your car helmet has a few features that DON'T work for M/C use.
The Eye port is smaller for car use.
The helmet has a thinner shell than a M/C one
A M/C helmet's face shield is TWICE as thick.
If you can afford a BMW bike you can afford a Fine Helmet
I own 3 helmets, 2 car/ kart use the other is M/C
it's like buying a gun, you hope that you don't need one, and you hope that when you do need it, it's in the right place to save your life.
and hope that you don't buy one that won't do the job.
(see the ammo price thread if'n you don't get it)