I have just been polishing my crystal ball, and it took me to the TT2015 - "classes are for two-wheeled machines powered by solar energy, with a maximum permitted speed of 90 kph, and a seat length of 30.5cm or less, or for the Senior event, use of pedal assistance is permitted. All machines must have a maximum noise level of 30db, except in designated areas near hospitals and nursing homes, where this must not exceed 25db"
A second polish took me back to the 60s - "up to 50cc/125cc/250cc/350cc/500cc, 2- or 4-strokes, no silencers, any number of gears, cylinders"................Ain't progress wunnerful?