Your Inseam and Frame Size

We'll talk primarily about fitting a road bike, and make notes about fitting a mountain bike where appropriate.

The best frame size for a cyclist is as small vertically as possible, with enough length horizontally to allow a stretched out, relaxed upper body. This frame will be lighter and stiffer than a larger one, and will handle better and be more comfortable than a smaller one.

A Determine Your Proper Frame Size

To determine your proper frame size, you’ll first need to get an accurate inseam measurement . Stand with your back against a wall, your bare feet 6" apart on a hard floor, looking straight ahead. Place a book or carpenter’s square between your legs with one edge against the wall, and pull it up firmly into your crotch, simulating the pressure of your saddle while riding. Have a helper measure from the top edge of the book to the floor, in centimeters. (You can convert inches to centimeters by multiplying inches by 2.54.) Repeat two or three times, for consistency, and average the results to get your inseam length.
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Frame size refers to the length of the frame’s seat tube. Pro frames are measured along the seat tube in one of two ways, center-to-top (C-T) or center-to-center (C-C). C-T measures the distance from the center of the bottom bracket to the top of the top tube or seat lug (See image bellow). C-C measures from the center of the bottom bracket to the center of the top tube. Since C-T measures to a point higher on the frame, a frame measuring 55cm C-T would also measure roughly 53.5–54cm C-C, a difference of 1–1.5cm.

To size your C-T road frame, we use a guideline of .67 x inseam length. For example, if you have an 85cm inseam, your C-T frame size would be .67 x 85cm, or 57cm.

LeMond’s formula, from his former coach, Cyrille Guimard, establishes C-C size by the formula .65 x inseam length, which yields virtually the same frame size when you add the 1–1.5cm difference between C-C and C-T.

Larger riders (6'0" and up) may find that this formula puts them on a too small, and uncomfortable, road frame. A taller cyclist who wants a more comfortable frame may be better off selecting a frame 27–28cm less than inseam length, C-T.

For a mountain bike, we start by recommending a frame in the range of 10–12cm smaller than you take in a road frame. For example, if you ride a 55cm C-T road frame, look for a 43–45cm (17–18") C-T mountain frame.

With the right frame size, you’ll be able to set your correct saddle height, which will be within a centimeter of .883 x inseam length, measured from the center of the bottom bracket to the low point of the top of your saddle. This allows full leg extension, with a slight bend in the leg at the bottom of the pedal stroke.

LeMond recommends that you then shorten this length by 3mm when using clipless pedals. Also, you might consider a slightly taller saddle height if you ride with your toes down and your heel raised. Most importantly, make any changes in saddle height gradually, and give your body time to adapt to the new position.

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Next, put your bike in a stationary trainer, raising the front wheel to level the bike. Check your position with a video camera or mirrors in front and to the side, or with the help of a friend. Position your cleats with the ball of your foot directly over the pedal axle, and in line with any natural "toe-in" or "toe-out" foot position. Warm up by riding easily for 8–10 minutes.

Then, as you continue to look ahead, slowly stop pedaling, and bring the cranks to rest at horizontal, parallel to the ground. Check the position of your forward knee relative to the pedal spindle —for a "neutral knee position," you’ll be able to drop a plumb line from just below the front of the forward kneecap, and have it bisect the pedal spindle and ball of your foot below. Remember to not raise or drop your heel or hip as you check this. Then, move the saddle fore or aft, as needed, to achieve this neutral position.

The neutral knee position serves as a good starting point for most cyclists, though many adjust it from here: long distance (stage) racers and mountain bikers often move the saddle back by 1cm or more, for power, and sprinters may move it forward, “getting on top of the gear” for quick acceleration.

One more note on frame size and geometry: if you cannot move the saddle back far enough to get your knee to the neutral position, you should look for a frame with a more relaxed seat tube angle, or consider a slightly larger frame; if you have trouble moving the saddle forward enough, look for a steeper seat tube angle on your next frame, or consider a smaller frame. For most frames, a seat tube angle shallower by 1° moves the seat lug 1cm back, relative to the bottom bracket.

Finally, recheck your saddle height. If you’ve moved your saddle forward or back, you’ve effectively shortened or lengthened your saddle height, and will need to readjust it.

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Next, dial in the correct "reach" to the bar, or horizontal fit. Proper reach gives you easier breathing, better neck and lower back comfort, and better weight distribution and bike handling.

That "ideal position" varies here more than anywhere else for cyclists, depending on riding style, flexibility, body proportions, and frame geometry, among others. And, your upper body position will evolve with more hours in the saddle. That is, you may find that you develop a lower, longer position as your fitness and flexibility improve. As Phinney notes, though it may be difficult to achieve a truly flat back, we cyclists should all strive to be “longer” across the top of the bike.

Unfortunately, there is no formula for sizing the top tube and stem that works as well as the inseam method. One indicator comes from glancing down at the front hub while riding in the drops; your view of the the front hub should be obstructed by the handlebar. LeMond recommends that your elbows, bent at 65–70° with your hands in the drops, should be within an inch or two of your knees at the top of your stroke.

Measure your current bike’s top tube and stem. Then, decide how you’d like to alter that fit; add the top tube length to the stem length to get your overall top dimension. The very reason we stock stems in 1cm increments, from 7cm to 14cm, is just to let you dial in your best top tube and stem length.

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Road handlebars come in several widths and bends. Most cyclists select a bar that is just as wide as their shoulders, measured as the distance between the shoulder joints. A wider bar opens the chest for better breathing and more leverage, but is less aerodynamic. You’ll need to find your own balance between the two.

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Choosing the right crank arm length, like the rest of this, comes down to your size and riding style. Longer cranks give you more leverage, helpful if you like to push big gears at a low cadence (climbing, time trialing, etc.). On the other hand, if you like to spin at a high cadence, you’ll do better with standard cranks. As a starting point, we often recommend 170mm cranks on 54cm C-T road frames and smaller, 170–172.5mm cranks on 55–61cm frames, and 172.5–175mm cranks on 62cm frames and larger. Mountain bikes generally get cranks 2.5–5mm longer than road bikes; that is, you might want a 175mm crank on your mountain bike if you’re set up with a 172.5 on your road bike.

We offer a wide range of gearing options for chainrings and cassettes. Depending on your strength and terrain, you’re welcome to specify the close-ratio gearing generally used for racing, or wider ratios for hilly terrain or more recreational riding.

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