Have you ever heard of near-field telemetry? This is a process in which measurement data is transmitted wirelessly over short distances from moving machine parts to a stationary receiver. This is also how the latest Powertap generation 2.4 works: With this power measurement device, electrical signals from the hub are transmitted to the bike computer via a transmitter, where they are displayed as power in watts. Powertap is the first device with this transmission technology.
The Powertap system for watt-controlled training has been around since 1997, and the functional principle has not changed since then. Like its competitor SRM, the system works with strain gauge strips. However, unlike SRM, these are not integrated into the crank, but into the rear wheel hub. Strain gauges are small sensors that react to deformations by changing the electrical resistance. Electrically amplified, these signals are then digitally transmitted to the computer which, together with the cadence, calculates the wattage. In contrast to earlier models, this is now coded and takes place wirelessly over a distance of a good one metre from the hub to the handlebars without any tangled cables. After two years of testing, the Powertap SL 2.4 is now on sale. Our first practical test focussed on criteria such as installation, operation, power consumption, precision and, of course, error-free data transmission.
Installation takes just a few minutes if you buy the hub already spoked into a wheel. Put the sprocket on the rotor, clamp the wheel, fix the holder for the computer to the handlebars or stem with two cable ties - and you're done. Setting up the computer is then a little awkward. Not all pre-settings can be transferred from the PC to the bike computer.
The computer with the three-line display is easy to operate using the two buttons "Mode" and "Select". All values are easy to read, the lines in the display can be customised during setup and even the repetition rate of the speed or cadence display can be changed, making the display more or less sensitive to changes in measured values. You have to spend more time setting intervals: The display does not provide any clear information as to whether an interval has started or ended. Once the interval has started, the "INT" display disappears, so you have to memorise when an interval is running.
The quality of the wireless transmission on the test device was absolutely convincing. After the computer was woken up from "sleep" mode, it always recognised both the hub and the heart rate belt reliably. There were never any failures during the ride, even during long passages along overhead power lines or suburban railway lines, the correct values were always displayed. There were no fanciful values for heart rate or power, and no incomplete data sets.
To determine their precision, we rode a precisely measured incline several times and compared the power displayed by the Powertap with the value that can be calculated from the difference in altitude, distance, system weight, journey time, rolling resistance and wind resistance. On steep inclines, the power can be determined to within one to two per cent. The result: The Powertap values were just under four per cent below the calculated values, but were reproduced time and time again - a good result overall. Why one button cell ran out of juice during the first practical test over 500 kilometres with a lot of playing around on the computer could not be conclusively clarified. It may be due to the wireless technology, which consumes more power than the cable version, or to the low temperatures at the time of the test. According to the operating instructions, the battery should provide 400 hours of power.
The scope of delivery includes PC and Mac-compatible, slightly improved evaluation software. In contrast to the previous version, some evaluation instruments are integrated with which you can precisely analyse your watt or pulse distribution. It is annoying that recorded tours are fragmented when they are read in and have to be painstakingly put together using the date and time. Cycleops has announced that it will get to grips with this problem by the autumn
SHORT AND SHORT
Powertap SL 2.4 is a smart power measurement system that can not only be elegantly mounted on a road bike thanks to wireless technology, but can also be used on several bikes as the transmitter is located in the hub. The test device worked perfectly, its precision is sufficient for demanding amateurs. Only the power consumption has increased slightly and the data readout could be smoother.
Distribution: TriDynamic, phone 08387/924420;
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