Xanti - Back arm bearings

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mds141
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Re: Xanti - Back arm bearings

Post by mds141 »

"Rear arm bearings can be visually checked, raise the car to top and then stand a few Feet or meters behind and look at the back wheels, if the bearings are bad they will be leaning in slightly at the top!"

Shouldn't that be 'full low'? If you put the suspension on full low and look at the rear wheels, then they will be leaning inwards like this /\ if the bearings in the radius arms are shot. Sorry to be pedantic.
Mark Smith.

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Re: Xanti - Back arm bearings

Post by lancia58 »

Hi Simon

Can you write some technical explanation for what is the damper valves and how it effects the stiffness ?

Thanks
Zohar
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Re: Xanti - Back arm bearings

Post by Mandrake »

lancia58 wrote: Can you write some technical explanation for what is the damper valves and how it effects the stiffness ?
I'm not sure that I'm qualified for that Zohar. :lol: I will try though.

A damper valve is there to control and prevent oscillations of the suspension - without a damper valve the car would bounce up and down dangerously over the slightest bump. Imagine a weight hanging from a spring - pull it down and release and it will bounce up and down maybe 10 times or more with slowly decreasing amplitude. A damper valve is something that dissipates energy through friction when the suspension tries to oscillate, which causes the suspension to quickly come to rest after a disturbance without excessive (or any) oscillation.

In conventional suspension this is done by the shock absorber - which contains a hydraulic ram, oil, compressed gas and a valve, all of which work to restrict movement. You might find the following interesting, in particular where it talks about a "blow off valve":

http://www.autospeed.com/cms/A_112605/article.html" onclick="window.open(this.href);return false;

In the Citroen system many of the things needed for a shock absorber already exist - a hydraulic ram, (the ram that connects to the suspension to lift the weight of the car) oil - (the oil which connects the ram to the sphere) and gas. (the gas that works as a spring in the sphere) All that is missing is a flow control valve, which is inserted into the neck of the sphere.

Here is an example of the damper valve in a sphere, its actually the layout used in the very early models like the DS, but it is functionally identical:

Image

There is a small "bypass" hole which is always open - to the left in this picture, on modern spheres it is down the centre of the valve. There is then a block in the middle with a number of larger holes drilled through it and a number of washer like metal discs clamped on either side of it.

Again the details vary between this image and modern spheres but the point is that the holes and discs are arranged such that if the oil tries to flow through in one direction the washers on the far side have to bend to allow the exit hole to open. For oil to flow in the washers on the inner side have to bend and lift up, for oil to flow out the washers on the outside (the visible ones) have to bend and lift up.

This is a threshold valve, although the article I linked to calls them a "blow off valve". It takes a certain amount of hydraulic force before the washer will start to bend - below this force no oil can flow through the main part of the valve, it can only flow through the tiny bypass hole.

The valve strength is tuned so that if you hit a large bump the force is great enough to open the washer valve thus the initial impact of the bump is absorbed well because the oil can flow relatively freely into the sphere. ("high speed bump" in that article) When the bump is passed and the suspension tries to rebound, the rebound force is weak enough (due to the very soft springing employed) that it is below the threshold of the washer valve thus the rebound can only flow - slowly - through the bypass hole, ("low speed bump" in that article) so there is a smooth and slow return to normal height without overshoot or oscillation.

One difference between Citroen dampers and most shock absorbers is that conventional shocks tend to have stiffer damping in the rebound direction than they do in the compression direction as described in that article - standard hydropneumatic generally has 50/50 damping where the damping in both directions is the same, which gives better ride but is only practical with soft springing.

A hydractive strut sphere has two differences to yours - the washer valves are made stiffer so that they have higher opening thresholds, and the rebound damping is also made stiffer than the compression damping. Secondly the bypass hole is reduced in area by about 4 times (half the diameter) which dramatically restricts it.

The end result is that the car will not respond to soft/small movements - you almost can't push it down by hand when trying to bounce the suspension and even when you can it takes a couple of seconds for it to return to normal height - because the rebound is flowing through the very small bypass hole. The spheres are tuned heavily for handling and stability at the expense of a hard ride. Large bumps are absorbed and the car will be very stable and grounded feeling, but small bumps will case the ride to feel "fidgety".

In an actual Hydractive car there is a third sphere with soft damping valves switched into circuit when the car is in "soft mode" which gives you soft smooth ride over small bumps, but without that extra sphere your car will effectively be in the "hard mode" all the time.
Simon

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Re: Xanti - Back arm bearings

Post by lancia58 »

Hi Simon

Excellent explanation !!! Thanks a lot

Regards
Zohar
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Re: Xanti - Back arm bearings

Post by lancia58 »

Hi Simon

As you said the car is very stable I tested it on sharp curves at 45Mph. Tracking wise the feeling is like the car rides on rails.
Thanks
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Re: Xanti - Back arm bearings

Post by citroenxm »

If its riding on rails there's a good sign the rear bearings are fine as the rear lean in can and does cause over steer and can throw the back end out..
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Re: Xanti - Back arm bearings

Post by Stickyfinger »

citroenxm wrote:If its riding on rails there's a good sign the rear bearings are fine as the rear lean in can and does cause over steer and can throw the back end out..
100% agree, I noticed this before I had the "crack of death" in my old C5, she tucked under at the back really badly in the end.
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Re: Xanti - Back arm bearings - Solved

Post by lancia58 »

After a year the Israeli importer of I.F.H.S finally import the correct spheres. He says in Israel not more than a 100 Xantias are still on the road , so only once a year the local I.F.S.H importer brings spheres stock. The Citroen genuine costs a fortune, so I never mount them.

Just mount them and the Xantia rides as it should.

Thank you all again for you help
Zohar
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