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  #6  
قديم 16-12-2010, 09:30 AM
cars admire
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عاجل رد: Citroën SM,STUNNING!!




VW Passat W8 4Motion: The most needlessly complicated car ever



I spied one of these at the local Bed, Bath & Beyond parking lot. (Don't I lead an exciting life?)

It's one thing to own a Citroen SM or a 450SEL 6.9L with a complicated suspension. But to stuff a mid-range Volkswagen with a W8 (two V4s sewn together), 4Motion, a Tiptronic transmission, and hundreds of gremlins-feeding electronic gizmos? It's sheer insanity.


VW went through all that trouble and this is the result:
  • A car that is 75% more expensive than a base Passat.
  • Extreme front weight bias.
  • A FOUR liter eight cylinder capable of a whopping 270 hp.
  • Less than 500 were sold in America in 2004.
  • It weights almost 3,900 pounds.
  • No power below 2500 rpm, max torque just 250 rpm later.
Nevertheless, there is a still a small and rabid cult that worships this car. Go figure.


Hydropneumatic Suspension
[hydropneumatic]

Since the late forties, Citroën have been running a fundamentally different system to the rest of the auto industry. Its called hydropneumatic suspension, and it is a whole-car solution which can include the brakes and steering as well as the suspension itself. The core technology of hydropneumatic suspension is as you might guess from the name, hydraulics. Ultra-smooth suspension is provided by the fluid's interaction with a pressurised gas, and in this respect, its very similar to the hydragas system described above. Citroën pioneered the system in the rear suspension of the 15 (Traction Avant) model, and it has been fitted to many of their cars since. Because of the complexity of the system, the rest of this section gets a bit wordy but hopefully not so much that I'll lose you half way through. Because this page is about all types of suspension, for clarity I decided to concentrate on the simplified version of this as installed in the "BX" model. If you're desperate to know every last nut and bolt of hydropneumatics, just do a google search for it. On we go....

The system is powered by a large hydraulic pump, typically belt-driven by the engine like an alternator or an air conditioner. the pump provides fluid to an accumulator at pressure, where it is stored ready to be delivered to servo a system. This pump is also used for the power steering and the brakes, and in the DS for the semi-automatic gearbox.
The BX was a major turning point in Citroën's history as it was the first car to be produced under the company's new Peugeot management, following the 1970s take-over. As a direct consequence of the Peugeot influence, the car was somewhat more conventional than its bulkier predecessors like the CX. This Peugeot-enforced "normalisation" of the design makes it fairly easy to examine as an illustration of how hydropneumatic suspension works.
Apart from the pump, the two most obvious components in the system are the spheres on top of each suspension strut, and the struts themselves. The spheres are like the springs in regular suspension, and the struts are the hydraulic components that make the fluid act like a spring.
The spring in this suspension system is provided by a hydraulic component called an accumulator, which is gas (typically nitrogen) under pressure in a bottle contained within a diaphragm. This is effectively a balloon which allows pressurised fluid to compress the gas, and then as pressure drops the gas pushes the fluid back to keep the system's pressure up. In the image here, the nitrogen gas is represented in red and the LHM fluid is represented in green. As the pressure in the fluid overcomes the gas pressure, the nitrogen is compressed by the diaphragm being pushed back. Then as the pressure in the fluid reduces, the gas pushes back the diaphragm which expels the fluid from the sphere, returning gas and fluid to equilibrium. This is the hydropneumatic equivalent to the spring being compressed and then rebounding.

So how can the interaction of compressing gas, hydraulic fluid and a diaphragm form a spring? Simple(ish): The pressure of the gas is the equivalent to the spring weight. The inlet hole at the bottom of the sphere restricts the flow of the fluid and provides an element of damping. By replacing the spheres for ones of different specifications, it's possible to adjust the ride characteristics of these cars.




Before we go any further it is pretty important that you understand where the fluid acting on the diaphragm in the sphere gets its force from, and to do that we are going to have to look at the operation of the other key component in the Citroën system - the strut.
The sphere in these systems is actually mounted at the end of the strut. The strut itself acts like a syringe to inject fluid into the sphere. When the wheel hits a bump it rises, pushes the piston back and this squeezes fluid through the tiny hole in the sphere to let the gas spring absorb the energy of the bump. Then when the car is over the bump, the gas pushes the diaphragm back out, pushing the fluid down to the strut, pushing the wheel down to the ground.
Some interesting possibilities were opened up when Citroën decided to use this system to spring their cars. One or two of the more obvious ones are that since the system is hydraulic, the ride height can easily be altered; Citroën put fancy valves called height correctors in the system. They are designed to correct for long-term/static errors in height. To do this there is a clamp on the middle of each roll bar connected by a linkage to the height corrector. This linkage varies by model - on DS, CX, GS, BX it is a simple torsion bar about 8mm diameter and about 400mm long, on the XM and Xantia it is a coil spring assembly with a double acting override linkage, but the functionality is the same. By measuring the height at the middle of the rollbar, it automatically takes the average of the left and right wheel height on that axle, and therefore cannot detect body roll. This prevents it from spuriously trying to react to body roll, as it can't do anything to counter it anyway - it can only make both sides go up or down together.
Additionally the height correctors have a hydraulic damping chamber in them which restricts and delays their movement - typically it takes a suspension movement of at least 20mm in one direction for at least 5 seconds before the height corrector will respond. Even fully bottoming the suspension still takes at least 5 seconds for a response.
This works as a simple averaging system and prevents the height correctors from responding to bumps or road undulations, (which would be undesirable). The slight exception here is the rear suspension which is subject to squat due to acceleration because of the front wheel drive. Prolonged heavy accleration of more than 5 seconds (particularly noticable on an automatic) will cause a height correction response - an undesirable side effect. (Hydractive 2 models take steps to try and avoid this response by stiffening the suspension during heavy acceleration).

Another noteworthy feature of Citroën system is its ability to "pre-set" a car for bumps in the road, keeping the car on an even keel. This is a result of the cross-piping between left and right struts on the same axle. They are connected permanently via a 3.5mm pipe, (except in Hydractive and Activa systems). The height corrector connects to a T-junction of this cross piping, but when the height corrector is "closed" (which is nearly all the time while driving) it represents a dead end, so only the piping from left to right comes into play. When the wheel on one side hits a bump some oil will flow into the sphere on that side via the damping valve, and some will flow across to the other side and extend the wheel on that side, which gives a slight roll stabalizing response. This tends to make the car more steady in the roll axis, and reduces the side to side rocking motion on transverse undulations.
A side effect of this cross piping is that it gives the suspension very soft compliance for "warp mode" movements, as the suspension spheres (springing) don't resist slow roll movements like conventional springs do - only the rollbar does. (This improves traction a lot at very slow speeds over very uneven ground) In fact without the rollbars the suspension would be completely unstable on the roll axis - you could sit on the left and it would go right down and the other side would go right up...
The downside of the cross connection is the same - the long term roll stiffness is provided only by the rollbar - and there is no damping control of the flow of oil from one side to the other, other than some restriction caused by the small pipe diameter - hence the tendency of older Citroëns to have a lot of very slow body roll.
Hydractive 2 overcomes these shortcomings by modifying the side to side connection - it is increased from 3.5mm to 10mm, but at the mid point there is a unit with an additional sphere, an on/off valve, and two damper valves. In the "soft mode" (selected dynamically by computer) this additional middle sphere is connected in circuit and provides additional springing, via the two damping valves in the unit. The system effectively has two parallel paths for the oil to flow for each bump, with different damping rates. The damper valves in the struts spheres on Hydractive 2 are very stiff, while the ones in the middle unit are softer, giving a net result of 3 stage damping in the soft mode, and 2 stage damping in the hard mode. Any body roll requires oil to either flow into and out of the very stiff damping valves in the strut spheres - where the opening thresholds are above that produced by roll movement - or to flow from side to side - where it must pass through two damping valves in series in the centre unit.
This means roll movements are hydraulically damped in Hydractive systems, unlike Hydropneumatic. This contributes towards the reduced roll on later models like XM and Xantia. Because of the large gauge of pipe there is the potential for greater instantaneous flow when hitting large bumps, so the roll axis stability of the car is actually improved over older models.
In the "hard mode", again selected dynamically by the computer based on inputs such as steering wheel angle and road speed, the central unit is isolated, completely blocking the cross-flow of oil and isolating the middle sphere, giving stiffer springing, much stiffer damping, and much reduced body roll.
The Activa and Hydractive- 2 refinements / developments were quite effective although only the Xantia has been fitted with it. The main setback was that ride comfort was even worse than a BMW (although cornering speeds were fantastic) which did not go too well with the traditional Citroën clientele. The current adjustable systems (computer controlled) lack this anti roll characteristic, and there are owners who always prefer the "comfort" setting rather than the "sporty" one, because again, that is not what Citroën is about.

A further mechanical advantage of hydraulic suspension is that the car is able to link its braking effort to the weight on the wheels. In the Citroën BX, the rear braking effort comes from the pressure exerted on the LHM fluid by the weight on those struts. This means that as the weight travels forward under braking, there is less pressure on the back suspension. The suspension then exerts less pressure on its fluid, and as weight and grip diminish on the wheels, so does the braking effort, thus the hydropneumatic system prevents rear wheel lock ups.

In addition to these benefits, Citroën pioneered computer controlled suspension in the early nineties by inserting a computer to take readings from the cars' chassis and control systems and let the computer make informed decisions about how to handle the cars suspension. The computer could then effect these decisions by things like servo valves, and offered benefits like soft suspension for cruising, but stiffer, sportier suspension for faster harder driving, allowing the driver to cruise in comfort and still enjoy a responsive car. It also moves substantially towards eliminating body roll and if used for a sportier driver will save tyre wear as well (they claim).



Its worth noting that when Mercedes launched their latest 600 SLC version with a computer controlled anti roll system, Auto Motor und Sport then proudly claimed that to be the first such anti roll system in world, only having to correct that one issue later by having to mention a French invention.
Rolls Royce was the only company ever to buy the patent and they used in in the rear suspension of the Silver Shadow. When Citroën was the owner of Maserati some of their cars were also hydropneumatised.



التعديل الأخير تم بواسطة cars admire ; 16-12-2010 الساعة 09:40 AM
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  #7  
قديم 17-12-2010, 03:33 AM
cars admire
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افتراضي رد: Citroën SM,STUNNING!!

sm_ citroen dieseL




1970 Citroën SM Automatic










manuaL
gearbox



LOOK AT THE POWER STEERING
IN THIS VIDEO INSPITE OF THE ENGINE IS COMPLETELY SHUT DOWN!!!





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  #8  
قديم 11-03-2011, 05:34 AM
د/مهدي نصار د/مهدي نصار غير متواجد حالياً
 
تاريخ التسجيل: Mar 2011
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د/مهدي نصار is on a distinguished road
افتراضي رد: Citroën SM,STUNNING!!

برجاء اي احد يدلني
هل الاكسنت الشكل الجديد نزلت مصر ولا لسه ولو نزلت سعرها كام؟
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  #9  
قديم 11-03-2011, 04:06 PM
ahmedromance ahmedromance غير متواجد حالياً
 
تاريخ التسجيل: Feb 2010
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ahmedromance is just really niceahmedromance is just really niceahmedromance is just really niceahmedromance is just really nice
افتراضي رد: Citroën SM,STUNNING!!


المشاركة الأصلية كتبت بواسطة د/مهدي نصار مشاهدة المشاركة

برجاء اي احد يدلني
هل الاكسنت الشكل الجديد نزلت مصر ولا لسه ولو نزلت سعرها كام؟
حضرتك ده قسم ستروين وليس هيونداي ممكن تسأل فالقسم المناسب والاخوة هيدلوك ان شاء الله
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  #10  
قديم 22-03-2011, 05:53 PM
hyundai2005 hyundai2005 غير متواجد حالياً
من انا؟: احمد حمدي
التخصص العملى: محاسب
هواياتي: قياده ومتابعه السيارات
 
تاريخ التسجيل: May 2010
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hyundai2005 has a reputation beyond reputehyundai2005 has a reputation beyond reputehyundai2005 has a reputation beyond reputehyundai2005 has a reputation beyond reputehyundai2005 has a reputation beyond reputehyundai2005 has a reputation beyond reputehyundai2005 has a reputation beyond reputehyundai2005 has a reputation beyond reputehyundai2005 has a reputation beyond reputehyundai2005 has a reputation beyond reputehyundai2005 has a reputation beyond repute
افتراضي رد: Citroën SM,STUNNING!!

sure i like it
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