Power Meter Considerations/Comparison
A version
of this article was originally
published in Triathlete Magazine
My coach wants me to start using a power meter when riding so that we can track my progress in the coming season in hopes that I can race more consistently. The two main options that I have found are PowerTap and SRM. There looks to be a pretty big price difference between these two. What are the big differences and what really matters? Thanks.
Jason , PA
Dear Jason,
Power training is becoming to the new millennium what heart rate training was in the 1990’s – the most effective way known to train. By combining heart rate and cadence data with power and performance data, athletes can learn a lot about technique and how their body performs. When done properly, power training not can not only help to make you a stronger rider, but it can also help make you a smarter rider.
There are a few popular options to record power and they each have potential benefits and limitations. While I’m only going to address units that were designed to specifically address power and that are accurate within +/- 5%, it is worth noting that companies like CicloSport offer price based cycling computers that provide estimated power output and Polar offers a basic power system that can be used with some of their heart rate monitors.
How Does a Power Meter Work? Most power meters use calibrated strain gauges to detect power output. Strain gauges are usually small metallic or wire grids whose electrical resistance varies proportionate to the amount of resistance (strain) put against them. The number of strain gauges in combination with the materials, construction and alignment used, determines how accurately a device reads.
What are the important things to consider?
Location of Unit – Strain gauges require a surface where there is pressure (strain) to gather data. On the drivetrain, there are three primary places that are used: the hub, the bottom bracket or the crank itself. Each of the three most popular power meters use a different location.
PowerTap places the strain gauges inside the hub body of the wheel. The primary benefit to this is that you can use any crank you want and, with an inexpensive additional receiver unit, a PowerTap wheel and computer head can be switched between multiple bikes. The potential limitation is that you need to use a rear wheel built around a PowerTap hub and each additional wheel you want power information from needs to be built around a PowerTap hub (starting at $420 for the hub only).
Ergomo uses a bottom bracket design. The benefits of this design is that you can use any rear wheel you want and the power reading is taken from the bottom bracket, which is close to the power source (the rider’s feet). The potential downside is that it is not easy to swap between bikes (you need to remove the bottom bracket) and while the new 2006 design is scheduled to be available in square taper, Octalink and ISIS crank compatible versions, it will not be compatible with the most recent generation of external bearing cranks (FSA Mega-Exo, Shimano Hollowtech II, etc…).
SRM is a crank mounted design. The primary benefits are that the power reading is taken directly off the crank (where the power is being produced), you can use any rear wheel you want, and switching between multiple bikes just requires an extra sensor kit and the ability to swap the crank to the other bike (under 5 minutes). The potential downsides are that some frame designs with uniquely shaped chainstays can require some creative mounting of the sensor and you need to use a crank with an integrated SRM unit in it. This being said, SRM offers a wide selection of cranks, including Octalink and square taper compatible designs as well as two of the latest and best external bearing designs on the market (Shimano’s Dura Ace and FSA’s Carbon Mega-Exo with standard or compact gearing).
Accuracy – All three brands claim accuracy that is within the needs of any athlete. PowerTap +/- 1.5% on their units, Ergomo +/- 2% and SRM +/- 5% to +/- 0.5% (depending upon unit). While there are some other differences in construction, in general, the higher the degree of accuracy a unit has, the more strain gauges it uses to read the output.
On Board Display – Each power meter computer head offers different features in regards to the data it displays and collects. All units include a full functioned cycling computer with cadence, while advanced functions like altimeters are also included with some units. Visit each company’s web site for specifics:
www.cycleops.com (PowerTap)
Dependability and Service – The units improve yearly in this regard. PowerTap had some moisture sealing issues on some earlier units, but anything available now uses an improved design to address this. While PowerTap has had some delivery delays historically, they have always stood behind the product. SRM started working with power meters in 1986 and their units are known to be dependable. SRM also has a U.S. based Service Center which turns units around efficiently and quickly if they need factory service. Ergomo recently changed distributors in the U.S. to Gita Sporting Goods and this change should help them handle any issues efficiently.
Weight – All power meters are going to add a little weight to your bike, but the information they offer more than makes up for the minor weight gain. Over a standard full Dura Ace equipped component group, Ergomo or SRM will add approximately 300g and PowerTap will add from 240g-400g, depending upon model.
Software – While all of the power meters come with software that allows you to download data into your computer, many of our clients prefer aftermarket software like Cycling Peaks (www.cyclingpeakssoftware.com). Ergomo includes a Cycling Peaks software package.
Price – If you just need to use your system with one rear wheel, PowerTap offers the least expensive options with the standard system starting around $800 and the lighter PowerTap SL systems starting at $1300. However, if you plan on using the system with more than one wheel, you need to add in the price of any additional wheels ($1500+ for a Zipp rim version). The latest Ergomo hub system is $1600 and SRM units all include a crank and range from $2100 up to $5200 for the extremely accurate (+/- 0.5%) Science version.
Power meters have gone from “high tech” concepts available only to the top pro athletes to readily available training equipment applicable to any serious athlete. All three designs offer functional and reliable power data so it comes down to where you want the data to be gathered (hub, bottom bracket or crank), how many bikes and wheels you want to use the power meter with, and what other components you want to use in conjunction.
Train hard and train smart!
Ian
Want more information on PowerMeters or want to order one of the systems discussed above? Contact us toll free at 866-833-4FIT or e-mail info@fitwerx.com.
Originally published March 2006/Copyright © 2006
