C5 X7 rear hydro shock specs for use in hotrod?

[Josh of Ohio]

Hello everyone.

Wondering if someone has knowledge of or access to a C5 X7 rear shock for a hydractive suspension.
I've got some odd questions and I'm hoping you might entertain my curiosity.

I'm contemplating if I might be able to use some Citroen components to do a custom hydro-pneumatic suspension on my 1946 Cadillac. I was trying to figure out a way to incorporate a hydraulic cylinder linked in series to a coilover like a cantilever suspension when I found the Citroen system. Now I'm enamored.
I want the ride quality and height adjustability, and I'm debating if I could design a system to make the car lean into corners for some added fun.

I wont be able to use the hydractive brain, but I think I've got an idea how I can have basic height control reusing the stock pump and valves and an air ride controller and sensors.
After digging through various Citroen and Toyota systems I found the C5 X7 rear shock hydro unit that seems to be of an eye-eye mount configuration which I might be able to use it on all 4 corners.
I'm not likely to find one here in the US to study, so here is my list of questions if anyone would be so kind as to lend a hand I would really appreciate the effort. I've already found a lot of information snooping around the archives.

best picture i can find has the coil, not the hydractive


Questions about how it is setup in the C5:
When installed in the C5 and sitting at normal ride height, what is the approximate angle of the shock to the long #9 lower link? (looks 45°ish)
What is the length of that number 9 link from center to center of the mount points? (maybe 300mm ish?)
How far inboard from the number 9 link outer eye is the shock mount center if you draw a line up from the shock eye to the link bar how long is the short end of the link? (looks something like 50mm)
I'm trying to calculate what the shock extension to suspension height relationship would be. For example if it is 45°, 300mm, 50mm then each 2 units of shock extension will get 1.2 units height change body to wheel.

Questions about the shock itself:
What is the extended length from the center of one eye to the other?
What is the compressed length from the center of one eye to the other?
What is the thread size and form for the Sphere? Does it match C6, because I might have to run separate dampers?
Any idea what the piston diameter is OR shock body OD of the main tube if no-one knows the piston?

Does anyone have a removed C5 rear shock they have no use for that they might want to sell me? Doesn't need to be usable, just intact and extendable.

Also working on a way to get the nitrogen pre-load in the sphere right for the load in the car.

Apologies for the flurry of questions.
Josh

[Stickyfinger]

Interesting.....

Here is a drawing of the rear strut/cylinder...

Now to burst your bubble............sorry, but I think you are thinking that it is all down to the strut and the sphere, these really are just parts of the working system.

The reason the Citroen suspension system works is because it is complicated, valve blocks, height sensors, electro control valves, even the internal diameters of various pipes to control flow etc. Add to all of that the method of control by the various ECU's which is milliseconds via the electric solenoids on the valve control blocks.
ALL of that is needed to run a system that responds in the way it does, none of that is remotely possible with the relatively simple and slow valve control of an Air-Bag type control unit. I also suspect that such systems would also not work, even explode, under the very high pressures required in the Citron systems.

"Sphere gas pressure" does not control ride height, spheres control the shock absorption rate by both gas pressure and the sphere neck valve/bore along with the fluid. Height is controlled by the (to simplify a lot) the amount/pressure of the fluid in the system and this is constantly adjusted by the valve blocks releasing it from the pump/accumulator part of the system. Fluid return is also controlled via these valve blocks.

You may wish to consider a more analogue system using earlier types of the suspension.
http://www.citroen-ds-id.com/ds/Landrov ... nsion.html

[steveclv]

I think that you should also be aware that the Hydroactive system operates at over 1,000 psi - jury rigging it together is probably not a good idea.

[Josh of Ohio]

Thanks for the input guys, I appreciate the feedback.
Yes you are correct. I am not attempting to reproduce the full Citroen suspension, just the most basic spring design with height control. I won't get all the eloquence of it with all the cool feedback control they have. I think ill run a seperate damper so I'm not constrained to the stock Citroen rates. If i can fit a damperless sphere.

I want the basic gas spring over hydraulic for packaging and mostly want the hydraulic height control without compromising ride. Airbag systems change height with pressure which compromises ride. Hydraulic systems have no accommodation for spring and damper. I could series a hydraulic cylinder and coiliver in a cantelever design, but the Citroen seems like a more interesting idea integrating them into a small package.
I'm starting understand the basics behind the system well enough. Gas pressure needs to be tuned to corner weight/piston-diameter/mech-advantage from mounting to be in an ideal volume fill in the sphere at static load to be in middle of diaphram stroke. The spring rate changes due to domed sphere geometry and diaphram engagement area and compression of the gas during stroke with height driven from the volume of oil in the hydraulic cylinder. It's a really intriguing system with a gas spring in series with a hydraulic cylinder both operating off the same oil cavity.

I can handle 1000 psi safely. 1000 psi is very low for hydraulic systems. I've designed/tested with systems up to 10,000 psi for work for many years. If i had to engineer something i can, but its all run of the mill and off the shelf type stuff for those pressures. Car brake lines are operating at peaks up to 2000 psi... but no potential energy of accumulators there, so less risk than the suspension.

Any thoughts or insight are greatly appreciated.

I may need to just bite the bullet and buy some stuff to figure out dimensions. Wish I had a US source.

[Josh of Ohio]

Oh I reread your message again.
No intent to use air valves with hydraulic.

My comment was to use an air control brain driving a stock Citroen pump and valve pack. Well, 2 citroen valve/pump packs so I have a hydraulic solenoid valve pair for each corner instead of just front back.

From the air system I would only use the height sensors and output leads through relays to the Citroen solenoids/pump. The control scheme is probably similar enough for basic height control that i wont need to try to write my own programs. At least that's what I'm hoping.

The citroen pump and valves will be right for the shocks.... I've found one example where a guy was running the pump manually with switches.

[Paul-R]

The sphere provides both springing and damping. Reduced to basics, the pressure in the sphere is the spring rate and the diameter of the orifice provides the damping. So, if you're going to have a separate damper unit, then drilling out the orifice to a large diameter will take the damping effect out.

Ride height is determined by a separate hydraulic suspension strut.

All thoughts my own and nothing guaranteed!

[Stickyfinger]

Hi....

I like guys who do not give up ..... off the top of my head I will throw some thoughts your way.

OK....so for "local" (in Yank-Land) contacts try here....
https://citroencarclub.us/about/

For front McPherson strut "type" you are looking at the following cars (parts are reasonably cheap/can be found)
BX, XM, Xantia, C5 Mk1/2 and the much more complicated (and $$$) C5x7 & C6 cars...... BX (all and some early XANTA/XM's) have components that do not have Hydractive 1or2 systems (ECU control) added and are much simpler in their design.
The above used swing-arm rears with short "stubby" type pistons...... but ?? fronts used as rear shocks for example.
You will NEED to use LHM oil for all such components/spheres to make them work correctly and prevent rot to sphere diaphragms/seals

The C5x7/C6 use strut type pistons front & rear......these are expensive, use LDS fluid (not LHM) and are now getting hard to find.

You may wish to consider the simple BX design as a starting place for your design.
http://www.citroenet.org.uk/miscellaneo ... ics-9.html

Rather than trying to reinvent it, you can still source all of the components/valve blocks etc (even if 2nd hand) and run your own lines to them.
The purchase of Citroen size piping and a Citroen pipe flair tool (you will need this to fit to ALL Citroen items (3.5mm & 4.5mm pipe with a [tailed] bubble flair).

All low pressure return lines can use air suspension tube and push fit connections IF they have LHM safe seals..... I can point you to a supplier (food container filling machines use similar push-fit fittings to air suspension but have oil seals that are mineral oil safe.

NOTE: All lines from the Pumps should be made in steel tube as Copper/Nickel WILL fracture due to the design of the pump/output pluses
NOTE: A "Simple" CX/DS type pump my be better, those of a BX (early) Xantia pump might be able to be adapted.

I would think the above would save a lot of time and $$$'s...........all depends on your budget/cost of your time.

[steveclv]

Chinese knock off C5 struts can be bought relatively cheaply from Aliexpress and spheres are available from https://www.sphere-discount.com/ who I am sure would ship to the US.
LDS tanks are available new from Poland via eBay - pumps are available used but not cheap - you might be able to source used ones but give them a strip down and clean first.

[Stickyfinger]

BUT, controlling them (leak back etc), would be hard work/need valve blocks under ecu control etc ?...?? not sure ??

[Josh of Ohio]

The OE valve block has 2 solenoid valves per circuit. 2 front and 2 back. First one is supply that also triggers the pump to run, and the second solenoid valve drains to tank. I can remove the OE control board and reuse the valve pack with the leads from the solenoids. I won't need the quock response time of the OE system since it will just be occasional height changes, so I think i can skip the accumulator sphere.

Air systems work the same way with 2 solenoid valves per corner in a control loop with ride height sensors. That's why I think I might be able to use their controller for simple height control.

It would be close to how the hydractive 3 system functions, not the 3+.
Except I'll plan to control each corner independently to simplify control, but at higher cost. The height sensors on the air systems are designed to work on control arms instead of the single sensor on each sway bar the hydractive design.

I'm a little worried about ride stiffness. I'll have to permanently choose either stiff or soft. For soft I'd have to rig 2 spheres per corner. But it won't have the crossover like the 3+, so the handling in soft won't take nearly as much detriment. Damping will also be static. I'm guessing the implications of the hydraulic crossover is why they have the complicated valving dynamically switching between stiff and soft while driving. You don't want all your hydro going to the wrong side through the crossover in a turn. That's why they close the crossover valve while turning.
2 spheres would also keep the strain in the diaphram low benefitting longevity.
I know for air systems a bigger bag area with lower pressure is preferred for ride. Not sure if that will equate but the logic is similar. And it matches the Hydractive engaging the second sphere, but I need to take a closer look at the pre-charge pressures so figure what they're doing.

I'm thinking to use the c5 rear shocks with either the c6 spheres or using center spheres on the shock because both of those dont have dampers integrated.
Depending where the mechanical advantage ends up I'd prefer to use stock pre-pressurized spheres. Otherwise I'll have to vent and add a fill valve to set a custom sphere pressure. Might be necessary anyway to keep aftermarket spheres in spec with the N2 leaking.

It's fun trying to figure out.

[steveclv]

Why not have the second sphere on each corner controlled through it's own solenoid? Switched in would be soft and out is hard - same principle as H3+.
Overall control would be pretty straightforward with an arduino - you could play some pretty tunes with it.

[Kees]

@Steve:
Sorry, but you are wrong.
The stiffness regulator on the C5 switch off the middle sphere when 12 V apply to the coil, so the suspension is hard and soft without the supply.
I have disconnect these stiffness regulator on my C5 3.0/24. So my car is always in the soft mode.
Greetings,
Kees.

[Kees]

@josh:
The 3 and 3+ system have no cross over system like the XM or Xantia.
The only thing in the 3+ system is the stiffness regulator with switch off the center sphere.
Greetings,
Kees.

[steveclv]

@kees - yes, you are correct - it was just the semantics of in/out

[Josh of Ohio]

I'm mainly using these videos for reference. Seems to be citroen technician training videos. They're great for trying to understand the suspension at a high level. This is my kind of entertainment.

Hydractive 3 and 3+


Hydropneumatic


The hydractive 3 does not have the stiffness regulator cross over, but the 3+ does. The 3 is permanently in stiff/sport mode.

In soft or comfort mode each wheel is exposed to all 3 spheres on the axle through 3 different damping levels all in parallel.
In sport mode each wheel is only exposed to its single sphere and damper as the stiffness regulator closes the crossover circuit and blocks off the 3rd sphere.
I also think of sport mode as safe handling mode because turning with the crossover open would not be good.

Sphere charge pressure sets the preload to get the diaphram in the desired position within its flex range after you load it on the car. N2 Preload doesn't change pressure in use (unless you exceed the pressure the car puts in the oil from loading, but then you no longer have an effective spring)
The resulting pressure in all the spheres in use on the car is dictated by vehicle weight per axle which causes the oil pressure. Exposure to additional spheres at the same in-use loaded pressure will increase diaphram surface area exposed, which reduces the effective dynamic increase of spring rate making the ride softer. More diaphram surface area means lower deflection of the diaphrams by the same oil displacement from the shock. Less diaphram deflection, because you're deflecting more diaphrams, at the same pressure result in a slower decrease in sphere gas volume which is what drives the non-linear spring rate increase during compression. With more spheres the spring doesn't increase stiffness as fast during wheel movement.

A similar reason makes higher precharged spheres increase stiffness more slowly and ride softer. The initial in-use loaded gas volume is larger at higher pre-pressure of the N2, so the same wheel movement is a smaller percentage of gas volume compression which is the non-linear curve for spring rate increase.
That makes sense now why the center stiffness regulator spheres are higher precharge.
These things are soo cool.

I could run 2 spheres per corner and be somewhere between sport and comfort for the spring rate curve. Maybe start with 1 and go from there.
Damping I expect to use a Manually dual-adjustable seperate shock absorber so I can dial in what I like for compression and rebound.

I'm unclear how the H 3 suspension isolates driver from passenger and yet only has one solenoid for height per axle. There must be a way to leak slowly from one side to the other after check valves or something. Irrelevant if I run 4 solenoid valves with 4 height sensors.

Thanks for all the thoughts!
You're making me think deeper.

I love the arduino idea. Part of me is dying to add a lot of cool complication. But maybe I need to keep those dark thoughts to future experiments after I get something that can hold the car up.