What makes a bike a "59?" Or a "58?" Or a "57?" Etc…

[XXtwindad]

The recent Kirk in the PSA section caught my eye: https://sellwoodcycle.com/collection...-road-frameset
Lovely bike, obviously. By why is it a "59?" The ST is 59, and the TT is 57.

Conversely, the super cool Alliance that auto_rock is selling (pictured) has a ST of 590, and a TT of 57. And it's a "57."

My Firefly (which fits like a glove...pictured here: https://forums.thepaceline.net/showthread.php?t=256533) has a ST of 550 and a TT of 580. It's a "58."

I know there are many people on the Forum much better educated than myself in these matters, so I'd appreciate some insight.

[charliedid]

[EB]

Some builders and companies size bikes by the seat tube measurement. This is more traditional and I associate it with Italian bikes in particular.

Some size bikes by the top tube. As sloping top tubes became common, sizing by the seat tube didn’t tell you much, so brands and builders switched to using the top tube length.

Top tube is more common than seat tube, but builders don’t always choose the most helpful number. Mike Varley of Black Mountain Cycles sizes by seat tube even though his frames have sloping top tubes, which makes the numbers confusing to compare to other brands sizing.

Since most brands have stopped making frames in 14 sizes, other than a few legacy Italian brands like Colnago and Pinarello, I find the t-shirt sizing more helpful for stock frames, along with stack and reach numbers.

[joevers]

Same thing that makes a shirt a "small" or a pair of pants a "4".

The medium sizes around 53-57 usually are lucky and get a top tube that lines up with what the size is nominally called, but bigger or smaller than that it's usually extremely roughly based on the seat tube, or what the seat tube would be if the top tube were roughly level. Top tubes get longer or shorter much less quickly that seat tube length changes.

Really, it dates back to when frames were lugged, and the nominal size was usually an exact measurement of the seat tube. There was much less flexibility with frame design when lugs dictated angles. Top tubes were level, or very close to level, angles were around 73*, and stems and bars didn't seem to vary nearly as much as they do now.

Brands calling their size a "56" is just giving riders an idea of what size a bike is in their own opinion, and is in no way universal or even a measurement of something on the bike. It's a starting point, and something to call it that isn't "medium/large".

For a road bike, stack and reach is the only way.

[buddybikes]

Firefly doesn't have "sizes" they just fit the (first purchaser), number be who cares for the user

[robt57]

Well, you are in luck. We can get the horse's keyboard in here for that particular frame, Kirk that is...

[XXtwindad]

Originally Posted by steveandbarb1

Firefly doesn't have "sizes" they just fit the (first purchaser), number be who cares for the user

Yes, that's true. It was marketed (and sold to me) as a "58."

[HTupolev]

Originally Posted by XXtwindad

By why is it a "59?"

When horizontal top tubes were ubiquitous, the number usually referred to the seat tube length. Different manufacturers defined "seat tube length" in slightly different ways, so even this has some wiggle room.
Nowadays it's often an abstract size number that corresponds to whatever the manufacturer thinks matches up with the market's understanding of what a certain size number means: a 59 is a 59 because it sizes about how someone looking for a 59 expects a bike to fit.

[mhespenheide]

Before mountain bikes and the Giant TCR, essentially all road bikes had horizontal top tubes. In that era, the size of a bike was referred to as the length of its seat tube. That wasn't perfect -- because some people measured center-to-center, others center-to-top, and also because different seat tube angles interacted with different top tube lengths. But it was good enough in most cases to get you in the right neighborhood, and you'd go with a slightly longer or shorter stem and live with it.

After sloping top tubes and long seat posts started to proliferate, the (effective) top tube length began to be more recognized as the better single number defining fit. Still, though, that discounts the effect of different seat tube angles and head tube lengths.

So some people started to use stack and reach as a better system, but they do need to be used as a pair of numbers and not a single number.

So: there's no good single number that really works. Some people are stuck in the deeper past of "what size seat tube would this be if it were a horizontal top tube". Some people are a little farther ahead and are describing "what top tube would this be if it were a horizontal top tube". Bike geeks like those on this forum want to see the entire geometry sheet. Someone like Mike Varley figures it's all a mess and no single number really works, so why not refer to a frame size as actual seat tube length and figures that his customers with looks at the entire geometry.

¯\_(ツ)_/¯

[Mike V]

I blame compact, sloping bikes.

[Steve in SLO]

This is why I like to see a ST x ETT in the title, but for slopers just an ETT does well.

[Velocipede]

Originally Posted by Mike V

I blame compact, sloping bikes.

Actually, Klein is the one who really started it back in the 80's. But Giant coined the whole "sloping top tube" monicker in the 90's. But older Klein frames had funky sloping geometry and you needed to check the ETT for proper fitting. And back then, he made custom frames including in mountain bikes. It's one of the reasons he had such long extended seattubes for awhile until "sloping" frames really took off.

[Mark McM]

Originally Posted by Velocipede

Actually, Klein is the one who really started it back in the 80's. But Giant coined the whole "sloping top tube" monicker in the 90's.

Actually, the monicker Giant used was "Compact" frame. While cynics like to point out that the sloping top tube was used as an excuse to produce fewer road frame sizes, it also provided benefits to the user. The main one was that they used longer seat posts, which flexed more to provide more ride compliance. While there are practical reasons that lugged frames used horizontal top tubes, there was no particular functional advantage. While the sloping top tube came to the fore with the popularity of MTB, other types of bikes had been using non-horizontal top tubes for decades (balloon bikes, BMX, etc.).

[unterhausen]

This is why some people are going with stack and reach, but nobody has gotten used to it yet.

Kona has sloping top tubes and they go by center-center distance so their sizes are way off from what you would expect. 5cm on a big frame.

You can adjust the seat post if the seat tube length is off, and get another stem if the top tube measurement is off, but if the top of the head tube is too tall, there is very little you can do. So stack is a pretty important number, and nobody cares what it is.

[Velocipede]

Originally Posted by Mark McM

Actually, the monicker Giant used was "Compact" frame. While cynics like to point out that the sloping top tube was used as an excuse to produce fewer road frame sizes, it also provided benefits to the user. The main one was that they used longer seat posts, which flexed more to provide more ride compliance. While there are practical reasons that lugged frames used horizontal top tubes, there was no particular functional advantage. While the sloping top tube came to the fore with the popularity of MTB, other types of bikes had been using non-horizontal top tubes for decades (balloon bikes, BMX, etc.).

I worked for a Giant dealer in 2000 and the rep used to use the term sloping all the time. Yes, it was compact as in TCR - Total Compact Road but everyone used the term sloping and they really coined it. All the ads and everything mentioned it being a sloping toptube.

Regarding non-horizontal toptubes on bikes for a long time, true. But fat tire bikes and BMX bikes weren't normally measured in frame sizes like road bikes. Fat tire/cruisers were listed in the tire size. BMX, it was M, L, XL, XXL and that was the length. And most times that was for Mini and Micro or standard styles of racing which also corresponded to the tire size.