While your initial hand-drawn wheels were actually pretty good all things considered, for the purposes of this lesson, and the next one, it is a fair trade-off to just stick to the ellipse guides and make some wee little wheels. For all my purposes here, the kind of detail you were able to get into at that scale was perfectly fine.

As a whole, your constructions were consistently well done. Each wheel was solid, the rims were well constructed, and you captured all of the complexity of that form (not simply constructing a cylinder, but pinning down the slight bulge through the middle of each tire) nicely.

When it comes to tire treads themselves, the shallower ones (with simple grooves) were laid out and stretched across the curving surface quite nicely. You've shown a great deal of patience and care in their application, and handled the patterns well.

The one I do want to talk about however is this one, with its chunky, protruding tire tread. I'm very happy you included at least one of these, because it is here where I can usually gently remind students of some concepts they may have forgotten.

Tire treads are made up of smaller forms that wrap around a larger one - meaning they're a texture - and while that doesn't come much into play with the shallower tire treads, there are relevant aspects to how we talk about texture back in lesson 2 that become relevant when we have these chunkier ones. The key is that we want to avoid situations where we have to fully construct these textural forms as they come off the surface, and to a point, that is what you've done here. You've defined each chunk individually, and filled in the side planes of those chunks to help distinguish them from their top planes.

In the future, instead of filling in the side planes, always focus on cast shadows. They're the bread and butter of texture. As shown here, on the left side simply filling in the side planes isolates the given form. It allows us to kind of understand how it exists in 3D on its own, but dropping cast shadows on the surface around the form allows us to define the relationship between that form and its surroundings, while still capturing the planes of the form just as effectively by capturing the corners of its silhouette (without touching its internal edges). This saves us from having to draw additional marks (which is useful when working at smaller scale, and in general it reduces the amount of visual noise), and provides the viewer with more useful information with which to understand the relationships between all the relevant forms in space.

So! With that laid out, I'll go ahead and mark this challenge as complete.