This just in from Frank Turner - all you single speed novices take note.
Frank: If using single speed with 700c x 23/25 wheels the following chainring/sprocket sizes are appropriate for the medium gear.
46-17 --- 71’7”
48-18 --- 70’7”
51-19 --- 71’1”
With multi-speed gears the machine must be set to prevent the selection of a ratio higher than specified. The ratio selected can be determined by using the following calculation.
Number of teeth on ring divided number of teeth on block multiplied by 26.5 (700c x 25).
Alternatively, hold the machine with one crank vertically down, mark the point immediately below the pedal, wheel the machine BACKWARDS in a straight line until the same pedal is at the bottom, mark the second point. The distance between the marks must not exceed 18 ft 10 ins (5.74 metres).
It may be possible to achieve this by adjusting the limit stop on the rear mechanism to prevent the higher ratios being selected and/or adjusting the stop on the front mechanism to prevent the large chainring being selected.
Prior to the start of the medium gear machines will be checked when signing on, this can take some time. Riders intending to enter are requested to ensure that ratios above 72 inches are locked out or a reliable indicator is attached to the gear cable run to show if high gears have been selected after the start. Arrival after 8am may mean you are out of luck!