Choosing the right size road bike can be tricky as they don't all fit the same. Two companies models listed as the same size can have vastly different measurements. Depending on the company, I ride anywhere from a 52 cm to 56 cm frame. All of these have the same length top tube. Don't think that because you ride a given size from one company that you can just get the same size from another and have it fit the same.
When a company lists the size of a road bike frame it is some variation of the length of the seat tube. Measurements all start from the center of the bottom bracket but can be to different points up the seat tube. That point can be the center of the top tube, top of the top tube, top of the seat tube or some virtual point that isn't even on the frame. Add in sloping top tubes and this can get really complicated.
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To compare the size of frames you should look at the effective top tube length and headtube length. This will give you an indication of the reach and height you can get the bar to. With a road bike as long as you can stand over the top tube with your feet flat you have enough crotch clearance so the reach and stack height are much more important.
As road bars don't have rise you need to make sure a given frame and fork combination will allow you to get the bar high enough in relation to your seat height. You can use stems with some rise but there are only a few options with much more than seventeen degrees of rise, particularly for the over sized handlebars that are standard these days. If choosing between two models that have a slight variation in top tube length you can compensate with a longer or shorter stem. A point to bear in mine is that carbon steerer tubes can only be spaced up around 30-40 mm before you risk damaging it. Too high and you risk the fork snapping off while riding.
When figuring out a frame size for a client I will take measurements of inseam length, torso length, arm length and shoulder width. Then I will do a structural assessment to look at flexibility and core strength. From this data I can calculate pretty closely what frame geometry will work best. Once the bike is built we take care of the fine tuning during the custom fitting session.
When you are trying to find a good size on your own, find a bike shop that will take your measurements and figure the reach and stack height you need. If you aren't buying a bike from them you will probably have to pay for this service but it is well worth it. You can also search for online size calculators that instruct you on how to take your measurements and calculate the correct size.
How to Choose the Right Size Road Bike
If you want to ride comfortably and as fast as your potential will allow you need to be fit properly and this starts with your frame. For more information on bike fitting and sizing go to my site http://www.cyclecambridge.com.
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