Power Consumption Overview

Power Consumption Overview

A version of this article was originally
published in Triathlete Magazine

After speaking to a number of people, I’m still confused about what really matters in regards to bikes and speed. Most commonly, I’ve been told that aerodynamics should be by far the most important thing to me and that I need to make sure that I get the most aerodynamic bike possible. However, I’ve read elsewhere that weight and stiffness are really important too. What is the real story?

Ted , WI

Dear Ted,

There are a number of concepts that relate to your equipment and performance. Some of these concepts are quantifiable (like aerodynamics and weight), while others are less tangible (like the handling and mechanical condition of your equipment). I’ve provided brief explanations of the most influential variables (based upon estimations from studies within the cycling industry) and categorized these variables by approximately how much rider energy they consume on a flat course without wind. For example, if you burn 1000 calories during a ride, a variable rated at 15% of energy consumption would use 150 of those calories.

First, the tangible concepts:

Aerodynamics: Rider aerodynamics is approximately 50% of total energy consumption and the aerodynamics of your equipment is approximately 15% of total energy consumption.

How aerodynamics relates to you and your bike is often one of the most simplified and misunderstood subjects in cycling and triathlon. Wind resistance is usually the largest obstacle a rider has to overcome. However, many people tend to approach aerodynamics backwards by trying to maximize the aerodynamics of their equipment without addressing the aerodynamics of their riding position.

If you compare your surface area to that of your bike, it is easy to see how you have a much greater impact on overall aerodynamics than your 20 lbs. bike. The aerodynamics of your body is about 3/4 of the total aerodynamics equation, while your equipment is the remaining quarter. Therefore, working with a qualified fitter to develop an optimal aero riding position that is within your body’s individual capabilities should be job one before focusing on the aerodynamics of the bike itself. The most aerodynamic equipment available will help very little if you, the rider, are not able to remain aerodynamic while using it.

An “ideal aero position” does not simply mean that you must have as low a riding position as possible. It means that you must have as aerodynamic a riding position as possible that is within your biomechanical capabilities and that you can maintain for as long as possible without compromising comfort or power. Trading power production and comfort for aerodynamics is rarely a good trade and an aerodynamic riding position is only effective when you are able to maintain it.

When it comes to the aerodynamics of your equipment, wheels and the components at the front of the bike (fork, bars, cable routing, helmet…) are the most influential. While ideally shaped aero frame tubing can make a small aerodynamic difference (estimated at 2-4% of total energy consumption), aero tubing often does not ride as comfortably and is not as torsionally stiff as high quality round tubing. If it comes down to a choice between a frame that helps you as a rider be more aerodynamic by maximizing your comfort and power in an aero position or a frame that is more aerodynamic on its own, but not as comfortable, the frame that helps you maximize how long you can stay in your aero position (comfort) is almost always going to help you ride faster.

Click here for more detailed information on aerodynamics in cycling.

Bicycle Stiffness: Approximately 15% of total energy consumption.

The stiffness of your frame and components relates to how efficiently and directly power from your body is transmitted to the road. In general, as a frame gets stiffer and more responsive side-to-side it also becomes vertically stiffer, thus transmitting more road shock to the rider. If a bike is too stiff for the rider, it can be uncomfortable and this will compromise the rider’s ability to remain aerodynamic and fresh throughout the ride. This being said, modern material technology and design has helped bridge the gap between torsional frame and component stiffness (responsiveness and power transmission) and vertical compliance (comfort and vibration damping). As the quality of a frame or component’s materials and design increases, it will be more likely to enhance torsional stiffness, while simultaneously enhancing comfort and compliance. The best frames in this regard tend to be handcrafted frames built with tubing that is individually selected to match the rider’s weight and ride quality requirements.

Click here for more detailed information on lateral stiffness and power transfer in cycling.

Rolling Resistance: Approximately 10% of total energy consumption.

Rolling resistance is the amount of friction between your tires and the road surface. In general, the more vertically compliant and vibration damp a product tends to be, the less rolling resistance it will tend to have and the less energy it will consume. Tire, wheel and frame designs impact rolling resistance the most.

Click here for more detailed information on rolling resistance in cycling.

Bicycle Weight: Approximately 10% of total energy consumption.

Like aerodynamics, bicycle weight is often an exaggerated and misunderstood concept. While you want a reasonably light bike, small weight differences between bikes are going to have negligible effects at best on most courses. Keep in mind that a 160 lbs rider on an 18 lbs bike has a gross vehicle weight of 178 lbs and the same rider on a 16 lbs bike weighs 176 lbs – not even a 1% difference.

Weight plays its largest role when it comes to acceleration, which is why it is a more influential variable when climbing than on the flats. Because rotating mass (wheels, for example) has a larger impact on momentum and acceleration that static weight (frame, for example), focus on reducing rotating weight before static weight. Regardless, never ride products that are built too light for your size and power.

Click here for more detailed information on bicycle weight.

Intangible Items:

Intangible items like frame geometry, nature (side winds, rain…) and mechanical quality are nearly impossible to quantify, but can effect your performance anywhere from 0% to 100% during a ride. For example, if the wind catches the cross section of your deep aero wheels or aero frame beyond what you can control, your riding day could end early. From the quality of the materials and design of your equipment to the mechanical condition of your bike, intangible items are either non-issues or are the entire issue. Make sure you use products that are suitable to your individual situation and choose your bike and bike shop/technician accordingly.

Click here for more information on mechanical reliablity and other intangibles that can impact performance.

Cycling is a big picture sport and (with the exception of intangibles) it is rare that a single variable is going to make or break your performance on a given day. By carefully reviewing the needs of your individual situation with a qualified technician, you will be able to find products that fit you well and will maximize your potential by addressing your individual needs best.

Have a great season!

Ian

Originally published April 2006/Copyright © 2006

About Ian

From first time riders to Olympians, Ian has helped thousands of athletes achieve their cycling and triathlon goals. Ian develops much of the Fit Werx fitting and analysis protocols and is responsible for technology training and development. He is regarded as one of the industry leaders in bicycle fitting, cycling biomechanics and bicycle geometry and design. He is dedicated to making sure the Fit Werx differences are delivered daily and provides Fit Werx with corporate direction and is responsible for uniting our staff and initiatives.

Find out more about Ian Here

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