We have converted an HP Velotechnik Gekko FX 26 to use trigger shifters as an experiment. It seems to work well.
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Recumbent gearing calculator
This Terratrike Rambler fits right inside a Toyota Prius.
Patterson Metropolis 2-speed geared crankset extends gear-range of 8-speed Terratrike Rambler
Using the Shimano Nexus internally-geared 8-speed hub on any trike gives you the enclosed reliability of planetary-geared hubs, but it's a bit slim on gear range -- particularly with a small 20" wheel on
In a sight that's becoming quite common at the shop these days, some bluebirds tried out a Catrike 700. [In reality, only one bluebird tried the trike while the others watched.]
He liked the color and the lines, but found it too difficult to shift. Also, the headrest was too big.