what is this?

[rennie]

I've never had a shaft drive before!

What is the small, rusty, perforated ring for? It spins with the wheel

(not the brake disc the smaller 1)

only thing I can think of is something to do with ABS?

[Marino]

It is abs, for abs , there must be a sensor pointing to it somewhere

[husoi]

Exactly what [mention]Marino[/mention] says, the ABS ring.

The sensor will be hidden away behind the shaft drive near that alum bit showing behind black bit.

[rennie]

I guessed correctly then

I was going to try to clean it up a bit but I think I'll leave well alone

[husoi]

Leave it for now.

If you at some point have to remove the wheel then you can clean it.

For that you should use a small brass brush.

Clean the sensor too then.

I wouldn't dismantle it just to clean the ABS ring and trying to clean it in place won't make a difference.

[Grumpy Old Git]

You should have a similar ring on your front wheel as well.

The ABS circuit senses the difference in rotation when the brakes are applied and prevents (continuous) lock up.

The down side to this tech is that bleeding the brake system is a right pain and seems to take forever. Also, you not be able to lock the front wheel to 'show off'.

[husoi]

The most complicated brake system I know is the Honda's CBS.

To help out with the bleeding I've made this wee tool.

The end with the small ball bearing goes to the caliper