The second article in a seven part series in partnership with Verve Cycling - the official power crank supplier to the Great Britain Cycling Team - looks how power meters work and why not all are equal.
All of the reliable power meter systems utilise strain gauges. These are very simple and surprisingly cheap devices that were invented in the late 1930’s. A strain gauge is composed of a foil pattern mounted on an insulating backing. These are then placed in locations which are subjected to strain, such as the cranks, and an electrical charge is put through them. Flex of the part containing the gauges cause them to deform, alters their electrical resistance and this can then be measured and expressed as the rider’s power output in watts. This information is then transmitted to a compatible head unit using either ANT+ or Bluetooth.
Repeatability and accuracy
Although strain gauges aren’t expensive, they are subject to a huge number of variables, including temperature and the stress and wear of regular use. Getting them to produce consistent and reliable data from one ride to the next and from year to year requires major investment in research and development and the use of the highest quality components.
Double vs single-sided
There are a number of single sided power meters but these simply measure on one side and then just double it up. This will always be an approximation as there are very few cyclists who have perfect left/right balance the whole time. A dual sided power meter also allows you to identify and work on any imbalances.
Smart trainers will have their own internal power meters but, because of drivetrain inefficiency, will typically read up to 5% lower than a pedal or crank based system. This isn’t necessarily an issue as the reading will still be consistent but it will make comparisons to your outside power data invalid and your Zwift avatar will go a bit slower! Fortunately with Zwift and other training platforms you can choose to use your on bike power meter to control the trainer and propel your avatar.
Power meters vs power estimators
A number of cheaper systems that don’t utilise strain gauges are available but these will only give a derived estimation of power. If you are unable or unwilling to invest in a reputable strain gauge based system, you’re probably better off sticking with heart rate to monitor riding intensity.
With many new brands appearing, be careful about jumping in at the deep end with unproven or first generation systems. Although the savings can be appealing, wait until some trusted long term reviews become available of both the units and the customer support available and whether any early teething problems get ironed out with firmware updates.
The Verve InfoCrank
The Verve InfoCrank is used by the UCI and the Great Britain Cycling Team. It suffers from no drift or data deterioration with time, is not temperature sensitive and does not require constant calibration to maintain accuracy. The Verve InfoCrank measures on both the left and right sides. Battery life is up to 200 hours of riding time and the batteries can be replaced by the user. With two road and a track model, an InfoCrank can be found for all common bottom bracket standards.