Modern vs retro frame sizing

Morning all

Apologies if this has been asked before, I've done a couple of searches on here and haven't found anything.

How do retro frame sizes compare to modern "compact" frame sizes?

For example - I currently have a M/L size Giant that I commute on, which has a 53.5cm/21.1" seat tube.

Does that translate to me needing a 21" sized "retro" frameset? Or should I be looking for a slightly larger frame size?

For reference I'm 5ft11" tall with a 32" inside leg, and I'm on the lookout for a frameset to build a singlespeed commuter with

Thanks

Jim

i'm an inch shorter than you, i'd probably buy an M/L giant if i was going to, and I ride "classic" bikes in about a 56/57cm

you need to use the effective top tube length rather than the seat tube length to compare to an old frame, and lots of old frames are either "square" (seat tube and top tube the same) or close to it

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cheers CCE, good to know

much more choice if I can buy something that would be delivered rather than looking local!

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Giant sizes must be odd as most modern or retro frames that are medium/large would be 18"ish centre to top. :?

21.1" sounds bike for someone your size too. 21"+ is usually XL.

Retro wise I found that up until a few years ago the frame sizes were pretty similar to modern as long as you fitted the correct length retro stem. It changed recently though with the move to shorter seat tubes to accommodate longer dropper posts and longer top tubes. If it's a modern, modern bike you need to be looking further in to the geometry than simply seat tube length.

Saying all that, I'd imagine most 18" retro would fit you at your height.

Roadie stuff....

It used to be that the nominal size of a steel frame was expressed as the length of the seat tube measured from the centre of the bottom-bracket to the top of the seat tube. This was a good starting point but obviously didn’t say much about the length of the top-tube. Now, manufacturers adopt a diverse range of ways to describe the size of their frames and people like time-trial riders look for the reach and stack measurements (use google).

I am 5’ 10” with a 33” inside leg and my two steel frames are 58cm and 58.5cm ctt. I have a fair amount of seat post out and most original posts are too short. Both have 110mm stems. My modern Cervelo (R3) has a 56cm frame which is 52cm ctt and my (slightly older) Specialized time-trial bike measures 48cm (and fits me well but wouldn’t suit someone less bendy).

Bottom line is probably to throw a leg over a few to see what suits you best- or measure reach and stack on your current bike and translate that across.

One thing I’ve noticed is that from 1980 on people have ridden with saddles relatively higher. I would get knee pain riding like most did in 1950s photos.

Sorry about the mixed measurements.

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I ride a 54cm in modern geometry terms but on a horizontal TT classic, I'm a 22 1/2" or 57cm in new money

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Cheers peeps, will aim for 56/57cm then at a guess

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slartibartfast":oae9kpqv said:

Cheers peeps, will aim for 56/57cm then at a guess

Click to expand...

No need to guess. 0.67 x your cycling inseam will give you a sporty "Eddy" fit, 0.70 a more relaxed French fit. I like the latter. This is a 63.5cm frame, which is 0.71 x my 89cm cycling inseam:

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just had to google cycling inseam, didn't even know that was a thing lol

will do that later on to see what I get! got my eye on a Schwinn frame on ebay just now, waiting for the seller to get back to me with some more info