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Riding with a power meter on your bike

Installing and using the 4iiii Precision Powermeter

Aug. 28, 2019 -

After I created a minimal road bike, I decided to install a power meter to find out what's it like to measure the power output of my legs. What can you learn from a power meter? Is it difficult to install? Is it worth it? Read along to find out.

Power Meters

Most power meters use strain gauges that deflect slightly when force is applied. By measuring this torque and combining it with angular velocity, power (measured in Watts) can be calculated. In addition to speed, cadence and heart rate sensors, power meters are a perfect tool to quantify your (training) performance.

There are different types of power meters available, differing in accuracy, ease of installation, maintenance requirements and bike portability. Power meter types:

4iiii Precision Powermeter - used by the professional cycling team Quick-Step / Bora Hansgrohe
4iiii Precision Powermeter - used by the professional cycling team Quick-Step / Bora Hansgrohe

4iiii Precision Power meter

A very popular pedal power meter is the 4iiii precision powermeter. Like the popular Stages power meters, the 4iiii power meter replaces your existing crank. Selecting the right power meter model is as easy as looking at your existing crank, you'll find the model and crank arm length near the pedal bolt

The crank arm has it's model and length written on it, on my bike that's: FC-5800, 172.5MM
The crank arm has it's model and length written on it, on my bike that's: FC-5800, 172.5MM

Installing the power meter

The 4iiii Precision Powermeter comes in a small box with a clear 5-step instruction printed on it.

Quick start guide installing and using the power meter
Quick start guide installing and using the power meter

Use a crank tool to remove the exising crank from your bike. I have a Shimano groupset and used the TL-FC16/FC-M960 crank tool. It's a (cheap) piece of plastic that allows you to remove the bearing cap from the crank. Use an alley key to loosen the two bolts, there is a small plate between the crank ends that you should popout to remove the crank.

Use a crank tool to remove the crank (Shimano TL-FC16 / FC-M960)
Use a crank tool to remove the crank (Shimano TL-FC16 / FC-M960)
You should be able to loosen the plastic cap with just your fingers/hand, no extreme torque is required
You should be able to loosen the plastic cap with just your fingers/hand, no extreme torque is required

When installing the power meter crank, you should pay attention to the instruction indicating the right amount of torque. Tighten the bolts evenly and equally using a torque wrench.

Tightening the crank arm using a torque wrench
Tightening the crank arm using a torque wrench
Make sure the battery is installed and the covering door is locked
Make sure the battery is installed and the covering door is locked
4iii Precision Power meter installed on my bike
4iii Precision Power meter installed on my bike

To use the power meter you should pair it with a compatible bike computer or your smartphone. I use an app as bike computer and therefore pair the power meter to my phone. Download the 4iiii app and follow the instructions to start a Bluetooth scan.

Bluetooth scan from the 4iiii app
Bluetooth scan from the 4iiii app

Once you have paired with the power meter, you should check for firmware updates. I had to install a few that improve the power meter's battery life, reliablility and accuracy. This can take a few minutes to complete.

Zero offset calibration is very important for accurate measurements
Zero offset calibration is very important for accurate measurements

Once you have connected and updated the power meter, you should calibrate it. This will allow it to detect its 'zero value' by performing a measurement in vertical position. The app will guide you through this, calibration will take a few moments. Once calibrated you can use the power meter in any of your favourite apps, including Strava and Cyclemeter.

Connecting the 4iiii Precision Powermeter to the Cyclemeter app
Connecting the 4iiii Precision Powermeter to the Cyclemeter app

Riding with a power meter

Using a power meter you can see how much energy your legs are applying to the pedal(s). It's a direct quantification of your efforts on the bike. It will take some time for you to get a feeling with the Watts measured by the power meter.

Wahoo RFLKT+ showing realtime data from my bike computer app - obviously I was standing still here... all the zero's (except my heart rate)
Wahoo RFLKT+ showing realtime data from my bike computer app - obviously I was standing still here... all the zero's (except my heart rate)

I use a Wahoo RFLKT+ in combination with the Cyclemeter app. It shows me the realtime power output, measured from my crank arm. This realtime feedback is interesting while riding your bike as you'll be able to see the (huge) differences in Watts measured.

Situations like going through a corner with the angle of the wind changing, can clearly have significant effects on the measured power output (when you maintain a certain speed / cadence). Or, when you change your position on the bike, like holding the handle bar from the top or bottom. Or, how much harder it actually is when you go faster (hint: that's not a linear relation!)

Compare the power meter to the heart rate sensor and you'll note that it's much more direct. It goes up and down instantly as you change the intensity of your pedalling. The combination of power meter and heart rate data is interesting. It allows you to find out what your maximum sustainable power is. Everybody can go (very) fast, but speed depends on external factors like wind. By looking at power data you'll be able to more accurately compare different rides, track your training progress and compare your efforts to other cyclists.

Data from the power meter, compared with speed, cadence and heart rate data
Data from the power meter, compared with speed, cadence and heart rate data

Look at the data above, it's from one of my rides with the power meter. Here you can see my heart rate, cadence, speed and power meter data in combined graphs. I cycled 23 KM's that can be divided in four segments:

Conclusion

Fitting a power meter on your bike is not difficult using the right tools. Connecting and calibrating the sensor is easy using your smartphone and the right app. Once connected and calibrated you can use the power meter with various apps.

Riding with a power meter offers you extra insights in your performance. You'll be able to quantify the intensity of your ride by looking at actual delivered energy. Training with power data is well worthy of another blog post that I might write in the future.

Do you need a power meter? Unless you're a pro athlete, I think you don't really need one. But if you're interested in performance data beyond metrics such as heart rate, speed and cadence - you might find a power meter useful. Just remember to enjoy your ride, whether with or without a power meter!

With or without a power meter: don't forget to enjoy your ride!
With or without a power meter: don't forget to enjoy your ride!

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