When bookings are made via the online booking wizard (OBW) they automatically come into your system as 'Generic Bookings' meaning that they are allocated generic (Product Line) rather than specific (Item) bikes.

This means that you then have the flexibility of assigning a specific bike at a later stage - for example when the customer comes to pick up their bikes. (This requires the optional GBAL module)

However, this flexibility to allocate later also brings a bit of 
complexity for your users.  If you would rather have BRM automatically allocate a specific bike each time it gets a booking via the OBW, then you can use the 'auto-allocate' setting:

Settings > Online > Advanced 

When this setting is ticked 
BRM will choose the first available specific Item that belongs to the Product Line in question, and allocate it.

Please note default is AUTO-ALLOCATE ON and can only be switched to off when you have the optional GBAL Module.