Australia's most widely-read bike magazine Size matters When it comes to sizes bikes seem to have their own mysterious rules. Jon Miller unravels the complications of sizing up a bike. Bikes are traditionally sized along their seat-tube, from the centre of the bottom bracket to the centre, or top, of the top tube. Photo: On Track Images In the frame It used to be simple. Traditionally, with limited choice, complicated measurements were never a problem when sizing a bike; you'd just stand astride the top tube, and if three fingers fitted snugly between your crotch and the top tube, then it was a fit. Things have changed, and choices in every conceivable bike dimension are enormous, but there are still some basic measurements you can use as a guide when shopping for a bike. You'll normally see road-bike sizes given in centimetres; for example, a 59 centimetre bike. This usually refers to the distance between the centre of the bottom bracket to the top of the top tube, where it meets the seat tube. In cases where the top tube isn't horizontal, such as ladies' bikes with step-through frames, or compact frames with sloping top tubes, it's a little harder. In this case, the same measurement is taken from an imagined horizontal top tube. There can be slight variations, however. Some manufacturers measure to the centre of the top tube, and not its top, and some bikes can have bottom brackets of varying heights. This means it's possible to have