How Crank Length Setting Affects Power Pedal Accuracy

In recent weeks, thanks to procuring a new (used) gravel bike while also testing a new smart trainer, I’ve spent more time with power meters, smart trainers, and dual recordings than usual. This includes swapping my Favero Assioma Duo power meter pedals (read my review here) between bikes with two different crank lengths, while also testing the accuracy of the smart trainer (a Wahoo KICKR CORE 2) in various Zwift activities.

In doing this, I realized I’ve never written a post about how crank length setting affects power readings on popular pedals like the Garmin Vector 3 or Favero’s various offerings. And while this may not be a topic many care about – most will just set the crank length to whatever matches their setup – I believe the info below will help explain why high-end Zwift races require riders to use certain smart trainers as their primary power source, vs a power meter.

Let’s dive in.

Test Setup

While simple math could deliver theoretical results, I wanted to prove the math using real equipment. So I created a simple Zwift workout which included three sets of the same three ascending 2-minute intervals.

The idea was to use a different crank length setting for each of the three sets and record the pedal power data to my head unit. Zwift would be recording my smart trainer’s power data, which should remain consistent across the 3 sets. Then I could compare the smart trainer’s data to the power pedal data to learn how much the crank length setting actually modified the power readings on the pedals.

Simple enough, right? Let’s look at the test results…

Test Results

You can see the full dataset on DC Rainmaker’s Analyzer tool, but I’ve summarized it below. I rode the first set of intervals with the crank length set too short, the second set with it too long, and the third set with it just right.

(Note that for each 2-minute interval, I took a 60-second sample of the data from the middle of the interval to arrive at my average power values below. This reduces data funkiness at the start of the interval as ERG targets change and trainer resistance adjusts.)

Set 1 (Crank Length: 167.5mm)

Set 2 (Crank Length: 177.5mm)

Set 3 (Crank Length Set Accurately to 172.5mm)

Takeaways

In simple terms, power is calculated by pedals like the Favero Assioma Duo as torque * angular speed (cadence). Torque is force * crank length.

Based on the math, changing crank length from 167.5 to 177.5 should increase power readings by 177.5/167.5, or +5.97%. And in fact, that’s what my test actually showed. The differences in power readings between the 167.5mm and 177.5mm crank length settings were:

• @160W Target: 5.87%
• @210W Target: 5.72%
• @260W Target: 5.96%

(These particular test numbers line up with the math quite nicely, although comparing other data like 172.5 to 177.5 doesn’t yield quite as clean a result. That’s sort of the nature of power testing, though. Between variations in power meter accuracy, pedalling style, drivetrain efficiency, and which data is actually being read and recorded by each recording device, you’ll easily see a variation of 1% from what the math says your results should be. And that’s OK.)

You may be wondering: in set 3, if the crank length is set accurately, why are the readings still higher on the pedals? I’d chalk this up to drivetrain loss, which most experts in the cycling world say runs between 2-3%. Every step of the way between your pedals and your hub, a bit of power is lost. Cranks, spiders, chainrings, chain, cassette… they can flex a bit, get worn, or be poorly lubricated. All those losses typically add up to a 2-3% reduction in power from what you’re putting into the pedals to what is actually delivered to your rear hub.

Power Meters and Zwift Racing

If power at the smart trainer is typically 2-3% lower than at your pedals, wouldn’t you want to use your pedals as your power source in Zwift? Well, yes, if no other factors were taken into account. Because we all want to put out more power, right? (Even if it’s unrealistic on some level, since outdoors, it’s the power at your rear hub that is driving your bike forward.)

There are other factors, though. Sticky watts is one factor that makes using a power meter on Zwift less desirable, unless you’re trying to cheat.

But I would submit that the biggest factor is that it’s very easy to change your power meter’s settings so it overreads power. While most modern mid and high-budget trainers are auto-calibrating, you can easily manipulate power data from a power meter.

Because of this, it’s become pretty standard for high-end indoor bike races to require riders to pair their smart trainer as the primary power source, even though they also require dual recording with a power meter.

Interestingly, I recently learned that Zwift Racing League leans this way as well, while stopping short of a hard requirement. From their rulebook:

6.1.3. Where a Smart Trainer is used, both its Power Meter and Resistance MUST be connected directly to the game as primary source. Participants using other forms of power meters when they have a smart trainer available may be penalised.

While Zwift allows event organizers to enable “hardware enforcement” that requires a power meter or smart trainer to be paired as the power source (no classic trainers/ZPower allowed), perhaps it’s time for Zwift to enable an even stricter setting, where only auto-calibrating smart trainers are allowed as a power source?

Your Thoughts

I hope you found this post interesting. Got questions? Thoughts on power meters or accuracy as they relate to Zwift racing? Share below!