TECH ED’S CHOICE
UNSPRUNG MASS AND BOOST
I often hear the term “unsprung mass”. I understand what it is; however, what effect does it have on a vehicle, particularly of the SUV kind?
Secondly, a lot of cars are turbocharged these days (BMW M5 and Mercedes-amg E63 S, for example) and, in some instances, have similar capacities but different power outputs. Is this a result of a boost difference? Would there be any merit in publishing the different boost figures in the roadtest section? IAN SHAW Durban
Unsprung mass
To clarify the term “unsprung mass” for our readers: it’s the mass influenced directly by road surface undulations without a spring-damper interface supporting the mass. The unsprung mass therefore does not include the vehicle body but does include wheels, brake assemblies, hubs and so on.
Any mass in motion must overcome momentum and inertia when making a directional change. Take, for example, a wheel travelling over a bump, compressing the suspension of the vehicle. In an ideal scenario, the wheel weighs little and easily follows the bump down on the other side with the spring-damper system pushing it against the surface at all times, never losing contact.
However, when the wheel’s mass is increased, the momentum of the wheel moving up over the bump prevents the suspension pushing the wheel down in time on the downward slope and the wheel loses contact with the surface for a brief moment. The more unsprung mass, the greater this effect, leading to all sorts of handling issues on all types of vehicles.
Boost figures
Power output is directly related to the mass of air flowing through the engine as oxygen is the limiting factor for combustion in petrol engines. Therefore it is safe to assume, if two modern turbopetrol engines are of similar capacity and produce the same amount of power, they are running close to the same amount of boost pressure. In the case of the M5 and E63 S, the BMW has a 4,4-litre engine compared with the Mercedes-amg’s 4,0 litre unit. The Mercedes-benz, however, produces 450 kw versus the BMW’S 441 kw. As the E63 S has a smaller engine capacity but produces more power, it is running higher boost.
We’d like to state the boost pressures for each turbocharged vehicle we test but, unfortunately, these figures are seen as proprietary information and not always disclosed by the manufacturers.