# Power meter vs Speed and Cadence Sensor

Hello everyone,
I’d like to know how different is the mesure of a power meter against the power estimation that comes out of speed and cadence sensors?

Does products like garmin vector, which is way to expensive, worth it?

Thanks

For Wheel-on Classic Trainers:
Zwift takes the wheel speed broadcast by your speed sensor and the power curve (see example below) of the trainer you select on the Pairing Screen and converts that to virtual watts, also known as “zPower”.

Your in-game speed is determined by:
• zPower watts
• Your weight and height
• In-game bike frame and wheel set
• In-game drafting (Tri and TT bikes get no drafting boost)
• In-game road surface (Mountain bikes are fastest on dirt roads, but slowest on pavement)
• In-game virtual elevation changes.

Activating a Power Up such as the Truck (draft boost), Feather (weight reduction), or Helmet (aero boost) can further alter your in-game speed for a short duration.

This is not a real trainer curve, only to demonstrate the concept.

Example: Your wheel turns at 40 km/h then Zwift will use the curve and determine your Power as 200w. Using a set of complex equations taking your weight, height, road incline, rolling resistance and other factors in to account Zwift will calculate your virtual Speed.
So that 200w can be 45km/h on a flat road or 12km/h on a hill.

For Smart Trainers and Power Meters:
Your in-game speed is determined by:
• The power/watts broadcast by your trainer/power meter
• Your weight and height entered
• In-game bike frame and wheel set
• In-game drafting (Tri and TT bikes get no drafting boost)
• In-game road surface (Mountain bikes are fastest on dirt roads, but slowest on pavement)
• In-game virtual elevation changes

Activating a Power Up such as the Truck (draft boost), Feather (weight reduction), or Helmet (aero boost) can further alter your in-game speed for a short duration.

*****Also note that there are no stop signs, cars, or wind, and you don’t need to slow down for curves within Zwift.

This will be made a Pinned Post soon.

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Claudio - the question if a power meter (PM from here) worth it or not, highly depends on your personal targets with your training. If you simply enjoy cycling and want to pass the time while you can’t ride outside, a wheel on smart trainer will do it.

If you want to get into more serious training with quick reactive trainers because the short interval work with big power differences means something to you, than invest in a direct drive trainer (DD).

If you want to have consistent results, accountability, and investing long hours into (in most cases) race preparation, than PM is the way. For example Vector 3 / Favero Assioma / Pwer2Max NGeco / Stages Dual / etc.

So it start from your needs. I know it’s not the answer to your original question, but it is a different aspect to consider before buying a PM.

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i have a wheel on trainer, where you have to be consistent with fitting and tyre pressure to get consistent and accurate results, s with spin down tests.
I now use garmin V3s ( left side only) power pedals ( keo fitting) which need little calibration, and give consistent results. (They’re BT and ANT+) ( can monitor on garmin head unit at same time as zwift)
It also means that i can transfer the pedals to my road bike, when i’m riding that.
Not cheap but i do find much more consistent and less frustrating.

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I agree with SteveK, consider power meter pedals. Even though I’m on a Kickr Core now, I’ve been using Favero Assiomas since I had a wheel-on trainer. I just swap the pedals between bikes (takes not even 5 minutes) when I go outside and have the benefit of using same device for consistency.

For example, I’m starting to dabble outdoors again in the midst of the pandemic & I use my power meter to truly go easy as I promised myself no hard training outdoors to minimize risk of injury. Yes, you could certainly do this by “feel”, but a power meter will literally keep you in your zones.

A little pricey yes, but a worthwhile purchase if you cycle a lot and want to track your progress.