Power Tools

Excerpted from “Training and Racing with a Power Meter” by Hunter Allen and Andrew R. Coggan, PhD. Copyright 2005- VeloPress.

  Once they hear about the power meter and its benefits—and see their competitors racing and training with power meters on their bikes—the first thing that most cyclists and triathletes ask is this: Which one is right for me? Other questions quickly follow:

Which type of power-meter technology is the best?
What about price? Which model is most cost-effective?
Do all the models have the same features and ease of use?
Which type has the fewest problems?
Does the athlete have to have a degree in computer science or exercise physiology to understand what all those graphs mean?

  We won’t specifically answer these questions because we believe that each power meter has its own merits and faults. We will, however, briefly explore the power meters on the market today and the pros and cons of each.

The Hardware: Four Different Approaches
The power-meter hardware is currently very different from one company to the next. That is because the four companies currently offering power meters all take very different approaches to the technology. Basically, the four methods have resulted in: (1) a crank-based integrated system; (2) a hub-based integrated system; (3) a chainstay-mounted sensor; and (4) a bottom bracket sensor.

SRM
  The first commercial power meter—and the one that started this technological revolution in cycling—was made by the SRM company (Schoberer Rad Messtechnik) in Germany. Ulrich Schoberer, a medical engineer before inventing the product, brought the measurement of wattage to the world in a product that can be used by the masses.

  Schoberer developed his first prototype in the 1980s by taking old cranks, cutting off the “spyder” portion (the part between the right crank arm and the chain-rings themselves), and then replacing it with a power meter, about the size of a small saucer plate, that consisted of a series of strain gauges embedded into the “plate.” The front chain-rings were then mounted onto this plate to allow for the measurement of power as the rider applied force to the pedals. As this force is transmitted, there is a twisting, or torsion, within the plate; the strain gauges measure the amount of twisting from normal. This torsion information is then sent to a microprocessor in the bicycle computer and converted into wattage. Uli Schoberer spent the ensuing years working on his ideas and building newer, more advanced models. The first models that he put on the market were incredibly pricey, reaching upward of $10,000 per unit. Greg LeMond was one of the first Americans to use a power meter.Already known for forging his cycling successes partly through the adoption of new technology, he again paved the way here.

  The SRM crank power meter, called the SRM Training System, which includes a crank and chain-rings, has become the “gold standard” by which all other power meters are measured. It certainly has been around the longest, has been updated and improved the most, and tends to be one of the most reliable of the power meters. Measuring wattage in the spyder of the crank is a very good place to measure it, as the data incorporate the force from both legs and measures it in the place where it occurs, right at the crank. Since the power meter is built as one unit with the crank itself, it is integrated into the bike, becoming just part of the equipment. It is also very weatherproof. SRM has models for road riders, track riders, and mountain bikers and also offers a scientific version for the lab. The company also produces indoor “spin” bikes for use in fitness and performance centers. The computer controller, called the power control, is rechargeable and mounts in front of the handlebar at handlebar height. The user can view all the necessary data while riding, including wattage, heart rate, cadence, elapsed time, and clock time, all on the same screen. This allows the cyclist to keep track of his or her effort while riding and pace the effort accordingly. The power control can be customized to a certain degree—for example, you set your own recording rates for later download, with intervals of up to 60 seconds between saved data points, and you can choose among different options for the display.

  Each time the rider uses the SRM, he must create a “zero-offset,” or “zero-point,” to assure that the wattage will be zero when there is no load on the power meter. This is a five-second procedure and can be done throughout the ride with no detrimental effects to your data Since the SRM uses a strain gauge to measure torsion, the readings can be susceptible to changes in temperature, so if you are riding and all of a sudden the temperature changes drastically and your watts appear to be different from what you would normally expect, then the SRM might need to be “re-zeroed.” Since the metal of the crank will change in size with changes in temperature, it is important that you achieve a zero-offset when the crank is at the same temperature as the ambient outside temperature. This only tends to be a problem when a bike goes from 70 degrees Fahrenheit while parked inside a house to a much colder 40–50 degrees outside. It is a good idea to park your bike outside for ten or fifteen minutes before beginning your ride so that the metal of the crank and the strain gauges can adjust to the correct temperature.

  The SRM computer is one of the best-designed models available and shows the experience of over twenty years in development. The ability to set your own intervals increases the amount of data that can be recorded, and the SRM has a very easy-to-use method of “marking” these intervals while riding. (When you start [and stop] an interval, you press the “set” button on the power control, and this creates a small “time stamp” in the data, so that when you download the data to your PC, you will see immediately where the interval started and stopped). The handlebar mount of the SRM is one of the best and most secure; the screen is easy to read but unobtrusive, and because it is placed flush and just in front of the handlebar, it is somewhat protected from damage in crashes. The user can also easily review data while riding and thus determine when it is time to head home and when it is time to do just one more hill repeat.

  One of the downsides to the SRM is its cost. It is currently the most expensive of the power meters. Another drawback is that the battery inside the power meter itself must be replaced by the factory when it is dead, and this requires some downtime for the user while you mail it in and wait for it to be returned. Fortunately for U.S. residents, SRM has opened a service center in Colorado Springs, so downtime is minimal.

  Another problem that occurs with the SRM is that over time, as the strain gauges age, the watt measurements are susceptible to “drift.” If this occurs, the SRM may need to be recalibrated to ensure its accuracy. The user can do this at home by following the recalibration procedure laid out in the SRM manual, but this can be confusing for the first timer. The SRM also does not output torque in the controller for download. This is unfortunate because the meter has the best ability to receive the correct torque of any model on the market. With torque for download, cyclists and coaches could analyze how torque loads might be affected by different types of cycling efforts.

PowerTap
  The PowerTap is a complete rear hub for the back wheel of a bicycle that houses a power meter. The hub contains a “torque tube” with strain gauges similar to those used by the SRM. These strain gauges measure the torsion inside the hub as it twists from the load that is applied to the pedals by the rider. The bicycle chain wraps around the cogs on the hub and, as it moves, causes small twists in the hub itself. This torque is measured and then converted into wattage at the PowerTap computer.

  The wattage that is measured in the PowerTap is the wattage that is actually getting to the road, as it has to “go through” the drivetrain from the crank. This causes the wattage to be about 5–10 watts lower than what would be measured by the SRM at the crank. The PowerTap takes measurements sixty times per second, averages these figures over a 1-second time period, and then records the data at intervals as short as 1.26 seconds.

  The PowerTap was created back in 1997 by a company called Etune, which consisted of a group of four hardworking partners with a vision toward the future. Unfortunately, like so many visionaries who start small companies, they were not able to keep the cash flow going while they dealt with the normal issues that come up when developing hardware of this nature. Thankfully for the rest of us, Graber Products Group, now known as the Saris Cycling Group, bought the company in 2001 and has since poured thousands of development dollars into improving the unit. The computer that mounts on the handlebar (PowerTap Pro model) has been redesigned with more memory, more functions, and upgradable firmware. The harness also has been upgraded in order to include “real” cadence (the older harness only allowed the computer to estimate cadence based on the pattern of torque pulses within the hub). The release of the new SL hub in 2004 has really brought PowerTap into a leading role in the marketplace in a big way. Previous complaints about weather-proofness and weight are now a thing of the past, and bike geeks will surely like the appearance of the PowerTap units, which incorporate carbon-fiber bits.

  One disadvantage of using the PowerTap is that you are locked into the wheel that the hub is laced to. If you want to use your super trick wheels for racing, you’ll need to get an additional hub for racing and then keep your standard wheel for training. PowerTap has yet to come out with a disc-wheel solution for time trialing, although many riders use CH Aero Wheel covers over their PowerTap wheel to achieve aerodynamics similar to that of a disc wheel. Though mountain bikers can use a hub to gather important data, the PowerTap is not yet compatible with disc brakes. Track riders are also out of luck at this point; although some prototype track hubs are in use, it is unlikely they will see production because the market for them is limited.

  There is also a slight problem with the way that the PowerTap computer records the data for download after your ride. Since the computer records at 1.26-second time intervals, the power reading may fluctuate up to 40 watts (20 watts above and below the true wattage level) from second to second. Though it is possible for such fluctuations to reflect actual changes in power output, when this occurs with the PowerTap it is more likely to be the computer that is creating the problem. Once the data are downloaded, the fluctuations are reflected in the graphs by a very jagged wattage line. This problem is called “precession” or “beat frequency” and can be demonstrated by a simple test that entails riding at different rpms: If you attempt to ride at a very smooth perceived wattage using a PowerTap for 2 minutes at 85 rpm, then at 95 rpm, and again at 105 rpm, you will find that at 95 rpm, the problem cancels itself out—the wattage display at that point becomes very smooth on the computer and also in the download. The company came up with a way to work around this issue in the computer display by allowing the owners of the PowerTap Pro to “smooth” the display at 5-, 10-, 15-, 30-, and 60-second intervals. This has the effect of canceling out the precession issue on the display, but it still can be observed in the downloads of the data.

  The future of the PowerTap is bright. Saris continues to make strides in hub innovations and also with its software. In 2004, the company released a spin bike with the PowerTap technology integrated into the flywheel, allowing indoor spin-class participants to have the benefits of using wattage data. This is a new market for Saris and certainly is a large one if the firm can break into the fitness world. It would be great to have power measurement in clubs and gyms all over the country, bringing power measurement to hardcore spinners, busy moms, and gym rats. Although the pricing on PowerTap power meters is very competitive, the PowerTap is still a sizable investment, especially considering the fact that you may still have to build a wheel around the hub.

Polar Electro
  The lightest and also the least expensive unit on the market, the Polar Electro power meter features a unique and interesting measuring system. The technology was developed by J. J. and Alan Cote and John Croy out of New England, who then sold their patent to Polar. The Polar systems measure chain tension via a chainstay-mounted sensor that detects vibrational frequency; just like a guitar string, a chain vibrates faster as its tension increases. This frequency is translated into an amount of force, which is then multiplied by chain speed, as measured by a magnetic sensor mounted on the rear derailleur, to derive power output: Power (in watts, or W) = chain tension (N) times chain velocity (in meters per second, or m/s).

  Although there have been reports of inaccuracies with the Polar system on the road, the system’s good accuracy and consistency have been demonstrated in numerous tests against the SRM and PowerTap systems. On an indoor trainer, the Polar system may or may not be accurate in all gears. Reports of inaccuracies with the Polar system are often related to improper installation of the system’s chainstay-mounted sensor, which some users feel is difficult to install correctly. Accuracy issues with the Polar system are not due to bumps in the road, but to an interference signal that occurs with some combinations of gear ratio, power, and cadence. The Polar unit also has many little wires that can easily be broken and wrapped around the drivetrain, as one sensor must be mounted onto the derailleur.

  The Polar system does have many advantages, however. One is that the computer portion of the unit also doubles as a wrist-mounted heart-rate monitor. Polar is best known for its heart-rate monitors, and this top-of-the-line monitor has so many features that many riders will buy the Polar power meter just for these alone. Most owners of the Polar unit were owners of the heart monitors first and bought the power option later. This makes the Polar unit the least expensive of the four units reviewed here and also the most attractive to the triathlon market. A multisport athlete can use the unit for measuring and recording heart rate during a swim, have wattage measurement for cycling, and then use the heart-rate data again when running, thereby having a steady supply of data throughout a race or training session and for review afterward. The Polar watch is very weatherproof and durable, though unfortunately the power unit has had some problems with these two essential features.

  The Polar power meter also contains an altimeter and is one of only two meters on the market with the ability to record altitude changes along with power data (the other is the Ergomo model). Although this information does not help coaches and athletes improve their training methods, it does help them to gain an understanding of how wattage and altitude relate to each other, and this can be of great importance to cyclists living or racing at higher altitudes. It also comes in handy to know the altitudes when reviewing downloaded data because it makes it easier to track where you were in a ride or race when the other data were recorded.

  The Polar model measures power at forty samples per second (i.e., it samples vibrational frequency several hundred times per second, chain speed slightly less often, and calculates power values forty times per second) and then averages these measurements over a complete pedal revolution for display to the user. The Polar display is updated every 2 seconds, with a displayed number captured and stored every 5 seconds. This timing can be problematic because the power data produced on a bicycle are highly variable. By not averaging the 2-second samples, the unit can create some imprecise data in the information that is saved for later download. Since the cadence-based averaging interval works out to close to 5 seconds, the data results are not ideal, and the stored numbers tend to have short power peaks that are rounded off.

  If Polar Electro can correct these problems, it stands a chance of becoming the industry leader with a small, accurate, and inexpensive unit. For those who are interested in power-measuring technology, it is definitely a company to watch.

Ergomo
  The newest power meter on the market is the Ergomo, made in Germany. The Ergomo was invented by Siegfried Gerlitzki as a way to measure the twisting of any spindle or axle. An avid cyclist, Gerlitzki set out to create the unit when he realized that wattage could easily be measured by a sensor placed on a bicycle’s bottom bracket. The Ergomo has been in production since 2001 in very limited quantities. The company has only recently been able to break out of the small business mold to begin offering a large number of high-quality units for sale to the general public. With the introduction of a smaller, more robust computer, the Ergomo stands poised to become a major player in the power-meter market. In many ways, its model has the simplest design of all the power meters, and because of this the company may be able to take advantage of economies of scale that no other manufacturer can realize.

  The Ergomo measures power differently from the units reviewed above in that it measures the torsion, or twisting, of the bottom bracket spindle. Every time you pedal, there is a small amount of twisting that occurs on the spindle, similar to the twisting that might occur with a wet towel if you twisted it into a whip. The Ergomo contains an optical sensor that allows it to measure the distance the axle twists, and from this distance it calculates the torque and the corresponding watts that are being applied in order to make that caliber of a twist. A small wire coming out of the bottom bracket shell sends the signal to the handlebar-mounted computer for calculation and display to the rider.

  One of the benefits of this system is that it allows the rider to use any square taper (ISO) crank and any wheel set. For Shimano customers using the new two-piece design or the older octalink spindle, Ergomo offers a carbon-fiber crank set as an option that was specifically designed for the Ergomo bottom bracket. By placing the power-measuring device essentially inside the bicycle frame, the designers came up with a way to protect the sensor itself from the elements and simultaneously created the potential for a future mountain bike unit. This innovation also could very well position Ergomo to retrofit the many indoor spin bicycles that exist and enable the company to bring power training to the masses in every gym and fitness club in the country.

  Ergomo released its new Pro computer, a major revision to its first model, the Sport, in fall 2005. The Sport model was quite large, and the handlebar mount was problematic. The new computer features the ability to view the power data in watts per kilogram once the user enters his or her weight into the computer. The company has also incorporated the ability to see Training Stress Score and Intensity Factor while riding (concepts discussed in depth in Chapter 6). Also new to the Ergomo Pro computer are coded heart rate and an altimeter. One feature that will be appreciated by every veteran of intervals is the ability to view watts in real time and average watts on the same screen while in interval mode.

  The most blatant disadvantage of this system is that because the sensor measures power at the bottom bracket spindle, it only measures the side that twists—the left side. Since the right side of the spindle is attached to the drivetrain, there is no significant twist on the right side. As a result, the Ergomo can only measure the rider’s left leg power accurately. The computer takes the power output from the left leg and doubles it in order to get the wattage. Although initially this may seem to be a significant problem, in reality it has not proven to be one for 97 percent of the users. Only riders with a large discrepancy in the strength of their legs would have inaccurate readings from the unit. Every rider has a small discrepancy in leg strength, but in the majority of people, this discrepancy is less than 5 percent; when on the bicycle, this would result in a difference of less than 10 watts between what an Ergomo unit would report and what the SRM or PowerTap would report. If a rider does have a large discrepancy in leg strength—from an injury, for example—then the Ergomo can be adjusted to still provide the rider with a very accurate picture of his or her wattage.

  One concern with the Ergomo has been the longevity of the bearings inside the sensor, how easily they can be replaced, and the cost of replacement. Ergomo includes a warranty on the unit for up to 10,000 miles and states in its literature that the bearings may last longer than 15,000 miles. Whether this will be the case with all owners remains to be seen.

  The future of Ergomo also looks very bright. With a new computer and a refined design of the sensor, the company looks to take over a lion’s share of the market. Giving the user the ability to choose from a wide range of wheel sets is definitely a plus, and the unit also currently comes in at a lower price than the SRM, making it a great choice for many riders.