How to measure bike lean angle
You can estimate bike lean angle with a phone, a dedicated inertial sensor, video or speed and corner radius. These methods measure different things. A useful result starts with knowing what the angle refers to and how the measurement behaves while moving.
Here, bike lean means the frame's sideways tilt from vertical. It is different from road camber, steering angle and the rider's body angle. A number shown to one decimal place is a display resolution, not proof of accuracy. Comparing methods requires the same reference and the same moment in the turn.
A phone in your pocket or on the handlebars
A phone is convenient for an initial experiment. Depending on its hardware and app, it can record accelerometer readings, gyroscope readings or an orientation estimate. Android's motion-sensor documentation distinguishes these outputs; an app labelled “tilt” does not tell you which it uses.
In a pocket, the phone follows your body and can move inside the fabric. That cannot directly establish frame lean. A rigid handlebar mount gives a more repeatable relationship, but steering changes its orientation relative to the frame. Mount alignment, vibration and the app's recording behaviour still matter. An accurate tilt reading while stationary does not establish accuracy through a corner.
A dedicated IMU such as the BNO055
An inertial measurement unit, or IMU, combines motion sensors. The Bosch BNO055 includes an accelerometer, gyroscope, magnetometer and onboard sensor fusion. Fusion combines sensor information into an orientation estimate rather than treating one acceleration axis as an angle.
A dedicated logger lets you control mounting, timestamps and what gets recorded. Its disadvantages are extra hardware and responsibility for calibration and interpretation. Sensor axes must be related to the bicycle's axes; a convenient “roll” output is not automatically frame lean. Neither the chip's resolution nor a calibration indicator specifies the accuracy of the complete system on a moving bicycle.
Camera and video analysis
Video can provide an independent view of the frame and rider. For a simple measurement, use a fixed camera looking approximately along the bicycle's direction of travel, with a known vertical reference. Compare the visible frame plane with that reference at the relevant frame.
This is a geometric estimate: an oblique viewpoint changes the projected angle. Lens distortion, camera tilt, motion blur and obscured wheels add uncertainty. A moving action camera needs its own orientation reference, and image stabilisation can change the apparent horizon. Tracker's coordinate-system guide explains setting the image reference axes. Video is useful for selected moments, but careful geometry and synchronisation are essential.
Calculating lean from GPS speed and radius
For a steady turn at constant speed on level ground, an ideal balance model gives:
θ = atan(v² / (g · r))
Use speed v in metres per second, turn radius r in metres and g = 9.81 m/s². At 30 km/h and 15 metres, the result is about 25.3°. The lean angle calculator lets you explore the relationship. OpenStax explains the underlying centripetal-force balance.
This estimates the tilt of the line from the tyre contact region to the combined bike-and-rider centre of mass, not necessarily the frame angle. Rider movement and changing corner conditions limit the comparison. Radius estimated from sparse or noisy GPS points can be particularly uncertain; GPS.gov describes reception errors near buildings and trees. GPS alone does not observe frame orientation.
What about cycling computers?
A cycling computer can bring ride data together, but its screen is only where the number is displayed. For any lean-angle field, check where the number comes from: an internal sensor, an external sensor or a calculation. Also check whether it saves the data for later analysis. GPS, gradient and heading fields describe different quantities. A product's sensor list alone cannot establish a usable lean-angle measurement.
Comparing the methods
No universal accuracy figure applies to these categories. Each result depends on the implementation and conditions.
| Method | Main advantage | Accuracy constraint |
|---|---|---|
| Pocket phone | Easy to carry | Body and pocket movement obscure frame lean |
| Mounted phone | Accessible recording | Mounting, steering and app processing |
| Fixed IMU | Repeatable sensor position | Alignment and dynamic estimation errors |
| Video | Independent visual reference | Perspective and timing |
| GPS calculation | Uses speed and route data | Radius uncertainty and model assumptions |
| Cycling computer | Convenient ride interface | Depends on the underlying data source |
Why readings can be wrong
Acceleration is not just gravity. In a turn, centripetal acceleration affects the accelerometer signal. A gravity-only tilt calculation can therefore report a wrong angle even through a smooth, steady corner. Filtering vibration does not remove this sustained effect. Analog Devices explains this limitation in AN-1057.
Gyroscope estimates can drift. A gyroscope measures rotation rate; integrating a small bias accumulates angle error. Fusion can help, but its other references also have limitations. Android's gyroscope guide describes noise and drift compensation.
Mounting and calibration are separate jobs. Sensor calibration addresses sensor errors. Alignment establishes how the sensor sits on the bike. A shifted mount invalidates that relationship; road slope complicates a simple upright zero. Vibration can disturb readings, while aggressive smoothing can delay a turn's apparent peak. Where magnetic measurements are used, nearby magnetic interference adds another concern.
Why Krengo uses a fixed IMU
Krengo's approach is a dedicated BNO055-based logger with a fixed relationship to the bicycle. That makes the mounting relationship repeatable and keeps it independent of a phone moving in a pocket. The purpose is to record data that can be examined after a ride.
This design choice is a starting point for validation. A useful check compares known stationary angles first, then time-aligned measurements against an independent reference during motion. Document the mount and inspect repeated passes, not just the highest number. Read the DIY logger guide for the architecture and the guide to road-bike lean for the physics. An angle log does not measure remaining tyre grip.