Cycling Power Measurment

| April 1, 2010 6:01 pm

by Franz Kelsch, updated 4/1/2010

Why Measure Power?

There has been an evolution from using only using heart rate measurements for training purposes to measuring the power output of the cyclists. To meet this need training method several approaches have been taken to develop power meters than read out the power in watts, being applied to the pedals.  To read about the science being cycling power read this other post.

Two Components

Most power meters are in two components. There is the mechanism to make the reading using something like a strain gauge. This is either built into the crank, the hub or some other part of the bike where power being applied to the pedals can be measured or estimated. Then there is the computer head which is mounted on your bicycle handlebar where you get the reading. The computer heads offer other cyclometer features including speed, distance, heart rate and even GPS on some models. The communication between the two components is either via a wire or wireless.

ANT+SportTM is a 2.4 GHz wireless network with standardized communication between devices including bike power sensors, speed sensors, cadence sensors and heart rate straps. This allows separate manufacturers to independently develop sensors and computers, allowing you to pick and choose your favorites to create a system that meets your needs. For example if you have a Garmin Edge 705 GPS based cyclometer, or the newer Garmin Edge 500, which offers no power measurement sensor, it could be coupled with the SRM or Quarq crank to read out power and could be used instead of the computer heads offered by those companies.

Offerings

PowerTap:

This offering is from the Saris Cycling group (makers of the CycleOps cycling trainer). It uses a special hub and handle mounted computer to measure and display the power at the pedals. It requires a rear wheel with the PowerTap hub. There are wireless options. It adds weight to the bike due to the hub and the need to use a wheel that can accommodate the PowerTap hub. It is also expensive, especially for the wireless version. Although there is a 2.4 Mhz wireless option, it does not support ANT+SportTM so you are stuck using the PowerTap computer which really looks looks like a prototype built by an engineer for development and not a product ready for the mass market. It can be used on a bike trainer to measure your power output. Moving to another bike will require moving the wheel and the computer head.

Best For : Those looking for a reliable approach to measuring power and are willing spend considerable money and take a weight penalty.

SRM:

This offering is from Schoberer Rad Messtechni. It uses a special crank to measure the power to the pedals. There are a variety of versions to match some of the popular cranks, including DurAce and SRAM. SRM supports ANT+Sport so you can either use it’s computer head (which kind of looks like a prototype) or another device that supports ANT+SportTM such as the Garmin 705 Edge or iBike.

Best For: Due to the extreme expense it is geared toward professional cyclists or those who are prepared to spend as much measuring power as they might spend on a good quality road bike.

Polar:

This offering is from Polar, famous for heart rate monitoring. It uses a device to measure chain tension on the bike and transmit that to certain models of their heart rate monitor or cyclometers. It is can be difficult to calibrate and but once it has been it can have a reliable output. You do the power level read out on compatible Polar heart rate montiors (such as the 725i) or Polar Cyclometers. When used with the latest CS600 cyclometer, you can get an efficiency readout on the cyclometer. This efficiency is an estimate of what percentage of the calories being burned by your body go into moving your bike along, a step beyond just measuring power.

Best For: Those who use Polar advanced cyclometers or heart rate monitors and are looking for a lightweight and less expensive approach and are willing to go to the effort get it properly calibrated.

iBike:

This offering from Velocomp is a relatively new approach. It is different in that it does not directly measure your power output as is done with the above products. It measures the parameters of what you work against and then calculates what your power would need to be to achieve the speed you are going. Except for the front wheel pickup it is all contained in a well designed computer head that is about as large as the computer for the PowerTap and weights 100g. You can also easily transfer from bike to bike. It has some negatives. It can not be used to measure power on a bike trainer. It also seems to have a short battery life due to using a non rechargeable watch type battery. It needs to be mounted so it gets a clean air flow since it uses wind speed in it’s calculations so use with aerobars can be difficult. Cost: Moderate to Expensive.

Best For: Those who are looking for a less expensive and very lightweight approach to estimating power. Those with aero bars should look elsewhere.

Quarq:

This offering is from Quarq and uses a special crankset to measure the power, similar to the SRM approach. It offers ANT+SportTM compatibility. It’s computer head is one of the more sophisticated cyclometers you can buy and has a GPS option. Cost: Expensive

Vector:

A new type of power meeter called the Vector is being developed by Metrigear.  Vector is an embedded high-resolution force meter that calculates a cyclist’s power by measuring force applied to the pedals. It will use Ant+Sport to communicate with the user supplied head unit or GPS unit that supports Ant+.   It is an interesting concept that could have a significant impact on the market for cycling power meters.   This device has not been released as of April 2010 so real world tests are not yet available.  For those interested, please contact the Metrigear Webstie.

Comparison Reviews:

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