Analysis and Material Selection of a Continuously Variable Transmission (CVT) for a Bicycle Drivetrain

Abstract

There has been much research and deliberation on what the optimum cadence for a cyclist is to achieve maximum efficiency and hence maximum performance. Research to date has shown that the ideal pedal rate varies based on the particular cycling task, such as course geography and duration. Several components of a CVT bicycle were analysed using SolidWorks finite element analysis to determine the von Mises stress. These results were used to determine and examine candidate materials for the components of the drivetrain by using Granta Design software. Technical feasibility study was carried out on the drivetrain where a v-belt was subjected to a pretension load which is uniform around the belt before pedalling. As the cyclist pedals, one side of the belt tightens whilst the other side of the belt slackens. However, the sum of the tension in the belt never changes and hence the load on the pulley. For a professional cyclist starting their sprint, 300 Nm applied torque at the highest gear ratio, the required pretension to prevent slippage is roughly 1500 N. The simulation yielded an initial maximum von Mises stress of 1250 MPa in the corner of the spindle. This stress was reduced to 1059 MPa by adding 0.7 mm radius to the corners of the cut-outs. Cadence is the rate at which a cyclist is pedalling/turning the pedals. Higher cadence in the order of (100–120) rpm would improve sprint cycling over shorter distances where the muscle fatigue is reduced and the cycling power output is increased. A well-maintained chain drive system with a derailleur setup can achieve an efficiency of 98.6%, whereas a well-maintained v-belt drive 97%. The reduced efficiency of a v-belt CVT drivetrain plus design constraints will be a hindrance to its adoption into professional cycling racing; however, a market for the recreation cyclist exists. The design will require further iterations to reduce the number of parts, improve integration with a standard bicycle, increase efficiency and will also require an extensive testing/commissioning phase. Results from the FEA indicated that materials with yield strengths exceeding 500 MPa would be required. Due to the nature of loading, materials with properties such as high specific stiffness and strength would be necessary.

Glossary

Cadence

Cadence is the rate at which a cyclist is pedalling/turning the pedals. It is the number of revolutions of the crank per minute (rpm). The wheels in turn spin proportional to the cadence and, however, change depending on the gearing setup. It is an important factor, which influences the power output, energy burnt and development of fatigue during cycling.

Cassette

Cassette is the set of multiple sprockets that attaches to the hub on the rear wheel. It works with a rear derailleur to provide multiple gear ratios to the rider. Cassettes are a variety and newer development of cogsets.

Continuously variable transmission (CVT)

A continuously variable transmission (CVT) is also known as a single-speed, step-less, pulley and twist-and-go transmission. This is an automatic transmission where a continuous range of effective gear ratios can be achieved seamlessly. CVT offers higher functional power, improved fuel efficiency and a smoother driving practice than a traditional automatic transmission.

Sprockets

A sprocket is a profiled wheel with teeth/cogs around the outer edge that fit/mesh into the holes in a chain or a length of film or tape or other perforated or indented material to move it round.