I definitely agree that working with an ellipse guide (even the smaller master ellipse templates most students use, since the whole ellipse sets get pretty expensive) would have helped a great deal. The most important benefit to using ellipse guides is not just that they make that part of the exercise easier - it's that they actually open up more room for your brain to focus on other things, ultimately helping you absorb the concepts from the exercise more effectively.

That said, despite that you still have managed pretty well with the construction of your wheels. You've worked with a number of ellipses to define more than just a basic cylinder - instead of focusing just on an ellipse at each end, which would end up looking quite stiff and hard, you did a great job of capturing the "inflated" impression of the tire by focusing multiple ellipses at each end, and including a larger one in the middle. You've also paid a good deal of attention to the rims/spokes themselves - I noticed particularly that you gave them a good bit of depth/thickness/dimension, rather than just focusing on the "top" face pattern and leaving it as a flat cut-out. Nice work.

Now, what you experienced in terms of the difficulties with tire tread textures is all completely normal and expected. In fact, this challenge is set up as a little bit of a trap. The majority of students by this point are so far removed from Lesson 2 that they've largely forgotten the textural markmaking principles, so I find dropping this challenge just before the last lesson serves as a nice little reminder that there's more to drawing than simply outlining everything in such explicit terms.

I can see that throughout the challenge you did struggle to varying degrees with capturing the tire treads through implicit means, but you have as a whole done a pretty good job. I'm especially pleased with certain cases - like number 3 and number 8. There is however something I want to highlight, to help keep you on the right track.

With the chunkier tire treads, it's easiest to really understand the textural forms that are present - after all, they stick out from the surface, and so it's easier to think about how they cast shadows. With the less overt tread textures, however - like those composed largely of grooves cut into a surface - it can be a lot more difficult. Fortunately it's also easier in such cases to get away with focusing on drawing the grooves. Still, it is always important to think about how the structures are built up in three dimension. A groove can be seen as a cut into the surface of an object (so negative space), but it's more useful to try and remind ourselves that the same texture can be interpreted as just having a lot of raised forms jutting out from that surface. What it all comes down to is understanding that the marks you're drawing to represent the texture are always the result of some raised area of form casting shadows upon a lower surface.

The difference is that in one case, we're simply drawing the grooves themselves - defining them as lines or shapes. In the other case, we're thinking about three separate elements - the form that casts the shadow, the surface that receives the shadow, and the shadow shape itself which does not necessarily fill in the receiving surface. When we just focus on drawing the grooves themselves, we end up filling them in completely. Often times there's no difference in the result (which is why it's easier to get away with shallower tire treads), but how we think about the spatial problems and whether or not we recognize all the elements at play determines how effectively we can apply the same principles in different situations.

The last thing I wanted to call out is just that in some cases - like 6 and 7 - you have a tendency to construct the textural forms (the big tread chunks) in explicit terms, and then determine the shadows they'll cast. While that is a step in the right direction, it's still a crutch and it's important that you keep pushing yourself to maintain the understanding of how those individual textural forms sit in space without drawing them on the page. Always force yourself to convey them using the shadows they cast, even if doing so is harder and yields more mistakes. After all, these are just exercises - this is where those mistakes should be made, so we can learn from them.

Anyway, all in all your work is still really solid here. I strongly encourage you to pick up a master ellipse template for Lesson 7, but I'll go ahead and mark this challenge as complete.