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Fit Factors To Consider Before Purchasing A Bike

Once you’ve found your ideal position on an existing bike or through a new fit, you can mimick that position on a new frame.

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Photo: John David Becker

Before your next bike purchase, consider these important fit factors.

Once you’ve found your ideal position on an existing bike or through a new fit, you can mimick that position on a new frame or cockpit using a tape measure and a calculator. Translate the fit accurately and your new ride will be comfortable immediately.

A fitting will tell you the locations of the contact points on a bike–saddle, pedals and aerobar–using stack and reach dimensions to describe the location of the elbow pads. Between your body and your bike frame are aerobars, a stem and headset spacers, and changing the shape and size of these pieces adjusts the fit of your bicycle. In order to connect fit geometry to frame geometry, all these intermediate pieces need to be accounted for.

At the conclusion of a fitting, you should be provided with the coordinates of your contact points that determine your position (or measure an existing bike with the instructions in red). Pay the closest attention to arm pad stack and arm pad reach:

– Pad stack: the vertical distance of the arm pad surface above the bottom bracket

– Pad reach: the horizontal distance from the back of the arm pad to the bottom bracket

Note: These numbers are different from frame stack and reach. If you already have a bike that fits you well, you can measure pad stack and reach from the bicycle. Keep in mind that the saddle position is important when setting up your bike to fit correctly, but not very important when considering if a bike will fit because most modern triathlon seat posts have a wide range of adjustability.

If you are the owner (or the prospective owner) of a triathlon bike with an integrated front end such as the Cervélo P5, the Trek Speed Concept or the BMC Time Machine, check with each manufacturer (or look on the company’s website) to see how your fit requirements align with their bikes.

Translating Pad Position

Aerobars: Many of them are shaped and sized uniquely and have different adjustability ranges. The stack height of the arm rest pads above the basebar center and the arm pad setback behind the basebar center impact a bike’s fit options. The arm pad and extension width are also things to consider when picking an aerobar. Many manufacturers publish their aerobar pad stack and pad setback ranges; the data of a few of the popular bars are listed in the chart below.

Stems: As a general rule, it is desirable to buy a tri bike that fits well with a stem that is 80–110 millimeters long. Aside from adjusting your aerobars, changing the stem can really change the way a bike fits, handles and looks. Stems that are too long or short will make the bike handle poorly. It is also desirable to buy a bike that fits well with a stem that has an angle between –17 degrees and –6 degrees, but you can stray a bit if needed. Stems are available in a wide range of angles and can be installed pointing up or downward so a –6 degree stem can be flipped over to become a +6 degree stem.

Headset spacers: They are rings that sit above your headset cap and below the stem, propping the aerobar higher. You can adjust the height of your aerobars by adding or removing spacers below the stem. It is generally considered a safety hazard to use more than 30mm or 40mm of spacers with today’s carbon forks.

Measure stack and reach on your own bike

From the floor, measure the height of the top of the arm pads, and to the middle of the bottom bracket. Subtract the second number from the first to get arm pad stack. To measure arm pad reach, you will need to cast a plumb line through the bottom bracket. The easiest way to do this is with a carpenter’s laser level, but a plumb bob will work as well. Once the plumb line has been cast, measure from this line horizontally to the back of the arm pad.

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Find the right frame size

These formulas can help you calculate the required frame stack and reach based on pad stack and reach data. Start with your pad numbers, then pick your preferred aerobar. For the stem and spacer we’ll use the coordinates of a 90mm stem at -6 degrees with 30mm of spacers. If you have a specific combination in mind, view our charts at to get the proper coordinates to use in the formulas.

Required frame stack = Required arm pad stack – arm pad stack of selected aerobar – StemY (use 37.7) – SpacerY (use 28.5)

Required frame reach =Required arm pad reach + arm pad setback of selected aerobar – StemX (use 81.9) + SpacerX (use 9.3)

As an example, the arm pad stack and reach required of a triathlete with particularly long legs and a short torso:

Required arm pad stack: 674mm

Required arm pad reach: 427mm

Based on using a Profile Design T4+ aerobar with the arm pads in the 60mm setback position, with a 90mm stem in the –6 degree position and 30mm of spacers below it, we would calculate the required frame stack and reach as follows:

Required frame stack = 674mm – 60mm – 37.7mm – 28.5mm = 547.8mm

Required frame reach = 427mm + 60mm – 81.9mm + 9.3m = 414.4mm

With this information on hand, you can now peruse geometry charts to find a bike that will match your fit requirements as closely as possible.

Plug And Play: Zipp’s VukaFit app easily translates pad stack and reach dimensions into options for a complete bike, no calculator needed. The only catch is it only includes Zipp aerobars.

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