Nicely done! Starting with your wheel constructions, I can see that you've made good use of an ellipse guide here. Through the shifting of your ellipses' scales and degrees, and the use of more than just two (one for each end), you've captured the impression that the wheel itself is inflated, puffing up through the midsection. You've also done a fairly good job building out the various rims/spokes - especially as you pushed through the set. The later ones are definitely more consistent and evenly spaced than some of the earlier ones like number 7.

Now, this challenge serves as a bit of a trap - many, and frankly, the majority of students who reach this stage have kind of forgotten about the principles back in Lesson 2's texture section. In your case, however, I can see that you've definitely tried to apply a good deal of what you remember from Lesson 2, although there are some issues that I can still address.

If you look at 13 and 14, these are cases where the tire tread texture features more significant, "chunkier" forms. Their size actually doesn't change things, however - you ended up drawing them using explicit, constructional techniques (defining those internal edges and such), but you still should be employing implicit drawing techniques instead. That means implying their presence by drawing the shadows they cast, rather than outlining them. The only place they should be outlined is where they break the silhouette of the wheel structure itself.

Obviously, that's pretty challenging - it means you have to hold this textural form in your mind, and understand how it relates to the surrounding surfaces, without actually drawing it, in order to draw the shadow shape.

The next issue I want to talk about is most visible in 12. Here, for the most part you filled the negative space in between the textural forms. Basically you already had shapes provided to you (those that existed in the gaps between forms), and you filled them in. But this is incorrect - because it means that your shadows take on some other arbitrary shape. Instead, our cast shadows must be independent shapes that define the relationship between the form casting the shadow, and the surface receiving it. Of course, with all these forms present, you'll no doubt end up with lots of cast shadows merging together. That'll no doubt happen, but we have to make sure that every new shadow we add are drawn with the form casting them in mind. Ultimately this will decide the outer shape of whatever collective, complex shadow shape results.

Finally, there is one last issue that can arise - where one ends up mixing up cast shadows and form shading. It's easy to end up filling in the side planes, but in doing so, we put a focus on the top plane as a flat shape. As shown in this diagram, leave the textural forms' silhouettes empty, and leave it to the corners in the silhouettes to establish the form as a three dimensional entity.

That covers the main issues that arise in varying places throughout your work - though as a whole I am very pleased to see that you have clearly tried to figure out how to work with cast shadows. Cases like number 6 especially show that you're trying to figure out how to capture these textures implicitly.

Do keep in mind though - our goal is not to capture any kind of local colour, as you tried to with number 8. There you were definitely trying to capture the dark colour of the rubber tire. In general, treat the objects you're drawing as though they're covered in the same white colour.

Anyway! All in all, you're doing quite well. You've got a few things to keep in mind, but they're pretty much par for the course. I'll go ahead and mark this challenge as complete.