mounting spare on roof rails (disco)

Hi

i'm busy trying to finish off preps for the big trip to morocco and need to
mount a secound spare some where.

My thoughts are on the roof using the factory fitted roof rails with some
cross braces added to spread the load.

so questions are:

* how heavy is a wheel and tyre? much differance with steel/alloy?
* whats the max load the roof bars will support?
* is there something out there already i can buy to do this?


I've many roof boxes mounted on the rails so i should think there would be a
weight issue.


Ta Mark