Seat tube measurements still doing my head in

I'm having another old-git moment about frame sizes, specifically seat tube measurements. Perhaps I need to stick this on the middle-aged thread.

European and US frames were historically measured c-c, centre BB to centre of top tube. UK and Japanese frames were historically measured c-t, centre BB to 'top'. But which top? Some builders quoted to the lowest point of the lug; others to the highest point; others still to the top of the top tube.

I grew up with Raleigh and Geoffrey Butler measurements, so all was fine until foreign bikes entered the stable. As long as I remembered or checked which was which, all was well. Sean Yates got it wrong with his first 7-11 Merckx and ended up with a frame larger than he wanted. I did the same with my first Merckx, but luckily a low-profile saddle made it look OK.

Then came Freuler 'hi-pro' geometry, rendering c-c measurements useless.
Then came various experiments, such as massively extended seat tubes, micro lo pros and, of course, compact geometry, which stuck.

Now we have a confusing mix. For instance, modern steel Colnagos are measured c-t; classic Colnagos c-c. And most ads for used bikes - here, eBay, everywhere - don't specify which measurement the seller is quoting.

It still throws me when someone offers a classic British frameset and measures it c-c; or measures an NJS frameset c-c; or measures an Italian frameset c-t. That's why I sympathise with Eddy in the pic, who's probably wondering who the daft English pro is who can't measure a bike properly.

I always quote the original, as-sold, measurement, and add the other to avoid any doubt. But I'm a dinosaur and older than most of my bikes.

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I have fallen foul of buying frames that have ended up being the wrong size a couple of times but these were when I assumed that I knew which measurement the seller was providing.

I now ask how a measurement has been taken if there is any doubt.

When selling I tend to provide c2c and c2t (of seattube) sizes.

The relevance of seat tube measurements became less and less meaningful when compact road frames with sloping top tubes came along. When I look at a chart of frame geometry to see what size will fit me, I look at the ‘effective top tube’ length and compare it to my older horizontal top tube frame. I ignore the actual seat tube length.

Absolutely. Seat tube length is virtually irrelevant for compact geo. 90 per cent of my bikes are classic geo, so it remains just as important as top tube for me.

When I take measurements for people I include a photo of me taking that measurement. That way they know actually what they are getting.

so it remains just as important as top tube for me

It is still not as important since quill stems are height adjustable. Assuming that taller seat tube equals taller head tube and you're not already slammed on shorter frames. Useful to cast a glance at to check this but top tube is by far the more important measurement that will determine whether the bike physically fits you or not. Seat tube only dictates how much seatpost you have on show but doesn't change where your saddle goes.

Yes and no. Seat and top tube are both intrinsically linked to reach, fit and ride. A taller seat tube puts the start of the top tube back, reducing reach. Most manufacturers have slacker seat angles for longer seat tubes, reducing the reach even more.

For example, with Corsa Extra geometry, I can get away with a 56x56, 57x56, 57x57 and 58x57 (all c-c). But a 57x57 suits me best, and a 58x57 ain't as good. And a 58x56 would be a non-starter. (I'd like to try a 58x58, but never seen one.)

Reach and stack are obviously the superior and more accurate way of measuring frames, both of which take into account how long the seat tube and top tube are. Obviously not all frames are listed with these figures, especially not second hand ones but you can use an online calculator if you know the figures.

I appreciate that ambiguity about where the other measurements are taken is still a problem....

Not on traditional frames with equal angle seat and head tubes. The height of the top tube is irrelevant given a quill stem essentially extends or shortens the head tube. A taller frame with a slammed stem will have saddle and bars in exactly the same position as a lower frame with equal length top tube if the stem and seatpost have more length exposed.

For a 73° parallel and same top tube, the reach is 0.3 cm less (cos 73°) for each 1cm increase in seat tube length. And many trad framebuilders also slacken the seat angle for bigger sizes.

For a 73° parallel and same top tube, the reach is 0.3 cm less (cos 73°) for each 1cm increase in seat tube length.

That's the reach to the top of the headtube. Which again, is irrelevant because the bars don't move back as the headtube increases length assuming you lower the stem by the same amount.

Fair point if you're keeping the same bar/stem set-up across bikes. For various reasons I tend to use deeper drops on my taller bikes - drops in similar position, tops higher.

The point about variable geometry matters either way, though. A 60cm c-t Master X-Lite has the top of the seat tube just over 2cm further behind the BB than a 56cm one (measured to the same height, 60cm). For an identical bar/stem set-up and height, the top tube is 3cm longer, but the equivalent reach, just 1cm more.

All part of the lottery of finding old frames that suit.

This is all out of the window with modern bikes anyway with non matching angles, cut steerers, stems of different angles etc.
Stack, reach and seat tube angle are the most critical these days but as was said before, you won't get that on 2nd hand ads without doing research.

Absolutely