As per your question, I'm going to tackle this critique in reverse, so we'll talk about the texture stuff first. Your question - which is a matter of, "if a shadow is being cast by a form, then we understand the location of the light source and the orientation of the different surfaces relative to it, and would that not demand the presence of form shading as well". But here's the thing - we aren't drawing what we see directly. As explained here in Lesson 2, we're observing our reference in order to first and foremost understand the nature of the forms that are present. We then take this understanding and use it to make our own decisions on how it's going to be depicted.

Cast shadows are the result of a form blocking a light source from reaching a different surface. Form shading is the result of a given surface facing away from the light source, and thus the light not being able to illuminate it. So yes - in terms of being bound by the laws of realism (which we are not unless we choose to be), if you have a box casting a shadow onto a ground plane, then the sides of that box which are on the same side as the shadow is being cast would, realistically, have form shading on them. But what we're doing in this course is not creating a series of realistic drawings. Rather, each and every drawing is an exercise focused on helping develop our understanding of the nature of these forms as they exist together in 3D space.

So, since form shading and cast shadows are related but fundamentally different visual elements, we are able to choose to depict one and not the other, as shown here - left one's all form shading, right one's all cast shadows. Depicting both simultaneously puts us in a precarious position where it blurs the boundaries between each component. So, picking one or the other avoids that issue, bringing us merely a matter of picking one or the other. Form shading focuses on the relationship between the form and the light source, but in otherwise isolation. Cast shadows however help us consider the relationships between this form and its surrounding surfaces, which is more in line with building up that understanding of spatial reasoning.

That said, textures with holes and grooves are notoriously tricky, for the simple reason that we have a tendency to focus on the grooves or holes themselves as being the textural forms in question. But they're not - they're negative space, an absence of form. The actual textural forms are the ones that make up the walls of the grooves, as shown here in this diagram.

I think this should help given the fact that while you're clearly thinking about working with cast shadows, you do have a tendency to end up filling in the negative spaces that already exist in your structures. Sometimes that is exactly right, but it's best to always ensure that you think of cast shadows as being a new, separate shape that has to be designed itself, based on the relationship between the form casting it and the surface receiving it. If that shape happens to take up the entirety of a hole or groove that already exists in the structure, causing you to "fill" that existing shape in, then that's great - as long as you've thought it through.

Alrighty! Onto the other half of the challenge, there isn't too much to say because you've done a great job. You've been mindful of building your basic wheel cylinders with a bit of a bump through the profile, to make them appear more inflated and bouncy, rather than just solid and unyielding. You've also paid attention to defining not only the outward face but also the side planes of your rims/spokes - although be careful with where you end up placing your line weight in this regard. If we look at 22, some of the internal edges of your rims (the ones separating the outward face from those individual side planes) are quite thick, which breaks up the cohesion of the form somewhat. Avoiding adding any additional line weight to the internal planes would definitely help with that.

And that's about it - I'll go ahead and mark this challenge as complete. There is certainly room for improvement in your use of cast shadow shapes in the textures, but I expect some of the advice I've offered here should help, and the fact that you're thinking about it and asking those questions means that you are headed in the right direction.