All Indoor Bikes have Different Resistance. Does it Matter?

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Two bikes with Different Resistance

Consider these common scenarios. You have:

  1. Bought a new indoor smart bike / trainer.  
  2. Two bikes / trainers of the same brand and model.
  3. Moved a trainer from one bike to another

Something is wrong. The same physical or virtual gear does not feel the same as your other bike.

If you take a single set of power meter pedals and measure the power produced by each bike at a given cadence for the same virtual gear, you can easily confirm different power readings.

Why is Resistance Different?

Imagine for a moment, all the indoor trainers and bikes available on the market. When doing a race or free ride, each bike/trainer is controlled by an app like Zwift in the same way. Zwift will say to the bike: "Produce the appropriate resistance to simulate a 70 kg rider going up a 5% incline on an asphalt road surface" where 70 kg is the rider weight set in Zwift, and incline and road conditions are determined by Zwift according to the riding conditions it wants to simulate.

You will note, that Zwift is not asking for a specific resistance level. It is asking the indoor bike to do an internal calculation and set its resistance accordingly. Every indoor bike/trainer on the planet will calculate a slightly different resistance value for the exact same request from Zwift.

Why Resistance Doesn't Matter

Let's say Bike A decides to resist at 50 Newton Meters (the measure of twisting force resistance or torque) and Bike B decides to resist at 60 Nm (about 20% more difficult). Does that means that Bike A will go faster? No. 

Power = Speed in Zwift

More power means more speed in Zwift.  It is roughly calculated as:
    Power = Resistance x Cadence 

To move at the same speed, the rider on Bike A is pushing against 20% less force. They must pedal 20% faster compared to Bike B. Does this mean one rider has an advantage? Not really. If Rider A wants to pedal slower and go just as fast, they simply need to pick a bigger gear.  Similarly, Rider B can pick a smaller gear and pedal faster to continue at the same speed.

The Good News

When asked by the Zwift app to produce the same amount of resistance, one bike bike will always resist more than the other. You can either change cadence or gears to get your match your power to the other bike.  

The Bad News (and it can be really bad if you are competitive)

If you are using a trainer or any bike without the power meter, this resistance issue has big implications in a competitive scenario. Power is calculated based on the amount of resistance the bike thinks it is creating.  When the bike/trainer is new, it is assumed to match factory tolerances. The problem is: resistance drifts as the bike gets older. Electronic brake efficiency and accuracy will change a parts move in relation to each other.  Power supply voltage and current can also vary. For more details, see Your Indoor Bike isn't as Powerful as you Think.  

These issues mean calculated power based on expected resistance will also drift. Some bikes will  measure low. Others will be high. Unscrupulous riders can also adjust things like magnet position to make the trainer think it is resisting more than it is.

The best solution today is to use pedal or crank-based power meters.  These devices measure your effort against whatever resistance is produced by your bike.

But I Calibrate My Trainer Regularly

Sorry. This simply resets the resistance inputs to the original factory level expectation. Trainer accuracy is reduced by so-called "spin down calibration" when the actual bike resistance drifts.  

Why Doesn't Zwift Help Us?

Zwift doesn't want trainer accuracy to be widely understood. Forcing everyone to use power meters is bad for business. 

You can make your setup more accurate by understanding how indoor bike resistance works. The people you race against will have bikes and trainers that produce widely inaccurate power estimates as they age. You will win some and likely loose several more.

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