There are probably hundreds of apps that will turn your iPhone into a mobile fitness device, using the phone’s GPS to track your speed and from there derive calories burned, route taken and so on. Some of them even connect wirelessly through a dongle to heart-rate monitors and the like. It’s like having a coach in your pocket, only he doesn’t wake you up in the morning by shouting at you.
Now, though, things are going to the next level. Wahoo’s new Blue SC speed and cadence sensor talks direct to the iPhone via low-powered Bluetooth 4, letting it communicate directly with your bike.
Speed is measured by counting how many times the wheel turns, and knowing the outer circumference of the tire. It’s a lot more accurate than GPS, which can get confused if it momentarily loses track of the satellites.
Meanwhile, cadence is measured by counting how many times you turn the pedals per second. By putting the sensor on the rear chainstay (the chainstays are the thin tunes that run from the bottom bracket to the rear wheel) it can measure both at the same time.
Wahoo makes its own app to talk to the Blue SC, but it’ll also work with your own app of choice. As long as that app is Strava or Cyclemeter.
But best of all is the cost. Even a cheap standalone bike computer with speed and cadence sensors is more than the $60 that the Blue SC will run you, and you won’t get all the other neat iPhone-provided functions. Just remember, it will only work with the new iPad and the iPhone 4S — these are the only models with Bluetooth 4.