Renault R31

[timbo]

Think we need to be more careful about the word "absorb" vs "dissipate" in these discussions.

Or better avoid both. Absorb is simply misleading, dissipate only applies to a conventional damper.

[PlatinumZealot]

Here we go, this principle has been discussed at length in this thread.

viewtopic.php?f=6&t=10270&start=30

[mep]

You seem to be as confused as me.

Well, call it damper or not that's not the point.
Also we don't need to discuss if its absorbing or dissipating.
Maybe we could say forces cancel each other out.

The device itself is very simple but with the theory behind it things become tricky.
I guess very few actively contributing to the forum do fully understand it including all the math.
And that think with the hydraulically amplifying is also not completely clear.
I would call it hydraulically gearing.
We need a new topic for that.

[dren]

It stores and releases energy to smooth out vibrations.

The J-damper stores and releases energy through a mass spinning. This new version stores and releases energy through a flowing/moving mass of mercury (heavy as a fluid).

[aral]

It stores and releases energy to smooth out vibrations.

The J-damper stores and releases energy through a mass spinning. This new version stores and releases energy through a flowing/moving mass of mercury (heavy as a fluid).

Are you sure that mercury is in use? I would have imagined that its use would be banned due to its high toxicity.

[DaveW]

Not sure that I understand the Renault device (yet), but it looks to me rather similar in principle to the Hydraulic Engine Mount, first developed (and understood) by Freudenberg, I believe. A physics based model is available in Adams. For those interested it (the HDM) can be modelled with reasonable accuracy with a two pole all-pass filter.

Edit: The earliest patent I can find is US4422779, filed in 1981. It doesn't look like much like the Renault device, but I'm reasonably sure that all the functional elements are present: spring, damping & fluid inertance.

[DaveW]

My guess would be that mercury is mentioned in the Lotus Renault patent only as a "catch all".

I believe there is a difference between "mechanical" & "fluid" inerters. The "mass" is logically connected directly between the end fittings in the former case, but via springs (representing fluid compliance) in the latter. The patent maintains that the fluid is "incompressible", but my experience would suggest otherwise. I would need to test an example to determine if the difference is significant, & whether it is a benefit or otherwise.

[mep]

I agree there is maybe a difference but I don't think it is because of the fluid being compressible.
How high would the forces need to be so that compression of the fluid accurs?
Probably higher than the 500kg mentioned.
I think the difference lies rather in the "hydraulical gearing".
As far as I understand it is a change in acceleration causing a pressure change in the small diameter tubes running around the device. This pressure change is then acting on the larger diameter of the piston and creates a accordingly higher force which is feed back to the rest of the suspension.

[ringo]

My guess would be that mercury is mentioned in the Lotus Renault patent only as a "catch all".

I believe there is a difference between "mechanical" & "fluid" inerters. The "mass" is logically connected directly between the end fittings in the former case, but via springs (representing fluid compliance) in the latter. The patent maintains that the fluid is "incompressible", but my experience would suggest otherwise. I would need to test an example to determine if the difference is significant, & whether it is a benefit or otherwise.

There are no springs necessary in the inerter itself. The key is literally the inertia of the mass.
In both cases the mass has radial displacement, speed and acceleration.
You need to have a radially moving mass that is constantly connected to the terminals in some way.
That is all it really is.

Springs is more to do with a system with a inerter as part of it.

[DaveW]

mep, ringo: Hydraulic fluid has compliance which is realistically modelled as a stiffness. Stiffness values of a damper range from around 4KN/mm for a typical cylindrical damper down to around 350N/mm for a rotary damper.

Does it matter? Stiff damper settings coupled with high compliance (low "installation" stiffness) usually loose control of the hub mode, which results in unpredictable handling. A track test with a FR car, demonstrated consistently that complaints occurred whenever installation stiffness dropped below 1 KN/mm.

http://papers.sae.org/2005-01-2411 contains a paper about modelling hydraulic engine mounts. Figure 3 shows the model, complete with series springs, & the example listed in figure 4 suggests a spring stiffness of 438 N/mm - I suggest that is not totally out of kilter with my estimates above. It is likely to soften the "attack" of the inerter.

[Javert]

FEE with this design would have been simply evil