To answer your question first, french curves are really just a collection of a ton of different kinds of curves, laid out in ways that one can often smoothly transition into an other. The general idea is that you'd line up the template with the curve you're looking to draw, and trace along it to a point where that template's curve levels out so you can take over with a ruler, or another curve. Keep in mind however - french curves do not replace ellipse templates, they serve different purposes.

Of course, ellipse guides are expensive, and the more affordable alternative is to pick up a master ellipse template, which has its limitations. We're hoping to be able to sell our own full ellipse guide sets for far cheaper than the hundred-plus dollars they generally cost (we're hoping to aim for around $30 or less) but that's going to be a ways off yet.

Anyway! Starting with the structure of your wheels, overall you're doing well, though I do have some suggestions. One point I noticed was that you don't tend to vary the ellipses as you slide down its width, which can definitely result in a much stiffer impression. Part of this no doubt comes from the limitations of the ellipse guides, so the lack of degree shift in 7 for example is understandable. That said, what we do want to see is more of a "bump" through the midsection, which gives the wheel a sense of being inflated rather than just overly stiff. This point depends though - sometimes wheels are just very straight across, due to having extremely thick rubber, but most tires have that even bulge through the center that makes them appear bouncier and "inflated".

Actually, just in case I should correct myself a bit. I noticed here on 13 that you noted down the degrees of your ellipses. I can't read it perfectly, but it looks like 30deg, 45deg, 35deg? I honestly don't think those numbers are correct, because that would imply that the farther ellipse is of a lower degree, which it definitely isn't. So that's a bit perplexing. So, probably not an actual issue since the visible result is fine, but those numbers are a bit weird.

When it comes to your rims/spokes, you've done a great job of paying attention not only to the outward face, but also in establishing the side planes, which help to establish how these structures exist fully in 3D space, making the whole wheel appear much more solid.

The other aspect of this challenge is texture, and there we run into a bit of a trap. It's very common for students to, being so far removed from Lesson 2, forget about the principles of implicit vs explicit markmaking techniques and as a result, end up drawing all of their tire tread textures with explicit marks, generally relying on outlining things.

Now, this is primarily more noticeable in tires with chunkier treads (of which you have only really number 9 and number 3). Those with narrower grooves tend to get by more easily with explicit marks, but it is still easy to fall into the trap of simply having a tire (or really any piece of texture) draw way too much attention when it's part of a larger drawing. All of that visual complexity results in a focal point, drawing the viewer's eye, whether you mean for it to or not.

Textural markmaking techniques are all centered around the idea of conveying that texture accurately, providing the viewer enough information to understand its nature, but without having to put down quite so much ink and linework. As shown here (the example is of bush viper scales but they're very similar), we can reduce how much ink we put down without changing the nature of the texture - but only when working implicitly, putting down the shadows those textural forms cast, which are free to get larger or smaller per our own intent and desire.

One useful way to think about it is a sundial. A sundial features only the one big form, but the nature of the shadow it casts depends on the time of day (and therefore where the sun is in the sky). It'll cast a long shadow when the light source is low on the horizon, but a smaller shadow when it's high in the sky. As we have control of our own light source, we can determine just how big those shadows need to be.

All that said, while shallower groove-type textures can fly under the radar in this regard, it is worth still thinking of them in terms of these textural concepts. I won't get into it too much, but I did want to provide you with this explanation on how to think about holes and grooves. It's easy to mistakenly think of the grooves as being the textural forms, which leads to subtle (often unnoticeable, but still important) mistakes. But of course, grooves are empty space - they are not forms. It is the walls and floor of the grooves/holes which are the relevant forms we must consider, in terms of what's casting the shadows, and what surfaces are receiving them.

Anyway! As I mentioned, this stumbling was expected, and it's very common. I'll still be marking this challenge as complete, just be sure to review the texture material before heading forward.