Power Meter Crankset Product

Overview

A power meter crankset measures the mechanical power a cyclist delivers — in watts, in real time — by instrumenting the crank itself as a torque transducer. Power is torque multiplied by angular velocity; the crank already carries all of the rider's torque and rotates at a measurable cadence, which is why the crank and its spider became the dominant measurement site. The principle is the same as an industrial torque cell: bond Foil Strain Gauge to metal where load produces predictable elastic strain, read the resistance change, and convert. What distinguishes a cycling power meter is doing this to ±1–2 % on a component that is hammered by weather, pressure-washed, and expected to run a year on a coin cell.

Measuring torque with strain

The Strain Gauge Package is the measurement core. Foil gauges — metal grids whose resistance changes about 0.2 % at full pedaling load — are bonded with heat-cured Gauge Adhesive to the Instrumented Spider or to the crank arms, in matched pairs oriented at ±45° to the axis. That geometry is the trick: torsional shear strain adds across the pair while bending strain and thermal expansion cancel. Wired as full Wheatstone bridges through the Bridge Wiring, the gauges produce a differential signal of a few millivolts at full scale that is inherently immune to supply drift.

The structure being measured is the Crank Arms & Spider itself — the spider or arm is simultaneously the drivetrain lever and the sensing spring, so its stiffness is part of the calibration and every unit is factory-characterized with known masses to establish its individual slope in N·m per volt. Spider-based designs gauge the Instrumented Spider because all chain torque passes through it regardless of which leg produced it; arm-based designs gauge the Non-Drive Arm (single-sided, doubling left power) or both arms for true left/right balance. A Temperature Sensor feeds firmware compensation for the residual thermal drift the bridge geometry cannot cancel; the pre-ride zero-offset the rider performs handles whatever remains.

From millivolts to watts

The Electronics Pod amplifies the bridge output in a low-drift Instrumentation Amplifier, digitizes it with a 24-bit 24-bit ADC at 50–200 Hz, and pairs it with cadence from the Cadence Accelerometer — a MEMS accelerometer that watches the gravity vector rotate once per revolution, eliminating the frame-mounted magnet of earlier designs. The Microcontroller integrates torque over each revolution, multiplies by angular velocity, and hands per-revolution power to the ANT+/BLE Radio, which broadcasts the ANT+ power profile and the BLE Cycling Power Service simultaneously at 4 Hz to head units and trainers. The whole pod lives in an IPX7 Pod Housing molded onto the spider or arm.

Power for all of this comes from the Battery System: classically a CR2032 Cell CR2032 behind an O-ring-sealed Battery Door, lasting 100–400 hours because the radio duty cycle, not the measurement, dominates the budget. A micropower Voltage Regulator holds the bridge excitation steady as the cell sags from 3.0 V toward 2.0 V; rechargeable variants swap the door for magnetic Charge Contacts so the seal is never broken.

Mechanical platform

Below the instrumentation, the product is a conventional high-end crankset. The Chainring Set bolts 50–53T and 34–39T rings to the instrumented spider with Chainring Bolts at about 12 N·m; because measurement happens upstream of the rings, spider-based meters tolerate ring swaps without recalibration. The Spindle & BB Interface passes a hollow 24–30 mm Spindle through the frame's Bottom Bracket, preloaded by the Preload Collar and clamped by the Crank Fixing Bolt at 35–50 N·m. One subtlety hides here: bottom bracket drag is downstream of a crank meter but upstream of a rear-hub meter, which is why crank-based units consistently read 1–2 % higher than hub-based ones on the same ride — both are correct about what they measure.

Accuracy in practice

The error budget is dominated by three terms. Slope stability depends on the Gauge Adhesive bond — a creeping epoxy joint is the canonical long-term drift mechanism. Thermal transients matter on fast descents, where the metal cools faster than the compensation can track; riders are told to re-zero after large temperature swings. And single-sided meters carry a systematic error equal to the rider's actual leg imbalance, commonly 2–6 %, which no calibration can remove. Within those limits, a maintained crank meter holds ±1–2 % — tight enough that training zones, FTP tests, and race pacing are all built on its numbers.

The category's significance is larger than the hardware: the power meter converted cycling training from speed- and heart-rate-based estimation into direct measurement of work, and the crank-based strain gauge design — first commercialized by SRM in the late 1980s — is still the reference architecture every newer approach (pedal spindles, hub torsion, optical deflection) is validated against.