A speed and cadence sensor helps roadies keep the pedals moving, and helps trail riders monitor their speed in deep canyons. The technology is old and reliable: All models use magnets on your wheel and crank arm to trigger a reed switch every time you spin around. Like power meters, they pair with your existing computer. The Garmin GCS 10 is the ANT+ standard; Wahoo Fitness sells the Blue SC if you want a Bluetooth LE version for your phone. Polar uses model-specific proprietary options and requires you to buy separate speed and distance sensors, unlike the integrated Garmin and Wahoo models.
A cycling website to track your data is essential. Use whichever one comes with your cycling computer, or visit TrainingPeaks for deeper (paid) data analysis. Especially useful if you train with power, Training Peaks provides an insane amount of metrics and analysis, and is the choice of multiple pro cycling teams.
Perfect your home "pain cave" with a bike trainer. Aside from allowing you to ride no matter the weather, a trainer can help you ramp up your fitness improvements in completely controlled indoor conditions. Starting around $1000, trainers such as the Wahoo KICKR automatically vary resistance (with brakes or magnets) to simulate hills, wind, and other race conditions. The KICKR connects to a computer, tablet, or smartphone via Bluetooth LE for precise power control with different apps, but you need to take your wheel off to mount it directly to the gears.
The CycleOps PowerBeam and TacX Genius also precisely control resistance and power, but they use a more-traditional friction approach in which you lock your skewer into a bracket and leave your wheel on. CycleOps and TacX are closed systems, but they offer some top-notch virtual riding software. The Kurt Kinetic InRide is a $200 power sensor/adapter for the excellent Kurt Kinetic fluid trainers that also pairs well with a Bluetooth LE-capable computer, tablet, or smartphone. It lacks resistance control (you manage resistance by changing gears), but it's cost effective, and Kurt trainers have a great roadlike feel.
Even if you own a cycling computer, Strava is a must-use app for hard-core cyclists who want to race the rest of the world virtually and battle pros and amateurs to be the king of the mountain. The service (which can also take exports from cycling computers) compares all historical rides on the same routes and ranks everyone just as in a real race.
The Wahoo RFLKT is a Bluetooth-powered bike computer that relies on your iPhone to feed it data. This arrangement keeps the cost down while giving you a full set of features--or, at least, the features you can get via compatible iOS apps, since the RFLKT lacks GPS or any sensor connectivity itself.
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