As the n900 has a infrared sensor, i'm asking myself, how hard it is to build a small device that has a magnet sensor, for the magnet in the spokes of a bike and a infrared led to send a signal per turn of the wheel.

You know, the N900 has a built in GPS receiver which can track your speed and mileage after installing an app such as Carman.

Probably quite easy. But why not go for a retro solution?

1. Clip playing card to forks so that it makes a nice noise when a spoke hits it.
2. Use microphone on N900 to pick up noise and software to count hits

But the gps has to fix, needs i guess more power, and doesn't precisely show the speed when accelerating. Also you could use such a device as cadence sensor.

I like the idea.
check http://www.youtube.com/watch?v=D9zicb-bo48 for more retro solutions

As far as I know, it has an IR transmitter (for remote control of consumer electronics), not a receiver.

That would be bad.

Seems like someone already had that idea, and it seems to work with palms: http://www.engbedded.com/images/proj...loace/vaif.pdf (second page)

I prefer GPS personally, as you can never be sure you calibrated exactly right using the magnet on wheel solution.

sroll down this page to see the bluetooth solution
http://velocomputer.com/products/
the only way you seem to be able to get his speed sensor is if you sign an agreement only to use it with java. otherwize it would be great

There is actually software for the N900 that's designed for this - eCoach. According to the info on Maemo Select:

http://maemo.nokia.com/maemo-select/...ations/ecoach/

"Keep track of all your sports and outdoor activities with the GPS-based eCoach application. With eCoach you can get the most from your workouts by monitoring your speed and distance in real time. You can also use the built-in training diary feature to track your progress as you work towards your specific training goals.

eCoach supports Open Street Map, Google Earth, and Virtual Earth, enabling you to view your routes on the map and share them with friends at a later point."

No cadence, but is it that critical for you? As for battery life, I guess we'll have to wait and see, but you could always attach an external power source, such as a battery or generator hub (perfect for those epic audax rides).

EDIT: According to reports of the eCoach presentation at the Maemo Summit, the program's also compatible with Bluetooth heart rate monitors (straps). Not that they're easy to find, but still.