Earlier in 2009, I wrote about how the wireless bike speedometer I got to replace an older wired one was a case where an RF link can beneficially take the placed of even a short wire link, see "Wireless or not? When are short-distance, point-to-point RF links worthwhile?". At the time, I was thrilled.
Well, times change and thrills fade. It turns out that the wireless set-up is just not as reliable as a wired one was. Whether it is this particular unit, or this model, or wireless speedometers as a class, I can't say. The handlebar-mounted readout shows the right speed much of the time, but often drops down to about half or a third of the actual speed and then recovers, even when I am going at a fairly constant rate.
I know the sensor pickup is OK, since I can hear the clicks of the internal reed switch as the spoke magnet goes by. But after that, I don't know where the unreliability is. It could be that I am at the border of the claimed 70-cm wireless range, or that the frame geometry is somehow in the way, or maybe the fork-mounted sender and/or the handlebar-mounted receiver/display don't like colder weather. Whatever the cause, it's frustrating. (And yes, I have put fresh batteries in both units.)
But the underlying issue and subsequent lesson goes beyond this non-critical application. To save power, reduce size, and keep costs down, the wireless bike speedometer is a simplex (unidirectional) design. There is no reverse channel and acknowledgement from the handlebar unit to the sender. And that's the weak link, so to speak. There is no way for the fork-mounted unit to know that its clicks were received (it's a fairly slow repetition rate, 0.2 clicks/sec per mile/hour speed).
Unlike a wireless mouse or TV remote control, for example, where the user can see a problem, and thus (in effect) implement the ACK/NACK protocol, and repeat the action if the signal was not received (frustrating, for sure, but at least doable), I don't have that luxury on the bike. Either the switch pulses are received and totaled, or they are not.
While low-cost, unidirectional wireless links may seem attractive—and they are in many cases, since they eliminate connectors and cables—be sure to take time to understand what the impact of a lost message will be. Regardless of the cause, whether it is distance interference, weak batteries, or other, the consequences can span minor to major. And if they are more than minor, you may want to embed a most costly, complex half- or full-duplex link, with associated protocols. Or, you may decide a wired link, with its greater reliability and immunity to outside factors, is a better choice after all. ♦
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