Inspired by the current poll. It means nothing – both for the reason that there is no standard definition of what you are measuring, and even if there was it has little to no bearing on how a frame will fit….

next week’s poll should be…
“do you know what reach/stack measurement your main bike is?”

Because thats how basic sizing is set out. It is a little vague and often depends on how the measurements are made. But its a basis for comparison.
Plus thats what test rides are for.

I agree that it’s completely rubbish and pointless.
Nobody actually buys the bike by the seattube length, so S/M/L makes more sense. Reach even more.

Stack and reach is helpful, particularly for bikes with non-straight tubes. You might as well just have a poll of ‘how tall are you?’

It’s not how basic sizing is set out, ETT is a vastly more useful measure for getting a rough idea of whether a bike will fit.

It’s not even a basis for comparison – some measures are centre to centre, some centre to top, some centre to top of junction, some centre to effective horizontal centre’. Even if there were some standardised definition of how to measure seat tube length it’s almost no help at all in choosing a correctly sized bikes. I ride bikes with seat tube lengths (centre to centre) between 45cm and 60cm and they all fit me.

It’s pretty straight forward, you measure the seat tube CTT, the top tube, CTC and if it’s about the same size as one you’ve already got, it’ll probably be the right size….

I always go on seat tube centre to top in inches .Im really tall and most compact road frames wouldnt fit without changing to a 400mm pin which I dont want .MTB frames I need at least a 20 inch centre to top .

It’s only meaningful if you have the geometry chart. But you need some kind of size to order or differentiate between frames so you could have size 1, 2, 3, etc but S, M, L, XL or 53, 54, 55 etc are going to give you a closer idea about fit.

Me, I know my effective top tube measurement and how seat angle and head tube length influence the cockpit so correct sizing is done instantly by looking at the geometry. So much better than anecdotal Internet sizing proffered on Internet forums which is mostly bullshite.

Meh, you might as well say “What’s the point in shoe sizes? I just need to know how wide they are” 🙂

Flashes – what you’ve got isn’t always right though.

Try being 6ft 4 and looking for a timetrial frame .There is no clue from the information as to whether the aero seat post will go up enough to fit me ,Surely its important to know the max height on a bike from centre of the bottom bracket to the seat rail to work out if a bike will fit or not

Me, I know my effective top tube measurement and how seat angle and head tube length influence the cockpit so correct sizing is done instantly by looking at the geometry.

Exactly the point I was trying to make

Meh, you might as well say “What’s the point in shoe sizes? I just need to know how wide they are”

If you want to use a shoe analogy, call it heel height – it affects how it looks and performs but not how it fits.

Try being 6ft 4 and looking for a timetrial frame….. its important to know the max height on a bike from centre of the bottom bracket to the seat rail to work out if a bike will fit or not

The TT frame being something of an exception (and not an especially common one this being an mtb forum) then yes, it’s useful to know the overall max height of the saddle, but to determine that you need to know more than seat tube length.

My general point is borne of a frustration with people mailing me saying “what size is closest to a 19 inch” or whatever, then seeing that very narrow nomenclature of sizing used in a poll here. Indeed even just S, M, L is more instructive.

The TT frame being something of an exception (and not an especially common one this being an mtb forum) then yes, it’s useful to know the overall max height of the saddle, but to determine that you need to know more than seat tube length.

The main thing I need to know is will a 400mmpost be long enough the front end I can fix with stack height ,rise lenghth.I ride xc so my bikes always have 110 mm to 150mm stems .I have never used the little short ones as even on large frames I would be cramped

Top tube length/reach is a much better indicator. On my own bikes a medium with a 17″ seat tube is the right size. My other bike has an 18’5″ seat tube and it’s only just big enough.