Confused: frame measurement?
 

My bike is a Masi Gran Criterium S. It has a sloping top tube and has no indication as to frame size. Back in the day I rode a Maza 54 cm C to C frame based on a variety of fitment guides in various books and magazines and my 32.5" inseam. Seemed to work just fine. Then I gave up riding for 8 yrs. When I came back to riding, bikes were no longer steel, there were triple cranks and sloping top tubes. Gone were my beloved Athena friction shifters. The Masi was a 60th birthday present from my gf and after 5 yrs. of riding it I still love the bike. Of course, since I had lost an inch of height (all in my upper body I assume) I had to replace the 110 mm stem with a 100 mm stem. Then I got curious. "What is the size of my bike?" So, I tried to measure it using the "virtual" lines people tell me are appropriate. Found a level floor and using a builder's level measured the virtual TT length and seat tube. Strangely both measurements came out 58 cm C to C. That seems pretty big to me. Am I doing something wrong with my measurements?

Is 32.5 your pants inseam or your cycling inseam? (floor to pubic bone)

If that is your pants inseam, 58 seems about right. Are you around 6' tall?

The bike does look about right for a 58 based on the amount of head tube between the top tube and downtube.

Cycling inseam. Barefoot crotch to floor using the book against the wall method. Measures out to 82.55 cm. Using the LeMond method of 65% it recommends a 54 frame.

Your method of measuring does sound about right to me- but don't worry if your bike comes up the wrong size. Frame size doesn't bother me- or rather the quoted frame size. What does worry me is how it fits.

I have found that I like a top tube length of 535mm. With that length TT- I can get the saddle position correct on Fore and aft and can get the reach right with possibly a change of stem length--So to me that is my critical length for a bike. On Boreas that equates to a 51cm frame and on the Giant is an "S" frame. But on other manufacturers I have found it can be as low as a 49 or as high as a 55.

Then you have to take in the peculiarities of how a rider likes his bike set up. I know of two riders- One my size- height and inseam that reckons my bike is too short. And another who is 5" taller than me that other than raising the saddle by 2"- can ride my bike and he says it fits him perfectly.

So does your bike fit you and is it comfortable? If yes to both these questions- then you have the right size bike for you. But may not be for other similar sized riders.

I thought is was centerline of crank to top of seat tube, but maybe not. Apparently some advocate center to center.

It's a matter of convention, not advocacy. Most Italian builders measured center to center, everyone else center to top. With conventional steel frames you added 1.5 cm to c-c to get c-t.

With sloping top tubes the main fitting criteria are standover (can you straddle it comfortably), virtual top tube length (horizontal between top of head tube to seat post--is the reach right for your torso and arm length), and head tube height (does it put the handlebar at a height where you want to reach it?).

Sizing a sloping top tube bike is usually virtual. It's usually the intersection of the distance between the center of the bottom bracket up the seat tube and seat post with a horizontal drawn from the top of the head tube. Except for LeMonds, which drew the horizontal from the center of the top tube where it met the head tube, and bikes with extended head tubes like the Specialized Roubaix, which draw the horizontal from the top of the top tube where it meets the head tube.

Just eyeball a couple sizes and try 'em on.

Frame size using the seat tube length can be open to errors and the same with top tube length.

If you get onto the Spec sheet on line for your bike and look at Geometry- there is one measurement that can confirm the frame size and that is Head Tube length. Top to bottom length and then transpose to Frame size and you can tell what frame size you have.

That's what I was thinking too. Looking at the photograph that looks to me like a head tube for a 58 cm frame. The seat to handlebar drop isn't as extreme as is commonly seen today either.

Does it matter? How does the bike feel when you ride it? It might be one of those things that's like Chinese food - if you like it, don't ask what it is.

I'm pretty comfortable on the bike but I've always "felt" that it's a bit big. I can't really quantify that... it's just a feeling. If I ever get some free cash to buy a second bike I will definitely look into a 56 C to C. With the Masi the seat is set at 74.5 cm height and is in a neutral pedal position. In that position when I am on the tops the front hub is starting to show behind the bars. In the drops it is perfectly obscured. I have been thinking about moving the seat a bit forward to help my spinning.

I stand corrected. I guess this works for some of the better modern frames which don't seem to use round top tubes any more. "Usually virtual"...I like that, especially for the ones I've seen lately with the flat top curved tampered sloping top tubes. When I came out from under my rock I did notice that things have definitely changed on the design of the better bikes.

The location of the saddle should be not be moved forward to shorten the reach, assuming that the seat location is already correct for your build. The reach chould be shortened by using a shorter stem and/or using a compact handlebar. You might just consider a compact handlebar to bring the reach to the hoods and the drops further back.

It isn't my intention to move the saddle forward to shorten the reach. I have already changed the stem from 110mm to 100mm. What I was referring to is the time honored method of enhancing spinning as differentiated from moving the saddle slightly rearward to enhance climbing.