Starting with the structural aspect of the challenge, you're doing a great job. You've made excellent use of your ellipse guide - limited as it may be - to build out the basic cylindrical structures with very gentle, subtle bumps that help promote a sense of being inflated, as though hitting the ground will cause it to bounce, rather than land with a thunk. There are some where it gets subtle enough to feel perhaps too cylindrical (like here) but overall you're doing a good job with this.

When it comes to the rims/spokes, I can see that you are frequently paying attention to both the outward face as well as the side planes of the forms, to give them an appropriate sense of thickness and make them feel more solid and 3D. There are a couple places - like this one - where your rims lack thickness, causing them to appear a little more flat. Always be sure to include those side planes, and don't fill them in with any sort of hatching. In general, hatching shouldn't be used in any of the drawings of real objects we do in this course, except in some really specific situations, as the textural concepts from Lesson 2 have us working specifically in cast shadow shapes and avoiding form shading in general. Similarly to not adding the thickness at all, filling in a side plane also interferes with our perception of that area as defining the thickness of the given form. So for example, with this wheel, leaving the inner surface with all of the hatching blank would have been better.

By and large that's mostly nitpicking, and as far as the structural aspect goes, you're doing well. This brings us to the second aspect of this challenge, which pertains to actual texture. In this regard, your work is a bit mixed. I like to use this challenge as a bit of a trap, given how far removed we are from the textural concepts from Lesson 2, it serves as a nice reminder for those who haven't applied any of its principles when tackling their tire treads. Tires are, after all, an excellent example of a texture - their tread patterns are made up of forms arranged along a cylindrical surface.

Now, there definitely are situations with shallower grooves that allow us to get away with using explicit markmaking (which is exactly what I do in the challenge's demonstration), but it's when students tackle the tires with larger, chunkier treads that the decision between working explicitly and implicitly really shines through. Unfortunately though - and by no fault of your own, as I'm playing a rather precarious game of "give them enough information but not too much information so the trap can be used as a more effective learning tool once it's sprung" - you don't appear to have drawn any tires with really chunky treads.

I can however see cases like this where you have definitely attempted to work more implicitly - so I'm going to focus on that, because while your intent was correct, your execution could be improved upon. The main concern comes from the fact that you're using lines, rather than shadow shapes. The thing about lines is that while they can certainly vary in line width, it's much harder to get them to taper to a very narrow point - whereas a shadow shape, which is a shape you actually design intentionally, outlining the boundary of that shape, before filling it in - can achieve far more dynamism, as shown here in this example involving cracks.

So where our shadow shapes can taper enough to appear to just gently fade away, our lines have a tendency to just suddenly stop.

Furthermore, when you're drawing those lost-and-found lines, you're focusing on the grooves of the texture itself. But a groove is not a textural form - it's negative space, the absence of form. The actual textural forms in question are the ones that define the grooves' walls and floor, as shown here in this diagram on how to think about holes in textures, and so they are what we need to be thinking about, in terms of designing shadow shapes based on our understanding of how those forms sit in space. You can read more about what exactly we're focusing on in these reminders from the texture notes. Ultimately, every mark we put down is going to be the specific shadow being cast by a specific form. Don't try to draw them right from observation, we need to think about what the things we see actually represent (in terms of the forms that are present), and understand them, before inventing the shadow shapes on our own.

The last thing I wanted to mention is exactly why this use of shadow shapes, over lines, is so important. The thing is, when we draw our tire treads - even the super chunky ones - using explicit markmaking, employing constructional principles and all that, we end up with a lot of marks densely packed together. When the wheel is floating in the void, that's not a problem. But when it plays a role as part of a vehicle construction, all of a sudden we have a really densely packed area of ink which draws the viewer's eye, making a focal point whether you want to or not.

Conversely, if we're working with shadow shapes, we can control exactly how much ink needs to go down, without changing the nature of the texture being depicted. As shown here (it's not a tire tread, but these african bush viper scales work under the same principles) we can make those shadows deeper or really blast them away without changing what is conveyed to the viewer. Thus, we can avoid unwanted focal points.

So- I'll leave you to review the Lesson 2 notes on texture, but as a whole you're still approaching the overall construction of your wheels well, so I'll go ahead and mark this challenge as complete.