Thanks, I'll have a measure along the seat tube when I get home tonight. There
is no measurable top-tube as this is one of those low step-through jobs. I
cannot lower the seat much because I'm already having discomfort on the pedals;
what I have had to do is to move it quite far back, so that my legs have more
length to extend forwards, but this makes the distance across the top a little
too gaping. Raising the handlebars, in turn, makes an already very squirrelly
bike even more unstable.

EFR

Arthur Harris wrote:


I think you mean 56 and 53 cm. Frame size is specified by seat tube length.
Different manufacturers use different methods of making this measurement. It
is always measured FROM the center of the bottom bracket. It is measured
along the seat tube TO either the top of the seat tube, the top of the top
tube, or the center of the top tube. If the frame has a sloping top tube, it
may be measured to an imaginary point where the top tube would intersect the
seat tube IF it were level. As you can see, it's not easy to compare sizes
between different bikes and especially different manufacturers.