There’s nothing worse than riding a bike that doesn’t fit well, but nothing more satisfying than pedalling a bike that does fit well. While there is no substitute for test riding a bike to experience how it actually fits and feels, learning a bit about road bike sizing and fit will help you narrow down the range of sizes to test for the next time you’re road bike shopping.
There are three common types of road frames: traditional, compact and semi-compact. Traditional frames have a horizontal top tube and more conventional geometries. Compact frames come with a top tube that slopes downward from the headtube to the seat tube; they tend to have a smaller and thus stiffer front triangle and more standover clearance. Semi-compact frame designs fall somewhere in between traditional and compact frames.
For any of the three types, there is a wide range of frame geometries offered by different manufacturers to suit an equally wide range of riding styles and purposes.
Some road bikes are designed for riders who prefer a more aggressive racing position while others let riders pedal more casually in a more upright position. Generally speaking, racing-oriented riders will like to ride bikes with longer top tubes and more drop between the seat and handlebars while more casual riders will ride bikes with shorter top tubes and less drop to the bars.
Effective Top Tube Length
Although many measurements play a role in bike sizing, the most important measurement is effective top tube length. On a traditional frame, this will be the length of the top tube. On a compact or semi-compact frame, this will be the distance, measured horizontally, between the head tube and the seatpost; it will not be exactly the same as the top tube’s length.
Check out this BikeRadar article for a chart giving rough guidelines for sizing your bike by effective top tube length: http://www.bikeradar.com/us/beginners/gear/article/road-bike-sizing-what-size-bike-do-i-need-40459/.
Dialing In The Fit
Once you’ve used effective top tube measurement to narrow down your choices for what size road bikes may fit, it’s time to hop on them and hone your position.
First, check that you can stand comfortably straddling the top tube. For both safety and comfort, you should have several centimeters of clearance.
Next, set your seat height. It’s reasonable to start with your seat about the same height as your hip bones are when standing next to the bike. Then sit on it and pedal. Check that your hips and low back do not rock from side to side. If they do, lower your seat until there is no rocking. Typical knee angles on the extended leg side range from about 25 to 30 degrees, although exact ideal knee angle is often a matter of personal preference.
Once your seat is at the right height, adjust its fore-aft position. Put your pedals in horizontal position, then drop a plumb line from the front center of the kneecap of your forward leg. It should fall roughly over the center of the pedal axle. If your knee is in front of the axle, move your seat rearward and vice versa.
Finally, you can adjust your bike’s reach by changing both the height of the handlebars and how far fore/aft they are positioned relative to the seat. Most bikes will come with spacers that you can move above or below the stem for some instantaneous bar height adjustability, but you may need to swap to an entirely different stem to achieve a significant position change reach-wise. One common rule of thumb is to pick a stem length and angle so that when you sit comfortably on the seat, with hands on the hoods, and look down toward your front wheel, the top of the handlebar line up with the front wheel axle.
Your local bike shop can help you with dialing in your bike positioning and swapping out stems.
Don’t just rely on bike sizing and fitting rules of thumbs. Take your candidate road bike(s) out for a test spin. There is no substitute for actually riding a bike to see how well it fits and handles as you pedal at speed on real roads. Your bike may technically fit “perfectly” but still not handle in a way that suits your riding preferences.