SB20 - What's Inside (2026)?

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The Stages SB20 indoor smart bike was released in 2020. Let's take a look its electronics and drivetrain components.

Updated in February 2026 🎉





An external power supply routes power to different bike components through a series of printed circuit boards (PCBs). The bike also includes two battery powered crank-based power meters.  Key internal drivetrain components include a bottom bracket and electronic flywheel brake. 

Here's an overview of the basic components:
  1. USB Board - Wedged vertically under the phone tray. It powers 3 USB ports.
  2. Upper PCB - Fastened horizontally under the phone support rail. This System On a Chip (SoC) handles wireless communication and controls the bike itself.
  3. Switch Board - This wire distribution board sits inside the aluminum handlebar stem. It routes connections for the brake level controls  and under handlebar tape "blip" shifter buttons. 
  4. Lower PCB - This mainly analog circuit board is attached to the flywheel / electronic brake unit. It controls bike resistance and sends power to other PCBs.
  5. Power Meters - A pair of battery operated power meters are bonded to the cranks. They wirelessly send cadence and power measurement data.

  6. Bottom Bracket - Bearing cartridge that supports the cranks .
  7. Flywheel - Electronic brakes that provide pedalling resistance.

Let's take a look at each of the components in more detail.

USB Board Location



The uniquely shaped USB Board sits vertically in the phone tray stand.

A pair of USB-A ports extend from the front of the bike. Each is rated at 2.5 amps. 







USB Board Details




The USB Board is responsible for regulating incoming voltage to the 5 volts needed for USB charging. The board has a buck controller for voltage normalization, and an array of MOSFET transistors for power output.There is also short circuit and current limiting protection.

Another interesting fact about this board: It has an extra output cable with a USB-micro connector that is not attached to anything. Stages clearly had plans for additional device. I am guessing some type of gear or status display.

🔬 I abandoned the built-in USB ports five years ago. I added an external 120 watt charger to simultaneously power a pair of iPads, a wireless charger and a Garmin computer.  I removed the USB board and power input cable.  For more details see: USB Upgrade.

Upper PCB Location



The main processor board is located under the plastic rail cover that sits between the display arm and  handlebar clamp. 

Connectors are mounted on the underside for output power to the USB PCB plus incoming power and control wires from the Lower PCB.   Shift button wires from the Switch Board are read via the large connector on the right .

🔬 An unused 4 pin I/O connector is also present. It is likely for diagnostics or the once-planned display.







Upper PCB Details


The PCB Board is contains the primary SB20 processor.  This System On a Chip (SoC) handles wireless communication and bike control functions. The top of the board has a wireless antenna used for both Bluetooth LE (BLE) and ANT+. 

With the RF shield removed we see a Nordic Semiconductor nRF52832 SoC.






The nRF52832 is a single chip wireless device controller. Common applications are: remote door openers, cell phones, home automation, NFC credit card terminals and of course smart bikes.

This board is similar to Arduino and Raspberry Pi systems used by prototypers, embedded system developers and hobbyists. The primary function for the embedded software is setting bike resistance in response to commands received from apps like Stages Cycling or Zwift.  The other main job is processing  the different brake lever and "blip" button states.

ANT+ and Bluetooth LE communication is handled by software libraries shipped by Nordic Semiconductor. 

Switch Board Location and Details 









The Switch Board is a passive PCB housed in the handlebar clamp. It distributes wires from the brake lever and "blip" buttons to the Upper PCB. 

The SB20 has capacity for 4 additional buttons routed through expansion plugs in the Switch Board.







Lower PCB Location and Details 

The Lower PCB  is attached to the flywheel assembly.



24 volts from the external power supply is modulated to control the eddy current Brake Coils to create resistance.  Power is reduced to 3-5 volts and then relayed to the Upper PCB along with a handful of control wires. The Optical Sensor is there to measure flywheel rotational speed.

🔬 For more information on the braking system, see Eddy Current Brakes: Your Indoor Bike isn't as Powerful as you Think.

Connectors between Upper and Lower PCBs 

Loose connections between the upper and lower PCB are common.  This can cause maximum resistance or inconsistent erg mode performance.    

Remove the plastic cover under the handlebar support and have a look.



Power Meter Location and Details


The SB20 has a power meter bonded to each crank. It has same the nRF52832 processor that is used on the Upper PCB.  The main difference with this board: It is powered by a 3 volt CR2032 battery.

Other hardware features: 
1. Strain gauge for measuring crank deflection
2. Accelerometer for reading crank angle and determining cadence.

Main functions include measuring torque via the strain gauge plus reading cadence and crank angles.  All are used to determine power that is sent approximately once a second in Bluetooth LE and ANT+ updates.  





Bottom Bracket



Like most bikes, the SB20 has a bottom bracket that allows the cranks to spin.  It can serviced or replaced if it fails.  Learn more at SB20 - Replacing the Bottom Bracket.

Flywheel



The flywheel provides pedal resistance to make indoor riding feel realistic. It has an electronic brake stator controlled by the Lower PCB. The flywheel is spun by the Drive Cogs. The flywheel has 3 pairs of bearings that can serviced or replaced. 

Also inside is an optical sensor that detects rotational velocity. Data is fed into the Upper PCB via the Lower PCB to regulate the eddy-current brake resistance at different speeds.
  
Here are posts describing maintenance and replacement.


Axle Bearing and Cogs Removal: SB20 - Flywheel Cog and Axle Bearing Service

Servicing the Large Flywheel Bearings: SB20 - Advanced Flywheel Servicing

Replacing the Large Flywheel Bearings: SB20 - Flywheel Bearing Replacement

Making Flywheel Bearing Service Easier: SB20 - Simplifying Flywheel Bearing Service

Drive Belt Tensioning and Alignment: Stages SB20 - Drive Belt Adjustments 



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