Unsprung mass of an inboard / rocker suspension design

[Locoblade]

Hi All

I was having a discussion with an associate of mine regarding the merits of an inboard rocker suspension system for a track car we were analysing. It started with the following statement (from him) something along these lines: "Inboard suspension generally allows you to achieve less unsprung mass. In a conventional outboard damper suspension design, the part of the spring/damer attached to the lower wishbone is unsprung mass. Using an inboard suspension design with single pushrod and a rocker actuating the damper, the only masses considered unsprung are the pushrod and the rocker, the damper is now considered fully sprung".

I believe this to be incorrect, the rocker/pushrod are certainly additions to the unsprung mass, but you also still need to consider the partially unsprung mass of the damper/spring regardless of where it sits, therefore using the same components, the inboard design will alway have more unsprung mass.

He argues that as the damper/spring is SPRUNG weight, it can't be unsprung. I replied that I think he is mixing weight and mass up, and although it is sprung weight in so much that the entire weight of the damper is supported by the chassis when mounted inboard (not partially by the wishbone as an outboard setup), in this instance unsprung WEIGHT and unsprung MASS cannot be considered one and the same. One end of the damper is still attached directly to the wheel so it's still unsprung regadless of how many linkages its connected by, so will still effect the suspension performance like any other unsprung component.

The crux of it is he seems to think we should be talking unsprung weight, whereas I have said it should be unsprung mass. I have tried explaining this but he still insists I'm wrong but cannot explain why. Can anyone clear this one up for me please, Im happy to be proven wrong but I always like to know why!

Regards

Chris

 

[Fabrico]

You are literally asking about a dozen questions here all at once.

Conventional vs remote?
Mass vs weight (static vs dynamic?)
Unsprung mass?
Unsprung weight?

There are many other factors that enter the picture. The two systems allow different construction of the suspension and chassis. So it's not that simple. Leverage is a big factor with inboard suspension and changes everything.
Neither one of you are completely correct, but your associate sounds closer to it.

 

[Warpspeed]

If it moves when the suspension moves, it is effectively additional unsprung mass. That will be modified by motion ratios, but the principle still holds.

 

[GregLocock]

Consider the easiest linkage type suspension, where the body mount is half way along the arm and the spring is at the far inboard end. Obviously the unsprung contribution from the spring is the same as if it were mounted at the wheel. Likewise for a shock.

You need to consider the motion ratios, in order to add the right proportion of the mass to the unsprung mass.

Confusingly for beam axles the unsprung mass varies depending on whether the car is rolling or bouncing.




Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[NormPeterson]

Chris - a suspension is a spring-mass vibration system, not the position of some linkage when it's at rest. Your associate appears to be thinking only in static terms.

As has been already noted, the mass effect in the overall system depends on more than just the weights of the components. And strictly speaking, there are rotational inertia and acceleration terms, too.


Norm