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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, half-shafts & the suspension pieces themselves.
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), and front-wheel drive.
The classic design of rear-wheel drive through a live axle gives quite a high unsprung weight, which is one reason why it has fallen out of favour.
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.
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