On designing and building the Rat Racer

As you might have guessed from the pre­vi­ous post about attic rats, I had a few left­over rat traps. And what does one do with left­over traps? I sup­posed you could do some­thing cool with ping pong balls, but I did not have nearly enough traps or ping pong balls.

I have a vague child­hood mem­ory of peo­ple mak­ing “cars” from mouse traps. I do not think I ever saw one in per­son, but I some­how know the design. I imag­ine it graced the pages of Boys Life at some point. As I remem­ber it, the basic design is this: four eye­let screws, screwed into the mouse trap base to hold the axles in place. Two pen­cils or wooden dow­els, used as axles. Four wheels, maybe cut from wood? A piece of string or wire. You would remove the “trap” mech­a­nism from the mouse­trap — the “cheese plat­form” and retain­ing bar — leav­ing you with just the snappy part on a piece of wood. That becomes the spring-loaded mech­a­nism, like a badass pow­er­ful wind-up car. Tie one end of the string to that, pull the snappy part over and hold it in place, wrap the other end of the string around the axle, put it on the floor, and let go. The snappy part pulls the string, spins the axle, and makes the wheels go.

What I have are rat traps, which are a bit more hefty, snappy, and dan­ger­ous to the fin­gers than lit­tle mouse traps, but I thought I would give it a try. I live in the 21st cen­tury and have a Replicator in my lab. I didn’t feel like mess­ing around with pen­cils, screws, and saws. Also, I felt my Thingiverse account was get­ting a bit stale and needed a new upload. I set out to design 3D-printed rat trap attach­ments that pro­vided the required ele­ments.

I took my calipers to a cou­ple of Victor brand rat traps to get a gen­eral sense of size. They are made of wood, so have minor incon­sis­ten­cies between them. I then designed end caps, axles, and wheels.


The design appeared to be sound, but the print­ing was iffy. What I ended up with, well, the web­site Hack-a-Day has a weekly fea­ture called Fail of the Week. It chron­i­cles great ideas that are brought to near-completion, but that just don’t work. There is then the hope that some­one in the com­ments (or oth­er­wise) can either learn from it or pick up the idea and improve it. To build a bet­ter mouse­trap, as it were.

My Replicator has some ther­mal prob­lems with large or long prints. Different regions of the lay­ers cool at dif­fer­ent rates, caus­ing some warp­ing. With cen­tral prints, every­thing is good, but prints with far-reaching foot­prints end up curl­ing and warp­ing. The end caps looked and fit great. The axle was warped and the wheel was seri­ously warped. I aborted.







At some point I might try is Zheng’s ABSynthe print­ing method. Up to this point, I have been a MakerBot purist and stuck with the stock parts and meth­ods. I hap­pened to have a few mouse­traps on hand thanks to an impulse buy after the warped print­ing. I thought I might refine the design with the smaller traps. The parts are smaller and eas­ier to print ver­sus work­ing with the rat trap’s larger, slower to print, and more prone to error parts. Another fac­tor in this deci­sion was that I was dubi­ous of the rat trap’s spring. I thought it might have too much torque. It’s a pretty nasty spring. The smaller mouse­traps also have a lot of power, but I felt they might be more work­able than the rat trap.

This, then, leads to the Rat Racer’s younger brother, the Mouse Car.


The Rat Racer parts in their cur­rent form can be down­loaded from Thingiverse.

To be con­tin­ued…

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Brian Enigma

Brian Enigma is a Portlander, manipulator of atoms & bits, minor-league blogger, and all-around great guy. He typically writes about the interesting “maker” projects he's working on, but sometimes veers off into puzzles, software, games, local news, and current events.

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