A new 3D model of a continuously variable transmission (CVT) for bicycles shows how additive manufacturing can contribute to the development of cost-effective, mechanically simple drive solutions. Developer and YouTuber Robert Murray-Smith has made the model and design data available for free on the Thingiverse platform. The implementation uses common components and is based on well-known principles of mechanics – with minimal use of materials.
The gearbox presented converts a rotating input movement into a variable rotary movement at the output via a lifting rod. The core element is a combination of a sliding rack and a so-called Whitworth mechanism, which was originally developed for forming machines. The special feature: by changing the position of a pivot point, the length of the stroke and therefore the transmission ratio can be adjusted – a basic function of a CVT.
Unlike conventional CVTs, which are often based on friction disks or hydraulic systems, this model has a purely mechanical design. The entire construction consists of just a few components that can be manufactured using FDM printing. Simple steel pins, plastic gears and structured carrier plates are used. The idea is to create a lightweight, low-cost and low-maintenance CVT variant that can potentially be used in series production for bicycles.
According to Murray-Smith, marketability was a key concern during development. Although complex, high-precision transmission solutions are technically impressive, they are hardly feasible in the mass market if the efficiency gain is not in proportion to the costs. The prototype presented therefore allows a practical evaluation of whether the concept meets the requirements of the bicycle sector despite its simplicity.
Although the model still has functional limitations – such as uneven movement due to the mechanics used – it shows the potential to be transformed into a practical solution through further iterations. The model is available under a Creative Commons license for developers who are keen to experiment.
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