come from the edges of the textural form that exist in the "depth" dimension of space (so if your horizontals were X and your verticals were Y, those diagonals come from that which exists in the Z dimension).

And that covers it. Of course, since the 'trap' was entirely intentional, this is not something I hold students back over. I'll go ahead and mark this challenge as complete - but be sure to revisit the Lesson 2 texture section, especially these reminders, before continuing forwards.