How does a wireless bike speedometer/odometer work?

Question

I mean the wireless signal - how does it work?

Answer 1

Basically, there is a transmitter/sender unit that you attach to the front forks(usually) and this generates a radio wave of whatever frequency the manufacturer designs.  A magnet attached to the spokes on the wheel pulses this signal and it it picked up by the receiver/head unit usually mounted on the handle bars.

It is the head unit, that really does the magic!!!

It counts the number of pulses it receives (ie. wheel rotations) and compares this with an internal clock, from this information (and the wheel size programmed into it by the rider) it calculates speed and distance covered.

Many also have a facility to measure heart rate, this is achieved by a strap around the riders chest (with electrical contacts on the skin) that sends another wireless pulse to the head unit and calculates your heartrate in beats per minute.

Some of them go a stage further and have a magnet on the crank and a wireless sender unit fixed to the chain stay, this will measure your pedaling speed or cadence (in RPM).

From this information you can, monitor your progress and train more effectively.

I use a Polar which you can upload the information onto a chart via a PC and tailor your training accordingly.

Clever stuff eh?

Hope this helps mate.

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Answer 2

The passing magnet on the wheel generates an electric signal in the transmitter which is sent by radio to the head unit, which is nothing more than a calculator/watch. It counts the clicks over time and calculates speed. Because it knows the size of the wheel it can calculate distance etc.