Moulton Hydragas and Creuat: coupled hydropneumatic springs
Moulton Hydragas and Creuat: coupled hydropneumatic springs
(OP)
After Greg Locock mentioned it, I decided to go dig up Alex Moulton's 1979 SAE paper on the Hydragas system. I was startled by the similarites to Creuat's paper of 23 years later (attached), even though the coupled hydropneumatic units are "wired" differently. (In the Creuat, the hydropneumatic compliances are configured to induce common-mode motion in coupled wheel pairs, rather than differential mode motion in the Moulton).
What was compelling was that both address the global dynamic behavior of the vehicle. Moulton went so far as to use multi-modal frequency domain analysis (quite a trick in the late '70's) as well as time domain.
It would appear to naifs like myself to be an elegant and powerful solution (and if BMC and BL bought into it, cheap, too.)
My question is, why hasn't anyone else picked up the ball? It seems BMW let the MGF revert to coil springs, and brushed aside Moulton's proposal for Hydragas in the Mini revival. Is this simply a case of NIH? That sad thing is, that according to Moulton, all the Hydragas tooling has been destroyed. Good ideas can sometimes disappear for bad reasons.
It would seem to me that the diagonal coupling that Creat advocates has some advantages (roll stiffnesses can be 3-4 times higher than bounce), particularly if one is not particularly concerned with smoothing out very short wheelbase vehicles (aka Mini)
(I should add that, as a teenager, I had a pleasant summer in the '60's tooling around in a car equipped with the predecessor of Hydragas, the Hydrolastic MG1100)
What was compelling was that both address the global dynamic behavior of the vehicle. Moulton went so far as to use multi-modal frequency domain analysis (quite a trick in the late '70's) as well as time domain.
It would appear to naifs like myself to be an elegant and powerful solution (and if BMC and BL bought into it, cheap, too.)
My question is, why hasn't anyone else picked up the ball? It seems BMW let the MGF revert to coil springs, and brushed aside Moulton's proposal for Hydragas in the Mini revival. Is this simply a case of NIH? That sad thing is, that according to Moulton, all the Hydragas tooling has been destroyed. Good ideas can sometimes disappear for bad reasons.
It would seem to me that the diagonal coupling that Creat advocates has some advantages (roll stiffnesses can be 3-4 times higher than bounce), particularly if one is not particularly concerned with smoothing out very short wheelbase vehicles (aka Mini)
(I should add that, as a teenager, I had a pleasant summer in the '60's tooling around in a car equipped with the predecessor of Hydragas, the Hydrolastic MG1100)





RE: Moulton Hydragas and Creuat: coupled hydropneumatic springs
I suggest the main reason no other makers "picked up the ball" is simply the cost of paying Moulton to use their system when makers like BMW can engineer one of their own and not have to pay anyone to use it.
One practical aspect of the Moulton hydragas / hydrolastic set up is the very limited ability for adjustments and modifications which means the appeal for sports car owners is very limited.
The MGF Hydragas variant has a reputation for quality control issues and some of this related to the suspension. I am guessing when BMW took control the idea of a small , mid engined sports car appealed but little else did hence the reversion to conventional coil springs and gas shocks.
As an aside I owned a Morris 1100 with Hydrolastic suspension...bouncy , bouncy !
Pete.
RE: Moulton Hydragas and Creuat: coupled hydropneumatic springs
My main point was not to advocate that everyone should start paying Moulton royalties; there are other approaches to coupled suspension: Creuat, and perhaps most majestically, the Citroen DS19. (No "Bouncy-Bouncy" there, although having my brief encounter the the MG1100, I quite know what you mean.)
Hydraulics is simply an approach that facilitates coupling, and, by extension, an approach to global control of the vehicle's attitude in varying dynamic situations.
I would argue that the problems lie not in the concept, but in the execution.
RE: Moulton Hydragas and Creuat: coupled hydropneumatic springs
Incidentally the 2CV originally had anti pitch bars, and the funniest rear shock absorbers.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Moulton Hydragas and Creuat: coupled hydropneumatic springs
(reportedly) horizontal hydraulic
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or friction type
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RE: Moulton Hydragas and Creuat: coupled hydropneumatic springs
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RE: Moulton Hydragas and Creuat: coupled hydropneumatic springs
RE: Moulton Hydragas and Creuat: coupled hydropneumatic springs
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Moulton Hydragas and Creuat: coupled hydropneumatic springs
They were interconnected to deal with pitch, but further benefits may be obtained by cross connecting them to deal with all four modes, again passively.
RE: Moulton Hydragas and Creuat: coupled hydropneumatic springs
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Moulton Hydragas and Creuat: coupled hydropneumatic springs
Greg - I think you meant to say Kinetic with a "c" - and it was Tenneco that acquired them.
The key benefit of interconnected suspensions is the ability to reduce unecessary individual wheel rate on a given input by separating out the suspensions operating modes. Same reason you use stabilizer bars - just on the next level..
RE: Moulton Hydragas and Creuat: coupled hydropneumatic springs
The in/in (which I prefer to call common-mode) situation you describe produces the same two-wheel bump problem, whether it's connected side-to-side as in an ARB, or back to front. In the former case, the one wheel bump becomes partly expressed as "pitch".
The really interesting case is in cross-coupled ARBs, which, due to their common-mode behaviour, provide increased compliant resistance to both roll and pitch (or anti dive/squat), for a given individual bump rate.
The downside would be that the two-wheel bump is espressed diagonally, and I confess I'm not sure what the implications of that are.
Any thoughts?
RE: Moulton Hydragas and Creuat: coupled hydropneumatic springs