How to choose the correct size of road bike
Monday, June 21, 2010 at 4:48PM I had originally planned to publish Part 4 of The Five Most Common Bike Fitting Issues, in which I was going to cover proper reach to the handlebars, but realized there is simply no way to address this topic without first discussing proper frame sizing.
Most of what you'll read here is adapted from my Road Bike Fitting Guidelines: The Essentials for Proper Fit. You can download it from the link below. This is a PDF document, so you must have Adobe Reader installed on your computer. I've also made some changes to it recently, so if you have an older version, you should replace yours with the current one (dated 3/14/11).
Download link for Road Bike Fitting Guidelines 3.14.11 (I've noticed that the link doesn't always work, and I'm not sure why. I think it's got something to do with browser settings. Anyway, if you can't download, just shoot me an email with "Request for bike fit paper" in the subject line, and I'll email you a copy. I'm at velofitter@yahoo.com)
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.
Athough it is true that a bike's front end steering characteristics are largely defined by it's 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. As a rule of thumb for general purpose road riding, a longer and lower position, provided it's comfortable, is preferable to one that is high and short; that's going to be hard to accomplish if the top tube is too long for you.
Determining the correct frame size
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. In the case of compact/sloping geometry this number would be for the effective, or virtual, center-to-center seat tube measurement. I actually multiply the inseam by .655 for a slightly more conservative length. Now, here's the important part, the top tube should also be the same length as the seat tube length you just determined (again, for sloping geometry this would be for the virtual, or effective, top tube length), provided your inseam is in proportion to your torso. My Road Bike Fitting Guidelines will provide further guidance on upper/lower body proportionality. I don't care what size the manufacturer may label the bike, the top tube length is far more important than some arbitrary sizing convention like Small, Medium, or Large. I've seen some "small" compact geometry frames that are far too large for many shorter cyclists.
The above formula applies to those who meet my "5Fs Rule": Fit, Fast, Fairly young (less than 40), Flexible, and Free of pre-existing conditions. If you have bad flexibility, only ride on the weekends, are 50, can't hold a pace of 15 mph while riding solo in the flats, and have a history of back problems, I would probably suggest a top tube that is perhaps 4 cm shorter than the above formula would indicate (in situations like this, it's probably best to consider a custom built frame). Sorry to be repetitive, but again, Road Bike Fitting Guidelines provides the details of "5Fs Rule."
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 shorter cyclists (5'6", 167 cm, or less) will find that they are on bikes that are horizontally way 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). It's another topic for a future article, but the problem is related to the designs of many smaller/larger frames which have steep/shallow seat tube angles combined with top tubes that are proportionately longer/shorter than average sized bikes. This is probably a good place to mention that toe overlap is simply going to be something to which shorter cyclists will have to adapt, especially if they are trying to stay on a bike designed around 700c wheels vs. 650c wheels.
Head Tube Length and Seat Tube Angles
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 3cm 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.
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. It's also something to keep in mind when you are comparing two frames for purchase that have differing seat tube angles, but the same top tube length. How do you know which one has the longer or shorter effective reach? As a general rule, each ½ degree change in seat tube angle works out to about a 7mm change in effective reach (again, provided the top tube length stays the same). A steeper seat tube provides for a longer reach, the shallower seat tube a shorter one. This topic usually gives the less technically inclined a headache, but it is important. For more on this subject, Nuevation has a good article with visuals: Nuevation Bike Geometry. Cervelo used to have an excellent tutorial on reach based geometry, but I can no longer find it on their website.
Where to get additional bike buying help
I have found that the Fit Kit measurement programs available at many bike shops really do a good job of getting people on the right size bike, and the numbers they come up with are very close to what I usually recommend. Unfortunately, many shops just don't use the tools anymore, or don't have anyone around who is trained in their use. Here is a link to the Fit Kit dealer listings: Fit Kit US Dealer Listings
You can also get in touch with me. I have a bike buying program for which I charge $75 for all of the following: a telephone interview, emailed instructions on how to take your anatomical measurements, determination of your ideal frame geometry, help with locating a bike/frame or custom builder within your budget, and if you want, I'll even try to locate a bike fitter in your area who can help with fine tuning your final position and pedaling biomechanics after you get the bike. If you live in my coverage area and want me to perform the final bike fit, I'll credit the cost of this program to my bike fitting services. Eric Bowen (858) 414 -7093 or velofitter@yahoo.com
Coming up in the next article - Part 4 of the Five Most Common Bike Fitting Issues...Reach to the Handlebars.
Reader Comments (4)
Hey, Eric
When you use the five Fs to modify the top tube length, does that modification get applied to the seat tube length, too?
When you use the level to figure out your inseam, should you expect a longer or a shorter measurement than your pant inseam?
I love the new service to help people who live out of your area. Great idea. I'm gonna link to and recommend that.
Hi Tom,
Great questions.
The Five Fs rule is only applied to the top tube length. Seat tube length (actual center-center for level top tube bikes, or virtual/effective center-center measurement for sloping/compact geometry) is 2/3 of measured inseam.
Most folks will discover that their measured inseam is usually about 1-2" (2.5- 5.1 cm) longer than their typical pant inseam.
Thanks for the plug, and keep up the great work at www.bikezilla.blogspot.com
great article Eric.
I've been reading an eBook called Bike Fit and my wife has a copy of Every Woman’s Guide to Cycling: Everything You Need to Know, From Buying Your First Bike to Winning Your First Race by Selene Yeager.
I noticed that both of these resources go into much more detail on the nuances with bike sizing for women that I thought you might be interested in covering also.
(I know my wife experienced quite a bit of discomfort initially with the way the local bike shop had set up her bike.)
Dear Bike-Books,
I can't vouch for Selene Yeager's book, as I have not yet read it.. Bike Fit by Dr. Arnie Baker is perhaps the best book on the topic available in the english language.
As mentioned in the article, I've got much more to say on women's specific bike fit issues, and there will certainly be some upcoming articles that will address this topic with the greater detail it deserves. Thanks for the kind words.