Stages SB20 - What's Inside?

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The Stages SB20 indoor smart bike was released in 2020. Let's take a look at the electronics that enable it  on platforms like Zwift.com and TrainerDay.com.




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.

Here's an overview the basic functions:
  1. USB Board - Wedged vertically under the phone tray. It powers 3 USB ports.
  2. Upper Board - 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 Board - 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.. 

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 Upper PCB 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 andditional device. I am guessing some type of gear or status display.

🔬 I abandoned the built-in USB ports two 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 Board 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 board plus incoming power and control wires from the Lower Board.   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 could be used for diagnostics or the once-planned display.







Upper Board Details


The Upper 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 Board Location and Details 

The Lower Board  is attached to the flywheel assembly.


24 volts from the external power supply is modulated to control the eddy current brake.  Power is also relayed to the Upper Board along with a handful of control wires.

🔬 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 Boards 

Loose connections between the upper and lower boards 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 Board.  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.  






Summary

That's it!  The SB20 uses relatively simple technology to create a good indoor riding experience.  If I have any concerns, it's age. This design was competitive 4 years ago.  In 2023, there are a few limitations:

1. Data Update Frequency - The SB20 sends data updates once a second.  New trainers - including the $500 Zwift Hub One  - now send and receive telemetry 10 times a second.

2. Battery Life -  Useable Power meter battery life is around 100 hours (see Batteries and SB20 Power Meters ). Competitors, like the Gamin Rally pedals, last 3 times longer.

3. More USB Power - 2.5 watts per USB-A port really doesn't cut it these days. 50 watts per USB-C port would be nice.

4. No built-in display - Not being able to see status and error messages is the source of many complaints on support forums. No virtual gear display is a significant limitation for many riders. Hardware on the USB and Upper Boards suggests that a built-in display was planned. Too bad it was never delivered.

And oh yeah: an Off/On switch would be appreciated.

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