Just to get this out of the way, I can see that you ended up freehanding your ellipses for this exercise. Ellipses are something we expect students will continue to practice long into the future, and mastery is definitely going to be a ways away. They're just difficult, and so with these last few lessons/challenges, we encourage the use of ellipse guides (even the smaller, cheaper, master ellipse templates which tend to have more limited ellipse sizes) just to take the difficulty of drawing those ellipses out of the equation, so students can focus primarily on the main core of the exercise. Working off a stronger core construction (that is, the ellipses defining the basic cylindrical form) also puts us in a better overall mindset to construct the rest of the object with more patience and general fastidiousness, whereas starting from a weaker construction tends to make us a little sloppier than we otherwise might.

These are all things I'll take into consideration in my critique, though I do hope that you are able to get your hands on a master ellipse template for the last lesson, in order to truly give it your best.

Aside from the ellipse issue, I do feel that you're largely aiming in the right direction with the constructions. Some of them turn out well - like number 13 - and I believe that is largely what you're aiming for. Do remember to execute those ellipses from your shoulder though, using the ghosting method, as the broad sweep with a larger overall radius will help you maintain a more consistent motion. Drawing more from the elbow is one major factor that can cause an ellipse to come out unevenly.

Looking at the spokes within the wheels' rims, you've largely done a pretty good job in thinking through each piece as three dimensional, and defining how they exist in space relative to the rest of the construction. Generally where the core construction gets weaker, these tend to suffer, but overall you are showing that you're clearly moving in the right direction.

The last point to discuss are the tire treads, which themselves are textures wrapping around the cylindrical surface. I did notice that at certain points early on, you did attempt to adhere to some of the textural principles from lesson 2, working using implicit drawing techniques and creating density gradients where the texture would be more prominent in some areas, and less so in others. This is the right direction to take, but one thing to always remember when it comes to texture is that working with clearly defined and designed cast shadow shapes is key.

There are a lot of situations where we'll be tempted to kind of "paint" on those shadows, line by line, stroke by stroke, building them up gradually, but this doesn't generally work too well. Instead, we need to think of each shadow as though we're designing a shape upon that surface, outlining it first, then filling it in to be a solid black. One of the big advantages of this is that this can help us think more about how the form casting the shadow exists in space, and how its presence in space relates to the way the shadow itself is designed. You definitely moved more in this direction with number 5, but it did seem somewhat arbitrary in terms of where you were laying those shadow shapes down, and didn't suggest you were necessarily thinking about each tire tread "chunk" individually.

For most of the others, you seemed to work more with line. This isn't actually too bad when working with the tire treads composed more of shallow "grooves" but it becomes less functional overall when dealing with beefier treads.

Anyway, all in all you're moving in the right direction. I'll go ahead and mark this challenge as complete. Keep in mind that for the next lesson, you will need to rely on the ability to execute ellipses more consistently. How you achieve that is up to you, with the ellipse template obviously being the easiest. Fortunately for the purposes in lesson 7, we don't actually need to be able to draw huge ellipses, so a full (expensive) set of ellipse guides isn't necessary. They are however pivotal in the techniques we use to constructing unit-grids, so again, I strongly urge you to get at least a master ellipse template.