Congrats on finishing the 25 Wheel challenge! I'll do my best to give you advice so that you can improve.

For the structural component (note I closed my tab on accidentally and ended up re-doing this so I may have left something out) I can see a great amount of improvement across the wheels especially since you've freehanded them. What started out as elongated cylinders turned more and more into bubbly wheels (where applicable) so great job on that. 25 is especially great because it nails both the structural component and textural component of the challenge. For the spokes and rims they have also improved and aside from a couple cases where they end up becoming flat (1, 2, 7) due to not adding a side plane the rest are fantastic. Furthermore it's great that you've used 2 lines for spokes on the bike wheels because most people end up using 1 which doesn't allow them to be seen as forms. Finally I'm not sure what happened with the spokes on wheel 11 but always make sure you are drawing confident lines from the shoulder otherwise the solidity of the forms is undermined.

I'll add more to this on the textural component but for the spokes you don't want to just fill the side planes with black (we can see this on 25, 7, 8, 19) because that is more akin to form shading which isn't used in this course. A good example of using black correctly can be seen on 23 which only shows the cast shadows as black.

For the textural component of the challenge, I can see that you've used a mixture of both implicit and explicit mark making. To start though, I did want to call out that in 12 you appear to have shifted strictly to using form shading for its own sake - meaning, the goal was to make the top/bottom darker to show the curvature of the surface. In general as explained here we do not use form shading for our drawings in this course at all. If we take a look at 5, 7 , 9 we can see that you've ended up filling the side planes (which would also be form shading) instead of drawing the cast shadow shape which would imply the presence of an elevated surface (i.e a texture). But if we take a look at 25 we can see that you have used implicit mark making to imply the presence of the grooves on the tire. Before I move on I want to address the areas of solid black that are seen on 15 and 24, which seem to be more like coloring rather than any shading. I think it's best to just leave solid black for cast shadows only so that you don't end up getting confused.

Here's how uncomfortable explains texture from the context of texture analysis from lesson 2

Here's a diagram that takes us step by step through how to think about a texture, in order to draw the gradient from that exercise. First, traced over the image of melted wax, I've identified some of the actual forms at play - individual globs of wax. This is what I'm doing when I'm looking at my reference - I'm identifying each form, one by one, and understanding how they sit in space in relation to one another.

In the row below that, I've represented how I'm thinking about how I might arrange those textural forms in the surface I'm using in my gradient. Remember that if we were to look at this surface from the side, it would not be flat. It would be as seen here (or alternatively here in case this version of the diagram makes more sense). These are not candle-wax specific, but rather just representing how forms are arranged on the surface, and how they relate to the light source, resulting in much longer, deeper shadows the further from the light source we move.

Now, it's with this understanding of how we're arranging these forms upon our surface that we can actually think about all of the individual shadows, one by one. Outlining those cast shadows is very important, because it's this step which allows us to design this shape, whose whole job is to establish the relationship between the textural form, and the surfaces around it. I've faded out the results of the previous row (where I outlined all of the candle wax blobs) - the hard part is that this needs to be done in your head. You can't draw those globs on the page. What you can do however is think about each individual glob one at a time, as though you're adding them in order, and with each one you're defining its cast shadow as you add it. So you're not thinking of every glob simultaneously - just one at a time, and maybe a bit about the ones around it.

And finally at the bottom, we have the actual results - we've filled in our designed shadow shapes, and due to the light source being at the far right, the shadows get deeper moving towards the left, eventually merging into that solid bar on the leftmost edge.

So, texture comes down to thinking about the actual forms at play, and doing so one at a time. Time is important, and if we rush ourselves to go faster than we're able, we simply won't pay attention to each form, one at a time. There is always a lot of temptation to just deal with randomness in this regard, but that's not solving the problem - it's giving up on it instead.

The last thing I wanted to offer you is this diagram. It's pertinent to the tire tread textures where we have a lot of shallow grooves, as most students are prone to thinking about the groove itself as though it's the textural form they're supposed to be thinking about. As shown in the diagram however, it is the walls that surround the grooves/holes which cast shadows upon one another, and upon the floor of the hole itself. This also applies to any texture with holes - so sponges, cracks, etc. Often the difference in the result between understanding the groove to just be empty space, and thinking of the grooves to be the actual form can be negligible - but given that these are all exercises, it still matters a great deal how we think about them.

Now, I will still be marking this challenge as complete. Given that it's a trap, it's not at all abnormal for students to fall into it - but it does mean that you may want to review the texture material from Lesson 2 prior to moving forward and completing the rest of the course.

If anything needs clarifying don't hesitate to ask!