Hack the String Car

String Car History

In the early 1980s, my college-aged brother designed a simple motor-plus-battery car that raced along a string between our carport post and the street-side power pole. For years now, we've been improving on his basic design, with the initial goal of keeping it really simple: one battery-driven motor, optional flashing lights, and repurposed materials whenever possible. The primary racing objective was speed and stability. The car ran at full speed until it found the end of the string -- then you had to be there to catch it or be prepared to repair it before the next time trial.

We talked about making the car smarter so that it could stop and reverse directions, and came up with lots of schemes and circuits to make it possible, but for many years were more focused on the high-speed "Gump" mode of operation. Run Forrest, run!

The first version was made from a reclaimed motor with a pulley already attached, a spent 9v battery, and a short length of fence wire to use as the chassis and string guides -- zero cost. Using fence wire as the chassis framework allowed for many design variants from sleek and compact to exceptionally stable stretch versions. The wire bends easily and can be formed into whatever the designer wants, as long as they think about center-of-gravity, balance, traction, stability, and connecting a simple electrical circuit. One challenge along the way was to create both the chassis and guides from a single length of wire, front-to-back with only bends, no cutting or brazing.

Here's an example of a "Gump" car from 2006 designed with an open chassis so that it could safely race along the edge of a windy lake. The tilt of the battery was an attempt to center as much of its weight under the pulley as possible to help with stability. For traction issues in damp and cold environments, we cut a very thin sliver of surgical tubing (resembles a miniscule rubber band) and snapped it into the bottom of the pulley's groove. We learned that a wider band will cause the car to pop off the string.

Of course, adding flashing lights for nighttime racing was an exciting improvement. This is a recently built version outfitted with flashing outrigger guide lights. Not cat-tested, however.

Over time, we upgraded the basic string car with an Adafruit Trinket microcontroller to detect end-of-string and autonomously prevent collisions using a variety of sensors and motor control -- akin to robots. The more sophisticated models learned about string length and applied predictive braking to keep the car from plowing into the tree and falling off the string.

For example, the Richter version replaced the microswitch sensors with a vibration sensor that detected when the car slammed into the end of string cushion. We had to run this one a bit slower so that it wouldn't bounce off the string!

For Richter, the Trinket was replaced with a custom ATtiny-85 microcontroller board to save space.

In spite of the seemingly endless options that come to mind, we continue to be inspired by the original design and, to this day, enjoy reclaiming old motors, turning our own custom pulleys, and devising faster, more stable cars based upon the idea that was hatched up in Grandpa's basement workshop.

We hope you are inspired to take on the challenge to build a better, faster, and smarter string car!

This guide was first published on Sep 01, 2017. It was last updated on Sep 01, 2017. This page (Hack the String Car) was last updated on Jul 22, 2019.