Jumping right in with the structural aspect of your wheels, I'm very pleased with how you've approached your work here. You've been mindful not only of laying out the basic cylindrical structure properly, but you've also done a good job of ensuring the center bulges a little in order to give that "inflated" appearance, which helps to suggest to the viewer that should the wheel be dropped, it'll bounce rather than land with a thud. You've also paid attention to defining not only the outward faces of your rims' spokes, but also the side planes of those structures to ensure they feel solid.

One thing I did notice - although I suspect this is not intentional and more the result of the line work being a little scratchy at times, or a combination of that and the smaller scale of the drawings (which is totally normal given the limitations of our ellipse guides) - is that in a number of cases it seemed like the side planes of those spokes were sometimes getting filled in, as though you were applying shading to them. Given that you frequently relied instead on cast shadows, I suspect this was more due to the size, but I'd also advise you to take a bit more care in the execution of those marks. Remember, the ghosting method is all about planning things out and practicing so you can execute marks confidently and cleanly, and it seems here that due to the smaller scale you may have fallen back to somewhat scratchier linework at times.

That said, 20 does appear to be a case where you drifted more into form shading, which of course in Lesson 2 is noted as something to avoid, but given the fact that this is the only concrete case of that it's easy to overlook. On the same note, I'd advise you to avoid filling negative spaces in with black arbitrarily (also not common in your work, but visible in the number 3, 11, and the second of the 3 bonus wheels), and limit your use of filled areas for cast shadows only.

Continuing onto the textural aspect of the challenge, this is something of a trap designed to catch students who forget about the principle of textural markmaking from Lesson 2, and end up tackling those smaller textural forms using explicit markmaking or other strategies that don't align with the course material. Many students fall into the trap, and it's largely expected for them to do so, given the span of time that's usually passed between Lesson 2 and here - but you seem to be purposely trying to apply those concepts, so I'm very pleased to see that.

For those students who do remember those concepts, it's not uncommon to see them slipping more towards filling in the side planes of their textural forms, which without the outlines of the form can look like it's correct, as the differences are incredibly subtle (for example as shown here). In your case, while there are some cases where you definitely are shading those side planes (number 8 is a good example of this), you jump back and forth, which shows me that you're working towards solidifying your understanding of the distinction. That's entirely normal, and it's what I hope to see - it does of course continue to take practice, where the general development of your spatial reasoning helps, but there's still some direct and intentional practice with this concept that is necessary, but you're definitely on the right track.

So! I am happy to mark this challenge as complete. Keep up the good work.