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@Emyr
Just bought an On-One 456 Evo 2, 20" to use as my only non-fat geared bike.
Looking at the geo chart, some of it doesn't add up.
http://www.on-one.co.uk/i/q/FROO456E2/on-one-456-evo2-frame
Stack: 597
Reach: 410
Seat Angle: 71.2
ETT: 618
tan(angle) = opposite/adjacent
tan(90-seat angle) * stack = ETT - Reach
tan(18.2 degrees) * 410 = 134.8mm
ETT - Reach = 208
Taking the angle as incorrect...
tan(90 - seat angle) = (ETT - Reach)/Stack
seat angle = 90 - tan-1((ETT - Reach)/Stack)
seat angle = 90 - tan-1(208/410)
seat angle = 90 - 26.90
seat angle = 63.1
So should I assume ETT and seat angle weren't necessarily calculated for the same fork length as stack and reach?