For this critique, I'm going to take into consideration the fact that you freehanded your ellipses - it's highly recommended that for this challenge and for lesson 7, students use ellipse guides, specifically because they help us focus on the core aspects of the exercise, rather than worrying about things for which we have other exercises to help improve upon. Ellipse guides can get expensive, but most students opt towards getting a master template, which is just one piece with a range of ellipses in many different degrees, but in a few limited sizes. While this means that students end up having to draw smaller, it's usually good enough for the purposes of these lessons - and of course the bonus is that a single master template is MUCH cheaper than a full set of ellipse guides.

That said, if for whatever reason it was impossible for you to get your hands on an ellipse template, then you ultimately must make do with what you've got, so I will be overlooking any issues with the execution of your ellipses.

Looking at your wheels, I can see that you're approaching the core construction fairly well, in terms of laying out ellipses that are smaller towards the outside and larger towards the inside. That said, when approaching the actual rims of the wheels, you tend to break away from the principles of construction we've covered up to this point, choosing instead to jump into later stages of construction way too soon, resulting in forms that don't feel solid or three dimensional.

For example if we look at wheel 20 (although this is actually something common throughout most of this set, I just decided to focus on one of the later ones), you jump right into a highly complex silhouette, trying to divide it into two distinct sets of faces (the front face of the rims and the side faces), instead of building the central portion with a single solid form, and then building out the individual pieces that extend out from it. You only really started approaching the rims in a more constructional approach when tackling the simpler ones in 22, 24 and 25 (building a cylinder in the middle and then box forms coming out from the side), though I see no reason to think that this was for any reason other than the rims themselves being much simpler.

Another area in which I noticed you broke away from the concepts covered in earlier lessons comes down to how you tackle the tire treads. It seems you've forgotten a fair bit of what we talk about in lesson 2's texture section. This comes up especially in treads with chunkier forms, like #15. There you've basically drawn partial outlines of those forms, rather than capturing the shadows they cast on their surroundings as individual cast shadow shapes. Similarly, looking at wheels like #22, I can see that you've attempted to create the sort of list-and-found edges that we get when cast shadows get blasted away by direct light, but the way in which you've broken up those 'shadows' (which again really are just lines) doesn't properly reflect which shadows would last longer than others when hit with stronger light. This is something I explain in these notes, specifically the bottom half of the diagram.

Since you're struggling with approaching texture with actual shadow shapes, and are still largely relying on outlines for those forms, I strongly recommend that you draw all textural marks using the two step process explained here. This will force you to deal with those marks as filled shapes, rather than just arbitrary lines.

As a whole I'm not super pleased with how these wheels have come out. While I understand that you may not be able to get your hands on an ellipse guide right now, having to deal with freehand ellipses can be very demanding, and can easily distract us from everything else involved in the work, in turn making us sloppy and haphazard. This means that we have to work that much harder, because a good bit of that effort is being siphoned off into something that isn't our focus right now. On top of that, the fact that the ellipses tend to come out more loosely reduces our standards and the care with which we approach the rest of the wheels. It's all very much psychological - if the basic construction of the wheel feels loose, why wouldn't we be loose when drawing texture?

Now, admittedly your ellipses are especially loose, which make me suspect that you may not have been as consistent in doing regular warmups consisting of exercises from the earlier lessons, but that is ultimately something that is left to you.

Before I mark this challenge as complete, I'm going to have you do some additional work to get you back on track with this.

As a side note, while this isn't directly related, looking at this demonstration I did in a critique of someone's texture challenge work, it may help you better understand how to think about approaching tire treads. These are of course the opposite - tire tracks, but similar concepts do apply in terms of working in shadow shapes rather than lines.