Why correct frame size is so important
Choosing the right size frame for your body is not only the first step in obtaining a comfortable and efficient position, but also ensures that your bike handles correctly. Generally, a road bike is not designed to be ultra stable, and behave like a loaded touring bike, and at the other end of the spectrum, neither is it supposed to maneuver like a super responsive track bike. Most road bikes are designed to handle in the middle ground, somewhere in between those two extremes.
Although it is true that a bike's front end steering characteristics are largely defined by its trail (definitely a topic for another day), trying to fix an ill fitting frame that's horizontally too long or too short by using a long or short stem, or by moving the saddle too far fore or aft, can compromise the handling characteristics. What do I mean by "handling characteristics?" Well, the bike can feel like it's fighting you when cornering, or may not want to track straight when you're your climbing out of the saddle, or can feel like it's too responsive (i.e. "twitchy"), or can shimmy when descending.
So, how do you determine your correct frame size? The really simplified rule on frame sizing is the same one that has been used by bike fitters and frame builders since the dawn of time - multiply your inseam by 2/3 to arrive at your correct center-to-center seat tube length for a traditional level top tube bike (inseam measured in centimeters). The top tube should be within 1 cm of your inseam length. In the case of the far more common compact/sloping geometry the seat tube length is going to be of little help in knowing the correct bike size. Instead, you should focus on making sure that the bike’s effective, horizontal, or virtual, center-to-center top tube measurement is within 1 cm of your inseam length. This formula only holds true if your upper and lower body are proportionate. I actually multiply the inseam by .655 for a slightly more conservative length. My Road Bike Fitting Guidelines will provide further guidance on upper/lower body proportionality. It doesn’t matter what size the manufacturer may label the bike, the top tube length is far more important than some arbitrary sizing convention like Small, Medium, Large, X Large. I've seen some "small" compact geometry frames that are far too long for many shorter cyclists.
The above formula may not apply to you if you have if you have a long or short torso vs. your inseam, bad flexibility, only ride occasionally, are new to cycling (especially if you are 40+), or have a history of injuries (especially back problems). For those with several of these issues, I might even suggest a road bike that has been modified with flat bars, like on a mountain bike or hybrid. Although rare, it may also mean that you will need a custom bike built.
So, you first need to measure your inseam, and no, it's definitely not the same as your pant inseam. To do this, you will need an assistant. Make sure you are in socks and cycling shorts. Next, set your feet about 9-10" (23-25 cm) apart and straddle a 2-3 foot (61- 91 cm) carpenter's level that has about a 2-3" (50-76 mm) thickness. Pull the device firmly into your crotch while facing a wall (leave enough space between you and the wall so you can hold both the front and back of the level, and your helper also has room in front of you to mark the wall). Make sure your level is perpendicular to the wall. Have your helper mark the spot on the wall at the top of the level. Measure from the floor to this mark and you now have a fairly accurate measurement of your inseam. I'd suggest taking the measurement three times, and using the average.
Short and Tall Cyclists
Many cyclists who are 5'6" (167 cm) or shorter will find that they are on bikes that are horizontally too long for them, and many taller cyclists (6'3", 190.5 cm, or more) will discover their bikes are too short for them (again, it's the top tube to which I'm referring). The problem is often related to the way companies will assign sizes to their smaller and larger bikes. As an example, a bike might be labeled a 51 cm, but actually has a 52 or 53 cm effective top tube. Also, the designs of many smaller/larger bikes have top tubes that are simply too long or too short for those at the extremes. If you follow the guidelines in this article, you will avoid this all to common problem.
Head Tube Length
Head tube length does enter into the equation of proper bike sizing, but it's not quite as important as the top tube length. A good rule of thumb is to figure that 3 cm of added front end height (either a longer head tube and/or added stem spacers) has about the same effect as shortening the top tube by about 1 cm. This ratio works in reverse, as well. For example, if you remove 3 cm of stem spacers, it will effectively lengthen your reach by 1 cm. Also, each 10 degree difference in stem rise is the equivalent of about a 1 cm change in reach. Many manufacturers are now designing bikes with longer head tubes to accommodate a more upright position; this is the same tactic that is employed in many women's specific models. Those designs are certainly a step in the right direction for those that need a higher front end, but a longer head tube isn't going to necessarily offset a top tube that is too long.
Seat Tube Angle
Now, to really complicate matters, if you are buying a new frame, and it has a different seat tube angle than your current bike, your reach to the bars can be affected; this holds true even if the top tube length is the same. If you are trying to keep the same position on your current bike relative to the bottom bracket of a new frame (what is known as the fore/aft position or setback), a steeper seat tube provides for the longer reach. Why? Since you will have to move your saddle farther back to maintain the same position, the reach to the bars lengthens. Conversely, a more shallow seat tube will provide for a shorter reach since you’ll have to move your saddle further forward to maintain your current position. As a general rule, each ½ degree change in seat tube angle from your current bike works out to about a 6mm change in effective reach (again, provided the top tube length stays the same).
Since all of this can get quite confusing, especially for those who are a bit mathematically challenged, you can get in touch with me. I have a bike buying program for which I charge $100 for all of the following: a telephone interview, emailed instructions on how to take your anatomical and current bike measurements, determination of your ideal frame geometry, and help with selecting a bike/frame or custom builder.