Sizing Up A Bike - Bike Geometry 101

In order to size your bike to fit you, awareness of the key measurements of a bike that affects a 'good fit' will be needed.  By gaining an understanding of how these variables can influence your performance and then optimizing them to achieve the desired outcome, you will get the most out of your cycling.

The frame component that plays the biggest part in ensuring a good fit is the seat tube.  The appropriate length of this tube can be determined from the measurement of the lower limb and adjusting to fit riding style and upper body length.  More detail is given below!

Top Tube Length.  Top tube length is also important to achieving a good fit, but this length is dependent on the length of the seat tube length.  Even on custom bikes, the top tube length is almost always within a 3/4 inch (3 cm) of the seat tube length.  Therefore the more important measurement is that of the top tube and the stem length combined.  Additionally, any differences in top tube length can be adjusted for by changing the stem length.  For road cyclists whose handlebar and stem are fairly standardized, this combined length will not change very much between athletes.  But for triathletes, where the choices in top tubes and stems are virtually endless, the decision about the proper proportions is much harder.  In order for the triathlete to size these components correctly, he or she must try to adjust the lengths in a manner which maximizes good riding posture.  And, hopefully, the following info will help.

Lower Limb Length.  To get the right seat tube length and seat height, proper measurement of the lower limb is needed.  The length of the lower limb is defined as the height of the top of the greater trochanteric (the large knob of bone at the side of the upper thigh) from the floor (see pic below).  Note that the measurement is not to where the bone sticks out the most, but to the top of the bone.  Once you have that measurement, use the chart below to determine the proper seat tube size.  From the chart you will get your approximate maximum seat tube length.  It is usually recommended that you don't go over that length but you are 'allowed' to go smaller if desired.  Many road racers opt for the smaller size since a smaller frame is often a little bit lighter and stiffer, and the shortness can be made up with the seat post.  Don't go too small though, because comfort and efficiency are still more important than a couple ounces of weight!

Seat Height.  For many years lab studies have shown that there is an optimum seat height which maximizes performance and minimizes oxygen cost.  We have also observed many of the sport's top cyclists and have seen that they too have virtually matched the lab-prescribed seat height.  For once, the scientists and the real world are in agreement!  As a result, we recommend trying to stay close to the prescribed height when determining how high to make your tush.

To get the optimal seat height a measurement of the lower limb will be needed as done above.  But this time, adjustments should be made to account for the type of shoe system being used.  The formula for the appropriate seat height is .98 (lower limb length + sole/cleat thickness) where the sole/cleat thickness is the distance from the upper surface of the inner sole where the foot makes contact, to the bottom of the shoe cleat that makes contact with the pedal.  If using traditional shoe cleats with toe clips and straps, the bottom is at the base of the slot in the cleat.

The height of the seat is measured from the bottom of a straight edge placed on the seat from the tip to the rear of the seat, to the crank axle, plus the distance from the crank axle to the to the pedal, a.k.a. the crank length (see pic).

Fore-Aft Position.  Since the bike seat tube is normally angled backwards from the front of the bike (normally at about 75 degrees), adjustments in the fore-aft position of the seat will change your seat height--so be careful. Moving the seat forward by 10 mm will effectively reduce you seat height by about 3 mm. Moving your seat 10 mm backward will effectively increase your seat height by 3 mm.

Seat Tilt.  If you desire more of a forward position, you may find yourself getting uncomfortable due to riding too much on you pubic area.  You can relieve some of this pressure by tilting the front of the seat downward.

Crank Length.  Scientists have yet to come up with a universal formula for the optimum crank length.  Considerations include the facts that longer cranks make it easier to generate power, but require more range of motion and shorter cranks are harder to turn but require less range of motion.  We believe comfort and 'feel' are key to choosing the appropriate length crank, but also believe that the following recommended lengths be considered when determining your crank length.

Setting The Handlebars.  Current handlebar technology has greatly improved the performance of triathletes everywhere.  The new 'aero bars' have significantly helped the athletes achieve optimal riding position while reducing aerodynamic drag, providing a more comfortable ride, and making the rider more stable.  If you are a triathlete, you must have aero bars on your bike!  Other than that, place those handlebars in a way that lets you get in as aerodynamic of a position as possible while maintaining the most amount of comfort.

Concentrating on these key components should help you get a bike that fits like a glove!