Jumping right in with the structural aspect of the challenge, you've handled this well. You leveraged your ellipse guide (which I'm glad you used - yeah, the tiny size can be quite limiting, but the exercise is far more effective when you don't have to also worry about executing perfect freehanded ellipses, even if you have to take a lot more time to handle smaller details) very effectively. I'm pleased to see that you worked with many concentric ellipses where required, and that you demonstrated a good deal of control over the nature of each wheel/tire - allowing those that were more rigid like 14 to be straight across, while including ellipses in the midsection for cases like 17, allowing you to make them larger to achieve an arcing profile to the wheel that conveys a stronger sense of it being inflated and bouncy.

While broadly you're also handling the spokes/rims well in terms of breaking them down into their simple components, being mindful of defining the side planes and not just focusing on the outward face, I did notice that when connecting the spokes to the inner tube of the wheel, you run into issues in terms of where your different lines connect, as shown here.

Continuing onto the textural aspect of the challenge, this is where things become a bit of an intentional trap. We're pretty far removed from Lesson 2, and so it's not at all uncommon for students to simply forget that we discuss at length concepts there relating to texture, and the use of implicit markmaking via cast shadows to convey it. Based on how you approached it here, it seems that you were mainly focusing on drawing what you see, and didn't necessarily think about how the techniques discussed earlier in the course might play a role. Rather, you seem to have remembered that we don't necessarily want to draw every little thing in our reference image, but ended up being more arbitrary in terms of deciding which details to leave in, and which to leave out.

Implicit markmaking relies not on making arbitrary choices, but rather on leveraging consistent rules, and it all hinges on cast shadows. 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. Even if we set aside worrying about where the light source specifically is and keeping it consistent (which does matter, but is not something we're worried about in the context of this course), we have a basis upon which to say that the same form in different locations can end up being drawn with entirely different marks, because of the way in which cast shadows work. So as long as we are focusing on every mark we put down being a cast shadow - whose shape we design based on our understanding of the relationship between that form and the surfaces around it in 3D space - we are freed from the burden of having to draw every single form in its entirety and ending up with super visually noisy mess.

To your credit, I did notice that you were attempting to keep in line with this kind of methodology earlier on - 4 is a pretty solid example of you relying on cast shadows, and others on that first page also appear to be trying for the same general idea. The further into the set you got however, the more I think you got discouraged with how it was going and ended up reaching for alternative methodologies - which is unfortunate, because at the end of the day things are just really challenging sometimes, and we have to struggle with them a great deal in order to make headway. Ultimately the further you got in, the better your wheels looked - but exercises aren't about results, they're about the process we employ and how doing so repeatedly changes the way in which we engage with those problems on a subconscious level.

Another point worth mentioning is that 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.

One last thing - I noticed that you had some trouble with the skateboard wheels, in terms of achieving the beveled edges with your ellipses. Here are some notes on it. The main thing you're missing is that it looks like you were drawing all of your ellipses on the same center position (the red ones and the blue ones technically have different centers, but rather than moving back along the minor axis line, the blue ones are centered on a point that's further up, rather than to the left). In order to get your beveled result, you need to shift the center of your ellipses back.

Anyway, I will be marking this challenge as complete, since the textural stuff is part of an intentional trap, really to provide students with a bit of a sharp reminder that there are concepts we've covered in the course that you may have allowed to slip through the cracks that should be reviewed (and that while texture is one such case, there may be other things so taking some time to reflect on that is well worth it). Just be sure to review the textural material (starting with these reminders) and also consider if there's anything else that may require further review as well.