Cybercab Elastic Drive

The motor is one loop of elastic cord and one finger on the rear axle. Roll the car backward a quarter of a wheel turn, set it down, and let go.

  • 180 mm nose to tail
  • 4.4 oz of PETG
  • 9 parts one of them bought
The finished shell, on a white sweep.
Direct side view of the printed car: a long gold wedge with a dark swept window and two black disc wheels
This is the side the whole design was judged on. Against the single reference photograph the brief supplied, the shipped silhouette overlaps it 0.903 — the gate that measured it would have failed anything under 0.9.

Eight printed parts and one that isn't.

The shell, the chassis, two axle-and-wheel pieces, two keyed end wheels and two window inserts all come off a printer. The ninth is a loop of elastic cord you buy. Keep scrolling and the shell comes off.

Press, slide 4 mm, lift.

A thumb latch moulded into the chassis floor bends 1.9 mm out of its striker. The shell then slides 4 mm toward the tail, clears the closed ends of four captured T-rails, and lifts straight off. Nothing unscrews.

The drivetrain is a piece of string.

It runs from a capped peg near the nose back to an open finger at the centre of the rear axle. Slack, its centreline measures 219.184 mm, and that is the entire power train.

Roll it backward.

The finger swings up as the rear wheels turn, and the cord has to take the long way around it. A quarter turn is 25.92 mm of backward roll and 10.447 mm of extra cord — watch the rule under the wheels.

Let go.

The cord shortens back to 219.184 mm and the same quarter turn runs forward. The finger is open on purpose: once the cord goes slack it drops off, instead of winding up the other way and driving the car back at you.

Shell back on.

Lower it onto the rails, hold the latch, slide forward. The cord is the consumable here — when it tires, lift the shell and hook on another.

What is under the shell, at size.

The shell is drawn transparent here so the drive shows through it: a flat chassis floor, two axles turning in short printed journals, and the cord running low down the centreline between them. Every figure below came off the sealed CAD rather than a tape measure.

Cutaway side elevation with the shell drawn transparent, showing the chassis, both axles and the red elastic cord running between them Dimensions in millimetres 180 114 60 ⌀33 219.184
Cutaway elevation rendered from the shipped assembly. Dimensions are millimetres; the cord is shown relaxed, in the pose the file was exported in.

  • Journals, not bearings

    Each 6 mm shaft turns in a 6.5 mm printed bore over a 5 mm land — 0.5 mm of clearance, printed in place, nothing to buy and nothing to press in.

  • Keyed end wheels

    A 3.4 mm square key enters a 3.9 mm socket, 0.25 mm a face. The key carries the torque; glue only stops the wheel walking off, and the run is explicit that it has not tested that bond.

  • Glazing, not paint

    Two 1.4 mm panels, 80 mm long, drop into 1.6 mm recesses with 0.1 mm of clearance and glue in flat. Printing them apart is why the windows are actually dark.

A quarter turn, measured at every step.

Rolling the car backward is the winding. The finger at the centre of the rear axle sweeps up through a quarter circle, the cord has to take the long way around it, and the path it follows grows by 10.447 mm. The build sampled that sweep every 15° and recorded both the cord's centreline length and how far the car has rolled, from the wheel radius alone: 16.5 mm times the angle in radians.

The table stops at 90° because the build does. A full rotation clears the frame geometrically, but nothing here has been run physically: cord force, grip, fatigue and how far it actually travels are all unmeasured, and the build notes set one quarter turn as the commissioning limit. Start there, and stop on any binding or whitening.

Sampled from the sealed assembly. A negative angle rolls backward; releasing to zero rolls forward.
Rear axleCord centrelineCar travel
0°219.184 mm0.00 mm
−15°219.597 mm−4.32 mm
−30°220.693 mm−8.64 mm
−45°222.388 mm−12.96 mm
−60°224.552 mm−17.28 mm
−75°227.025 mm−21.60 mm
−90°229.631 mm−25.92 mm

Service is a thumb and four rails.

  • 1.9 mmPress the latch under the chassis inward, until its tooth clears the striker in the shell.
  • 4 mmSlide the shell toward the tail. The T-rails are closed at their forward ends, so this is the only way it goes.
  • LiftStraight up and off. Hook a new cord over the front peg and the rear finger, then run the three moves backward.
Three-quarter view from behind, showing the faceted tail, the rear valance and the dark side glazing
The tail end. The overhanging deck above the rear wheels is what the shell hangs from once the latch lets go.
Close three-quarter view with a sliver of the red elastic cord visible behind the front wheel
The only place the drive shows from outside: a sliver of cord behind the front wheel.
Reference shell
1 printed
Service chassis
1 printed
Rear driven wheelset
1 printed
Front free wheelset
1 printed
Keyed end wheels
2 printed
Glazing inserts
2 printed
Drive loop
1 bought

Print it, or have it printed.

Overall
7.1 × 3.1 × 2.4 in
Weight
4.4 oz in PETG
On the printer
13.2 h
Parts
8 printed, 9 in the box
Bed needed
220 × 220 mm
Wheels
33 mm on a 114 mm wheelbase
Track
70 mm
Shell walls
2.4 mm sides, 3.6 mm roof
Drive loop
one ring of about 1.4 mm round cord
Assembly
adult, with glue for wheels and glazing
The car sitting on a pale wooden floor, ready to be rolled backward
A clear, level floor is where it runs. Small parts, so keep it away from children under three.
  • 6 unique files to print
  • 0 regions needing support, in any of them
  • 6 mm longest unsupported bridge

Checked at a 0.4 mm nozzle against a 0.8 mm minimum wall: 1.2% of the shell's surface faces down steeply enough to matter, and all of it bridges.

Ships within 14 days. SKU VB-G47JCTGN.