Citroen hydro suspension on high performance cars?
Discussion
Just thinking about this, and wondering if anyone has ever used citroen hydro suspension on seriously high performance cars? If not, why not?!
I've been sat here thinking about my suspension again. I dont like putting coil overs in from outer bottom to inner top mounting positions as it obviously fights the natural requirement for progressive springing. Obviously I could just have inboard coil overs which will partly negate this, but due to the rear sphere's just packing up on my xantia they popped into my head as another alternative. With the Active version, you can adjust the ride quality as well as height, so I cant think of a reason not to use them other than the minor complication of adding a pump and another resevoir in the engine bay. Advantages are you could instantly set & adjust ride height and stiffness from the comfort of your seat when you arrive at a track, and if you find a speed hump on the road a wee bit high, you can just pump up the height and off you go! I've never driven a high performance Citroen for obvious reasons but the ride is fantastic and handling pretty good even in standard setup on those I have.
Thoughts?
I've been sat here thinking about my suspension again. I dont like putting coil overs in from outer bottom to inner top mounting positions as it obviously fights the natural requirement for progressive springing. Obviously I could just have inboard coil overs which will partly negate this, but due to the rear sphere's just packing up on my xantia they popped into my head as another alternative. With the Active version, you can adjust the ride quality as well as height, so I cant think of a reason not to use them other than the minor complication of adding a pump and another resevoir in the engine bay. Advantages are you could instantly set & adjust ride height and stiffness from the comfort of your seat when you arrive at a track, and if you find a speed hump on the road a wee bit high, you can just pump up the height and off you go! I've never driven a high performance Citroen for obvious reasons but the ride is fantastic and handling pretty good even in standard setup on those I have.
Thoughts?
Edited by davi on Friday 7th July 18:39
weight... OK you have some minor added weight, but not a great deal.
Complexity... I'm not too phased by that side, other than powering the pump it's really simple to plumb in and set up.
cost... that was another of the reasons for thinking of it - £60 for the 4 sphere's and you can replace them in 30 seconds, and the pumps, resevoirs etc can be got from various places for a fraction of the cost of genuine ones.
I' more concerned with if I'd be able to get the suspension characteristics out of it needed. Definitely worth more think time I reckon.
Complexity... I'm not too phased by that side, other than powering the pump it's really simple to plumb in and set up.
cost... that was another of the reasons for thinking of it - £60 for the 4 sphere's and you can replace them in 30 seconds, and the pumps, resevoirs etc can be got from various places for a fraction of the cost of genuine ones.
I' more concerned with if I'd be able to get the suspension characteristics out of it needed. Definitely worth more think time I reckon.
GTM K3 and Mk3 and later Midas' were Metro based using hydragas units. Not as sophisticated but gives a pointer to what can be achieved. One mod was to replace standard fluid with Citroen equivalent.
Had a Mk3 Midas for a few years and a Xantia. Always wondered what a Libra would be like with a bit of fluid between the wheels and gas at each corner. Adjust the ride height from the drivers seat.
Had a Mk3 Midas for a few years and a Xantia. Always wondered what a Libra would be like with a bit of fluid between the wheels and gas at each corner. Adjust the ride height from the drivers seat.
Will your kit be about the same weight as a Xantia? If not, you'll have to experiment with the pressures in the spheres. As I understand it, the fact that the sphere is the shape it is means that you get a non-linear rising rate spring because the cross sectional area of the nitrogen bag that the fluid pushesagainst changes as it rises up the sphere. I'm not certain but I think the more sophisticated Xantias only have dual rate suspension - I don't think you can actually adjust the rate as such. There is a third sphere at each end and some solenoid valves. When the car senses that you're driving sedately, the valves are open and the third sphere is "in series" wit hthe other two. Like any springs in series, the rate gets lower. When you thrash the car, the valves close and the third sphere is taken out of circuit so that the spring rate is effectively increased. All the electronic gubbins for this might be quite hard to sort out. I think there is a steering wheel angle sensor, a raod speed sensor, a throttle position sensor, a yaw rate sensor and a lateral acceleration sensor. All these between them send the information to the central control unit to decide when to switch to "sport" mode.
Another thing to consider will be damping. I think the Citroen uses restrictors in the neck of each sphere. These have spring steel washers either side that bend out of the way to let fluid through the various galleries. I think you can alter the damping by adding or removing washers.
In adition to the Citroen pump, you'll also need the pressure regulator and main accumulator, I'd guess. Depending on what engine you're using, you might need a heavy flywheel to help it not to stall when the pump cuts in at idle (although the idle speed control valve might be able to react quickly enough). Also, to make it sit at whatever ride ehight you wanted, you'd need the height correctors and associated linkages to front and rear anti-roll bars.
Finally, you might need a pretty high bonnet line to accommodate the front struts.
Another thing to consider will be damping. I think the Citroen uses restrictors in the neck of each sphere. These have spring steel washers either side that bend out of the way to let fluid through the various galleries. I think you can alter the damping by adding or removing washers.
In adition to the Citroen pump, you'll also need the pressure regulator and main accumulator, I'd guess. Depending on what engine you're using, you might need a heavy flywheel to help it not to stall when the pump cuts in at idle (although the idle speed control valve might be able to react quickly enough). Also, to make it sit at whatever ride ehight you wanted, you'd need the height correctors and associated linkages to front and rear anti-roll bars.
Finally, you might need a pretty high bonnet line to accommodate the front struts.
some very valid points in there Avocet, just started reading a bit more detail on the Xantia setup. I've stripped/worked on various lower spec citroen systems before so it's going to be interesting to see the differences and what can and cant be done - will comment more after further reading!
My thougths are the front struts would be discarded, the sphere's positioned horizontally much like on the rear.
My thougths are the front struts would be discarded, the sphere's positioned horizontally much like on the rear.
davi said:
I dont like putting coil overs in from outer bottom to inner top mounting positions as it obviously fights the natural requirement for progressive springing.
In what way? Most double wishbone setups with an inclined spring have a natrualy rising rate, albeit a very small rising rate. Progressivley wound springs are the simple answer, or if you want to get complex have an inboard shock with push/pull rod and bellcrank giving you whaever rising rate you desire.
MR2Mike said:
Most double wishbone setups with an inclined spring have a naturally rising rate, albeit a very small rising rate.
It is absurdly early on the morning and my brain hasn't woken up, yet, but unless I'm very much mistaken, most double wishbone setups with an inclined outboard spring have a naturally falling rate. As the spring is compressed, the angle of the damper tends more and more toward the horizontal.
I used to have a few big old Citroens some years ago - 2 DSs and a couple of CXs. My wife had a pikey-spec Xantia so it didn't have any of the "hydractive" stuff on it but your project sounds really interesting. It's true you can get round the falling rate of a typical double wishbone setup with rising rate springs but this sounds like a cool thing to try just for the hell of it. I doubt it will deliver any real performance advantages otherwise major manufacturers would have used this sort of setup more often.
I can't remember on the Xantia rear but the CX and DS both had a bellcrank lever as part of the rear trailing arm that pushed against a pushrod which poked up inside the suspension cylinder. As far as I can remember, the pushrod only had a crappy bent wire clip holding it in position - i.e. if the wheel ever moved down faster than the cylinder could move, the pushrod would just drop out! Clearly, it was never a problem for Citroen but if you change the leverage ratio by much, it might be an issue for you. I suppose it also means that there can never be any rebound damping - in other words, the suspension strut would never be able to exert any kind of "tensile" force to prevent the wheel just "free-falling" into a pothole. The old Citroens had incredibly high leverage ratios - about 8 or 10 to one, so they never used to have rubber suspension bushes, they always had taper roller bearings in the pivots.
I can't remember on the Xantia rear but the CX and DS both had a bellcrank lever as part of the rear trailing arm that pushed against a pushrod which poked up inside the suspension cylinder. As far as I can remember, the pushrod only had a crappy bent wire clip holding it in position - i.e. if the wheel ever moved down faster than the cylinder could move, the pushrod would just drop out! Clearly, it was never a problem for Citroen but if you change the leverage ratio by much, it might be an issue for you. I suppose it also means that there can never be any rebound damping - in other words, the suspension strut would never be able to exert any kind of "tensile" force to prevent the wheel just "free-falling" into a pothole. The old Citroens had incredibly high leverage ratios - about 8 or 10 to one, so they never used to have rubber suspension bushes, they always had taper roller bearings in the pivots.
Sam_68 said:
most double wishbone setups with an inclined outboard spring have a naturally falling rate. As the spring is compressed, the angle of the damper tends more and more toward the horizontal.
Yup, that's why I dont like em, you are fighting a problem that needn't exist. avocet said:
I used to have a few big old Citroens some years ago - 2 DSs and a couple of CXs. My wife had a pikey-spec Xantia so it didn't have any of the "hydractive" stuff on it but your project sounds really interesting. It's true you can get round the falling rate of a typical double wishbone setup with rising rate springs but this sounds like a cool thing to try just for the hell of it. I doubt it will deliver any real performance advantages otherwise major manufacturers would have used this sort of setup more often.
I can't remember on the Xantia rear but the CX and DS both had a bellcrank lever as part of the rear trailing arm that pushed against a pushrod which poked up inside the suspension cylinder. As far as I can remember, the pushrod only had a crappy bent wire clip holding it in position - i.e. if the wheel ever moved down faster than the cylinder could move, the pushrod would just drop out! Clearly, it was never a problem for Citroen but if you change the leverage ratio by much, it might be an issue for you. I suppose it also means that there can never be any rebound damping - in other words, the suspension strut would never be able to exert any kind of "tensile" force to prevent the wheel just "free-falling" into a pothole. The old Citroens had incredibly high leverage ratios - about 8 or 10 to one, so they never used to have rubber suspension bushes, they always had taper roller bearings in the pivots.
I can't remember on the Xantia rear but the CX and DS both had a bellcrank lever as part of the rear trailing arm that pushed against a pushrod which poked up inside the suspension cylinder. As far as I can remember, the pushrod only had a crappy bent wire clip holding it in position - i.e. if the wheel ever moved down faster than the cylinder could move, the pushrod would just drop out! Clearly, it was never a problem for Citroen but if you change the leverage ratio by much, it might be an issue for you. I suppose it also means that there can never be any rebound damping - in other words, the suspension strut would never be able to exert any kind of "tensile" force to prevent the wheel just "free-falling" into a pothole. The old Citroens had incredibly high leverage ratios - about 8 or 10 to one, so they never used to have rubber suspension bushes, they always had taper roller bearings in the pivots.
Thanks Avocet. I think I'd probably want to design it with an "easy out" route if it was pants, but it's just one of those idea's that felt so right to try! It'll need some proper investigating first, but I think it could be very interesting. I'd not really be looking for any performance advantages either, as long as it was on par with a standard set up I'd be happy, the adjustable ride height would be a bonus to boot!
I was speaking for quite some time with a chap from Hulme today, their roadgoing version is to have adjustable ride height with adjustable stiffness from a selector control inside... he wouldn't say anymore than that unfortunately, but I'd like to know what they are doing!
Sam_68 said:
MR2Mike said:
Most double wishbone setups with an inclined spring have a naturally rising rate, albeit a very small rising rate.
It is absurdly early on the morning and my brain hasn't woken up, yet, but unless I'm very much mistaken, most double wishbone setups with an inclined outboard spring have a naturally falling rate. As the spring is compressed, the angle of the damper tends more and more toward the horizontal.
Sam,
Do the sums, it takes a fair ammount of tilt from the vertical to give falling rate over the first few inches. I was surprised just how much you needed. The way I did it was to take the first and the third inch of suspension travel and calculate spring length change for a unit change in wheel position. I found that a very small rising rate was more likely unless a big spring angle was used.
cymtriks said:
Sam_68 said:
MR2Mike said:
Most double wishbone setups with an inclined spring have a naturally rising rate, albeit a very small rising rate.
It is absurdly early on the morning and my brain hasn't woken up, yet, but unless I'm very much mistaken, most double wishbone setups with an inclined outboard spring have a naturally falling rate. As the spring is compressed, the angle of the damper tends more and more toward the horizontal.
Sam,
Do the sums, it takes a fair ammount of tilt from the vertical to give falling rate over the first few inches. I was surprised just how much you needed. The way I did it was to take the first and the third inch of suspension travel and calculate spring length change for a unit change in wheel position. I found that a very small rising rate was more likely unless a big spring angle was used.
That's true enough, though with most cars of this type, they tend to have quite a heavily inclined spring due to space, look at the ultima for example, the spring is very heavily canted over and there is definitely a falling rate (unless my sums are wrong, which is highly possible!!!)
striker 20v said:
There's a Guy in Holland building a seven type car (think stuart taylor) with the citroen suspension in it, he's allready to rolling chassis stage so.... it can be done added weight wasn't that many I recall,
Lets look up hi e-mail adres at the dutch kitcarclub.
grtz Thomas
Lets look up hi e-mail adres at the dutch kitcarclub.
grtz Thomas
If you have a link I'd be VERY interested to see more!
davi said:
cymtriks said:
Sam_68 said:
MR2Mike said:
Most double wishbone setups with an inclined spring have a naturally rising rate, albeit a very small rising rate.
It is absurdly early on the morning and my brain hasn't woken up, yet, but unless I'm very much mistaken, most double wishbone setups with an inclined outboard spring have a naturally falling rate. As the spring is compressed, the angle of the damper tends more and more toward the horizontal.
Sam,
Do the sums, it takes a fair ammount of tilt from the vertical to give falling rate over the first few inches. I was surprised just how much you needed. The way I did it was to take the first and the third inch of suspension travel and calculate spring length change for a unit change in wheel position. I found that a very small rising rate was more likely unless a big spring angle was used.
That's true enough, though with most cars of this type, they tend to have quite a heavily inclined spring due to space, look at the ultima for example, the spring is very heavily canted over and there is definitely a falling rate (unless my sums are wrong, which is highly possible!!!)
I've just checked it for a "typical" front set up of-
lower wishbone length 13.5 inches, spring pivot 2 inches from hub ball joint, spring chassis mount 8 inches in and above its lower pivot i.e. at 45 degrees and 11.3 inches as fitted with the lower wishbone parallel to ground.
Over the first inch of hub vertical travel the spring compresses 0.609 inches
Over the second inch of hub vertical travel the spring compresses 0.641 inches
So the rate is rising by over 5 percent over 3 inches even when the initial angle is 45 degrees
From memory of previous calculations you need 50 to 60 degrees to get falling rate.
Sam_68 said:
MR2Mike said:
Most double wishbone setups with an inclined spring have a naturally rising rate, albeit a very small rising rate.
It is absurdly early on the morning and my brain hasn't woken up, yet, but unless I'm very much mistaken, most double wishbone setups with an inclined outboard spring have a naturally falling rate. As the spring is compressed, the angle of the damper tends more and more toward the horizontal.
No, the angle of the damper relative to the ground is totaly immaterial. The angle of the damper with respect the the lower wishbone is the important part, and on most cars this actualy moves closer to perpendicular as the the suspension is compressed, giving the aformentioned rising rate.
Aren't you chaps forgetting to resolve the forces? You need a cosine in there and the angle keeps getting worse as you compress.........
While some would argue with a few things he says, Mr Staniforth has a nice simple explanation of why anything over about 17 degrees inclination is a bad thing (for falling rate).
While some would argue with a few things he says, Mr Staniforth has a nice simple explanation of why anything over about 17 degrees inclination is a bad thing (for falling rate).
CorseChris said:
Aren't you chaps forgetting to resolve the forces? You need a cosine in there and the angle keeps getting worse as you compress.........
While some would argue with a few things he says, Mr Staniforth has a nice simple explanation of why anything over about 17 degrees inclination is a bad thing (for falling rate).
While some would argue with a few things he says, Mr Staniforth has a nice simple explanation of why anything over about 17 degrees inclination is a bad thing (for falling rate).
Took Mr Staniforth's book on holiday with me to read while on the beach. His comment is that Mr Staniforth said:
"The forces that a bottom link can exert upwards onto the coil are at their maximum when the two are at right angles to each other....
...But as soon as that angle begins reducting when the coil begins to lean inwards the spring suffers a steady increasing disadvantage. It is compressed less for given wheel movement, can only exert less force because of this and is seen by the wheel as steadily weaker. The effect is very small to about 15 degrees of inclination, appreciable by 25 degrees and very seriously affecting wheel frequency by 40 degrees...."
...But as soon as that angle begins reducting when the coil begins to lean inwards the spring suffers a steady increasing disadvantage. It is compressed less for given wheel movement, can only exert less force because of this and is seen by the wheel as steadily weaker. The effect is very small to about 15 degrees of inclination, appreciable by 25 degrees and very seriously affecting wheel frequency by 40 degrees...."
He does go on to say it is possible to overcome this with very complex springing, but basically says why bother if you can just shift the geometry so you dont have to.
Yet more reading needed....
Edited by davi on Sunday 23 July 10:59
davi said:
CorseChris said:
Aren't you chaps forgetting to resolve the forces? You need a cosine in there and the angle keeps getting worse as you compress.........
While some would argue with a few things he says, Mr Staniforth has a nice simple explanation of why anything over about 17 degrees inclination is a bad thing (for falling rate).
While some would argue with a few things he says, Mr Staniforth has a nice simple explanation of why anything over about 17 degrees inclination is a bad thing (for falling rate).
Took Mr Staniforth's book on holiday with me to read while on the beach. His comment is that Mr Staniforth said:
"The forces that a bottom link can exert upwards onto the coil are at their maximum when the two are at right angles to each other....
...But as soon as that angle begins reducting when the coil begins to lean inwards the spring suffers a steady increasing disadvantage. It is compressed less for given wheel movement, can only exert less force because of this and is seen by the wheel as steadily weaker. The effect is very small to about 15 degrees of inclination, appreciable by 25 degrees and very seriously affecting wheel frequency by 40 degrees...."
...But as soon as that angle begins reducting when the coil begins to lean inwards the spring suffers a steady increasing disadvantage. It is compressed less for given wheel movement, can only exert less force because of this and is seen by the wheel as steadily weaker. The effect is very small to about 15 degrees of inclination, appreciable by 25 degrees and very seriously affecting wheel frequency by 40 degrees...."
He does go on to say it is possible to overcome this with very complex springing, but basically says why bother if you can just shift the geometry so you dont have to.
Yet more reading needed....
But what he does not do is to calculate how much the spring is compressed for each inch of travel. The lower mount is moving in an arc towards the top mount in two directions, vertically and horizontally and the spring is compressed more for each inch.
I posted the figures above in a bit of a rush and I assumed that the cosine issue wouldn't change things much as the change in angle is only a few degrees. It is however enough to offset that rising rate and to give a falling rate overall but only by a few percent.
A quick check for a 30 degree spring angle with an initial spring length of 12 inches with the wishbone horizontal gives a definite rising rate by a couple of percent over the first few inches including the cosine issue.
Previously I've convinced myself that a lowcost front end could be made that did not have falling rate and that angled springs were not the problem that Staniforth makes out.
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