Unsprung mass: what it is and why it is so much talked about by car connoisseurs

It’s not uncommon to hear the strange term “unsprung mass” from car experts and salespeople. Like most other specialized terms, the meaning of this concept may remain unobvious, and even not understandable for the vast majority of ordinary motorists. Well, it’s time to understand in simple language without complicated terms and concepts.

What is “cushioned” and “unsprung” mass

Leaf springs are the name of the elastic element (EE) of a vehicle’s suspension. The name comes from the French word “ressort” and literally translates as a spring. The main function of a vehicle’s resilient elements is to transmit the load from the frame or body to the vehicle’s undercarriage: wheels, tracks, etc. In addition, the UE is responsible for softening shocks and bumps when passing bumps. According to the type of the elastic element, suspensions are divided into spring (with leaf springs, as in old cars and trucks), spring, torsion, rubber, pneumatic and hydropneumatic.

However, regardless of the type of UE suspension, any “spring” divides the structure of the car into two parts, which are called sprung mass and unsprung mass. The cushioned mass includes everything above the “springs”: the body, engine, interior components, etc. The unsprung mass is predictably everything that is below the “springs”: wheels, brakes, the system of fasteners and arms, etc. However, this is where citizens have a question, why is it important in principle to divide the car’s structure into any two masses?

What does the unsprung mass affect?

The fact is that the ratio of cushioned to unsprung mass has a direct impact on the behavior of the car on the road. The mass ratio affects the braking distance, acceleration dynamics, the grip of the wheels with asphalt, the complexity of driving, fuel consumption. If we speak exclusively about passenger cars, the most optimal ratio of weights is 1 to 15. That is, the entire unsprung (lower) part of the structure of the vehicle should not be heavier than 1/15 of the cushioned (upper) part of the structure.

The indicator of this or that mass does not say anything in itself, it is the quality of their ratio that is important. Most often car owners change unsprung mass of their favorite “swallow” by installation of wheels with non-original diameter. For example, replacement of original 17-inch disks with 225/50/17 tires weighing 21 kg with 18-inch disks with 225/50/18 tires weighing 26 kg leads to increase in fuel consumption by about 4 liters, reduces dynamics of acceleration up to a “hundred” by about 2.5 seconds in a car with a 3-liter 250 hp engine. Besides, after such tuning, the car can start behaving as if a trailer was attached to it from behind!

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