How the Roadster Ramp Flip Works

Turn the crank yourself

This is the actual 3D design. Grab the crank and drag it around to run the whole thing. You can also drag the gear, the arm or the car. Drag the background to look around, and scroll or pinch to zoom.

How it works

The car is glued onto a 2 mm steel axle that runs through the end of a printed tube. That tube pokes through a slot in the black backdrop, and everything that moves the car rides behind it on a printed tray I call the carrier. One crank has to do four jobs:

1. Turn a big gear slowly (Part 1).
2. Move the car along its path. A swinging arm pushes the tube along a slot in the backdrop (Part 2).
3. Tilt the car to match the ramp. The first cam groove (Part 3).
4. Roll the car in mid-air. The second cam groove swings a rocker, and a braid loop and capstan turn that swing into a half barrel roll (Parts 3 and 4).

I wanted the crank on the side of the box so you can see it in the shot, which means it turns a shaft pointing sideways. A pair of 18-tooth bevel gears turns that 90° into the back plate. That spins a 17-tooth pinion, and the pinion drives a 153-tooth gear.

The whole run is about 8¾ turns of the crank. The big gear only turns 350° in that time, so it never quite makes a full revolution. The 9:1 reduction keeps the crank light and slows the car down enough that you can actually follow the stunt.

The big gear's shaft also carries an arm with a slot down the middle. The backdrop has its own slot, the track, cut in the exact path the car takes: along the road, up the ramp, over the top and down to the landing.

The tube runs through both slots, so it can only sit where they cross. As the arm swings, that crossing point slides along the track and drags the car with it. The arm needs a slot instead of a hole because the track gets closer to and farther from the gear's centre as it goes, so the tube has to slide in and out along the arm.

The arm and the track only decide where the car is. To control which way it points and when it rolls, there are two cam grooves cut into the back plate, each with a 4×10×5 bearing riding in it as a follower.

• Groove 1 sets the tilt. Its follower, wheel B, is fixed to the carrier. With the tube in the track and wheel B in groove 1, the carrier is pinned at two points, so the groove decides the angle. It's shaped so the car always matches the ramp, all the way to 170° at the top, where it hangs nearly upside down.
• Groove 2 times the roll. Its follower, wheel R, sits on the end of a 27 mm rocker that pivots on the carrier. For most of the run, groove 2 just follows along in step with the carrier, so the rocker doesn't move. Right after the car leaves the ramp, it bends away and forces the rocker through 75°. Then it falls back in step.

So groove 1 steers the car, and groove 2 is basically a timer that says exactly when to roll: between 6.6 and 8.0 turns of the crank. Every motion needs its own path, which is why it takes two grooves.

Drag the slider to run it. The dark blue stretch of the roll groove is where it bends away, and that's the only place the rocker moves. The numbered dots match the six pictures at the top; click one to jump there.

This is my favourite part. The car has to roll about its own axis while it's flying through the air on the end of a tube that's also moving and tilting. There's no room for a motor or gears out there, so the roll is done with a loop of braid.

Try it: swing the rocker and watch the braid turn the capstan.

How the loop is routed

• It starts on a 4.8 mm radius drum on the rocker.
• The braid is 0.2 mm black PE.
• A 1.5 mm steel pin turns both legs 90° to point up the tube.
• Both legs run up the 5.2 mm bore inside the tube.
• At the top, it wraps 1½ times around the capstan, a printed sleeve glued onto the car's 2 mm steel axle.
• Then it runs back down the tube to the drum.

Why a capstan

• It doesn't slip. 1½ wraps of braid grip the sleeve, and a dab of glue on the first wrap locks it in place.
• It fits. The capstan is only 3.8 mm across. You can't get gears or a shaft through a 5.2 mm hole in a moving tube, but two strands of braid go through easily.
• Tilting doesn't roll the car. The drum and the capstan both ride on the carrier, so when the carrier tilts, the whole loop tilts with it and no braid moves. Only the rocker swinging relative to the carrier pulls braid. That's why groove 2 has to follow the carrier everywhere except the roll.
• It works both ways. One leg pulls the car into the roll and the other pulls it back out when you crank in reverse, so there are no springs.

To tension the loop, one end ties off on a small peg. You turn the peg with a 2 mm hex key from the back, then lock it with a clamp screw.

Why it rolls at all: coming over the top leaves the car upside down and facing the other way. The half roll puts it back on its wheels before it lands.

There's a small lug on the back of the big gear and a pin on a post right next to it. At the end of the run, the lug hits the pin. Crank back the other way and the gear goes almost a full turn before the lug hits the pin from the other side.

I put the stops on the gear instead of on the carrier on purpose. With a 9:1 reduction, forcing the crank against a stop at the carrier would push that load straight through the printed tube. On the gear, the tube never sees it.

The pin is eccentric, so a twist with a hex key fine-tunes where the run starts and stops. Once it's set, it gets glued.

The whole run, turn by turn

Crank it backwards about 8¾ turns and the whole thing runs in reverse back to the start.

Why the back is a skeleton

The back plate is mostly windows and the big gear has spokes, because I want you to be able to watch the gears from behind. It saves a good amount of plastic too.

Pictures are Blender renders of the R03 design. The 3D model and the cam-path view come from the same design data the SOLIDWORKS model was checked against. Sizes are for the 1:50 model.