Thoughts on decoupled suspension. 2

[TMcRally]

I'm headed into retirement and want to plan a toy to build for fun, for me. It won't necessarily make sense to others but it will to me.

I have been around Subaru Impreza Tarmac Rally cars a lot and have a shed full of parts so I'll make use of those.

I'd like to design a space frame for the AWD bits and pieces with double wishbone independent suspension front and back if it will fit.

I like the idea of decoupled suspension because I like to see good, new? (at least the road less traveled), ideas put into practice and have some benefit, but mostly because I love to see the cause and effect of R&D but not so much if it's a total failure.

As a side note: I am a huge fan of the Porsche PDCC system where they power a hydraulic motor in line with the sway bars to control/override roll. Not a gimmick at all but a fantastic thing to drive on a twisty mountain road. So I imagine I could incorporate this to the hydraulic decoupled system by adding a pump, some measures and some software.​

This will end up a club circuit racer/show car.

Does anyone have experience or contribution to this discussion, either electric dampers or hydraulic.

Looking forward to hearing your comments.
Dave

 

[GregLocock]

Yes I was part of the rather small team that designed Lotus' SID which had any sort of suspension interlinks you want. Sadly it fell into a black hole when it was nearing completion but eventually got finished and used.


First of all define whet you mean by decoupled - zero warp and fully interconnected?

Check out UWA's FSAE-A car which had a suspiciously professional setup reminiscent of Perth's Kinetik suspension, later used in various Toyotas.

The advent of Fox type shocks with remote reservoirs has vastly simplified the hydraulic side, none of that MOOG stuff needed.

 

[TMcRally]

Thanks Greg.

This is a learning curve for me. I enjoy the R&D learning and inventing the most so I'll follow up your leads and see where it goes.

That must have been a great project, it must have been really frustrating when they hit the pause button.

Regards
Dave

 

[GregLocock]

Situation normal with research projects. https://www.vehicledynamicsinternat...-a-significant-active-dynamics-programme.html Richard occasionally posts on motorsports/dynamics related forums, John died a few years back. The other two are after my time there.

My particular interest in SID was "SI". That is we had the spine and the subframes at each end, and the suspension arms, and the body itself, . Alll the bushes for those were of identical size, and then a French elastomer company knocked up all sorts of bushes that would fit. So the idea was we could investigate whether isolated subframes (pah) gave better secondary ride than putting the isolation in the suspension, or whether we should have a rigid gokart with the body isolated from it (and so on and so forth). Sadly I left in the hiatus, and I think they ended up just running the original config.

This investigation was sparked by many experiences with the GM large rear wheel drive chassis (Opel Omega etc). I would routinely knock out the subframe bushes and fit nylon or aluminium pucks instead, for a vast improvement in secondary ride.

Fast forward 12 years or so and we launched a RWD car with a rigidly mounted rear suspension 'subframe', and it was just as excellent as I had expected, and my NVH partner in crime Mr Sherry had modelled. Bolts straight to the frame, vertically, not bushes. As you can see the drag link has a nice big soft bush for NVH, and it makes no odds for handling.


.

 

[dtimberlake]

"a vast improvement in secondary ride."

Is this a decent reference for a big dummy ?

What is primary and secondary ride?

In automotive NVH circles, ride behaviour is often referred to as primary and secondary ride, but what do these terms really mean.

blog.prosig.com

 

[BrianPetersen]

Wondering out loud whether the original poster had in mind a passive system (springs and dampers and linkages) or some level of active control (solenoid- or motor-actuated damper orifices, spring preload adjusters, or anti-roll bar preloaders).

Packard TorsionAire (motorised rear self levelling with mechanically front-rear interconnected torsion bars) https://www.hotcars.com/packards-amazing-torsion-level-suspension/

Citroen 2CV (mechanically front-rear interconnected spring and damper units) https://www.hotcars.com/extraordinary-suspension-of-citroen-2cv/

the simplicity of which is contrasted by the insanely complex oleopneumatic suspension of the DS http://www.mycitroen.dk/library/ds/red/hydraulics/Chas' Hydraulic Course.pdf

The Citroen Xantia Activa was the latest development of this system: https://www.theautopian.com/the-cit...that-could-out-handle-a-supercar-holy-grails/

BMC (British Leyland) Hydrolastic https://classicmotorsports.com/articles/quick-explanation-bmc-hydrolastic-suspension/

and later Hydragas https://www.aronline.co.uk/cars/mg-cars/mgftf/technical-hydragas-explained/

All of the front-rear interconnected systems rely on having some combination of anti-dive in the front suspension geometry and anti-lift in the rear suspension geometry so that application of the brakes doesn't lead to the front suspension slamming against the bump stops while the rear suspension tops out. The Packard and the Citroen DS and Xantia systems actively manage ride height to compensate (at least to some degree) for passenger and cargo carrying. The others just settle to whatever ride height they end up at.

 

[GregLocock]

Yes, Prosig's right. So primary is up to about 4-6 Hz, secondary is about 4 Hz up to 30 Hz.

 

[TMcRally]

Wondering out loud whether the original poster had in mind a passive system (springs and dampers and linkages) or some level of active control (solenoid- or motor-actuated damper orifices, spring preload adjusters, or anti-roll bar preloaders).

Packard TorsionAire (motorised rear self levelling with mechanically front-rear interconnected torsion bars) https://www.hotcars.com/packards-amazing-torsion-level-suspension/

Citroen 2CV (mechanically front-rear interconnected spring and damper units) https://www.hotcars.com/extraordinary-suspension-of-citroen-2cv/

the simplicity of which is contrasted by the insanely complex oleopneumatic suspension of the DS http://www.mycitroen.dk/library/ds/red/hydraulics/Chas' Hydraulic Course.pdf

The Citroen Xantia Activa was the latest development of this system: https://www.theautopian.com/the-cit...that-could-out-handle-a-supercar-holy-grails/

BMC (British Leyland) Hydrolastic https://classicmotorsports.com/articles/quick-explanation-bmc-hydrolastic-suspension/

and later Hydragas https://www.aronline.co.uk/cars/mg-cars/mgftf/technical-hydragas-explained/

All of the front-rear interconnected systems rely on having some combination of anti-dive in the front suspension geometry and anti-lift in the rear suspension geometry so that application of the brakes doesn't lead to the front suspension slamming against the bump stops while the rear suspension tops out. The Packard and the Citroen DS and Xantia systems actively manage ride height to compensate (at least to some degree) for passenger and cargo carrying. The others just settle to whatever ride height they end up at.

Thanks for this Brian.

My mind is all over the place on this, I started out thinking hydraulics then, jumped to electric, then back to hydraulics because I thought, maybe I could add some active component with a pump. Then I wondered how fast the hydraulics could act....maybe with a pump and some pressure sensors, I wouldn't need springs.

Thanks again for the links, I'll go through them.

 

[GregLocock]

Yes, hydraulics can hiit several hundred Hz, I used to have a hydraulic shaker for doing modal analysis on big things. However Lotus went the pure hydraulic route and not surprisingly found the tiny theoretical improvements were vastly out weighed by practicalities.

 

[TMcRally]

Situation normal with research projects. https://www.vehicledynamicsinternat...-a-significant-active-dynamics-programme.html Richard occasionally posts on motorsports/dynamics related forums, John died a few years back. The other two are after my time there.

My particular interest in SID was "SI". That is we had the spine and the subframes at each end, and the suspension arms, and the body itself, . Alll the bushes for those were of identical size, and then a French elastomer company knocked up all sorts of bushes that would fit. So the idea was we could investigate whether isolated subframes (pah) gave better secondary ride than putting the isolation in the suspension, or whether we should have a rigid gokart with the body isolated from it (and so on and so forth). Sadly I left in the hiatus, and I think they ended up just running the original config.

This investigation was sparked by many experiences with the GM large rear wheel drive chassis (Opel Omega etc). I would routinely knock out the subframe bushes and fit nylon or aluminium pucks instead, for a vast improvement in secondary ride.

Fast forward 12 years or so and we launched a RWD car with a rigidly mounted rear suspension 'subframe', and it was just as excellent as I had expected, and my NVH partner in crime Mr Sherry had modelled. Bolts straight to the frame, vertically, not bushes. As you can see the drag link has a nice big soft bush for NVH, and it makes no odds for handling.

View attachment 5358
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I watched a video of Alan Jones driving a Williams with no suspension. He wasn't too happy with the ride .

But it was quicker, I'm assuming it was too hard on the car to continue with and with little controls for individual track/driver setup. He said it might be ok with suspension under the seat.

I tried solid mounting the rear suspension in a Subaru and I was very surprised by how much difference it made, I spun on exit in the first turn on a warm up lap, caught me completely by surprise. I should have persevered with it to see where it led but didn't.

 

[gruntguru]

I tried solid mounting the rear suspension in a Subaru and I was very surprised by how much difference it made, I spun on exit in the first turn on a warm up lap, caught me completely by surprise. I should have persevered with it to see where it led but didn't.

Ouch - not much to do with frequency response. Simple case of roll moment distribution going near 100% rear.

 

[TMcRally]

Ouch - not much to do with frequency response. Simple case of roll moment distribution going near 100% rear.

Sorry, I meant solid mount the subframe, along the lines of Greg's post.

 

[GregLocock]

They might have used the subframe mounts to give some compliance steer, so you may have reduced the linear understeer margin.

That being said I prefer to do most of the understeer heavy lifting at the front end, but sometimes you just have to grab what you can. For Bronco we had some pretty aggressive understeer targets and I pumped in as much roll steer at the rear as I could. That's the only program I've ever worked on when we didn't have the traditional tear up and bun fight over restoring the understeer. Obviously you don't want roll steer on a rough road vehicle, but it all worked out in the end.