Measuring Frames
Mon, January 8, 2018 
Growing up in England, and taking up the sport of cycling there, later building frames, there was only one way to measure a bike frame and that was from the center of the bottom bracket to the top of the seat lug. It never occurred to me there was another way.
Italians measure frames from the center of the bottom bracket to the center of the top tube. However, I had very little exposure to Italian frames, there were few in the UK, I had no reason to measure one.
When I came to the US I continued measuring center to top as I had always done. One change I did make when I resumed building my own frames, I did switch from inches to centimeters. In England I had always built frames in half an inch increments. For example, 21”, 21.5”, 22” 22.5” etc.
I noticed Americans always spoke of frame sizes in Centimeters, so I switched. I always thought it strange that in a country so entrenched in Imperial measurement in every other walk of life, people would readily accept the metric system with regard to bicycles.
I continued to build frames and still no one ever questioned my method measuring, I was well into the 1980s and I gradually discovered I was in the minority, and almost everyone measured center to center. But now it was too late to change. I had hundreds of frames out there, it would have been chaotic to switch. I had no alternative but continue as I had always done.
If there is one instance in my life where I could go back and do things differently, I would have measured center to center when I started building frames again in California. It would have been no big deal. But no one told me, I didn’t know.
Working for Masi I never had to measure a frame. They had a series of “Jig Frames.” Frames built by Faliero Masi himself when he first opened his shop in California. One in every size. I would simply choose one in the size needed and use it the set the frame jig. This ensured that every Masi frame was built exactly to Masi’s design.
Also at the back of my mind I seem to remember the Masi frames were measured center to top anyway. Faliero Masi was always a bit of a rebel amongst Italian framebuilders, he used Reynolds 531 tubing for example.
Measuring center to top is not the wrong way, it is simply a different way. Where it become an issue is when frames are bought and sold. A seller lists it measured center to top, and the buyer assumes it is center to center, or vice versa. The buyer ends up with a frame the wrong size.
All frames I built were measured center to top and stamped that way under the bottom bracket. If you are buying one of my frames, and the seller doesn’t clearly state what size is stamped on the frame, ask. The center to center measurement is simply 2 cm. less than what is stamped. For example, a frame stamped 56 cm. will measure 54 center to center.
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Reader Comments (8)
Specifics of a sizing scheme mattered much less when every road bike had a level top tube made of steel roughly 1 1/4" in diameter give or take. It was relatively simple and required little more than fudging for specific brands.
New materials and geometry send us to stack and reach. Much like BB drop from the axle plane being more accurate than height, matters of fit are separated from externalities, in this case frame angles. Even if you don't understand geometry you can experience it and easily find similarly sized bikes with different ride qualities.
When I was much younger, all the frames were described in inches. During my early years of serious riding, it was the same. I don't recall every measuring a frame, I just took the description to heart knowing I would ride a 24" frame. This was in the late 60's through early 70"s.
30 years later, getting back into biking, everything was new. measurements were CM, the front brake was on the left, 700c was the dominant size wheel with everyone talking clinchers when I was use to tubulars.
Several years later I learned that some measure to the top of the seat tube, or lug and other measured to the center. With the variations of profiles of seat lugs , I lean heavily on c-t-c measurements. Of course one could argue that although c-t-c is a good consistent reference, tube diameter could make a difference, much like lug profile dimensions.
I must concede that either is acceptable. Not checking can make a difference if the bike being considered for purchase, is on the edge of your range.
It's understandable how your frames sizes came to be described as they are - it's part of the charm. I knew that typically UK frames were measured in inches and centre to top, with "Euro" frames in cm and centre to centre. But why choose the top of the seat lug and not the top of the top tube? This was before sloping top tubes (I think) and while the top tube's top surface has a certain definition, seat lugs vary - some ornate and others plain - yet their shape is much less important to fit. And where on the seat lug? Topmost point? Clamp collar?
Since you were consistent, none of this really matters. And the rule is - never, ever trust an eBay seller to measure a frame properly :-)
So with sloping top tube racing frames of today, how does a builder determine the head tube length?
Seems he can choose a long head tube, up to a limit, but there is a limit to how short he can go. The top tube angle will change with shorter, and larger frames if one chose a shorter, or longer head tube. Does the angle of the top tube determine head tube length, or does a builder choose the shortest head tube possible while maintaining some kind of slope to the top tube on larger frames?
Some frames even have seat tubes going all way up to the saddle.
How does a builder decide which gives preference on today’s racing frames: Head Tube length, or top tube angle?
Centre-to-top has the advantage of working for both gents' and ladies' frames.
At Mike Melton's shop in Columbia, SC, (late seventies), he, and therefore I, always measured to top of the top tube. In inches. Don't know whether he ever changed.
I was reading a blog post the other day on Measure bicycle, and I learned some amazing new details from it. The author had written quite well, and I found myself going to other sites and blogs for learning more about Measure bicycle There are some great resource out there that can help you learn this subject as well.
Someday, I would like as thorough an explanation of how Cannondales were measured. Go find a frame mark 56, and I guarantee, nothing will be 56!!