I prepared the simplified version of the program - here is the formula giving almost the same result as the exact calculation, which is more complex and includes iterative solution of a nonlinear equation:
p = dh * m * (m + mt) / (1.61 * V * mt)
where:
p .. sphere pressure at rest (bar)
m .. car weight over the font/rear axis (kg) - weight distribution front/rear is about 65/35%
V .. sphere volume (400, 450, 500 ... cm3)
mt .. test (your) weight on the bumper (kg)
dh .. 'dive' measured on the bumper (cm)
The procedure:
1. The car have to be empty (no passenger or luggage of significant weight inside) and stabilised on the correct height
2. Measure the height of some point on the rear bumper from the ground. It's a good idea to have an assistant
3. Sit on the bumper
4. After the car settles on the new, lower height, but before the hydraulics kicks in and it starts to raise (a few seconds later), measure the height of the same point on the bumper from the ground, and subtract the measurment from that, obtained in step 2, to get the value 'dh'. For lighter cars, like GS, with new spheres, it will be hard to get the correct measurment, as the rear end will probably sit to bump stops, after about 8 cm travel.
5. Calculate the pressure of rear spheres according to the formula
6. Repeat the procedure from 2 to 5 on the front side of the car, and calculate the pressure of front spheres
If the car has hydractive suspension, you can check spheres on the cylinders this way (not the middle one), but assure suspension is in 'hard' mode (switch off the engine, close the doors and leave the vehicle for at least 30 sec, I think).
Important: I didn't have the oportunity to check the program on the cars with brand new spheres, so I'M NOT SURE IT GIVES THE CORRECT RESULT !!!
Everyone interested in this discussion can check his/her car and write the results (including car type, his/(her) weight, spheres' volume or type - look at the identification number stamped on spheres, how old the spheres are, and measured 'dive'), so we will see.
You can download info about spheres (volume and correct pressure) for your car from the download section of this forum.
Alex
Edited by - alexx on 25 Dec 2002 13:25:10
Sphere pressure
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allmond
- Posts: 214
- Joined: 28 Feb 2001, 01:02
<BLOCKQUOTE id=quote><font size=1 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>
If the car has hydractive suspension, you can check spheres on the cylinders this way (not the middle one), but assure suspension is in 'hard' mode (switch off the engine, close the doors and leave the vehicle for at least 30 sec, I think).
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Alex
ALL the spheres on my Hydractive Xantia are brand new. If I put it in hard mode, though, and sit on it, I get virtually no measurable movement.
I posted a query on this a while ago and got nowhere. I'd be interested to know how much movement people get.
Merry Christmas
Jamie
If the car has hydractive suspension, you can check spheres on the cylinders this way (not the middle one), but assure suspension is in 'hard' mode (switch off the engine, close the doors and leave the vehicle for at least 30 sec, I think).
<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>
Alex
ALL the spheres on my Hydractive Xantia are brand new. If I put it in hard mode, though, and sit on it, I get virtually no measurable movement.
I posted a query on this a while ago and got nowhere. I'd be interested to know how much movement people get.
Merry Christmas
Jamie
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alexx
- Posts: 462
- Joined: 19 Nov 2002, 02:42
Yes, I forgot something. I also noticed the very 'hard' suspension on Xantias with HA in hard mode, if pushed down by hands (but don't know the condition of spheres). Assuming the ride height and other things are ok, the reason is in small central bores on HA spheres. Cross-section area of central bore is 5-7 times smaller than on non-hydractive models, so, in my opinion, the car will need much longer time to 'dive' under your weight, and suspension seems to be very hard.
Fortunately, central bore is important only on small wheel up/down speed. On the road, forces are much greater than you can produce by hands, two metal disc around central bore deflects and other 8 holes open. There are two discs on each sphere. One is visible, for extraction, opening 4 holes, and one is inside the sphere, for compression, opening other 4 holes.
Alex
Fortunately, central bore is important only on small wheel up/down speed. On the road, forces are much greater than you can produce by hands, two metal disc around central bore deflects and other 8 holes open. There are two discs on each sphere. One is visible, for extraction, opening 4 holes, and one is inside the sphere, for compression, opening other 4 holes.
Alex
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alexx
- Posts: 462
- Joined: 19 Nov 2002, 02:42
For those interested in the sphere pressure discussion/testing, I slightly modyfied the formula in my yesterday's post, so now it gives more accurate result. Please, look above.
Example for Xantia 1.8, with 1.5 yr old spheres (kerb weight=1200 kg), which I test drove a few days ago:
Front:
m = 1200 * 0.65 = 780 kg
V = 450 cm3
mt = 71 kg
dh = 3 cm
p = 3 * 780 * 851 / (1.61 * 450 * 71) = 38.7 bar (65 bar when new)
Rear:
m = 1200 * 0.35 = 420 kg
V = 400 cm3
mt = 71 kg
dh = 6.5 cm
p = 6.5 * 420 * 491 / (1.61 * 400 * 71) = 29.3 bar (40 bar when new)
I think, the results might be near the real values.
Alex
Example for Xantia 1.8, with 1.5 yr old spheres (kerb weight=1200 kg), which I test drove a few days ago:
Front:
m = 1200 * 0.65 = 780 kg
V = 450 cm3
mt = 71 kg
dh = 3 cm
p = 3 * 780 * 851 / (1.61 * 450 * 71) = 38.7 bar (65 bar when new)
Rear:
m = 1200 * 0.35 = 420 kg
V = 400 cm3
mt = 71 kg
dh = 6.5 cm
p = 6.5 * 420 * 491 / (1.61 * 400 * 71) = 29.3 bar (40 bar when new)
I think, the results might be near the real values.
Alex
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JohnW
- Posts: 131
- Joined: 09 Dec 2001, 03:40
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alexx
- Posts: 462
- Joined: 19 Nov 2002, 02:42
As it turned out in my analysis, the 'spring rate' depends only on the weight supported by the wheel, sphere volume and pressure in the sphere at rest. There are several other parameters included in the 'exact calculation' (my program), which didn't apear in the formula. I wanted to keep it as simple as possible, so I used some average values for these parameters, which are all comprised in the coefficient '1.61'. Hope we'll get some 'results' here, so we'll corrrect it up or down if necessary. Hope the formula will be somewhat usefull for the quick test of sphere condition. Of course, it can't be expected to get very precise results, but it's not needed anyway.
Interestingly, the spring rate seems to be the same no matter what piston diameter and where the strut is attached to the suspension arm, although the 'working pressure' is different (if I didn't overlook something really big, of course).
Important note: kerb weight according to DIN means empty car with full tank of fuel, so when checking the rear spheres this way, it's good to have the car in this condition. And the other thing: if the spheres are new, it doesn't mean that they are precisely on the correct pressure, especially after staying who-knows-how-long on the shelf.
The easyest way to check the sphere condition is, in my opinion, this one: after fitting new/renewed spheres, measure the 'dive' front and rear. Then check it once or twice a year, with aprox. the same amount of fuel in the tank (it's important!). When the 'dive' falls to 60-70%, it's the right time for renewing the spheres again.
Alex
Interestingly, the spring rate seems to be the same no matter what piston diameter and where the strut is attached to the suspension arm, although the 'working pressure' is different (if I didn't overlook something really big, of course).
Important note: kerb weight according to DIN means empty car with full tank of fuel, so when checking the rear spheres this way, it's good to have the car in this condition. And the other thing: if the spheres are new, it doesn't mean that they are precisely on the correct pressure, especially after staying who-knows-how-long on the shelf.
The easyest way to check the sphere condition is, in my opinion, this one: after fitting new/renewed spheres, measure the 'dive' front and rear. Then check it once or twice a year, with aprox. the same amount of fuel in the tank (it's important!). When the 'dive' falls to 60-70%, it's the right time for renewing the spheres again.
Alex
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JohnW
- Posts: 131
- Joined: 09 Dec 2001, 03:40
Alex,
I agree with all that. It seems to me that this provides a simple and elegant way of monitoring sphere pressures in a good enough, semi-quantitative way and, importantly, in a simple and cheap way.
I'll start tomorrow. My Xantia had its rear spheres recharged a year ago and the front ones were fine.
Thanks again for taking the trouble to pass this on.
Regards
John
I agree with all that. It seems to me that this provides a simple and elegant way of monitoring sphere pressures in a good enough, semi-quantitative way and, importantly, in a simple and cheap way.
I'll start tomorrow. My Xantia had its rear spheres recharged a year ago and the front ones were fine.
Thanks again for taking the trouble to pass this on.
Regards
John
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alexx
- Posts: 462
- Joined: 19 Nov 2002, 02:42
Ok, John.
If your car has anti-sink, it will be fine to check the rear spheres with engine runing (or short time after switching the engine off) AND after several hours, when the anti-sink sphere is cut off by the anti-sink valve. I think you will notice the difference, which will tell you about the condition of the anti-sink sphere.
I'll get my spheres checked somewhere else and report the results. But after festivities, of course. Hope the program wasn't just a waste of time, giving totally wrong results.
The ride in that Xantia I mentioned last time was superb, not much different than in GS, far better than in my car, although I was told by several people it has harder suspension than both BX and GS. Even without the math I don't see the reason why BX (1.6), as successor of GS and predecessor of Xantia, should have such a different suspension setup. But I'm planning tu buy a Xantia in the near future anyway.
Happy New Year to everyone on this forum.
Alex
If your car has anti-sink, it will be fine to check the rear spheres with engine runing (or short time after switching the engine off) AND after several hours, when the anti-sink sphere is cut off by the anti-sink valve. I think you will notice the difference, which will tell you about the condition of the anti-sink sphere.
I'll get my spheres checked somewhere else and report the results. But after festivities, of course. Hope the program wasn't just a waste of time, giving totally wrong results.
The ride in that Xantia I mentioned last time was superb, not much different than in GS, far better than in my car, although I was told by several people it has harder suspension than both BX and GS. Even without the math I don't see the reason why BX (1.6), as successor of GS and predecessor of Xantia, should have such a different suspension setup. But I'm planning tu buy a Xantia in the near future anyway.
Happy New Year to everyone on this forum.
Alex
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ravi
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