I'm just too lazy. Why should I fit yet another sphere that doesn't make any difference to the ride quality. I'll even have to drop the subframe and all that entails struggling with siezed bolts.
But now I read that it does affect the ride quality. Look at these posts from XM-L.
By the way I'm playing with this off-line yahoo groups downloader - pg offline.
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Quotes from XM-L follow
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The way my friendly, faithful local specialist did it was to have the car
up on the ramp, remove the rear exhaust box and heatshield (to do the rear
hippoactive sphere), and place a gearbox support (the big long supports,
work a bit like a trolleyjack, but about five foot tall.) under that side
of the subframe. Take the two subframe mount bolts out of that side and
loosen the other side, then just slightly lower the support. The subframe
sank slightly, just gave us enough to get the sphere out and the new one in.
Why does it make a big difference to the ride? Beats me! Apparently it has
the same effect on Xants, too. Change all the "normal" spheres, ride only
fair - change the anti-sink as well, and it's suddenly good....
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>> ...How does the anti-sink sphere affect the ride comfort?
The extra anti-sink sphere is connected as a sort of 4th sphere to the rear
suspension. On anti-sink cars, the rear middle sphere is really equivalent to
about 2/3 of the mid sphere, the anti-sink being the remaining 1/3. When it's
flat, the result is similar to a semi-flat ('harder') middle sphere.
Some time ago the relation of sphere pressure and spring constant was discussed
on this list at length. Lower pressure equates to a higher percieved spring
constant ('harder'), higher pressure equates to a percieved lower spring
constant ('softer'). This is quite easy to understand, if a bit
counter-intuitive. With a lower initial pressure, for a constant operating
pressure, the gas is compressed to a smaller volume.
A few examplea:
1)
400cc sphere is filled with 25 bar and working pressure is 50 bar. Since
pressure of LHM=pressure of gas, to have the gas at 50bar (2x initial) the
volume must shrink to 1/2, i.e. half of the sphere will be filled with LHM. Now,
if 50cc is added to the sphere (i.e. we hit a bump requiring that displacement
through the movement of the piston in the strut), the liquid coming into the
sphere will reduce the volume of the gas from 200 to 150cc, rising the pressure
(and the counter-force to the 'bump in the road'!) by 33.3...%
2)
Now let's see what happens for a sphere with lower pressure, say 15 bar. First,
at the operating pressure, 50bar, the volume of the gas is already reduced
further compared to the previous example - now the volume needs to be reduced
3.333... times to balance the operating pressure of 50bar - to 120cc. There is
280cc of LHM in the sphere. If we now add 50cc (hitting a bump), the volume of
the gas reduces to 70cc, and the pressure, and counter force to the bump
increases 71.4% - more than double compared to the previous example. In other
words, this sphere feels like a spring that is >twice as hard for this kind of
bump!
3)
Finally, what if the sphere volume is reduced? Say, from 400 to 300cc, starting
at the same 25 bar initial pressure. At 50bar operating, the volume of the gas
has to reduce double again, so there is 150cc of LHM in the sphere. However, now
the same bump (50cc) is proportionally bigger for the 300cc sphere, compared to
the 400cc. The pressure and the counter force increase by a factor of 50%, so
this smaller sphere feels 50% harder than the larger one. This is what would
happen with a dead anti-sink sphere.
Now, is there a 'sphere equivalence rule', perhaps if you wouldn't want to
bother with changeing the anti-sink sphere? Strictly speaking, no. However, in a
pinch a smaller sphere (factually or effectively) may be substituted but it has
to be filled with a higher pressure - as high as needed to get the proper volume
of gas at operating pressure. For instance, a 400cc sphere filled to 25bar can
be replaced by a 300cc sphere filled to 33.3...bar for a similar result. Keep in
mind, however that this simple math is not as simple as it may first seem
because we get both positive and negative displacement from a sphere. A real
calculation would have to take this into account ant attempt to find the
pressure at which the 'spring constant differential' has the least error
compared to an original sphere. In the above case, the smaller sphere would have
to be filled to a slightly lower pressure than the simple math suggests, and it
would also feel slightly harder. In general, larger spheres filled to higher
initial pressures feel softer. However, there are other rules to sphere
selection, such as balancing the effective spring constant change with the load
shange to produce a more or less constant resonant frequency. Initial sphere
pressure must never be more than operating pressure. There must always be more
liquid in the sphere than the max. displacement of a strut, etc. Even so, these
rules give a lot of leeway and once can, with some basic maths and lots of
patience, customise the suspension considerably.
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Full Version Anti-sink
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Hi Steve,
I won't pretend to understand the anti-sink system fully but there's bits of it you can understand perfectly just by inspection.
Think back to the pre-hydractive days of the CX, Xantia and even early XM's (I think !). The spheres on each wheel were fed hydraulic fluid by 3.5mm diameter pipes. When the wheel passed over a bump or pothole the wheel rose and fell in the wheel arch respectively. When rising, the fluid is compressed by the piston in the strut and its forced into the sphere. The same rise in pressure also tries to push fluid backwards through the 3.5mm feed pipe. However, the device at the other end of the pipe was the height corrector which incorporates a sliding spool type of valve. The rising pressure pushes against the side of the spool which is blocking the hole. Therefore by its nature the HC does not allow flow to travel backwards in the pipe unless it opens deliberately(which takes time). Furthermore the travel of fluid at that speed through such a small pipe would meet a high degree of pipe resistance. The conclusion to this is that high volume and therefore high speed fluid transfer was never designed or indeed happened through the small 3.5mm pipes.
Now consider cars with hydractive fitted. An additional sphere is fitted to the centre of each axle and is linked to each of the wheel spheres not by 3.5mm piping but by 10mm diameter piping. Now when the wheel goes over a bump the fluid can be pushed out of the piston in two directions. Some of the fluid will go into the immediate wheel sphere with the balance transferred along the 10mm pipe to the centre sphere. In which case, both spheres now share the absorption of the bump. The converse happens when hitting a pothole. Note that the hydractive system on each axle is still fed by a Height Corrector with only a 3.5mm pipe supplying it. ALLl hydractive systems have the larger 10mm diameter bore pipes.
What is very clear from the above rationale is that a pipe of 3.5mm diameter is neither designed nor destined to carry the types of fluid flow rates seen during the event of a wheel passing over a bump or pothole. You need a pipe of 10mm diameter or so to provide the required small amount of resistance to allow flow to occur in very small time intervals. The anti-sink sphere has only a 3.5mm connection and therefore does not take any active part in the suspension function. Furthermore the anti-sink sphere has no innards like a suspension sphere. It is exactly the same type as the accumulator sphere attached to the regulator. It has no jets or orifices. Compare this to a traditionally spriung car with springs but no shock absorbers. It would bounce all over the place if this sphere was part of the suspension.
The purpose of the anti-sink is to overcome the (natural) leakage through the brake valve. The rear brakes get their pressure from the rear suspension. A lightly loaded car provides little rear braking force whereas a bootful of cement will provide a lot of rear braking effort. When stationary the rear suspension pressure leaks away through the brake valve. To prevent this there are two features on the anti-sink. One is the anti-sink valve and the other is the anti-sink sphere. The valve prevents fluid being lost to the brake valve when the engine is stopped and the anti-sink sphere provides a reserve of pressure to the now separated part of the circuit. Hence it doesn't sink.
Having said all of that I still admit my anti-sink knowledge is not great. It is possible that the pressure retained in the anti-sink sphere somehow acts on another component in the system thus opening a hydraulic 'switch' which may explain the differences some of the people describe above. I have to say, personally, I doubt it.
Cheers
noz
I won't pretend to understand the anti-sink system fully but there's bits of it you can understand perfectly just by inspection.
Think back to the pre-hydractive days of the CX, Xantia and even early XM's (I think !). The spheres on each wheel were fed hydraulic fluid by 3.5mm diameter pipes. When the wheel passed over a bump or pothole the wheel rose and fell in the wheel arch respectively. When rising, the fluid is compressed by the piston in the strut and its forced into the sphere. The same rise in pressure also tries to push fluid backwards through the 3.5mm feed pipe. However, the device at the other end of the pipe was the height corrector which incorporates a sliding spool type of valve. The rising pressure pushes against the side of the spool which is blocking the hole. Therefore by its nature the HC does not allow flow to travel backwards in the pipe unless it opens deliberately(which takes time). Furthermore the travel of fluid at that speed through such a small pipe would meet a high degree of pipe resistance. The conclusion to this is that high volume and therefore high speed fluid transfer was never designed or indeed happened through the small 3.5mm pipes.
Now consider cars with hydractive fitted. An additional sphere is fitted to the centre of each axle and is linked to each of the wheel spheres not by 3.5mm piping but by 10mm diameter piping. Now when the wheel goes over a bump the fluid can be pushed out of the piston in two directions. Some of the fluid will go into the immediate wheel sphere with the balance transferred along the 10mm pipe to the centre sphere. In which case, both spheres now share the absorption of the bump. The converse happens when hitting a pothole. Note that the hydractive system on each axle is still fed by a Height Corrector with only a 3.5mm pipe supplying it. ALLl hydractive systems have the larger 10mm diameter bore pipes.
What is very clear from the above rationale is that a pipe of 3.5mm diameter is neither designed nor destined to carry the types of fluid flow rates seen during the event of a wheel passing over a bump or pothole. You need a pipe of 10mm diameter or so to provide the required small amount of resistance to allow flow to occur in very small time intervals. The anti-sink sphere has only a 3.5mm connection and therefore does not take any active part in the suspension function. Furthermore the anti-sink sphere has no innards like a suspension sphere. It is exactly the same type as the accumulator sphere attached to the regulator. It has no jets or orifices. Compare this to a traditionally spriung car with springs but no shock absorbers. It would bounce all over the place if this sphere was part of the suspension.
The purpose of the anti-sink is to overcome the (natural) leakage through the brake valve. The rear brakes get their pressure from the rear suspension. A lightly loaded car provides little rear braking force whereas a bootful of cement will provide a lot of rear braking effort. When stationary the rear suspension pressure leaks away through the brake valve. To prevent this there are two features on the anti-sink. One is the anti-sink valve and the other is the anti-sink sphere. The valve prevents fluid being lost to the brake valve when the engine is stopped and the anti-sink sphere provides a reserve of pressure to the now separated part of the circuit. Hence it doesn't sink.
Having said all of that I still admit my anti-sink knowledge is not great. It is possible that the pressure retained in the anti-sink sphere somehow acts on another component in the system thus opening a hydraulic 'switch' which may explain the differences some of the people describe above. I have to say, personally, I doubt it.
Cheers
noz
There is some discussion currently (I think on XM-L and not here) about the effect of pressure (or slight lack of it) on the hydractive system.
The theory being that a slight reduction in pressure will result in the electrovalves not operating fully and a harder ride. Now there is always leakage to the return system by design, but in an older car this increases due to age/ware and is causing problems with a hard ride.
I recon that could explain this phenomenon if it is right.
If we have a car that has increased leakage, but borderline for this effect. Have a new anti-sink sphere and we have an extra pressure surce to keep up the pressure in tyhe rear system when something gives a little extra spurt into the return from a bump or something. Flat anti-sink and we are relying on the accumulator to put the pressure back up which is probably also having to increase the presure in the front system due to the same bump causing extra leakage there, and a longer length of 3.5mm piping to push it through to get to the back.
Does that make sense to those who understand the hydraulics a lot better than I do?
Cheers,
Rowan.
The theory being that a slight reduction in pressure will result in the electrovalves not operating fully and a harder ride. Now there is always leakage to the return system by design, but in an older car this increases due to age/ware and is causing problems with a hard ride.
I recon that could explain this phenomenon if it is right.
If we have a car that has increased leakage, but borderline for this effect. Have a new anti-sink sphere and we have an extra pressure surce to keep up the pressure in tyhe rear system when something gives a little extra spurt into the return from a bump or something. Flat anti-sink and we are relying on the accumulator to put the pressure back up which is probably also having to increase the presure in the front system due to the same bump causing extra leakage there, and a longer length of 3.5mm piping to push it through to get to the back.
Does that make sense to those who understand the hydraulics a lot better than I do?
Cheers,
Rowan.
Hi Steve, That was a very interesting article. I had never considered the possibility that the anti sink sphere could theoretically augment the action of the normal and hydractive spheres. After worrying it over in my mind I eventually came to the same conclusion that NorrieNoz explained so well. (Wot you doing with only one post, Noz?)
There is also a bit of misinformation in the final paragraph of the XM-L article with regard to fitting smaller spheres at higher pressures. You may well have the same available sphere volume for springing, but you will not have the same extension volume. It's noticeable that the hydractive spheres run at higher pressures than the wheel spheres so that there is minimum sphere compression under static load, therefore maximum volume (and softness) available for springing. But the extension from normal is supplied by the wheel spheres. If the writer is only referring to the hydractive spheres in his suggestion, the possibility is that the sphere pressure may be higher than the static pressure, in which case the hydractive sphere would not give any assistance (extra volume) during small wheel diplacements.
Rowan, I may well be wrong but as I see it any increased leakage during a bump would drop the ride height by a few mm; the pressure in the system would still be the same as would the sphere volume available for springing. So ride comfort would not be compromised. N'est pas?
Derek
There is also a bit of misinformation in the final paragraph of the XM-L article with regard to fitting smaller spheres at higher pressures. You may well have the same available sphere volume for springing, but you will not have the same extension volume. It's noticeable that the hydractive spheres run at higher pressures than the wheel spheres so that there is minimum sphere compression under static load, therefore maximum volume (and softness) available for springing. But the extension from normal is supplied by the wheel spheres. If the writer is only referring to the hydractive spheres in his suggestion, the possibility is that the sphere pressure may be higher than the static pressure, in which case the hydractive sphere would not give any assistance (extra volume) during small wheel diplacements.
Rowan, I may well be wrong but as I see it any increased leakage during a bump would drop the ride height by a few mm; the pressure in the system would still be the same as would the sphere volume available for springing. So ride comfort would not be compromised. N'est pas?
Derek
What cars use anti sink? I have had four XMs, the last two VSX's, but none had anti sink.
Peter.N.
Peter.N.
Hi Peter
All 2.5's made were anti-sink but I have been surprised that my later V6's were not.
I see a high mileage 2.1 YBL maroon estate is on ebay currently at £300 and no bidders.
Regards
XMexc
All 2.5's made were anti-sink but I have been surprised that my later V6's were not.
I see a high mileage 2.1 YBL maroon estate is on ebay currently at £300 and no bidders.
Regards
XMexc
can i be enlightened as to the difference between anti-sink and rear centre sphere. i have a 2.5 from '94 but only one rear sphere apart from the wheel units.
I've recently checked out the ecu and i do have two mosfets so i assume i have a rear centre sphere with electrovalve, etc. Where then is the anti sink if its separate?
I've recently checked out the ecu and i do have two mosfets so i assume i have a rear centre sphere with electrovalve, etc. Where then is the anti sink if its separate?
Hi Gav
The 2.5 rear anti sink sphere is fixed to and above the rear passenger trailing arm around the mid point of the arm. It has a single 3.5mm pipe from its centre.
Regards
XMexc
The 2.5 rear anti sink sphere is fixed to and above the rear passenger trailing arm around the mid point of the arm. It has a single 3.5mm pipe from its centre.
Regards
XMexc
Hi XMexc
Thanks for the info - that at least explains why I have not come across anti sink! Thanks also for pointing the YBL out, it had so far escaped me. High mileage it is, although not anything like as high as my last one, but, did you notice that it has had a new engine - 400 miles! It has various problems but I think it may well be worth having a look at. I will email him and find out a bit more about it.
Peter.N.
Thanks for the info - that at least explains why I have not come across anti sink! Thanks also for pointing the YBL out, it had so far escaped me. High mileage it is, although not anything like as high as my last one, but, did you notice that it has had a new engine - 400 miles! It has various problems but I think it may well be worth having a look at. I will email him and find out a bit more about it.
Peter.N.
morning XMexc,
thanks for confirming that i have atleast replaced the correct sphere. I purchased an anti-sink sphere and fitted it a while ago where you describe. and it works!
cheers
Gav
thanks for confirming that i have atleast replaced the correct sphere. I purchased an anti-sink sphere and fitted it a while ago where you describe. and it works!
cheers
Gav
Hi Gav,
Am I missing a trick here? You say that you replaced the antisink sphere and it works. But its only function in life is to provide rear wheel brake pressure in the event of hydraulic failure, so how do you know it works?
According to the manual antisink was fitted to all cars from the 1995 model year on.
Derek
Am I missing a trick here? You say that you replaced the antisink sphere and it works. But its only function in life is to provide rear wheel brake pressure in the event of hydraulic failure, so how do you know it works?
According to the manual antisink was fitted to all cars from the 1995 model year on.
Derek
Hi Derek,
you make me wonder now. I replaced (what i believed to be) the antisink sphere in the hope that the car would keep its rear end up over night. It always scraped coming off the drive in the morning. With new sphere it does, but i also replaced all wheel spheres at the same time. I am still a bit confused by all the different names to the rear spheres as i only have one. Is it antisink, rear centre, rear electrovalve, rear accumulator, etc, etc. Although my car is mk2 it is '94 (orga 6540) hence what you say suggests otherwise. Recently i have been investiagting the lack of smooth ride hence investigating the electronics and electrovalves. I have two of those.
Gav
you make me wonder now. I replaced (what i believed to be) the antisink sphere in the hope that the car would keep its rear end up over night. It always scraped coming off the drive in the morning. With new sphere it does, but i also replaced all wheel spheres at the same time. I am still a bit confused by all the different names to the rear spheres as i only have one. Is it antisink, rear centre, rear electrovalve, rear accumulator, etc, etc. Although my car is mk2 it is '94 (orga 6540) hence what you say suggests otherwise. Recently i have been investiagting the lack of smooth ride hence investigating the electronics and electrovalves. I have two of those.
Gav
thinking about this again, do mark1.5s simple not have the additional hydractive spheres you are all refering to. but do have the antisink over the near side rear wheel!
i'll get a photo of under the rear of the car when i get the chance to confirm pipe sizes and sphere locations.
i'll get a photo of under the rear of the car when i get the chance to confirm pipe sizes and sphere locations.
Hi Gav,
Non-hydractive cars have three spheres at the front, one at the top of each leg plus the accumulator sphere; and two at the back, one on each wheel cylinder.
Non-hydractive cars with antisink have an additional sphere, the antisink sphere, at the back. As XMexclusive wrote, it's tucked away above the near side suspension arm.
Hydractive cars without antisink have an extra sphere at the front, fitted to the hydractive valve block, centrally and behind the accumulator sphere. They also have an additional sphere at the rear fitted to the valve block about centrally. These two are the hydractive spheres.
Hydractive cars with antisink have the additional eighth sphere, the antisink sphere, tucked away above the nearside suspension arm.
The easy way to check if you have antisink is to look for the rear antisink valve. Locate the rear height corrector (central or thereabouts). The antisink valve, if present, is fitted vertically a little to its right. Has four small diameter pipes going to it if I remember correctly. Usual precautions about supporting the car before you slide under. Of course if you have hydractive you'll have a sport/normal switch at the left side of the the gear lever.
If you have antisink and the rear end is still dropping, it's highly unlikely to be the A/S valve that is leaking as they are simple pressure valves and very robust. The A/S sphere is completely isolated from the rear suspension when the A/S valve is closed so cannot influence the rear suspension's ability to stay up.
Derek
Non-hydractive cars have three spheres at the front, one at the top of each leg plus the accumulator sphere; and two at the back, one on each wheel cylinder.
Non-hydractive cars with antisink have an additional sphere, the antisink sphere, at the back. As XMexclusive wrote, it's tucked away above the near side suspension arm.
Hydractive cars without antisink have an extra sphere at the front, fitted to the hydractive valve block, centrally and behind the accumulator sphere. They also have an additional sphere at the rear fitted to the valve block about centrally. These two are the hydractive spheres.
Hydractive cars with antisink have the additional eighth sphere, the antisink sphere, tucked away above the nearside suspension arm.
The easy way to check if you have antisink is to look for the rear antisink valve. Locate the rear height corrector (central or thereabouts). The antisink valve, if present, is fitted vertically a little to its right. Has four small diameter pipes going to it if I remember correctly. Usual precautions about supporting the car before you slide under. Of course if you have hydractive you'll have a sport/normal switch at the left side of the the gear lever.
If you have antisink and the rear end is still dropping, it's highly unlikely to be the A/S valve that is leaking as they are simple pressure valves and very robust. The A/S sphere is completely isolated from the rear suspension when the A/S valve is closed so cannot influence the rear suspension's ability to stay up.
Derek
Hi Gav
The anti-sink sphere hydraulic pipe connection is totally different to the other rear spheres in having a 3.5mm pipe connecting directly into the centre. All the other rear spheres screw into a large diameter threaded mounting valve or block and the hydraulic pipe connections are made to the mounting block.
I have photos of the back end sphere set up (with and without anti-sink) but cannot at present upload to the site. These photos were taken from above and show all the pipe work and wiring that is normally hidden from view. If you PM contact details I will print off a set and mark up the various bits and post to you.
Your 2.5 is a 95 model year car built on the 5 Oct 1994. My nearest one was built at the end of Nov the same year and that definately had anti-sink but the V6 built a year later is not fitted with it.
Hi Derek
The rear anti-sink has a primary function of keeping the rear end up in the air when the engine is switched off. The 2.5 on the drive has not been started for at least 14 days and the suspension is still up at ride height. Does the suspension on your V6 stay up overnight?
Regards
XMexc
The anti-sink sphere hydraulic pipe connection is totally different to the other rear spheres in having a 3.5mm pipe connecting directly into the centre. All the other rear spheres screw into a large diameter threaded mounting valve or block and the hydraulic pipe connections are made to the mounting block.
I have photos of the back end sphere set up (with and without anti-sink) but cannot at present upload to the site. These photos were taken from above and show all the pipe work and wiring that is normally hidden from view. If you PM contact details I will print off a set and mark up the various bits and post to you.
Your 2.5 is a 95 model year car built on the 5 Oct 1994. My nearest one was built at the end of Nov the same year and that definately had anti-sink but the V6 built a year later is not fitted with it.
Hi Derek
The rear anti-sink has a primary function of keeping the rear end up in the air when the engine is switched off. The 2.5 on the drive has not been started for at least 14 days and the suspension is still up at ride height. Does the suspension on your V6 stay up overnight?
Regards
XMexc
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