📏 Two-Scale Survey Cart: step-by-step build

Roll a measuring cart with linked fine and coarse indicators, then calibrate its distance scale.

The measuring wheels turn a three-to-one fine indicator, whose shared shaft drives a second five-to-one reduction. One coarse turn represents five fine turns and fifteen wheel turns. Use both indicators to distinguish a longer journey from a wrapped fine reading, then compare nominal wheel geometry with a measured rollout. This original straight-line instrument is not a certified odometer and does not remember complete coarse-pointer revolutions automatically.

  • Level 4 of 5 (tricky)
  • About 45 minutes
  • 13 steps, 40 parts
  • Powered by: Push
  • Series: Mechanical Wonders

What you will learn

Build these first

  • 🔢 Odometer Cart: Five turns of the wheels make the flag go round just once — a cart that counts! (level 1 of 5, about 15 minutes, 9 steps)
  • 🔬 Compound Counting Bench: Two five-to-one stages multiply: count 25 input turns for one output turn. (level 2 of 5, about 20 minutes, 7 steps)

Parts you need

  • 2 × Beam 11
  • 4 × Black pin (grippy)
  • 2 × Open frame 5×7
  • 2 × Axle 10
  • 14 × Half bush
  • 2 × Tiny gear (8 teeth)
  • 4 × Bush
  • 1 × Medium gear (24 teeth)
  • 2 × Axle 7
  • 2 × Thin beam 2
  • 1 × Giant gear (40 teeth)
  • 4 × Thin-tyre wheel

Steps

  1. Lay an 11-hole side rail on its edge. Its axle holes are the first and ninth holes; the tall counter frame goes on the inside.
  2. Pin a standing 5×7 frame onto the rail with two separated black pins through its bottom edge. Its right column is above the ninth rail hole. Two pins prevent the frame hinging.
  3. Fit the front and measuring axles through the first and ninth rail holes, and the rear one through the counter frame. Add half bushes outside the rail as the first shaft stops.
  4. Slide an 8-tooth gear onto the measuring axle at the front face of the centre gap. Seat a full bush behind it. The gear will meet the inside of the other frame after that frame is fitted. Also preload a full bush on each inner side of the front axle; these meet the side rails and retain its complete shaft.
  5. Slide the second standing 5×7 frame onto the measuring axle, opposite the first. Preload two black pins in its bottom edge, pointing outward toward the second rail.
  6. Slide the second side rail onto both axle tips and the two frame pins. Add half bushes immediately outside it to retain both complete axle stacks.
  7. Hold a 24T gear and an 8T gear together between the frames, then pass a 7-module shaft through both gears and the third hole up each frame column. The 24T faces the wheel gear; the 8T sits behind it. Seat a half bush outside the back bearing. Turn the whole keyed shaft 7.5 degrees to fit teeth into gaps.
  8. Seat a half bush outside the front fine bearing, then a thin 2-hole yellow pointer by its first cross hole, and another half bush against its outer face. The pointer and both gears share one shaft. Its one-spacing tip is a direction mark, not a handle.
  9. Hold the 40T coarse gear and a full spacer between the frames at the sixth hole up each column. Pass another 7-module shaft through them and both bearings; fit a rear half bush. Rotate the whole shaft slightly to phase its teeth with the fine shaft’s 8T.
  10. Retain the coarse shaft outside the front bearing with a half bush, a thin red 2-hole pointer and a half bush against its outer face. Both short pointers clear each other and the tyres throughout full rotations.
  11. Fit the two thin-tyre wheels on this side, seating their hub inner faces against the existing half bushes. These are the thin wheels, not the larger tyres. A cross axle makes the measuring wheels turn with the input gear.
  12. Fit the two thin-tyre wheels on this side, seating their hub inner faces against the existing half bushes. These are the thin wheels, not the larger tyres. A cross axle makes the measuring wheels turn with the input gear.
  13. Seat half bushes against all four outer hub faces. The recessed hubs determine the cap positions, not the tyre’s wider outside faces. Check each wheel and the whole axle cannot walk sideways.

Play time

On a level surface, roll slowly in a straight line. Use removable marks on the stationary frames as pointer references; record starting angles rather than twisting pointers against engaged gears. Three wheel turns make one reverse fine turn; five fine turns make one coarse turn in the wheel direction. Count complete coarse turns as well as the fractional reading. Calibrate against a tape measure with three trials over the same straight distance. Turning, slip and tyre compression can spoil a distance estimate. No certified accuracy, physical test or payload rating is claimed; stop for binding, sideways shaft movement or rubbing.

Credited demonstrations and component sources

  • LEGO ODOMETER - Technology and physics — Mechatronic: Independent wheel-driven distance-counter demonstration inspires the measuring task. Our original four-wheel cart uses different serial 3:1 and 5:1 spur reductions, supports and instructions; it does not reproduce the creator’s model or claim its calibration.
  • LEGO Education — Make It Move With Wheels: Primary teaching reference: distance equals wheel circumference times rotations under a rolling assumption. Our calibration experiment and two-scale interpretation are original.