Unsprung weight is a term used to describe that part of a vehicle's mass that is directly connected to the wheels, and not isolated through the suspension. Unsprung weights typically consists of the weight of the wheels, tires, brakes (if within the wheels), spindles, bearings, and a portion of weight of the half-shafts, springs, and suspension links.
Because this part of a vehicle is in direct connection to the road and therefore subject to all of its imperfections, it is important that this weight be as low as possible, or at least much lower than the weight of the sprung part of the vehicle, in order that the wheels maintain optimal contact with the road surface. The greater the unsprung weight, the greater the inertia of the suspension, which will be unable to respond as quickly to rapid changes at the interface between the road and wheel. Unsprung weight can be reduced by using light alloy wheels, independent rear suspension on rear and all wheel drive vehicles (where the weight of the rear differential is not part of the unsprung weight) (also obtained by the use of front-wheel drive) and the location of brakes inboard rather than within the wheels.
The classic design of rear-wheel drive through a live axle ("Hotchkiss Drive") gives quite a high unsprung weight, which is one reason why it has fallen out of favour for other than trucks. The use of independent rear suspension allows the handling and response under acceleration and braking to be finely tuned. The Hotchkiss Drive has one significant advantage in that very high power can be transferred to the ground with less tendency to induce wheel hop and so is favored in drag racing. This advantage also accrues to a similar rear drive suspension, the de Dion. The de Dion uses a rigid beam to connect the rear wheel bearings and uses half shafts to transmit power from a chassis-mounted differential, reducing the unsprung weight. A recent vehicle using this suspension was the Ford Ranger EV which had a an aluminum beam on leaf springs and a chassis mounted geared electric powertrain unit located between the rear wheels.
The basic balance is between the ability of the suspension system to supply reactive forces to the mass of the unsprung weight in order to counteract the forces supplied by road surface imperfections. Heavy unsprung weight can overload a suspension systems ability to react, leading to tire hop, slapping, poor traction & bad ride and handling.