Add me to the Nope! List. This ~might~ make conceptual sense, in the vaguest possible way, for the very casual biker. It would NOT make sense for a serious daily cyclist. Why?

As others have already pointed out, the purpose of lock and helmet, and construction of each are basically incompatible. Helmets should be light and crushable. Locks should be nearly impregnable without serious tools. No one material will have both these characteristics, or allow for the creation of one device with both characteristics.

Further, putting significant extra weight on the head is counter to good survivability in a crash, not to mention uncomfortable.

Lastly, the idea of a multi-piece helmet is not out of the question, but it adds a lot of compromises (Motorcycle helmets with movable chin bars come to mind). It necessarily adds weight, increases complexity of manufacture and use, and probably compromises comfort.

It seems to me a better solution would be to create a lighter-weight, high security lock which could comfortably mount on the widest variety of frames without the excessive girth of a typical U-lock. Of course, I'm sure the major players have been working on this for years... so good luck.

Or maybe a combination water bottle cage/ u-lock ;-)

I'd lose the helmet aspect, or at the very least the idea of the helmet also being a lock.

Helmets are awkward to carry around, maybe you could make a device that attaches the helmet to a lock securely in some fancy fashion.

It's not needed, since putting the lock through the straps work, but there are A LOT of useless products made for vanity or perceived convenience around. Make a "pretty" model as well as an industrial model and you'll get both sexes thinking it will make a cool accessory.

What would be truly revolutionary: a security cable made of some ultralight, super strong, space-age fibre materials. Few milliliters thick, few ounces light, un-cuttable with any known metals or diamonds, indestructible.

Did anyone read "Ringworld"? That's what I'm talking about :D

Teela Brown does not need a bike lock OR a helmet.