Bontrager DuoTrap S: Understanding Speed, Cadence, and Power Measurement

Does DuoTrap S Measure Power?

No, the Bontrager DuoTrap S sensor does not measure power. It is designed to measure speed and cadence, providing essential data for cyclists but lacking the necessary components to calculate power output.

What is DuoTrap S?

The Bontrager DuoTrap S is an integrated, frame-mounted sensor specifically designed for compatible Trek bicycles. It neatly fits into the chainstay, offering a clean, aerodynamic solution for tracking key cycling metrics. Unlike external sensors that strap onto the frame, DuoTrap S is discreet and protected from the elements, communicating wirelessly via ANT+ and Bluetooth Smart protocols to your cycling computer, smartphone, or other compatible devices. Its primary function is to replace traditional wired or external magnet-based sensors for speed and cadence.

Understanding Cycling Power

In cycling, power is defined as the rate at which work is done, typically measured in watts. It represents the actual output of the rider, reflecting the force applied to the pedals and the speed at which those forces are generated. Unlike speed, distance, or heart rate, power is an objective and immediate measure of effort, uninfluenced by external factors like wind, gradient, or drafting. For serious cyclists, triathletes, and trainers, power data is invaluable for:

• Precise Training: Structuring workouts based on specific power zones.
• Pacing Strategies: Maintaining consistent effort during races or long rides.
• Performance Tracking: Quantifying improvements over time.
• Efficiency Analysis: Understanding how different positions or equipment affect output.

How Power is Measured in Cycling

To measure power, a device must determine two key variables: force and velocity. In cycling, this typically involves measuring the torque (rotational force) applied to a component of the drivetrain and the angular velocity (rotational speed) of that component. Modern power meters achieve this through various technologies, most commonly using:

• Strain gauges: These tiny electrical resistors change their resistance when stretched or compressed, allowing the device to detect minute deformations caused by applied force.
• Accelerometers/gyroscopes: Used in some pedal-based systems to determine angular velocity and sometimes force vectors.

These measurements are then processed by the power meter's internal electronics to calculate power (Power = Torque x Angular Velocity).

DuoTrap S Capabilities: Speed and Cadence

The DuoTrap S sensor is engineered to provide two fundamental cycling metrics:

• Speed Measurement: The sensor works in conjunction with a small magnet attached to the rear wheel's spoke. As the wheel rotates, the magnet passes the sensor, triggering a signal that allows the device to count wheel revolutions. Knowing the wheel's circumference, the sensor can accurately calculate the bicycle's speed and total distance traveled.
• Cadence Measurement: Similarly, a small magnet attached to the crank arm passes a different part of the DuoTrap S sensor with each pedal revolution. This allows the sensor to count revolutions per minute (RPM), providing an accurate measure of pedaling cadence.

These two metrics are crucial for understanding ride dynamics and training, but they are distinct from power.

Why DuoTrap S Doesn't Measure Power

The fundamental reason DuoTrap S does not measure power is its design and the type of sensors it incorporates.

• Lack of Force Sensors: DuoTrap S does not contain strain gauges or any other technology capable of detecting the force or torque applied by the rider to the drivetrain. Its sensors are simple reed switches or hall effect sensors designed to detect the presence of a magnet, allowing it to count revolutions.
• Lack of Torque Calculation: Without the ability to measure force or torque, the sensor cannot perform the necessary calculations (Force x Velocity or Torque x Angular Velocity) to derive power in watts. It simply provides counts of rotations for speed and cadence.

While cadence is a component of the power equation, it's only one half. The other, equally critical half—force—is entirely absent from DuoTrap S's capabilities.

How to Add Power Measurement to Your Bike

If you're looking to add power measurement to your cycling data, you'll need a dedicated power meter. These devices come in various forms, each with its own advantages and installation considerations:

• Crank-Based Power Meters: Integrated into one or both crank arms (e.g., Stages, 4iiii). They measure the force applied to the crank. Single-sided models estimate total power from one leg, while dual-sided models measure both legs independently.
• Pedal-Based Power Meters: Built directly into the pedals (e.g., Garmin Vector, Favero Assioma). These are often easy to install and transfer between bikes, measuring force directly at the point of application.
• Hub-Based Power Meters: Integrated into the rear wheel hub (e.g., PowerTap). They measure power at the point where it's transferred to the wheel, offering a robust and often precise solution.
• Spider-Based Power Meters: Located in the crankset's spider (the part that connects the crank arms to the chainrings) (e.g., Quarq, SRM). These are known for high accuracy and measure total power before it reaches the chainrings.

All these power meters transmit data wirelessly via ANT+ and/or Bluetooth, making them compatible with most modern cycling computers and head units, including those that can also receive data from your DuoTrap S.

Integrating DuoTrap S with Power Meters

While DuoTrap S doesn't measure power, it perfectly complements a dedicated power meter. Many cyclists use a setup where:

• DuoTrap S provides reliable speed and cadence data.
• A separate power meter provides accurate power output in watts.
• A cycling computer (e.g., Garmin Edge, Wahoo ELEMNT, Bontrager RideTime) receives and displays data from both devices simultaneously, offering a complete picture of your ride performance.

This integrated approach ensures you have all the key metrics—speed, distance, cadence, and power—without redundancy or interference.

Conclusion: The Role of DuoTrap S in Your Data Ecosystem

The Bontrager DuoTrap S is an excellent, discreet, and reliable sensor for measuring speed and cadence on compatible Trek bicycles. It efficiently provides two fundamental data points essential for any cyclist. However, it is crucial to understand its limitations: it is not a power meter and cannot provide data on your power output in watts. For power measurement, a separate, dedicated power meter is required. When combined, DuoTrap S and a power meter form a comprehensive data ecosystem, empowering you with the insights needed to optimize your training and track your progress effectively.