Continually variable transmissions, or CVTs, are one of those black-box technologies in the automotive world—we don’t assume much knowledge about how they work, we just hope that they work. These transmissions are designed to adjust their gear ratios on the fly, rather than stepping through different, predetermined gear sets like a conventional automatic or manual gearbox.
If we tried to explain the inside of a CVT, you probably wouldn’t believe how simple it really is. Thankfully, there are brick layin’ mad geniuses with YouTube channels.
Sariel’s Lego Workshop is run by a guy named Paul, who builds incredibly complex machines with Lego’s Technic’s kits. His creations run the gamut from entire supercars to translations of everyday mechanisms into plastic masterpieces—like this CVT.
Paul’s build elegantly demonstrates of the core components of every CVT: the belt and its conical pulleys that the transmission uses to select its gear ratios. A shifting “fork” guides the drive belt along different positions between the two pulleys. This fork changes the relative diameters of the input and output pulleys, a process similar to swapping gear sets with different tooth counts in order to change the gear ratio.
Paul’s pure-Lego design also demonstrates one of the CVT’s greatest weaknesses: belt slip. Besides lukewarm consumer reception, the greatest roadblock to the CVT’s popularity is the belt drive’s relatively narrow contact patch across the pulleys, which limits the amount of torque that the transmission can support before overrunning the belt. Since a CVT is best suited to lower-torque outputs and also improves fuel economy, you’ll typically find this transmission design on compact commuter cars.