I'm glad my explanation for those intersections helped! As for your wheels here, you have by and large done a pretty good job - though I have a couple things to offer to help you get the most out of this exercise.

This challenge generally splits into two categories. First, we look at the constructions themselves, and with this I think you have by and large done a pretty good job. You're laying out multiple ellipses to capture the entirety of each wheel's complex profile (so for example, instead of drawing only working with a basic cylinder whose 'profile' would just be straight, flat edges, you've got a ton of wheels where you've got either one or multiple cross-sections down the middle that get bigger, giving us more of an impression of "inflation" for the tire, which further sells what it's supposed to be.

You're also generally doing a good job in building out the various spokes/rims on your wheels, but there are some areas where I think you may have stopped a little early. For example, 6 is an excellent example where you've got each spoke built out in its entirety - the front face of the rims, as well as each one's side plane to establish them fully in three dimensions. There are however other cases where you fall short of this. I'm not entirely sure where 5 falls, but it does look to me like each spoke was drawn as a single line. This would be okay if we're looking at something like wire - given that it might be a bicycle wheel, that could be appropriate. Skipping down to 22 for instance however, we do have the width of each "arm" established, each one's given a full face, but no side plane - so they still appear to be paper-thin in terms of depth.

Of course, the "correct" approach in each instance varies, but it is important that when you catch yourself representing something either as a line, or just a flat shape, make sure you think about why you're opting not to give it that little bit of thickness.

The second part of this challenge comes down to a big reminder of what many students tend to forget - that is, the textural markmaking principles from Lesson 2. Given how far removed we are from those early days of the course, it's really common for students to forget exactly how we're meant to approach texture - like the tire treads. Many students will just jump straight into explicitly outlining each and every form (which you did do on occasion, with examples like 19), so I'm pleased to see that you clearly did think back to that earlier lesson, and consider how exactly you could go about drawing these textures implicitly for many of these.

More importantly, another mistake students do often make is confusing the difference between drawing a cast shadow - which is its own separate, independent shape that defines the relationship between the existing textural form that casts it, and the surface upon which it is cast - and simply filling in the side planes of those textural tread chunks instead (which is more similar to form shading). The difference is quite subtle, as I demonstrated on this example I did for another student.

In 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).

Looking very closely at yours, however, I keep thinking that you're filling in side planes instead, but I consistently find signs that you are in fact trying to draw the shadows. The shapes themselves simply aren't refined enough - so the one recommendation I'd offer is to purposefully design your intended shadow shape first, then fill it in. That way you have clear boundaries for the shape, which can more strongly and specifically imply the presence of a particular form. When we end up making looser collections of marks, or "painting" strokes onto the page one at a time, things tend to get a lot more muddy.

So! With that, I'll go ahead and mark this challenge as complete. You've done quite well, so keep up the good work.