If you remember Project M85 Freeride, you'll see the same idea in this new concept bike from Derbi, the Derbi DH 2.0, a mountain bike sized motorcycle with a small engine and very light weight. It has fuel in frame like a Buell, an automatic transmission, a 100cc air cooled 4 stroke engine, underseat exhaust, inverted fork, air monoshock out back and a dry weight of 88 pounds! That's a lot closer to mountain bike weight than motorcycle. The DH in the name refers to downhill mountain biking. (They explain even the brand name Derbi originates from DERived from BIcycle. I didn't know that!) And if I read things correctly, the engine can even be quickly dismounted for a fast downhill with less weight.
This is a pretty high tech little machine which also shows the guys who built Project M85 Freeride might be on to something. Cool.