Starting with the structural aspect of the challenge, you're doing very well. You've put your ellipse guide to excellent use, and I'm very pleased to see that you didn't merely stick to a basic cylinder, but rather included as many ellipses as you required to establish every subtle aspect of that wheel structure - including a nice, gentle arc in its profile to capture the fact that the tires are indeed full of air, rather than solid all the way through. This contributes immensely to the realistic sense that the wheel would land with a bounce, rather than a solid and immobile thunk.

Generally speaking you are similarly conscientious with constructing your rims/spokes, especially in terms of fleshing out the entirety of those structures - not just the outward face, but also their side planes, so as to establish the entirety of the structure in three dimensions. There were a few cases where you did fall a little short on this though - 13, where you drew the holes along the outer rim of the hub cap, but didn't establish the inner side planes of those holes, causing them to appear paper-thin, and 5 where you were faced with the admittedly thin-as-all-hell bike spokes. While they are thin, they're still not paper thin - so you're left with a choice to either represent them as lines, or making them a little thicker than they actually are in order to convey them as forms - and the latter would probably still be the better choice.

This is of course nitpicking - as a whole you've handled this well.

The second part of the challenge relates more to texture, and in that sense it's something of a trap. It's very common for students to be so far removed from Lesson 2, that they don't even think to go back and review that material when dealing with the tire treads here, which are very much a textural problem. Texture is after all all about forms that rest along the surface of another object.

In your case, you certainly did fall into the trap, but not as entirely as others. In fact, even early on I can see that you're trying to wrap your head around the difference between the ways in which you could be using your filled areas of solid black. In number 4 for example, you fill in the side planes of your textural forms, and in number 6, you're actually drawing the shadows those forms appear to cast. The distinction is at times subtle and hard to identify (as noted in this diagram on another student's work), but you absolutely deserve a lot of credit for clearly thinking in terms of cast shadows at this point early on, and doing so as you continue through the set.

That said, as far as the distinction between implicit/explicit markmaking, you are definitely still dealing in explicit markmaking for the majority of these, given the fact that you're constructing each textural form in its entirety before delving into the use of solid black cast shadow shapes. It is worth mentioning that 11 definitely is more implicit, and thus more correct for our purposes.

There is one last situation I wanted to take a moment to discuss - textures involving holes and grooves, like the shallower grooves we can often see on tires. In such cases, it's easy to mistake the "textural forms" in question as actually being the holes, or the grooves. Of course they're not - these things are an absence of form, with the actual textural forms being the walls that surround them, which cast shadows upon one another and on the floor of the empty space itself.

Of course, when dealing with really narrow ones the distinction can be tricky, but it all comes down to how we think about the nature of the forms, even if it doesn't immensely change the nature of the marks we draw to represent them. These notes go into more detail on this topic, so give them a read.

And that about covers it! As far as the 'trap' goes, you've only really stumbled in it, rather than outright getting caught - but all the same, I don't hold people back over it anyway. It's more of a rude reminder that one needs to review notes like these reminders from Lesson 2. So, I'll go ahead and mark this challenge as complete.