Science Nerding

Maybe of help to anyone thinking about a power system. Quick chunk nabbed from my Lit Review I'm working on:

The SRM (Schoberer Rad Messtechnik, Welldorf, Germany) crankset was one of the first portable power measuring tools available to the cycling community as a large, albeit at a high price. The SRM system calculates power output from the torque and angular velocity generated at the bottom bracket of the bicycle (E. Farria et al, 2005). This is achieved through a system of strain gauges located between the cranks and the chainrings which measures the deformation between the two. This is proportional to the torque being generated during each pedal rotation. Several studies have validated the crankset and its test to test repeatability (A. Gardner et al., 2004; W. Bertucci et al., 2005; S. Duc et al., 2007;A Juekendrup et al, 2003) and it has been shown to be a valid system in both laboratory and field conditions. Variations of the crankset now exist for both scientific measurement (accuracy +/- 0.5%, weight 827g), Professional (accuracy +/- 2.5%, weight 560g), and amateur (accuracy +/- 5%, weight 640).

The PowerTap (Saris Cycling Group, Madison, U.S.A) is a power measuring device which is located within the rear hub of the bicycle. It calculates power in a similar manner to the SRM system with 4 strain gauges located within the rear hub. The advantage being that it measures the power that is transferred to the rear hub; this takes into account any power loss through the drivetrain of the bicycle. This gives an accurate measure of the power being applied to propel the cyclist. Like the SRM the Powertap has been validated and shown to be a reliable and repeatable source of power measurement (Gardner et al., 2004; Bertucci et al., 2005; Duc et al., 2007). However the price of the PowerTap is over €1,000 cheaper than the base model SRM, has a reported accuracy of 2.5%, and weighs 579g at the most basic model making it a more appealing option for amateur cyclists who want consistent power measurement.

The Ergomo Pro (SG Sensortechnik GmbH & Co, KG, Mörfeldn-Walldorf, Germany) is a system that is located in the bottom bracket of the cyclist bicycle. It measures power generated through a photo-interrupter system which measures the torque generated in the bottom bracket (Coggan, 2006; Duc et al., 2007). This system does not use a strain gauge system like the SRM and PowerTap and is free from any temperature related problems (Duc et al., 2007). However, the sensors that measure the torque generated are only located in the left hand side of the bottom bracket. In order to generate a total score for the torque the figures gained are simply doubled and then recorded by the system (Duc et al., 2007; Coggan, 2006). Although the Ergomo Pro has a purported accuracy of +/- 1% this calculation system inherently introduces a large measure of error as cyclists rarely generate the exact same force through both legs (Burke, 1996; Daly, 1976). This has been shown in studies comparing the Ergomo Pro to both the SRM and PowerTap systems (Duc et al., 2007). The next generation Ergomo Pro, due for release in autumn 2010, further reduces the weight below 344g, while aiming to increase the accuracy through new algorithms and design. Unlike the SRM and PowerTap the Ergomo suffers from no temperature related issues and is not affected by electronic or radio interference.