Originally Posted by
shelbyfv
You've been a member for almost 8 years. Do you not already have a bike that fits? If so, use that as a guide. 51cm effective top tube on a road bike sounds about right for someone 5'7" unless you are oddly proportioned. Publishing stack and reach figures is a relatively new convention. Ebay sellers of vintage bikes are not likely to know those numbers. I'd suggest you sort out whether you want a bike to ride or just enjoy noodling with geometry. Both types of folks here on BF.
I have a bike that I like quite a bit. Nothing fancy. But I don't have access to it right now. Pandemic dislocation. And I do like noodling about geometry. I think. But my interest is also practical. I know from experience that bikes can fit and feel very different. That won't be a surprise to anyone here obviously. It seems like geometry could help me understand what I'm getting. And also perhaps could be a way to compensate for not being able to compare with my existing bike.
Maybe here's a more specific question. This ebay seller said that the top bar and seat post lengths are the primary dimensions affecting fit. Can anyone here explain how to interpret these lengths? For instance, this 51cm Olmo frame has a top bar of 52.5cm and a 27.2mm seatpost. What if anything can I learn from those numbers?