Author, Publisher, Developer, Stitchcoder


Beginnings of a Whole Car

As reported in earlier blog posts, I've recently started selling "Mechanical Gifs", highly stylized acrylic models of simple mechanisms designed to illustrate how the things work—the same way an animated gif would, but in physical form.

The models currently for sale are one mechanism at a time: A lock, an engine, etc. But I've got a lot of other designs in my back pocket waiting to come to life (and to the store). Here is a preview of the inspiration I had a few days ago to combine several models I have been working on into a complete working car. (Well, more like the skeleton of the most fundamental working parts of an imaginary radial-engine car.)

Here's what it looks like if you push it across a table. The mechanism is smooth enough that the whole drivetrain turns even though there is almost no friction between the smooth acrylic tires and the smooth varnished table top.

Here is a video showing how the radial engine (a slightly modified copy of the one for sale at connects to the differential (an unfinished design not currently available). Yes, there is no transmission and no universal/CV joints. Simplicity is key here! This is essence of car, not reality of car.

Here is more of how the differential works. (I didn't have anyone to hold my phone, so I can' show you how it works when the driveshaft is turning and you hold one wheel. Which is the whole point of a differential.)

Here's a close-up of the rack-and-pinion steering mechanism in action (top and bottom views):

The differential and steering mechanisms are not final designs, but neither are they first-generation. Both of them have been through 4 or 5 cycles of design, laser-cut, build, improve. For example, notice the little wings on the sides of the green rack gear in the steering mechanism? They extend as far as the ends of the joints connecting the rack to the tie rods. Without those wings, the rack tries to twist out of parallel when it's turned all the way to one side or another. Of course I didn't realize it would do that before I built one without wings.

I have several ideas already how to improve this design. For example, I'm going to replace the flat table-top connecting all the parts with two vertical rails running down the sides of the model. This will save plastic and make the whole thing much stronger and stiffer.

I'm also pretty sure I'm going to add a 2-speed transmission before finalizing the kit. (I have a design for a transmission, and it sort of works, but it's definitely not ready for prime time yet.) I don't think I will add any kind of suspension or universal joints: these parts do not feel to me as fundamental as the engine, transmission, differential, and steering.

My goal in all of these mechanical gifs is to simplify down to the most essential aspects of the design, to focus attention on the principal motions. In this case I wanted to find the essence of an old-school internal combustion car (electric cars are a whole other can of worms).

Of course you can't have a car without an engine, so that's definitely in. Without a steering mechanism you can only go in a straight line, so that also seems pretty fundamental. And if you have only one engine, then you must have a differential, or you will tear up the tires and axles as soon as you try to go around a corner. 

The transmission you could live without, but all real cars have one, because internal combustion engines are only efficient in a fairly narrow range of rotational speed. You have to have gears to let the speed of the wheels vary over a wide range, while keeping the speed of the engine in a narrow range. (Electric motors don't have this problem, and they are small, so you can just put a separate motor on each wheel and get rid of both the transmission and the differential.)

The other nice thing about a transmission will be that it shifts the axis of the drive train, so I can move the axis upwards, allowing the engine to be in a more natural , higher position while the differential remains lower down.

Stay tuned, maybe in a week or two I'll have the transmission installed.... (Sorry, not much chance of this being for sale before x-mas.)

P.S. Brakes? We don't need no stinking brakes. Onward!

Theodore Gray5 Comments