I wanted to start off with something at least somewhat straight forward - this cab-over style of truck is pretty much a few boxes and a few cylinders. Unfortunately things do get complicated when we get into its detail (and I stumble on it myself, as you'll see) but the core construction of it as straightforward as can be.

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While the rest of this lesson will focus very heavily on proportion studies, for this one I wanted to jump right in, due to the relatively simplistic construction. I set in a simple box (which right off the bat I made too wide, so I had to cut it back a little on the right side of the drawing), a box underneath and the beginnings of a plane for the wheel. Notice that I defined one vertical line and two lines going off to the left, with no second vertical. This is because we don't yet know where that second vertical should go. We want this plane to be a perfect square (so it can contain the wheel, a perfect circle), so we'll be using the techniques outlined in the videos above.

So here you've got a cylinder with three concentric ellipses on each side. The reason for this is... well, my wheels weren't big enough to start with, and in my second attempt they still weren't big enough. Yeah, there's no magic here. The inner-most ellipse is the result of what we were doing in the previous step - I laid in the ellipse so that it fit snugly between the first vertical and the two horizontals, and I used a degree that made it so the contact points above and below (where the ellipse touches those top and bottom horizontal lines) align vertically. As explained in the videos, this only matches up when the ellipse accurately represents a perfect circle within the context of this perspective system. Once this has been achieved, we can draw the remaining vertical to close off the ellipse in a plane - which also happens to now represent a perfect square in 3D space.

Note: Freehanding ellipses with all of this business of specific degrees and matching up contact points is hard. You're not likely to get it down perfectly, so as I mentioned above, you are free to use ellipse guides, rulers, etc. to make your life easier. If you happen to do it freehand, that's fine as well, just don't get frustrated if you don't nail your ellipses. A bit of deviation won't ruin things, it's totally fine.

Another thing worth mentioning here is that unless your wheels are at an angle relative to the body of the vehicle, you should start the wheels on the same axle off as a single cylinder. You can cut them up later, but in general it's a lot easier to work through these phases with larger cylinders rather than squatter ones already cut to the size of each individual wheel.

Looking at my reference, I roughly judged the distance from the front wheel to the back wheel to be roughly 5 wheels. When dealing with wheeled vehicles like this, it's extremely helpful to use those wheels as units of measurement. Since they're perfect circles, we can fairly easily understand and apply their proportions in multiple dimensions, using them for length, height, and even sometimes the width of the object.

Here I've applied the technique to transfer measurements back in perspective (also described in the videos above).

Same deal - upon looking more carefully at my reference, I realized the height of my truck was way off, and that the whole thing is about 3 wheels tall. So, again, I transferred that measurement up and defined a more correct box.

At this point, most of my primary construction is done, and I can start refining those forms - that is, rounding off edges, carving more complex information, and so on. This truck's finer details were deceptively complex, and I ended up leaving a great deal alone due to its insignificance in the context of this demonstration. Honestly, I wish I'd left more of it alone, as it ended up being a bit of a rabbit hole that just kept going deeper and deeper...