Your Indoor Bike isn't as Powerful as you Think

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My Indoor Bike has 2200 Watts. I'm good, right?

Answer: Maybe.

Indoor Bike Resistance

Let's first understand how an indoor bike creates the resistance that makes riding more realistic.

🔔 In this post, I use the term indoor bike to refer to smart bikes and trainers. My indoor bike is the Stages SB20 smart bike. I use it in most examples.

Eddy Current Brakes


Modern indoor bikes use an "eddy current brake" to produce resistance. It's a near magical device. An aluminum disk spins past one or more magnets that creates resistance thanks to eddy current waves. Resistance increases when the disk spins faster or the magnetic field strength is increased. That's somewhat counter-intuitive when you consider that aluminum is not magnetic.

Eddy current brakes provide the smooth and progressive resistance that makes indoor riding so much better than even a few years ago.




🔬 Faster spinning = more resistance.  Keep this in mind.  Later, we'll see how lower RPMs can limit indoor bike power.

Peloton and and other Indoor Bikes

Peloton bikes - and others including spin bikes like the Stages SC3 or NordicTrack S22i - use a pair of permanent magnets on either side of an aluminum flywheel disk. Resistance increases when the magnets are moved inward with a small electric motor. 



These resistance systems are inexpensive and reasonably robust. There are some limitations:
  • Resistance changes can be slow since the magnets take time to move in and out.
  • Difficult to calibrate since small changes in magnet position can adversely affect resistance.
  • Inconsistent manufacturing tolerances mean magnet positions can drift over time.
  • Cheater's dream: Unscrupulous riders can get higher power in Zwift by blocking magnet movement (sometimes with just a turn of a limiting screw).    

My Stages SB20 Smart Bike and other high-end Smart Trainers

The latest generation of smart bikes and trainers use electromagnets in place of moving magnets. 

My SB20 bike uses a radial array of electromagnet coils to create eddy current resistance with the rotating aluminum outer rim that's connected to the drive belt. Resistance increases when more current is applied to the coils. 



Electromagnets are more accurate and reliable thanks to fewer moving parts. They also allow for faster resistance changes thanks to solid state current switching. This system is also easier to calibrate since resistance is proportional to the electric current used to energize the stationary coils.

Heat

Eddy current brakes generate heat. This is one place where metal parts are preferred. High-end bikes and trainers are built with steel and aluminum parts. Budget priced devices tend to have more plastic components. Aspiring olympic sprinters take note 😀.

2200 Watts is more than Enough! Isn't it?

Now that we know how Eddy current resistance is produced, let's see how it works in practice. 

From the Stages SB20 website - Maximum Power: 2200 Watts at 130 RPM

📣 FWIW: Most high-end smart bikes and trainers are rated from 2000w to 2200w maximum power.

Hopefully, you noticed the key attribute: Maximum resistance is produced at 130 RPM. Consider what we discovered earlier: higher RPM = more resistance. The inverse is also true: lower RPM = less resistance.    


Other methods for producing electrical resistance have a linear relationship with RPM. The dashed line shows how the resistance from a DC generator declines uniformly with RPM (since generators produce power immediately as RPM increases from zero). The blue line shows how eddy current resistance falls slightly faster due its dependency on non-magnetic disk/rim motion. This behaviour continues as RPM decreases. Eddy current resistance disappears altogether at low RPMs. 

Impact on You

To get maximum resistance from an eddy current brake, you must keep your cadence high. The red lines in the graph illustrate an issue for many riders: Maximum resistance at 70 RPM is just 1100 watts. A large number of riders can hit 1100w for a second or two. Competitive Cat I-II riders can average 1200w and higher for 30 seconds. 

This means riders can unknowingly exceed bike resistance limits when lugging or standing on the pedals in a sprint. Riders often start at a high cadence (110 RPM and higher) but descend to as low as 60 RPM at the end of a sprint. 

🔔 Note: Some SB20 riders report hitting a "wall of resistance" after sprinting @1100w+ for a few seconds. Other riders report "breaking through" the resistance and having power drop rapidly. 

Both assertions are difficult to verify since riders reporting problems don't always share RPM and power graphs. Not everyone can hit 1100w, so reports are relatively scarce. More data is needed.  

📣 There are rumours that Stages has a new SB20 firmware update to address sprinting issues. Once again, more data is needed.

🌟 Tip: Be like Mark Cavendish and finish your sprints at 120+ RPM. You will likely produce more power and your indoor bike will be more cooperative.

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