running gear

Short definition from Wiktionary

Noun

1. 1
   The wheels, suspension, steering, powertrain and usually the chassis of a motor vehicle.
2. 2
   Those components of a railway vehicle that run passively on the rails, unlike those of the driving gear. Traditionally these are the wheels, axles, axle boxes, springs and vehicle frame.
3. 3
   The wheels and axles of a carriage or other vehicle.