Honestly, there is a lot of voodoo to your question. Generally, sizing should work out in such a way that height will be a good predictor of somewhat "standard" frame sizes, while the Stack or headtube length allows options of sitting up or leaning over more. If you are at a height that is well within a the range for a particular size, and you get an appropriately high or low front end (Stack), then the rest of your fit should come from selecting proper bars, stem, etc.

Recommending a top tube length makes some sense, but if it comes on a bike that is too low, you haven't accomplished a good fit choice.

That said, real 51cm top tubes are rare and are very hard to find without "faking it" by the maker leaning the seat tube angle too far forward. Are you 5'3" or shorter? If not, or you don't have really short arms, I'd suspect the measurements you submitted might not be spot on.


I think what is likely to work for the most people is a size based on height, and a choice of low or tall head tube based on rider posture preference. If you're 5'10 and are moderately flexible, most everyone's 56cm frame should be a reasonable choice.