petertreglown
Devout Dirtbag
this has probably been asked before, but what is an 'old' equivalent frame size compared to a 'new' frame size ?
'old' versus 'new' frame sizing
- Thread starter petertreglown
- Start date
petertreglown
Devout Dirtbag
i am aware of the main design priciples of 'compact' geometry but the sizing seems a little arbituary when compared to 'traditional' geometry, is there a rough guide to equivalent sizing even if that is just converting metric to imperial?...for example 52cm versus 20 1/2 inches, which begs the question of why do classic/retro road race cycles seem so large 
I just bought a brand new road bike, by the German manufacturer Cube. The seat tube is 52cm centre to top. The top tube comes into the seat tube about 3cm from the top and slopes up to a tall head tube.
This is a "sportive" geometry frame which is what I'll use it for - long rides and lots of climbing. The equivalent tall head tube in an old 531 frame would be like having the front end of a 58cm bike.
The point is that to get the same riding position as my 53.5cm (21") 531 frame, with handlebars, saddle and bottom bracket positioning the same, I had to get the new frame in this geometry because you cannot move the handlebars up by more than the height of a few thin spacers. This is the biggest drawvbck of the A-Head threadless system, in my view.
So, if you like your quill stem extended a bit, you will need to look for a bigger frame size than normal or a Sportive type with a top tube that slopes up to a taller head tube. Or retro fit a new fork with a long steerer tube which you can leave long and stack spacers onto.
As far as sizing goes, my new German dream machine is, as I said, 52cm c to t but is called a "56cm". I checked with CRC by phone and they confimred this is indeed correct.
Go figure!
This is a "sportive" geometry frame which is what I'll use it for - long rides and lots of climbing. The equivalent tall head tube in an old 531 frame would be like having the front end of a 58cm bike.
The point is that to get the same riding position as my 53.5cm (21") 531 frame, with handlebars, saddle and bottom bracket positioning the same, I had to get the new frame in this geometry because you cannot move the handlebars up by more than the height of a few thin spacers. This is the biggest drawvbck of the A-Head threadless system, in my view.
So, if you like your quill stem extended a bit, you will need to look for a bigger frame size than normal or a Sportive type with a top tube that slopes up to a taller head tube. Or retro fit a new fork with a long steerer tube which you can leave long and stack spacers onto.
As far as sizing goes, my new German dream machine is, as I said, 52cm c to t but is called a "56cm". I checked with CRC by phone and they confimred this is indeed correct.
Go figure!
Peter, specifically on your original question, if you are buying a modern frame, I'd start from your ideal riding position as far as the dimensions of the saddle/bar/bottom bracket triangle goes, and try to replicate it with the new frame.
It may require you to think about stem length, seat post protrusion etc and maybe getting a tall head tube type, as I've described.
On the plus side, all new frames have very clear geometry data at online retailers, so it's easy to work out even if you can't get to see the actual bike.
The danger with saying you'll just convert 21" to, say, 54cm, is that you may end up with bike where the handlebars are too low with no way of heightening them, as I've described, unless you buy an expensive new fork. So geometry, particularly head tube length, becomes more important than it used to be on a conventional horizontal top tube design.
It may require you to think about stem length, seat post protrusion etc and maybe getting a tall head tube type, as I've described.
On the plus side, all new frames have very clear geometry data at online retailers, so it's easy to work out even if you can't get to see the actual bike.
The danger with saying you'll just convert 21" to, say, 54cm, is that you may end up with bike where the handlebars are too low with no way of heightening them, as I've described, unless you buy an expensive new fork. So geometry, particularly head tube length, becomes more important than it used to be on a conventional horizontal top tube design.
