A simple LEGO continuously variable transmission (CVT) uses two opposing cones on separate shafts and a rubber belt between them. Slide the belt along the cones to change the effective diameters driving each shaft—and therefore the output speed and torque. It is a useful demonstration of the principle, but smooth plastic cones can let the belt slip under high load.
How the cone-and-belt CVT works
In the basic arrangement described by Hackaday’s 2021 LEGO CVT demonstration, a motor turns an input shaft carrying a cone. A second cone sits on the output shaft, facing the opposite direction. A rubber belt loops between the cones.
Moving the belt along the cones changes the effective radius at which it drives each shaft. Because those radii can change continuously as the belt slides, this design can vary its ratio without switching between fixed gears. The demonstration illustrates the basic idea; it does not operate like every modern automotive CVT.
Build the mechanism
There is no verified, complete parts list or frame plan for this design, so use these steps as a layout guide and adapt the structure to the pieces you have.
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- Make a supported input shaft. Mount a cone on the shaft and connect a motor or other drive source. Support the shaft so it can rotate freely.
- Set up the output shaft. Mount the second cone facing the first and support its shaft. Leave enough room between the cones for the belt to travel across their surfaces.
- Fit the belt. Loop a rubber belt around both cones. It should stay engaged without being so loose that it slips immediately or so tight that it adds unnecessary friction.
- Check belt travel. Turn the mechanism slowly by hand and confirm that the belt can move between cone positions without striking the frame or nearby parts.
- Test the ratio change. Run the drive slowly and shift the belt toward different ends of the cones. Observe how output speed changes; avoid adding a heavy load to this plastic-cone demonstration.
LEGO Education’s pulley lesson explains the relevant general principle: a belt transfers motion through friction. A belt that is too loose can slip, while excessive tension wastes energy through friction at the axle and bearing.
What happens to speed and torque
When the belt rides on the smaller effective radius of the input cone and the larger radius of the output cone, the output turns more slowly and has greater available torque. Move the belt toward the opposite ends and the output turns faster, with less available torque. This is the speed–torque trade-off; no measured ratios are published for this particular build.
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The same direction of change appears in ordinary belt-and-pulley examples: a smaller drive wheel turning a larger driven wheel makes the driven wheel turn more slowly. The exact behavior of a LEGO build depends on its geometry and how well its belt grips.
Troubleshoot slipping or binding
- The belt slips under load: Reduce the load and check the belt’s fit. The Hackaday demonstration notes that smooth plastic cones limit friction, so this design is not suited to transmitting high torque.
- The belt slips even with little load: Check whether it is too loose. Belt-driven pulleys rely on friction to transmit motion, and slack can prevent a reliable grip.
- The mechanism binds or loses motion: Check that the shafts are aligned and turn freely, and that the belt is not overtightened. Excessive belt tension can increase friction at axles and bearings.
- The ratio changes in apparent steps: Some LEGO mechanisms imitate CVT behavior with discrete steps rather than a continuous change. A 2023 2in1 Bricking overview of three LEGO Technic CVTs identifies both continuous and stepped approaches, including cone- and ball-based concepts.
Choosing parts and comparing designs
A LEGO Technic parts assortment is a reasonable starting category, but the pieces required depend on the specific design. Neither LEGO’s educational pulley resource nor its official instructions page for the 772-piece, 2022 App-Controlled Transformation Vehicle (42140) establishes a complete parts list for this cone-and-belt CVT.
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Other LEGO CVT concepts vary in how they change the ratio and how much they ask of the builder. A useful comparison is whether a design is continuous or stepped, whether it uses a sliding belt on cones, balls, or a ratchet, and how it handles friction and belt slip. Available descriptions do not provide controlled performance comparisons, so there is no sound basis here for ranking designs by efficiency, durability, or torque capacity.
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