On one hand, you mention that you struggled and had plenty of difficulty - and I don't doubt that, because constructing wheels and really anything that relies on ellipses (especially freehand ellipses) is notoriously difficult. On the other hand, your results are always still exceptionally well done. That's not to say there aren't mistakes - there's actually a big one that I'll get into soon, but it's a mistake everyone makes, and it's one of the reasons this challenge exists.

All that aside, your constructions are extremely well done. You've done a great job of blocking in those ellipses, of aligning them to your central minor axis line, of building out the rims - of which you experimented with many different kinds, capturing each one's individual character, and playing with a wide variety of tire treads to a considerable level of detail. I can also see some definite improvement - your ellipses are considerably rougher towards the beginning, and there's a lot more confidence behind your linework through the middle and into the end, even if your ellipses aren't perfect. You certainly are using that ballpoint well though - in 24 I wouldn't have condoned redrawing your ellipses after the fact, but fortunately ballpoint is much more forgiving.

So, the major issue at hand is this: texture. Tire treads are made up of a lot of little forms that sit along the surface of a larger structure, following its contours, and therefore tire treads should employ the textural techniques and concepts discussed back in Lesson 2. This isn't really a huge deal with treads that have shallower grooves to them, because there's a lot less room to end up distinguishing between outlining of the tread 'chunks' and casting shadows into the grooves (obviously there is a difference, but the impact is lessened and you don't end up with needlessly noisy results if you don't follow those principles to the letter). The bigger the tread-chunk though, the more significant the issue becomes.

If we look at number 18 for instance - where I can see you already trying to navigate how you're going to handle those textural forms, using more implicit techniques to imply the forms using the shadows they'd cast would be quite effective. Where you did start blocking out more solid filled black shapes, you placed them on the side of the textural forms. A good shot, but technically incorrect - we can treat that more as form shading, rather than cast shadows, because it falls on the form in question rather than its surroundings. As shown here, the cast shadow should be placed around the form, and all internal edges should be left out. Instead, we can imply that form's three dimensional nature through its silhouette, or as much of its silhouette as we can capture using the various shadows being cast around it. That silhouette's corners still manages to suggest a three dimensional form just as effectively, but it saves us a lot of precious linework, and therefore decreases the overall contrast of that tire tread. It also allows us to control the textural density, as these implicit techniques generally do. In turn, if these wheels were part of a larger construction, we'd have the flexibility to ease up on how much they draw the eye, and avoid creating unintentional focal points.

Again, the ballpoint definitely helps lessen the impact of this overall - but if you were to draw them with a fineliner, having all those lines would definitely become visually taxing. Most students do forget about those principles by this stage, so this challenge serves as a wonderful reminder before heading off into the last lesson.

So! I'll go ahead and mark this challenge as complete. Keep up the good work, and don't forget about the all-important cast shadow shapes.