frame sizing nowadays

[jeff_o]

hi all. hopefully everyone's back home safe from a great sunday ride (i just did).

i went to bikebarn (paramatta) yesterday looking for a frame for myself. we were measuring the frames but then the guys told me that the seat tube length doesn't matter nowadays as some have sloping (compact) or even curved top tube. nowadays they're more particular about the top tube length.

so when you say a bike is 54cm size. is that the seat tube length from BB center to top of seat tube? or industry standards have changed now? i always knew 54 is from BB center to top of seat tube.

[wombatK]

or industry standards have changed now?

There is no industry standard. You have to translate the manufacturer's dimensions into effective top-tube length (i.e. as if it were parallel to the ground, and effective seat tube length (where the parallel to the ground top-tube would intersect the seat tube of seat post extending from it. Then compare bikes and/or assess whether its close to what you need. Different manufacturers have different ways of reporting the size of the bike in order to make the numbers meaningless.

They don't want you to make a decision based on actual facts (like measured sizes) - they just want you to buy their bike because it's in stock at the shop, looks good, or Cadel rides one etc.,.

[notwal]

^^^ so cynical

The old seat tube sizing was never an agreed standard. Some measured from the BB centre to the centre line intersections of the seat tube and top tube others measured to the top of the top tube. Still they were were pretty close I guess.

These days they often quote effective seat tube length even though they have sloping top tubes.

The important numbers for comparing frame fit are frame stack and frame reach. These numbers are never given but they can usually be derived from the size tables.This calculator can do the maths for you. I understand there is an error in it in that it overstates the stack by about 10 mm because it doesn't take account of the drop due to the fork offset.

Reach and stack don't take account of the seat tube angle but that is usually given.

[scirocco]

yeah, frame sizes based on seat tube length are meaningless these days. I have bikes called 50, 52.5 and 54 and they all fit about the same because of their different geometry.

Use top tube length as the main criteria and correct by 10mm per degree for seat tube angle. Not a perfect system but will do the job in comparing different bikes without calculating reach and stack which manufacturers won't tell you directly anyway.