Sprung vs Unsprung weight

In this brief article, we will discuss the Suspension Sprung weight,  Unsprung weight, and the significance of the Unsprung weight  parameter on suspension performance.

What is the Sprung and Unsprung weight of a Suspension?

Sprung Weight

A vehicle’s “Sprung” weight is defined as the weight of the vehicle that is supported by the Springs. The Suspension “Sprung weight” is defined as that portion of the vehicle’s weight (Corner weight) that is supported by the respective corner suspension.

By definition, in a car’s suspension, the Sprung weight would include everything that the suspension arms are connected to, meaning, the car body. In cases, where it is a dependent axle suspension, a portion of the leaf spring weight (usually one-third) would also be taken as ‘Sprung’ weight.

Unsprung Weight

The term ‘unsprung mass’ was first applied to an Automotive Suspension and explained as a concept by the mathematician Albert Healey of the Dunlop tyre company at the Institution of Automobile Engineers in 1924.

The “Unsprung” weight of a vehicle is that portion of the weight of the vehicle that is not supported by springs. This would imply, everything that the car body is connected to, like the suspension arms, springs, shock absorbers, wheel hubs and tires, brakes and part of the weight of the axle half-shafts. 

In the cases where it is a dependent type of suspension, the live axle, the axle beam, differential gearbox housing and axle-shafts would all be part of the unsprung weight of the suspension.

What is the importance of ‘Unsprung’ weight?

While engineering the suspension system, one of the objectives would be to have an unsprung weight that is “as low as possible”.

The reasons behind it are:

Effect on Ride Quality

A lighter unsprung mass implies that it is more responsive to road undulations. The unsprung mass natural resonant frequency is  inversely proportional to mass. So a lighter unsprung mass means a higher natural frequency and therefore less probability to resonate with the road frequencies which are generally in the low range. It absorbs more energy from the road vibrations and provides a better isolation to the sprung mass or vehicle body. The Shock absorbers and springs are subjected to a relatively lesser load when the unsprung mass is lower.

Effect on Handling Performance 

For the very same reasons why lower unsprung mass improves ride, it also improves handling response. The suspension systems ability to respond to road vibrations and cornering loads is enhanced by the lower unsprung mass.

Effect on Acceleration and Braking performance

A lower Unsprung weight is definitely an advantage when it comes to braking and acceleration. Lighter wheels and tires are easier to optimize for braking performance. Acceleration also improves in the process of lesser unsprung weight.

Reduction of Unsprung Mass

DeDion Axle

In the past, the Dedion axle was an attempt at reducing the  unsprung mass of a live rear axle, but at the same time providing the articulation ability. The approach was quite similar to the modern-day live independent rear axle, but the difference being that the DeDion axle retained the dependent axle tube connecting the 2 rear wheels but with a split rear axle. With the simpler and more advantageous Independent rear axle design, teh DeDion axle lost its relevance

Inboard springs

This was an approach developed for Formula racing and is still used to this day. The “Push-Rod” and “Pull-Rod” bellcrank lever type of suspension allowed the Springs and Dampers to be mounted in-board, or in other words, Chassis mounted.

Carbon Fiber

Since decades, F1 racing suspension control arms have been made of Carbon fiber for 2 reasons:

  • Reduction of unsprung weight coupled with the superior strength of carbon fiber
  • Better aerodynamics since the suspension control arms can be made slender and shaped like aerofoils to reduce wind resistance.

Challenges with Unsprung Weight reduction

Modern day F1 racing cars and road cars like the Mclaren P1,  have alternate electric “In-wheel” motor propulsion units that significantly increase the wheel weight as compared to a non-motor wheel. This introduces more unpredictability of the unsprung weight vibration response to road undulations. The remedy, in this case, is to use an Active Damping system that uses a “Sliding Mode Controller” strategy in the Damper electronic control unit.

Conclusion

In this brief article, we have discussed the Suspension Bar, its features, advantages, and applications.

References

https://www.jstor.org/stable/44717906
https://journals.sagepub.com/doi/10.1177/1461348418767096

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