Master ellipse templates indeed have their limitations - but ultimately it's a question of spending $15-20 or spending over a hundred. Fortunately we are already moving forward with our plans to manufacture and sell ellipse guides ourselves. I've already purchased a laser cutter for the guy who handles our pen sales, we've sourced acrylic, and we're going through the process of mocking them up, seeing what works. We're hoping that we'll be able to sell a full set up to about 3.5inch on the major axis for around thirty bucks, with a far greater range of degrees. Unfortunately I don't have any clear timeline on when we'll be in a position to actually start selling them, so I wouldn't wait on that to move onto Lesson 7.

Anyway, jumping into your wheels, starting with the structural aspects of these, you have generally handled them quite well. While you do have some that maintain an equal diameter across their width, this generally reflected what the reference required, with the majority of the others conveying a sense of "inflation" by having the midsection get a little bigger, giving it a sort of bump or "bubbly" effect as you called it. This is a good thing - it makes the wheels more believable, and gives us a better sense of how they'd behave were they to roll or bounce around.

For the most part you've also been fairly attentive to defining the side planes of things like the spokes/rims, rather than only focusing on the outward facing section, which helped to establish those spokes as being fully three dimensional, rather than just paper cut-outs stuck along the surface of the wheel (although in cases like 23 and 24 you did get a bit lax with this later in the set).

As to the second part of the challenge, we look at the actual way in which the texture of the tire treads has been handled. It's very common for students to actually forget about the textural principles from Lesson 2, given how far removed we are from that lesson, so this challenge serves as kind of a trap - or at least a reminder - for students to review those notes if they'd forgotten. Fortunately I can see that you did not forget, and you've made a considerable effort to try to work with implicit marks here, although there are some issues in how you've tackled it, which we can see in some of the wheels with the bigger, "chunkier" tread patterns - like number 8, number 9, number 15 and number 16.

Most notably, you're running into an issue where you're actually filling in the side planes of the textural forms, rather than drawing the shadows they cast. This is something that students are generally quite easily confused by, because the difference between them is not so clear. I've got an explanation I wrote up for another student who was running into the same issue (their work gave me a good opportunity to demonstrate the distinction) and I'll include that below, but before we get into that, I'll try to lay out the main distinction.

Cast shadows are cast from one form onto another surface, and it is the shape of the shadow itself which helps to define the relationship between them. When we simply fill in an existing shape - like the side plane of a given form - we're skipping the step of defining and designing such a shadow shape based on that spatial relationship, and so that information is not conveyed to the viewer. Instead, we're merely making a surface darker based on its orientation in space, which is more similar to form shading. As discussed here in Lesson 2, form shading isn't actually something we use for our drawings throughout this course.

As to the other explanation I did for the other student, take a look at this diagram and the explanation quoted below.

On the top, we've got the structural outlines for the given form - of course, since we want to work implicitly, we cannot use outlines. In the second row, we've got two options for conveying that textural form through the use of filled black shapes. On the left, they fill in the side planes, placing those shapes on the surface of the form itself, and actually filling in areas that are already enclosed and defined on the form and leaving its "top" face empty. This would be incorrect, more similar to form shading and not a cast shadow. On the right, we have an actual cast shadow - they look similar, but the key point to pay attention to is shown in the third row - it is the actual silhouette of the form itself which is implied. We've removed all of the internal edges of the form, and so while it looks kind of like the top face, but if you look more closely, it has certain subtle elements that are much more nuanced - instead of just using purely horizontal and vertical edges, we have some diagonals that come from the edges of the textural form that exist in the "depth" dimension of space (so if your horizontals were X and your verticals were Y, those diagonals come from that which exists in the Z dimension).

Because the differences are quite subtle, you may need to revisit that diagram/explanation a few times going forward, so don't worry too much if it doesn't make complete sense just yet.

And before I finish up this critique, I'll answer your question - although unfortunately the answer isn't going to be super satisfying. When we're stuck with a master ellipse template, as we've already discussed, we're saddled with some significant limitations - and if we need to draw an ellipse with a degree of 35 but we've only got 25 and 45, then the ellipse guide won't really be of much help, and we inevitably have to freehand the given ellipse.

But that doesn't mean master ellipse templates aren't extremely useful in Lesson 7 - it's just not for drawing the wheels themselves. Rather, it's for how we start out the constructions. In the lesson, I go over how we can leverage the relationship between a circle in 3D space, and being able to draw an actual cube in 3D space, and how that can then be extended to basically creating a 3D "unit grid" to replicate specific proportions for the length, height, and depth of the vehicle's overall bounding box. We use those ellipses to establish the initial cube, and then we can multiply those cubes in whichever dimension using the techniques we learned in Lesson 6.

Because this occurs as the first step, it also means that it is at this point that we can decide how the vehicle's going to be oriented. That means that we can work with whatever ellipses we have access to in our master ellipse template, and allow the rest of the construction to be built around that. So in that sense we may end up working with a very limited arrangement of orientations for our vehicles, but that's not really a big deal in the context of the lesson. What matters though is that the master ellipse template should be enough for us to establish this basic, foundational step, and then build upon it.

Conversely, the ellipses we draw for the wheels may not be perfect if we freehand them, but there's very little that is then built on top of them, and that requires them to be perfect, so it's not as big of an issue.

So! I'll go ahead and mark this challenge as complete. Keep up the good work.