Spool Car 2.0 -- a High Performance Take on an Old-Fashioned Toy

Quick Summary: This is a high performance take on an old-fashioned toy. Spool Car 2.0 is a rubber band "car" that anyone can make. It goes fast and straight. While the basic version isn't really a car, you can easily turn it into one if your situation requires 3 or 4 wheels. I am also including a higher performance version (3.0) that requires some 3-D printing but has some improvements that make it ideal for classroom use.

In The Classroom: I developed these spool cars to give my students something interesting that they could engineer, race, and measure. Compared to typical rubber band cars, they are simpler to make and much easier to steer straight. In addition to the base model, I have created a classroom set of interchangeable parts (Step 10, below) so that students can quickly test the effects of changing key variables, like wheel size, number of rubber bands, "wheel base," and front-wheel drive vs. rear wheel drive.

Historical Background: In case you're wondering about version 1.0, the name "spool car" is just one of several names for a rubber band-powered toy vehicle made from a sewing spool, a rubber band, and a few other simple materials that could be found (100 years ago) around any home. There are other Instructables that will show you how to make the original sewing spool version (Spool Car, Cotton Reel Tank). I was first introduced to these toys by Harold Warstler, a talented and generous woodworker who lived and worked in the Appalachian mountains, near Dillard Georgia. He called them "spool tractors.” This short video shows a classic example of what the author calls "toy steam roller," which is arguably a more fitting name.

Supplies

I will be providing two sets of instructions -- a "simplest" version, which is easiest to make but more difficult to modify for higher performance, and a "classroom" version that can be used year-after-year by students, allowing for quick modifications for higher performance.

Basic Spool Car 2.0

Cardboard
2 nuts. I used 1/4" nuts.
A big nail
2 paper clips
1 wire coat hanger
Wide rubber bands
Pliers
Wire cutter
Utility knife
Two push pins (or thin nail)
Hammer
Hot glue gun (and hot glue) -- you could use white glue or wood glue, but it would take longer
A work surface suitable for utility knife cutting and poking with push pins
Pencil

Classroom Set of Spool Cars (Version 3.0)

3D Printer (and the provided files -- in step 10)
1/4" square dowels -- sawed to 6" lengths
14 gauge (or slightly thicker) steel wire
4d nails (or something about that size)
Drill and 3/32" bit
Clipper (or small bolt cutter -- something for cutting nails)
Cardboard
Glue (hot, white, or wood)
Wide rubber bands
Two options for cutting wheels:
Laser cutter
Manual Method:
Utility knife
Two push pins (or thin nail)
Hammer
A work surface suitable for utility knife cutting and poking with push pins
Pencil
Optional Supplies:
Skateboard Bearings
A variety of rubber bands
Armor-All (rubber band airplane forums suggest using this to lubricate bands, supposedly improving their efficiency and lifespan)

Step 1: Check Out the Parts and Highlights

These figures show all of the parts and provide a summary of the construction process.

Step 2: Cut Out the Wheels

[Jump To These Directions In The Video -- starts at 0:05]

This vehicle is mostly wheels, so it pays to make good ones. Good wheels will make it easy for your spool car to travel straight and smoothly. You will need to make four identical cardboard circles and glue them to make two double-ply wheels. The method I'm sharing here is a surprisingly easy way to make perfect wheels. Personally, I find it very satisfying. You may find yourself looking for other reasons to make cardboard circles.

Decide how big you want your wheels to be. You will be stretching rubber bands around their edges, for traction, so it's easiest if the diameters are between 3" and 6".
Use the utility knife to cut a couple of cardboard rectangles of cardboard, big enough to make two disks from each.
Make a "compass" by cutting another rectangle of cardboard that is about as long as your wheel diameters and half that wide.
Push a push pin through your "compass," near one end. Then poke the same push pin (with compass attached) into your cardboard rectangle where you want the center of a wheel to be located.
Hammer your push pin into your work surface, pinning both pieces of cardboard in place. But don't hammer so hard that you break the push pin.
Poke the utility knife through your compass where you want the wheel edge to be. Poke through the compass so that you hit the cardboard below.
Keeping the utility knife in place with one hand, use your other hand to rotate the big cardboard below the compass. Apply enough pressure to begin scratching a circle outline of the wheel.
Keep rotating the bottom cardboard, gradually applying more pressure with the utility knife until you have cut all of the way through.
Remove the push pin gently with the hammer. I've broken a push pin by being too rough. They're just small pieces of plastic.
Remove the disk and repeat this process with the other three disks.

Step 3: Assemble the Wheels

[Jump To These Directions In The Video -- starts at 2:23]

Take a look at the grain of your cardboard disks. For strength and performance, it’s best to glue the disks so that the cardboard grain of one disk is perpendicular to the grain of the other disk. [That's how plywood is made.]
Prepare to glue the two disks together. You should apply a ring of glue near the edges of your wheels, because it prevents the rubber bands from slipping between the wheel plies. Be very careful to match the disks up as perfectly as you can! I match up the edges with my hands while I rest the edges on a tabletop to further align them. If you're using hot glue, you will have very little time, so practice first.
When the glue has set, enlarge the push pin hole to give the wire axle some more wiggle room. You can do this by poking a big nail or a pencil point through the hole and wiggling it around.
Finally, stretch a wide rubber band around the edge of each wheel. You can do it! But it may be easier if you find a helper. If you're worried about the bands slipping off, you can secure them with a little hot glue.

Step 4: Prepare the Frame Wire

[Jump To These Directions In The Video -- Starts at 3:27]

Straighten the coat hanger. You can use the pliers for this. It doesn't need to be super-straight, but if you want to get it super-straight, here's a cool tip for straightening wire. Just don't overdo the straightening, or you will break the wire.
If either end of the wire is twisty, cut that part off.
Bend the coat hanger in half, so that the ends match up. If they don't match, you can re-bend the wire or snip off the longer end.
Use pliers to bend each end into a short "U" shaped hook. Bend the two wires in the same direction.
Use pliers to make a right angle bend in each wire, about 1 1/2" from the hook end. Bend the two wires in opposite directions.
Bend the whole wire into the approximate shape that you'll want when it is attached to the wheels.

Step 5: Complete the Spool

[Jump To These Directions In The Video -- Starts at 5:37]

The “spool” is the two wheels plus a cardboard crosspiece. I like to size the length of the spool so that my rubber bands have a little bit of tension in them before they’re wound up. That tension keeps the bands in place and holds your sled together. To determine a good length, hook a rubber band to the two hooks of your wire frame, and spread them apart until the band stretches just a little bit. Then measure the distance between the wire’s right angle bends, and subtract about 1 inch. This will be the “length” of your cardboard rectangle. The other dimension of the rectangle should be somewhere between the radius of your wheels and the diameter of your wheels. I like to make it closer to the radius.
Draw your rectangle carefully on cardboard. It is important to make your rectangle's corners as "square" as possible, so that your wheels can be properly aligned. You can use a carpenter's square or a piece of paper to trace square edges.
Cut out your rectangle.
Prepare to fold your rectangle in half along its length, so that it will make the L-shaped crosspiece. Before you make the fold, draw a line down the exact middle of your rectangle, along its length. Then score along this line with your utility knife, cutting through only the outer paper layer. Then fold the cardboard so that the cut that you made is on the outside of the bend.
Hot glue one of the "L" ends of your rectangle to a wheel. Make sure that the angle made by the cardboard L "encloses" the wheel's center hole (see diagram for explanation). Hold the rectangle in place until the glue hardens.
Attach the other wheel (to the other end of the crosspiece) in the same way.

Step 6: Insert the Torque Pin

[Jump To These Directions In The Video -- Starts at 8:03]

The torque pin (a bit of paper clip, in this case) is what makes the rubber bands twist. Without it, the spool car won't go. The pin is held in place by holes in the cardboard, so you will need to poke a couple of holes.

Locate the holes. The holes should be at the midpoint between the wheels, and they should be positioned on opposite sides of the rubber band from one another. This is because the torque pin needs to go between the rubber band strands.
Poke the holes. They need to be big enough for your paper clip to pass through. You can poke them with a push pin and then do some wiggling. You can poke them with a nail.
One of your paper clips will become the torque pin. Straighten a section of paper clip. Bend one end so that it won't slip all of the way through the holes. Insert the paper clip through both holes. Bend the other end of the paper clip to keep it from coming out. Then trim off any excess paper clip length.

Step 7: Frame and Spool Assembly

[Jump To These Directions In The Video -- Starts at 8:29]

Thread the two 1/4” nuts onto the wire — one on each end.
The wire hooks won’t fit through the axle holes, so cut slits across those holes using the utility knife.
Insert the wire hooks through the wheel slits, but don’t insert the right angle bends.
Hook a rubber band (or two) to the two hooks. This will help hold things together while you glue the nuts in place.
Slide the nuts to the wheels. Then center the nuts on the axle holes and glue them in place. For best performance, try to keep the glue only on the sides of the nut next to the wheel.
At this point you should check to see if the wire is touching a wheel anywhere except at the nut. If it is, bend the wire so that the nuts are its only points of contact with the spool. Ok

Step 8: Test Drive and Steering Check

[Jump To These Directions In The Video -- Starts at 9:46]

Make sure that your torque pin is in place and inserted between the strands of your rubber band. Turn your wheels and confirm that the rubber band begins to twist. If it doesn't twist, re-insert your torque pin properly.
Wind up your car. Don't waste your time doing this by turning the wheels with your hand. Grab the coat hanger and roll your wheels across the floor. If you roll too far, you'll break the rubber band. If you don't roll far enough, you won't car won't go as fast as it otherwise could. Knowing how much you can wind your bands takes experience.
Place the car on the floor and let go. If you set it down one way, you will have a front wheel drive. If you flip it over, you will have a rear wheel drive.
You can adjust the steering by shifting the point where the wire touches the floor. Moving that point to the left will make it turn to the left. Moving it to the right will make it turn to the right.

Step 9: Break the "World Record" -- Increasing Speed and Measuring Performance

According to the folks at Guiness, the World's Fastest rubber band car covered a distance of 5m in a time of 2.82 seconds. You should be able to break that. I don't want to spoil your fun, so I won't reveal every tip (and I also don't have time), but I do want to explain how to deal with wheelies. As soon as you start adding more bands to your spool car, your car will begin popping wheelies.

[Jump to the wheelie part of the video 1:38] Wheelies are fun, but they make it harder for your spool car to steer straight. The best way to prevent wheelies is to lengthen your wire. Alternatively, you could wind your bands less, but that's no fun. You could also adding weight to the wire, but that will slow down your acceleration. Adding length to the wire doesn't add much weight, but it does add downward torque, and that's what you need.
If you want to see how your car compares to the world record, set up a 5m long "race course," and time your car. The world record is 2.82 seconds. My car covered the distance in 1.65 seconds. I think that mark could be broken by quite a bit. [Jump to the speed-measuring part of the video -- 3:26]

Step 10: Spool Car 3.0 -- a Classroom Engineering Platform

These 3-D printable parts represent an interchangeable system with some higher performance features. With this platform, students can quickly manipulate multiple variables, creating and testing many iterations of their designs. Assembly and operation are explained in the video. Here are some key improvements over Spool Car 2.0...

Quick wheel changes (I was able to remove the wheels and put on new ones in about 50 seconds)
Interchangeable parts (So different classes of students can use the same set of parts to make unique cars).
A release mechanism, so the car can keep rolling after the bands unwind
A low friction option with skateboard bearings.

**NEW DEVELOPMENTS:

After some recent testing by my students, it has become apparent that the "pin keeper" is more trouble than it's worth. It's better to just let the release torque pin pop out. If you put a piece of colored tape on it, it's easy to find, and it marks the spot where the "motor" of the car stops pushing. This is useful information if you are trying to find the point where the car reached its maximum speed. There are a lot of ways to make the torque pin, but one of the easiest is to make it out of wire with a coiled end. The coiled end will add some weight, which will give the head of the pin more angular momentum, which will help it eject when the bands are unwound. I have included a photo of the bare pin with a coil at its head, and another photo of the same pin (inserted into the spool car) with its head covered in blue tape.
I have also decided that it's nice to be able to add a front wheel. I have included two photos showing how the front wheel can be attached. I have also added an STL file for the wheel and its hub. The hub has a hole that's about right for a 14 gauge wire, but you can ream it out to a larger size if you want. The wheel increases the car's travel distance. It also makes the car easier to steer.