Finding a virtual frame size
Wed, September 25, 2013 
No more than 20 years ago racing frames were lugged steel, built by craftsmen. Many aspects of design like the diameter of the tubes for example had not changed for 100 years. Frames were built in one centimeter increments so there were as many as 18 different size frames to cover the complete range of different size people.
Top tubes on road bicycle frames were horizontal and exactly level. This gave a point of reference, and once a rider had determined his correct size frame, for the rest of his life he could then buy any brand of frame in his particular size, and it would fit.
No one ever talked about handlebar drop, (The distance from the top of the saddle to the top of the handlebars.) because if a person had the correct size frame and the saddle was set for their particular setting, then the height of the handlebars above the frame was automatically correct. It was limited by the amount of adjustment in the old quill stems. About 1 1/2 inches. (37mm.)
In the mid 1990s this changed mainly due to the influence of mountain bike design and road bike frames are now labeled “Compact,” which means they come in fewer sizes, and the greater differences between small, medium, and large are taken up by longer seat posts, and a variety of handlebar stems that come in different lengths and angles.
Choosing a frame size is the easy part because there are fewer sizes, but where do the handlebars and saddle go in relation to everything else? One answer is to find your virtual frame size and create an imaginary horizontal line that becomes a reference point, and the handlebars are then set a certain distance from that reference point.
Different manufacturers’ sizes will vary slightly, and once a person has decided on a particular brand, they should go by that companies recommended size. Generally speaking an XS frame will fit a person 5’0” to 5’4” tall. Small frame 5’3’’ to 5’7” tall. Medium frame 5’6” to 5’11” tall. Large frame 5’10” to 6’3” tall. And a XL frame 6’2” to 6’5” tall.
There is some overlap in sizes, notice that a person 5’10” tall could use either a medium or large frame. This person should choose the smaller medium size for racing, or the large for more leisurely riding. The larger frame will have a longer head tube, making the handlebars higher in relation to the saddle height.
For some time now people have been asking where they can find my frame sizing chart that used to be on my old website. I have posted a printable version as a PDF, you can find it here. Instructions are on the sheet, your virtual frame size will be in the “Center to Top” column, as it represents the top side the top tube on an of the old school level top tube frame.
For example, if the virtual frame size is 61cm. measure that distance from the center of the bottom bracket, and mark the seat post at that point with a piece of masking tape. The top of the handle bars should be between 9 and 12.5cm above the horizontal virtual line. An easy way to check this, on a level floor, measure vertically down to the floor from the piece of tape. Add the 9 to 12.5 cm then measure up from the floor to the top of the bars.
The chart was originally intended for a racing set up, and one thing to keep in mind is that modern handlebars are flat on top and the brake lever hoods are higher. There is an article here on saddle height, that some have found useful.
The frame size chart also has top tube length and stem length. If you take these from the same line as your virtual frame size, and add the two together, you will have the recommended distance from the center of the seat post to the center of the handlebars. Measured horizontally across. (You can ignore the 1/2 centimeters.)
No frame sizing method or chart is written in stone, it is intended as a guide to give a person a place to start from. There will probably need to be some fine tuning to arrive at the perfect position. However, for someone starting out, if you know your virtual size before you buy a bike, you will be more informed and have an idea if the bike you are considering is at least close to the right size.
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Reader Comments (9)
Dave, if one goes to a real bike shop to buy a bike, then they don't need to know what size to get because the staff will figure it out and maybe there will be a bile to test ride. However, these days when so many people are buying on the Internet sight unseen, your advice is very pertinent. As for the sizing chart you linked, it's all in centimetres. I've been accustomed to seeing inch size frames always being centre to top and metric sizes being centre to centre. Of course my bikes are vintage steel, not those plastic things.
Shouldn't the picture have labeled the line that runs down the seat tube to the center of the bottom bracket as the "frame size"? The picture suggests that the frame size somehow is measured from from the top of the wheel...
No doubt the interchangeability offered by hugely varying seat tube lengths, and easily variable stem heights and stem lengths has made it possible for manufacturers to make fewer sizes of frames that "fit" more people. I don't miss the days of of searching for extra long "seven" stems for frames that were too small.
I wonder however about ability of many bike shops to properly fit riders, especially new riders, to their bikes.
TimJ
It just happens that the left side of the line hits the top of the wheel, but it's the right side that you're measuring: The place the seat tube would end at if the top tubes were still horizontal.
Thanks for your post Dave, seriously. I can say that the LBS can be hit or miss as to whether they put you on the best fitting frame. My experience is that if you're a beginner they'll put on the best fitting frame they currently have stocked which might be too big or too small to begin with.
One thing i struggled with was seat tube angle and the corresponding seat set back. I had to finally just draw it out. I ended up figuring roughly 1cm/degree change on the STA. I know it might be asking a lot but could give some guidance on STA or general saddle position. The problem is that some companies use the same STA for all frame sizes and some use steeper STA for smaller sizes. This makes it so hard to compare the two and to find a good starting point for proper fit once you do get your bike.
Best,
Mike
Goleta, CA.
New frame angles opens the door for frame size confusion. Wouldn't you know it that I would be between M & L. Will stay with steel.
Interesting the position of the brake hood levers on the bars. Watching TDF and other races, riders now seem to mount the hood levers higher up so they can grip and pull on the levers. I have always preferred the lever hoods down on the bend of the bars
I dealt with this recently when trying to order a new frame based on its geometry. While top tube length seemed like the most important factor, differences in the head tube and seat tube angles mean that reach could vary quite a bit between frames with equal lengths but different angles. Ultimately I had to just make diagrams for the two different sizes and make the necessary calculations, given the two angles, using bottom bracket drop, seat tube length, top tube length, and my distance from bottom bracket to saddle which should stay constant from frame to frame. The final number, the horizontal distance from the top of the saddle directly above the seat post to the top of the stem, assuming that they're of equal height--which is where I tend to like them, gives me a way to compare the reach without worrying about stem length between a frame that I've ridden and one that I haven't.
As it happens the 51cm frame that I used as my starting point--a 51cm seat tube and 54cm top tube--is 3mm shorter in reach than the frame I was considering--a 52cm seat tube and 54.5cm top tube. Even though that 52cm frame has a slacker head tube angle and steeper seat tube angle, the difference in reach was consistent with the difference in top tube length. I ended up going with the next size down since I felt a bit too stretched out on the 51cm bike while using an 80mm stem.
In other words, when in doubt just do some basic trigonometry to verify. It isn't particularly hard if you already have a bicycle to use a frame of reference..
HI, just wondered if anyone could help me. I am in the market for a new bike and am thinking about a Colnago c59. I 188cm and have been told a 54s would be the right size for me, but am unsure. As think I may need a bigger one. It has a virtual Horizontal Tube is 55.3. If I get a long stem, do you think it would be suitable!?
Thanks, David.
Just verifying that the chart requires *cycling* measurements... :
Inseam Crotch to floor without shoes ... measured while feet about a foot apart ?
Height Actual measured without shoes, not estimated ... also measured while feet about a foot apart ?
Very comforting to find an easy to follow sizing guide like this, thank you Mr. Moulton