Originally Posted by John E
There are two generally-accepted ways to measure frame size:
1) center of crankshaft, along the seat tube, to the center of the top tube;
2) center of crankshaft, along the seat tube, to the top of the top tube.
(1), popular in Europe, makes sense when one is figuring out frame geometry and measuring everything center-to-center.
(2), more popular in the US, seems the most practical to me, as it gives the most direct assessment of things like standover height (which can still vary because of BB height) and leg extension.
Hi John E,
For classic frames, mid-1980s and older, the sizing approach was typically opposite of what you noted. Most European bikes used the center-to-top (C-T) method while most American manufacturers used the center-to-center (C-C) method. (Notice I said most.
)
I make this point in case someone is measuring a classic frame and starts getting a seat tube length with a 0.5 cm increment. If you get that kind of measurement then try measuring the other method. You should typically get a seat tube length in whole cm increments.
I ride a 53 cm C-T or a 52 cm C-C (which is a tad smaller) in a classic geometry. Here is my 1975 Bianchi Specialissima which is a 53 cm C-T:
My current Look KG481SL is a 52 cm and it seems that I ride a 51 cm "sized" bike in the sloping geometry format. However, the only sloping geometry bike that I've owned to date was a 2001 Bianchi XL Ti which was called a 53 cm.
It had plenty of seatpost showing:
Finally, top tubes have always been measured C-C but these days you always have to look at the virtual top tube length, not the actual. Also, top tube length has typically varied a bit for any given size bike because different manufacturers have different approaches to fit and handling. As noted above, you can adjust for this variation through stem length.
All of this is to say that different people like and fit well on different bikes because no one is "normal."
