Proper Bike Fit

Are you going to look for a new bike at the local bike shop? Or thinking of buying a used bike from a friend who is "about your same size?" I wrote this article as a primer to describe the many variables involved in matching the bike to your unique body. If you're a serious amateur or a professional, you're already educated; if not, I hope this helps! ...Chris

What is proper bike fit and how do you get it? Proper fit can be a lot more scientific than straddling the bike and making sure you “have a couple inches" (standover clearance).

This article will address only traditional bikes made of tubes welded into a triangular main frame. These frames have a horizontal top tube, an angled seat tube, and an angled down tube. There are smaller tubes called stays that hold the rear wheel, but these tubes are not critical to bike fit. The tube that production frames are measured by is the seat tube (the tube between the cranks and where the seatpost inserts). The measurement runs from the center of the top tube intersection to the center of the bottom bracket (the thing the crankarms spin on). Thus, a frame with an 18" seat tube is called an 18" frame.

A starting point to find your correct frame size is to measure your inseam. Do this by standing against a wall in your stocking feet. Find a book that is about 2 to 3 inches thick. Put the book between your legs and push up into your crotch with light pressure. It will help get an accurate measurement if you also keep the book in contact with the wall and have a friend measure from the floor to the top of the book. Multiply this measurement by .66 and you are close to your frame size! (Road bike only.) Multiply this same number by 109% and this gives you your approximate saddle height. (Measured through the center of the seat tube from the pedal spindle—extended in line with the seat tube—to the top of the saddle.)

Let’s get back to the tube you are straddling when you first climb on that potential new bike—the top tube. The length of this tube is actually the most critical dimension on the bike. The top tube determines your weight distribution and therefore the stability and the way the bike handles. Even more noticeable is the relationship between top-tube length and riding comfort: If you are too stretched out you’ll have back and shoulder pain. If you are too cramped you’ll have restricted breathing and reduced muscular efficiency.
With most production road bikes the relationship between the seat and top tube lengths is nearly constant and doesn't vary more than a centimeter (even between different bike companies). With this in mind you can get a better fit on a production bike if you follow these guidelines: Women typically have shorter torsos, therefore they should “err” towards a smaller frame size than a man of similar height. Regardless of sex take note of your body proportions relative to the norm, and if you have a long torso get a larger frame size. This may seem backwards because you’ll have less standover.
The old guideline of the front wheel hub disappearing behind the handlebars when you are in the drops is great IF your saddle fore-aft is properly set and you have some bend in your elbows. More important is that when you are seated, most of your weight should fall on the saddle and you should feel like you can comfortably support your upper body for hours. Additionally, your knees should be located directly over the pedal axle (when cranks are parallel to the ground).

Mountain bike fit is much more subjective. MTB wheelbases are longer and weight distribution less critical (at least until you are very experienced and know exactly what characteristics you are after). Mountain bikes are measured the same way as a road bike; by the length of the seat tube. However, it is almost impossible to gauge mountain bike fit or compare fit between makes by this parameter. Some companies drastically slope the top tubes for greater standover and maneuverability in extreme situations. This can result in a “16 inch” frame that will fit a six footer; because the top tube may be long enough. Conversely an “18 inch” mountain bike from another manufacturer may be too short for the same six footer. Mountain bike seatposts are extremely long and it is possible to “accommodate” nearly anyone on any mountain bike.

At this point you are probably thinking where do I start then? What I recommend is finding a road bike that fits you correctly. Despite the different riding riding positions and handlebars, I've found top tube lengths to be nearly interchangeable between road and mountain bikes for the same rider. Use the top tube length from a road bike that fits you and find a mountain bike that is similar in top tube length. You must measure mountain bike top tubes parallel to the ground—start at the intersection of the top and head tubes and measure to the center of the seatpost. This may seem like a roundabout way to fit yourself to a mountain bike but it’s also nearly foolproof. From there you can zero in on your perfect bike by taking the kind of riding you do into consideration. If you ride on wide-open fire roads a slightly longer bike will have greater stability. If you ride a lot of single track opt for a slightly shorter bike with more maneuverability. Get a perfect fit by changing the stem length slightly. Happy trails.