This took me by surprise, but I did some digging and you're right... I stand corrected.

I come from a BMX dirt background in which all frames of various sizes have extremely low if not similar stand-over heights - for obvious clearance needed for jumping and bunny hopping (same reason why MTB jump bikes and trials bikes have really short seat tubes as well). Like road bikes, BMX frame sizing is also primarily based on TT length because a taller rider would need more "cockpit" room (effective distance from BB to handlebar).

I had wrongly assumed all these years that MTBs are measured similarly by TT. On one sizing website, it even mentions that some MTB frames of the same seat tube length (same frame size) may have short or long TT length... how bizarre!

Oh well, we learn everyday...


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