What is this?

It came off a Mavic front wheel of a 1980/90s Peugeot road bike that I'm renovating.

It's about 2" (50mm) in diameter and has three prongs that clip through the spokes around the rim of the hub.

The grey bit is plastic and the brown bit is magnetic.

Anyone got any ideas?

http://farm3.static.flickr.com/2457/3941864636_12eee1c55a.jpg

Sensor for a computer?

That was my thought but there was nothing on the forks.

Why can't I get the picture to show in the text??

Avocet computer magnet that fitted around your hub. Early 90's. Fork kit must be missing.

Cheers hopster.

In the bin then. I've got a modern wireless one to fit in its place anyway.