The mass that form a vehicle’s total weight can be split in two types:
- the sprung mass are the one linked to the ground through the shocks, and consequently subjected to variations in the ground clearance: these are usually the chassis, the engine, the body and all those parts rigidly bolted to these.
- the un-sprung mass are the one wich are not (or shouldn’t be…) subject to any variation in ground clearance since they follows the ground pattern: typically these mass shall consist in the wheels and all the components bound to them, as some shock dampers and braking system parts.
The sprung mass changes its distance between the ground and the un-sprung masses by the shock dampers that, by compression and extension, absorbs the road’s bumps.
The un-sprung mass consists in wheels and oscillating suspension parts, that are in contact with the ground and then follow the road pattern, keeping a costant ground clearance.
Even with equal total mass, it’s important that the sprung to un-sprung mass ratio will be as high as possible: this determinates in fact to have a lower inertia of wheels and suspensions, that can promptly react to the road’s asperities; and thus a major inertia of the vehicle’s body that suffers to a lesser extent from the stresses transmitted by the shock dampers. In other words, the higher will be the sprung to un-sprung mass ratio, the lower will the frame’s transmitted vibrations be.