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Here is a link to the thread in the fsae forum that Harry mentioned: http://www.fsae.com/forums/showthread.php?8950-Suspension-Design/page25

Post #250 was rather helpful.

going back to the 50's, alex issigonis designed a suspension system for the trusty B.M.C. mini, using rubber springs, this however was changed to the hydrolastic system which inter connected the front to the back on one side and the same on the other, with no accumolators in the system.

in later years they were separated to prevent strange effects in heave, each "spring" was now filled independantly to a specific ride hight.

the fronts were acting vertically and the rears were horizontal, the front used a much shorter lever to operate than the back due to the front engine transverse design and the lack of much weight at the rear but essentially used the same "spring" although the pressures were not necesarily the same front to rear dependant on the amount of fluid required to raise the vehicle ride hight to the required setting.

essentially it worked like a modern air bag system but used the Citroen fluid.

to fill the individual chamber springs, we made a hand operated pump from a brake master cylinder with a remote resovoir but with a large windshield washer tank to feed it and a hand lever to operate the master cylinder, mounted in a frame.

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this story shall the good man teach his son,
and chrispin chrispian shall ne'er go by,
from this day to the end of the world.
but we in it shall be remembered.

@Andreas, you should read the whole thread, some really really useful insight and much food for thought on suspensions...

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Regards,
Harry

John, thanks for that info -- very helpful I revisited some sites I found before and learned a bit more too.

Harry, I did read the whole thread! I was just linking to the most relevant post for this thread. ....and yes, there is lots of good stuff on that thread -- my mind was opened up! Now I know more about the different suspension types. My car will be for the street mostly, but want it to do (much) better than the average street car on the track. I am still open to a suspension linkage type.

I came up with this idea, but I don't think it is unique - although maybe a variation of what has already been done. The solenoids would be activated depending on steering wheel, brake and accelerator pedal input along with speed. See the image below. The nice thing is that pressures and fluid levels can be changed to affect firmness and travel (to setup for road or track)....and on top of that you have firming of roll or pitch with steering or braking. You also get overall heave control and no warp dampening. At least, that is what I think will happen. Any comments are welcome.

active suspension really throws the book out the window, roll center and camber change it causes are no longer the same.

if a conventional system were used the sway bars would allow tuning in this area but that no longer applies, when throwing the book out the window, be sure to not throw the baby out with the bath water

_________________
this story shall the good man teach his son,
and chrispin chrispian shall ne'er go by,
from this day to the end of the world.
but we in it shall be remembered.

John, are you saying that active suspension is bad, my design is bad, or both need further study/design? I am fine with either, but over the internet it is sometimes difficult to get someone's meaning. I don't mind hearing my design is bad if I know why or where to look.

The design I sketched does not include the actual suspension linkages (double wishbone, etc) because I am not sure of their details for now. I just connected the tires to the hydraulic actuators so that the direction of their action could be visualized.

I also didn't use any flow restrictors in the diagram, but plan to....probably at least two one-way manually variable restrictors for each actuator so I can adjust rebound and bump.

Andreas,

the complexity of the proposed system is the problem, to react to the variouse steering inputs whilst handling the various road conditions will require a lot of computer power and i doubt that a solenoid system will be able to cope.

the complexity for the linkage will involve many components and the overall weight of the system will in my opinion be a non starter and trying to limit this has resulted in a system that is less than perfect.

this has been tried before by several manufacturers but almost all have returned to a basic spring and of late have introduced magnetic fluid in shock absorbers to control the springs for push button adjustments of the ride quality and the rapid and diverse adjustments required in a rapid frequency.

i commend you for attempting to design a better mouse trap, that is what this forum is all about and wish you good luck in your endevores.

having said the above, for ease of design and ease of manufacture i would suggest that the inter connection of the wheel units be dispenced with and you concentrate on a hydraulic chamber for each wheel connected to a nitrogen filled accumolator which can be pressurised externally either by a hand pump for static pressure or by an engine or electrical pump with an overall controll valve and hight limit "switches" to each wheel unit, the "spring" pressure would be controlled by the size of the accumolator for each wheel, however for ease of manufacture, the accumolators should remain the same for all four corners, likewise the hydraulic chambers and variations dealt within the linkage.

you might find interesting information by looking at the Mercedes load leveling system used in the 300TE from the 90's, this only operates on the rear but i am sure the principals are the same for all four wheels.

it must be noted that Mercedes still used a rear spring and a sway bar due to the rapid fluctuations needed to respond to the road conditions which the hydraulics could not supply.

_________________
this story shall the good man teach his son,
and chrispin chrispian shall ne'er go by,
from this day to the end of the world.
but we in it shall be remembered.

this is a BMC 1100

this system relys on the red actuators as the accumolators, they are made of rubber and the fluid causes them to expand in diameter when the vehicle hits a bump and compresses them axially.

the front and rear are inter connected but they are not connected side to side.

_________________
this story shall the good man teach his son,
and chrispin chrispian shall ne'er go by,
from this day to the end of the world.
but we in it shall be remembered.

Thanks for your response, John.

Production cars have different goals, but I will say that Citroen still uses a similar type of hydropneumatic suspension. It is more complicated than I need though and their system is set up more for varying loads and comfort. I have no problem adjusting air and fluid levels manually for balance and firmness so I don't need an onboard hydraulic pump, etc.

I also don't plan on anything complicated with the above system. If the driver is turning the steering wheel, the two sides are decoupled. If the driver is pressing the gas or the brake, the front and rear are decoupled. If you decoupled all four wheels you basically have a conventional suspension with a stiffer spring. With all of them coupled, a single wheel is (basically) sprung four times less since it can use up four times the air volume in the four accumulators before getting to the same pressure.

Mind you, I am no expert. I am willing to think about it and experiment to see if it works.

Ooops, now I see you have edited your post.

I did see the BMC 1100 before after you mentioned it. The two sides are connected which is great for roll as you can get twice the stiffness in roll as in an individual wheel (bump). The problem I see is when you press on the brake or gas and get a change of pitch.

Andreas, the primary goal of the BMC system was to make the short wheelbase ride over bumpy roads more like a longer wheelbase car. In this respect, they were very successful. I am no expert on these, however I believe that there is no cross connection left to right.


Linked image from: http://www.elevenhundred.com/hydro.php

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Chuck.

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I meant the two rights (FR, RR) were connected and the two lefts were connected (FL, RL). Oops, I see how it was misinterpreted.

Gotcha!

_________________
Chuck.

“Any suspension will work if you don’t let it.” - Colin Chapman

Visit my ongoing MGB Rustoration log: over HERE

Or my Wankel powered Locost log : over HERE

And don't forget my Cushman Truckster resto Locostusa.com/forums/viewtopic.php?f=36&t=17766

in later vehicles, like the metro and minis before they were built by BMW, they completely disconnected the front from the rear and just relied upon the deformation of the units.

just to re-enforce what i have said, all the competition cars now replace the hydro units with conventional springs and shocks.

the very early minis used aeon rubber springs and shocks, no hydrolastic, these were known as dry minis instead of hydrolastic which were known as wet minis, these dry cars were prefered over the wet cars for competition use.

_________________
this story shall the good man teach his son,
and chrispin chrispian shall ne'er go by,
from this day to the end of the world.
but we in it shall be remembered.

John, do you think the swapping out of the suspensions could be due to the low spring rate that is probably inherent in a rubber bladder type system? Removing the linkage between the wheels would stiffen the spring rate too.

The Citroen hydropneumatic system is closer to what I would be implementing and I think I read that they have used it in racing, but not sure how much. Also the McLarens mentioned earlier have interconnected hydro pneumatic suspensions, so I don't think it is necessarily inferior. Is it easy to implement? Probably not, at least not without some tinkering.