Different menus in left sidebar

Hi,

i would like to setup a business website. The business has 3 sections – for example cars, bicycles and skateboards.

On the frontpage, I am using three frontpage widgets with frontpage text.
If the user chooses one the section (for example: cars), I would like to implement a car-specific menu in the left sidebar (for example: Toyota, BMW, Porsche). If the user chooses another section, he should see the submenu of this next section in the left sidebar.

Any idea how to solve this issue?

Thank you very much.
michael