Summary

This may be a great project for anyone with extra time on their hands. Printing, sanding, and assembling this car takes a lot of time and effort, but it's good for someone like me that has no woodworking skills.

For best results, the body should be smoothed using resin and baby powder. (Google "3d printing smoothing with resin and baby powder" for videos and guides on how to do this.)

The Connaught Type A was a British Formula Two race car. It wasn't particularly successful, but it was a very pretty car with some distinctive physical characteristics that made it fun (and challenging) to design in Fusion 360.

What is a "pinewood derby car shell"?

A pinewood derby is an event created by the Boy Scouts many years ago. Each scout is given a pinewood derby kit typically consisting of a block of wood with slots for axles, 4 wheels, and 4 nails to be used as axles. The scout (and his dad, usually) will then carve the block of wood into the shape of a car, paint it and apply stickers, install the wheels on the axles, and then race it at the "derby", which involves a sloped track with a timekeeping apparatus.

An official scout derby will usually require that the parts in the kit be used for cars entered in the derby. Other organizations (including workplaces) often hold derbies, too, and some of them require the use of the parts in the box, including the block of wood.

These "pinewood derby car shells" are intended to be used with an official derby wood block, cut down to size, along with the axles and wheels included in the kit. A printable model of the shaped wood block is provided. You can either use the printed model or use it to accurately cut an official wood block down to size.

Resolution:

12 mm

Infill:

60%

Notes:

• Car body: The body consists of 5 parts, which allows for some nice curves without excessive supports. They require some work to get them to fit together smoothly. You can print the main section of the body on a raft if you like, but I uploaded a version that has a "miniraft" built in, which enhances adhesion to the build plate for the supports.
• Exhaust: The exhaust is the most difficult piece to get the supports to work well. I used tree-style supports and removed them very carefully, because the exhaust is fragile.
• Windshield: I printed the windshield using transparent PETG in vase mode.
• All the other pieces print pretty easily with minimal supports.

Post-Printing

Smoothing

This is the first car that I've smoothed with 3d printing resin and baby powder. You can find videos and guides online that describe the process, but here it is in a nutshell:

• Mix 1 part resin and 2-3 parts baby powder in an opaque container (so that it can be stored without hardening due to UV exposure).
• Using a paintbrush or cotton swab, spread a thin layer of the resin/baby powder mix over the body, wherever you want to smooth print lines and gaps.
• Use a UV flashlight (a cheap one--like the kind used to harden resin finger nail polish, which you can order from Amazon or buy in the pharmacy department of Walmart) to harden the resin.
• Sand the body smooth until there are no thick layers left.
• Repeat over any areas that are still rough.

Adding weights

Depending on the infill you use, you should be able to add tungsten weights to the holes in the rear of the body. If you plan well, you can maximize the weights and still end up right at 5 oz. I haven't raced this design, but it seems like it could be pretty fast if you put some effort into it.

How I Designed This

As with most of the cars that I design, the main body is a fairly complicated loft using 3 profiles and rails across the top and bottom and sides. The rails on the top and bottom are simple fit point splines, but the other rails combine fit point splines using intersection curves. It's pretty challenging to ensure that the resulting curves intersect all three of the planes.

Once I had the basic body shape, I refined it using extruded surfaces and planes to split the body and remove the pieces I didn't need.

The exhaust was really fun to design, using intersection curves. I added some simple extrusions to provide surfaces to attach it to the body, and also to make it easier to print.