SB20 - Bike Resistance Issues

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SB20 owners occasionally report that their bike is producing resistance that is too high or low.


Here's how to verify that your bike's resistance is correct.

What is resistance and what causes it to change?

Resistance is the pedal tension that you push against to generate power. The faster you pedal against a given amount of resistance, the more power you produce. 


Note: Erg mode behaviour is different. Here's a description if you are interested: What is Erg Mode?  

This discussion focuses on free ride/race/sim mode resistance.

What about Power Meters?

Power meters measure how hard you are pushing against the bike's resistance. Pedalling faster against this resistance, increases power (watts).  Pedalling slower, decreases power.

Power meters do NOT affect resistance. They only reflect increased resistance if you choose to pedal harder against it 💪.

Example: Zwift Resistance

Here's how resistance is set in a Zwift free ride or race:

  • Zwift sends a command to the SB20 telling it to resist by the appropriate amount for a given road surface, rider height + weight, wind + speed and road incline (slope).
  • Smart bikes and trainers with virtual gears add a further calculation to increase or decrease resistance based on the virtual gear you have selected. 
  • Increasing slope, raises resistance. Decreasing slope, lowers resistance. Changing gears, wind or road surface similarly affects resistance.  Zwift decides when to vary any of these parameters.


Other apps, like IndieVelo, Rouvy and FulGaz, behave similarly.  

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Bike Resistance Test Procedure 

The Stages Cycling app will be configured to do a set of resistance tests.  

Pre-test Setup

To do the test, you need to configure the SB20 for 25 Dream Drive gears.  


Summary:

  • Before changing the configuration, record your current gear configuration so you can return to it later.
  • Ensure Gradiant scale factor is set to 100%
  • Change Gear setup to Dream drive with Total number of gears set to 25 gears.
👍 I avoid having to switch between configurations by using the Stage Cycling app on my iPhone for these tests. The iPad that I normally use holds my preferred configuration. The SB20 automatically inherits its configuration from the last Stage Cycling app that you run. As such, I restore my normal SB20 configuration (Dream Drive 25 with SRAM button layout) by simply running the Stage Cycling app on my iPad once, after these tests are completed.  


Put Stages Cycling into Sim/Slope Mode



After connecting to the SB20, click the Run icon followed by Select Stages Bike Control mode.

Click Slope / Sim mode.  Ensure Slope is 0%.

You are now ready to start testing.

Run Resistance Test 

Start pedalling at 80 RPM.


Select gears 8, 10 & 12.  You should see power numbers should in the range shown.

Gear 08 @ 80 RPM = 114w to 118
Gear 10 @ 80 RPM = 184w to 188
Gear 12 @ 80 RPM = 263w to 267

🔬 These test are more precise when your power meters are accurate.  Power meters are verified with a torque test (see: Measuring Torque Against Factory Specifications).  While not essential, you could consider doing a torque test first.


Post-test Configuration

After completing the tests, put the Stages Cycling app back into External Control mode and optionally restore your previous gear configuration.


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FAQ

My resistance is off by 10%.  Does it matter?

Not really. Let's keep in mind there is only one way for you to go faster in Zwift: produce more watts. There are only two ways to produce more watts: pedal faster or shift to a larger gear.  

If your SB20 produces 10% less resistance in a given scenario, you will produce 10% less power at the same cadence as me. If you simply shift to a 10% bigger gear - and thereby increase resistance by 10% - we then produce the same power. 

Think of it like this: with 10% less resistance you might have an equivalent of a 11-36t rear cog where I could have a 10-32t rear cog. Which cassette goes up an 8% incline faster?  The answer is: Whoever is pushing hardest in the right gear.  You might be in the equivalent of the 4th largest cog. I could be in the 3rd largest cog. In different gears, we could very well be pushing against exactly the same resistance and be producing identical power at the same cadence.

Which is better: 10% low resistance or something else?

It's a tradeoff. With the equivalent of a 10-32t rear cog, I might run out of gears on the steepest climbs.  You have 10% more climbing gears.  This is where the Zwift Trainer Difficulty and Stages Cycling Gradient Scale sliders are used.  Move to the left and you get more gears for climbing, with wider overall gearing. Move the slider to the right and you get the equivalent of fewer teeth on the biggest cogs, with tighter overall gearing.

Does weight matter?

Yes but resistance isn't part of the equation. If you are producing the same watts (at any resistance level) but you weight less, then you will go up hills faster. 

Does this test require accurate power meter?

The Bluetooth/ANT+ spec that controls the bike does not support directly setting or retrieving "resistance level". So yes, we need to estimate resistance. The torque produced by a rider is measured by the PMs and is multiplied by cadence to get power. The test is not perfect. It uses the power calculated by the PMs as a proxy for the resistance level. If the PMs are inaccurate, this test will be inaccurate.

Aside: If you own any other trainer or bike on the market, you cannot verify resistance. Period. These devices don't have power meters. They calculate power based on what "they think" the resistance is (exactly what you are complaining about). Plenty of trainers have faulty resistance mechanisms (especially as they age) and give equally incorrect power readings. If you want to cheat, get one of these trainers. You can't get caught.

Summary: 10% resistance error does not matter so long as you have power meters to read actual power. You can just change gears to compensate. 10% power meter inaccuracy is a big deal. You should do a torque test if you suspect your PMs are faulty: Measuring Torque Against Factory Specifications

Comments

  1. OlmoOctober 25, 2024 at 8:54 PM

    my test gives 85 watts at 80 rpm.... however I tested my power sensors with a satisfactory result with a weight of 25 kg. Do you have any idea what is wrong?

    ReplyDelete

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