This is the Gravity Bike built by Jeff Tiedeken and it's helluva fast. It can reach high speeds when going downhill due to its lightweight design and lack of features like a chain and pedals. Just don't expect to climb uphill.
The seat is fitted over the rear wheel, and it has a low profile. The minimalistic design includes 26-inch Crossmax Lefty wheels, Avid Elixr hydraulic disc brakes and motorbike-style footpeg struts. The bike has reached speeds of 50 mph and is said to have the potential to achieve up to 70 mph.
But what good is a bike if it can't ride on a flat straight road, though, clearly, this bike doesn't have that in mind.