Folding bike helmet

The concept is interesting but don't think the space saving will ever be enough to make an attractive proposition. Perhaps an inflatable helmet would work, or bicycle airbags. I'll get to work on that as soon as I've finished with the design for my soup slicing machine.

You've been beaten to it.

http://www.designboom.com/weblog/cat/8/view/13023/hovding-airbag-helmet-for-bicyclists.html