Database Design / ER Diagram
A Google of "open source subscription software rent a bike" gives a number of interesting results, including this one. You can take and adapt ideas from some of these programmes - the beauty of Open Source.

Just a couple of thoughts about the ERD. Why include colour - that can readily change - and is it really relevant? I mean, how many people care about the colour of a hire bike? Here in Dublin, they are the same colour and have a uniform appearance - I think that this "branding" contributed in some way to the success of the scheme. You also have size - again, here in Dublin, a similar scheme uses 1 size (adjustable). 

All of your bicycle attributes (top left of ERD) should be in 1 table with PK Bicycle_ID - model, make, size and colour follow logically from the Bicycle_ID.

Also, one thing that struck me was this GPS stuff - are you sure that there aren't EU Data Protection issues with this? Has it been looked at by a lawyer competent in EU DP law? Finally, another thing about GPS - maybe you should have a GPS coordinate for the Terminals? People regularly now navigate using phones with GPS - they'll be able to find the nearest terminal more easily this way.