Sphere damping valve modifications??

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juraj
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Sphere damping valve modifications??

Post by juraj »

Hi all

Are damping valves the same apart from the central restriction hole - orifice?

Or is the arrangement of the washers different.

In other words can you take a sphere with a .6mm diameter orifice, from an hydractive XM or Xantia and drill it out to 1.5mm to suit - you guessed it - my BX.

I remember hearing of some people who wanted softer spheres so they experimented with drilling out the hole on their original spheres, thus they had a bigger hole but still the same damper washers.

But it cant be much worse than my current configuration, I am running rear spheres on the front. During my stay overseas my father had the spheres 'regassed' - more likely they just fitted rear spheres on the front.

bye all

j
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Mandrake
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Re: Sphere damping valve modifications??

Post by Mandrake »

juraj wrote:Hi all

Are damping valves the same apart from the central restriction hole - orifice?
No
Or is the arrangement of the washers different.
Yes, the thickness of washers vary, the thicker the stack of washers, the higher the opening "threshold" before oil can pass through this route.

Jounce and Rebound thresholds can be set independently - the washers on the outer edge of the valve set the rebound, the ones on the inner side (that you can't see) set the Jounce.

Most spheres have similar or equal thresholds for both directions, but Hydractive (strut) spheres are a notable exception - they seem to have a lot heavier damping for rebound than they do for jounce. (approx 70/30)

This is done for handling and cornering reasons - asymmetric damping like this tends to give better handling, while symmetric gives better ride comfort. (Hydractive 2 is asymmetric in hard mode and close to symmetric in soft mode incidentally)
In other words can you take a sphere with a .6mm diameter orifice, from an hydractive XM or Xantia and drill it out to 1.5mm to suit - you guessed it - my BX.
No, not if you want to get good results anyway. Hydractive strut spheres have a thicker than usual washer stack as part of the increased damping, not just a smaller bypass hole.

The relationship between the centre bypass hole and the threshold (thickness) of the washers is quite important, but I'm not quite sure how to describe it without going into a lot of theory.

Having a larger bypass hole and stiff washers (which is what the above mod would give you) is not the same as having a smaller hole but a thinner washer stack.

The way it usually seems to be tuned (although hydractive 2 is more complex) is that the thresholds are set high enough that that a "bump" impact can open the washer valves without too much difficulty, and thus be absorbed, but the rebound force (wheel displacement x springing rate) is too weak to open the valves.

The bypass hole then becomes the only path for the suspension to return slowly to the steady state condition and the hole size is optimized to fine tune the damping.

The problem with drilling the hole larger on a hydractive sphere is that because the thresholds are set high you'd have to drill the hole larger than normal to get an acceptably soft ride, but then you would have very little body control because the threshold valves will not be doing much.

Most likely getting a soft ride by only drilling out the hole would give "boat like" handling :lol:
I remember hearing of some people who wanted softer spheres so they experimented with drilling out the hole on their original spheres, thus they had a bigger hole but still the same damper washers.
You can do this, but most models have pretty optimally tuned damping anyway (except perhaps the C5 which sounds like it has very firm damping by default) so if you drill it out much you're going to end up with a boat like underdamped handling - the bigger the bypass hole, the less damping effect the threshold valves have, and they lose their ability to control small movements effectively. (Stability)
But it cant be much worse than my current configuration, I am running rear spheres on the front. During my stay overseas my father had the spheres 'regassed' - more likely they just fitted rear spheres on the front.
So why not get the correct spheres, if they are clearly the wrong spheres fitted ? Seems like a better idea than modifying the wrong spheres... 8)

Regards,
Simon
Simon

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juraj
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Post by juraj »

thanks Mandrake

Well I had already drilled them out by the time i saw your extensive reply and they are much better,

yes the ride is much boatier but is also much closer to the original feel,

i did it mainly because i have a spare set of BX spheres which need regassing and i wont get around to it for a few weeks if not months so in the mean time i wanted to see how these .6mm spheres would go when drilled to 1.6mm, and it is much better

thanks for all the info,

bye
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