Stages SB20 - Measuring Torque Against Factory Specifications

Torque Measurement

I previously documented how to compare SB20 power meters with a set of calibrated pedals. It gives you a good indication of how your power meters compare in actual riding scenarios.  

This test statically measures torque with a pair of fixed weights. It tells you if your power meters meet factory specifications.

This test does not provide the full calibration that you can perform with Garmin or Favero pedals. You can't change offset or slope parameters to get the Stages PM measure correctly in the future.

Basic Testing Procedure

The basic method involves two (big) steps:

1. Position the cranks horizontally to get raw measurements produced by hanging 25 lb weights from each crank.

2. Use a spreadsheet to process raw measurements and get an average reading for both cranks. Compare these readings against expected values (the list of expected values is shown later in this post).

The test setup looks like this:

With equal weights on each side, cranks can be easily rotated to measure force at each pedal spindle.  

Why 25 lb (11.24 kg) weights?  Bike power meters commonly measure the equivalent of hundreds of pounds of force. It is impractical to test with weights that heavy. 100 lb weights are ideal. Unfortunately, they are big and don't allow cranks to fully rotate. Conversely, weights lower than 25 lb may not fully stress the power meters. 25 lb is a good compromise. 

Some Assembly Required

To support the weights, you will need pedal spindles from an old (or new if you prefer) set of pedals.  Here's one of my disassembled 1985 SPD road pedals.  

I added tape to the spindles to help hold the weights. Zip ties help position the weight properly near the end of the spindle.

Use the spindles to replace your existing pedals.  

Don't forget, the left pedal has reverse threads. 

Here's how to remember: One Crank has Reversed Threads - Which one? 

Using the StagesPower app

Tests are initiated with the StagesPower app (available in iOS and Android app stores). 

Zero Reset

Before starting, do a zero reset.

Summary

1. Rotate cranks to wake them up. Set to vertical position. Open the StagesPower app. 

2. The main page is shown. Press the Connect with Bluetooth button.

3. The Connect page is shown. Select both cranks. Click Connect.

4. The Power/RPM page is shown. Wait for both cranks to be displayed as connected.

5. The Zero Reset page is shown. Press the Perform Zero Reset button. ADC numbers are updated.

Zero reset is now complete.

If you are interested, here's what zero reset does (it's not much): Power Meters - Zero Reset.

Weight Positioning

Weights should now be hung from both spindles. The center of each weight must be positioned 55m from the edge of the crank face. This alignment is critical.

Test Left Crank

Both cranks need to be tested.  Start with the left crank. 

Summary

1. Rotate cranks. Position the left crank horizontally in the forward position.

    a. You can optionally use a level to ensure the cranks are horizontal.

2. Return to the StagesPower Connect page. Select the left crank. Click Connect.

3. The Power/RPM page is shown. Wait for the left crank to be displayed as connected. Press and hold the blank area below the Tools button.

4. The Torque / Force page is shown. Press the Start button. 

5. Testing starts. The Torque and Force fields are repeatedly updated. Allow this test to run for 20 to 60 seconds and then press the Stop button.

6. Press the Email button. You can then send the .csv file created during the test to your email address. 

The left crank has now been tested.

Test Right Crank

Repeat steps 1 to 6 above for the right crank.

When complete, you will have .csv files for the left and right cranks sent to your email address. Each file will contains a (long) list of raw data samples for kilogram force (kgf) and torque (Nm).

Open .csv files in a Spreadsheet

Open both .csv files in the spreadsheet of your choice. Excel and Google sheets are good. I used Apple Numbers for this example.

Modifications needed:

1. Rename columns to Left kgf, Left Nm, Right kgf, Right Nm.

2. Create average calculations for each of these columns

3. Create overall averages.

Here's a copy of the above spreadsheet, complete with formulas: SB20 Power Measurements.csv

Expected Torque Values

The values for Torque vary depending on your crank length. Here's the related formula:

Torque (Nm) = Mass (kg) x gravity (9.81) x crank length (meters). 

For this test, the only variable is crank length:

Torque (Nm) = 11.34  * 9.81 * crank length

 Here are the expected torque values for each crank length: 

	Crank Length (mm)
	165	170	172.5	175
Torque (Nm)	18.5	18.9	19.2	19.5

Measurement errors are easily induced. Any value within -/+ 2% should be considered normal. 

Making Minor Changes

If your tests do not match the expected values shown, you can make minor changes by modifying crank length.

 Using the Stages Cycling app:

• You can raise power readings by increasing configured crank length. For example: if your actual crank length is 172.5 mm, you can try setting 175.0 mm. 
• You can lower power readings by decreasing configured crank length. For example: if the your actual crank length is 172.5 mm, you can try setting 170.0 mm or lower.

Depending on the crank length you physically use, most people will be able to adjust their Stages PM readings by about -/+ 5% using this method.

You must then re-run all tests to ensure you are getting accurate readings versus the expected values.  Repeat with different crank length settings until your readings match.

Can Stages Help?

Yes. Stages can do something here. 

A few SB20 owners have reported that the StagesPower app can indeed apply offsets and slope changes to power readings using an undocumented procedure. Until these steps are known, end-user adjustments are restricted to the minor crank length changes described above.

Reaching out to Stages is your best option if you are convinced your power meters are inaccurate.