Cycling may be the most geek-friendly activity out there. You can find more gadget options here than in any other category, but we suggest focusing on the basics. A good way to start is to choose a cycling computer (with a heart-rate monitor), a cycling website, an indoor trainer, and, if you can afford it, a power meter.
The best options
Your cycling computer is the single most essential piece of cycling tech, for the roads or the trails. A dedicated computer is easier to read than a smartphone, can include barometric pressure for better altitude, and lets you avoid killing your smartphone's batteries (helpful on those days when you flat out one too many times and need to call for a ride). Aside from keeping tabs on your speed limit, a cycling computer can track your distance and location (even more important when riding compared to running), and it can pair with heart-rate or power sensors to improve your training.
Skip the old wired models and go with a wireless, GPS-enabled unit such as the venerable Garmin Edge 500 or the Magellan Switch Up watch. If you also run, many GPS watches (especially from Garmin, Polar, and Suunto) will work for cycling too; bike mounts for these watches are widely available (and will keep your hands on the handlebars). Most models use simple maps that will display only where you've been, so step up to a Garmin Edge 810 if you want full maps. The Garmin Edge 510 and 810 can connect with your phone over Bluetooth and piggyback on the data connection to send out live updates and automatically upload your workout.
If you have the cash, and if you ride roads, get a direct-force power meter. While a heart-rate sensor tells you the effect your ride has on your body, a power meter directly measures your output using a strain gauge (and isn't affected by lack of sleep or too much coffee). A power meter is also valuable since it measures your output regardless of speed, allowing you to maintain the right effort on hills or flats. Studies show that training with a power meter improves fitness faster than using a heart-rate monitor alone. Although Bluetooth LE is starting to appear, most such units use ANT+ and can therefore connect with far more cycling computers.
The Stages Power Meter, which uses a modified crank arm with a strain gauge, starts at $699, but it may be less accurate than other models since it measures only one side of your pedaling (unless you buy two). CycleOps sells a range of hub-based PowerTap meters that integrate into your wheel, while SRM and SRAM are crank-based systems starting at over $1000. Pedal-based meters are hitting the market, with the Polar Keo available now and the Garmin Vector arriving later in the year. These items are easy to swap between bikes, and measure left/right power balance--but they command a premium price and are definitely for early adopters (as they are difficult to find right now).
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.
The TrainerRoad Web application guides you through workouts by connecting your computer to your power meter, heart-rate monitor, and other sensors. The $10-per-month service can even control the resistance of the Wahoo KICKR. If you prefer virtual racing and roads, the KineMap iPad app and service adds real-world street and race videos, tied to your pace.
Things to avoid
There's only one reason to go with a non-GPS cycling computer these days--and that reason is budget.
Stick with ANT+ computers and sensors for now, unless you connect to your smartphone. Bluetooth LE isn't well supported in cycling computers. That will change in the next year or two, but if you need something that works now, ANT+ is the answer.
We hate to say it, but unless you are a Polar die-hard, skip the proprietary wireless standard (and more-expensive sensors) and wait for the company's Bluetooth LE products.
Finally, don't buy a bike trainer without a flywheel. Without one to keep inertia, your wheel will screech to a halt if you stop pedaling.