Beware the Online Bike Fitter

Our bike editor tested a handful of new fit apps to see how they stacked up to the brick-and-mortar specialists. Let’s just say some worked better than others.

Bike fitting is perhaps the most underrated and overlooked aspect of cycling. No matter how expensive a bike you buy, if it’s the wrong size, or you don’t take the time to fit it properly, you’ll never reap the full benefits. 

I’ve experienced and recommended lots of bike fit systems over the years, including Retül, Specialized’s Body Geometry, Guru, and specialty shops like San Francisco’s 3D Bike Fit that blend techniques and expertise for a truly custom approach. Each of these methods works fine, presuming it's conducted by a skilled fitter, though they can be pricey, usually starting at $150 and going beyond $500. In the last few years, a number of internet fit systems and apps have come online promising to help get you set up on bike properly—minus the sticker shock. Since I’ve been fit a number of times, I decided to put a few of these tools to the test and see how their results compare to an in-person fit session.

Most of the online calculators are offered by retailers to help you choose the right size and fit when purchasing a bike, then to know your numbers when the bike arrives so you can set it up correctly. For the majority of these interfaces, you take a series of measurements—including height, inseam, shoulder width, thigh, shin, and arm length, sternum height, and foot size—and the program spits out the numbers for your optimal bike fit. For accuracy, I had my wife take all measurements, and then I compared the generated fits to my primary road bike, a size 54 Cervélo R Series, which was set up most recently at a Retül fit studio. I have ridden thousands of miles on that setup with no pain or injuries, so I assume it's a solid baseline.

The first thing I learned: Be wary of what sites and sources you use for online fitting. For instance, the e-retailer Backcountry’s fit calculator, which uses just three questions (including your gender) to determine what size bike you should get, recommended I look at a size 56-57cm road bike. If I took that suggestion, I’d get a bike that ranged from large to way too big for me. The calculator at JensonUSA, one of the larger online cycling retailers, didn’t even work at all.

Even more specialized fitters, such as Setthebike.com, returned some seriously wonky numbers. The site suggested a handlebar to saddle distance of 37.7 inches (my bike is set at 27.5) and a crank to saddle height of 26.6 (mine is closer to 30). I’m fairly certain that their proposed numbers aren’t even achievable on a retail bike.

I was more optimistic about Pedal Force, a consumer-direct bike brand whose fit calculator took all the right measurements and was calibrated down to the millimeter. The bike sizing recommendations were reasonably close: 53.6cm top tube, 39cm bars, 110mm stem, and 35.4-inch saddle-to-pedal are all within a centimeter of being correct. However, the set-up suggestions were pretty far off, including 1.5 inches shorter reach, a saddle height nearly an inch lower, and a 170mm crank instead of my existing 172.5mm. I tried several other small bike manufacturers’ fit calculators and came up with similar results. They put me in the right ballpark, but didn’t get me anywhere near the precise fit to which I’ve grown accustomed.

The best online fitter that I found came from bike e-tailer Competitive Cyclist. The system takes comprehensive measurements, offers detailed how to videos so you get data correct, and produces three separate fit options, ranging from the super race-oriented Competitive Fit to a more relaxed French Fit that values comfort over aerodynamics. My Cervélo is definitely a competitive fit, and even in its most aggressive recommendations, the calculator was off by a few centimeters. The biggest failing was the recommendation for a bike with a shorter top tube (53.1cm) but longer frame, which, if I were purchasing a bike, would put me in far too small a frame. The saddle height was also three-quarters of an inch lower than my standard bike. Out of curiosity, I set up a test bike (Trek Émonda Disc, size 54, the closest thing I have in house to the suggested 53.1cm top tube) to the fit particulars. While I could certainly tolerate the position, it's not one I would choose.

As a sanity check, I decided to book an appointment with Shimano at their Boulder, Colorado, offices to try out the company’s new physical fit system. The starts with static measurements using a proprietary set of tools and evaluation by your fitter, followed by dynamic, 3D analysis of your position while pedaling on a fit bike. Using a set of nodes that attach to pivot points on the body, much like the Retül system, your fitter can watch your cadence and power in real time as changes are made to your position. It’s a seriously involved and precise system, and my results came back only nominally different—I’m talking a matter of millimeters—from my setup on the Cervélo.

So far, I’ve been talking only about the actual measurements on the bike. But a fitter also brings other expertise. For instance, my Shimano fitter suggested I try a new cleat, which seems to have alleviated a bit of creakiness I’ve been having in my right knee. And at one of my previous sessions, at 3D Bike Fit, the fitter gave me wider spindles on my pedals and formed custom orthotics for my shoes. Of any fit adjustment I’ve ever had, these detailed tweaks have made the biggest difference comfort and performance through the years.

The experience with Shimano—especially the fact that it matched so closely to my existing bike fit, whereas the online fitters yielded wildly differing results—underscored my feeling that while an online fit system might be a good starting point for picking the right size bike, you need human expertise to home in on the particulars and make sure you are perfectly situated on your bike.

I won't say don’t use online fit calculators: they’re an OK starting point if you’re new to cycling. However, a good local bike shop with a competent fitter should be able to get you a lot closer and more comfortable. And while a trip to a dedicated bike fitter might be expensive, the years of comfortable riding that follow should make the money well spent.