Was it this ?that nice Citroen diagram which shows how the diaphragm area varies with displacement?
SUSPENSION QUESTION
Moderator: RichardW
Simon,
you are correct with observation about doubling the weight. If we somehow put 1600 kg in the middle of the car (= additional 400 kg per wheel), there won't be enough pressure on the regulator to raise the car from the bump stops to the normal height
But there is nothing wrong with the math. We can calculate with 20% greater weight and get 44% greater spring constant. To get precise result, we should also calculate with very small suspension travel, like 0.01 mm or so (because hydropneumatic spring is progressive, unlike ordinary steel spring), or use different mathematic approach
Most of the useful load in the car goes to the rear suspension, so there is higher change of resonant frequency in rear suspension. Therefore, resonant frequency of unladen car (with new spheres) is tuned to be lower on the rear than on the front (on the car with conventional suspension it is higher). Also, damping plays significant role here, so we have smaller central damper holes on rear spheres than on the front, resulting in higher damping force for movements near resonant frequency. And, in the case of C5, rear suspension arms are almost in contact with rear progressive bump stops when the car is on the normal height (on Xantia mkII, there is about 2cm clearance). This element plays very active part during the drive (like in many other cars) and its spring constant doesn't depend on the load
About second question: when you change the pressure of the gas, its temperature also changes. If you compress it, it will get hot. That's why turbo diesel engines are usually fitted with intercoolers. So, p V = const is valid only when the nitrogen in the sphere has enough time to transmit the heat to/from the surrounding steel. On quick suspension movements during the drive, there will be very little heat exchange, so p V^k = const should be used
Of course, you won't notice spheres becoming hot when you load the car with passengers - mass of the steel in the sphere is much higher than mass of the nitrogen
Speed of suspension movements is somewhere in the range 0-2 m/s and resonant frequency of the wheel is about 10-15 Hz as you mentioned (effects of this resonant frequency can clearly be seen on many steepy tarmac roads by the way)
About the dnyeys question - diaphragm shape has almost no effect in hydropneumatic suspension. If you move the wheel 1 cm up and piston diameter is 2.2 cm, it will displace 3.8 ccm of LHM into the sphere and compress the gas for the same amount, regardless the shape of diaphragm. You need some force to deform it, and this force depends on the diaphragm geometry, but it is much lower than force produced by the gas pressure. It's different story in pneumatic suspension, where geometry of the air spring does have significant influence to its behavior, because there is no hydraulic fluid in between
Anyway, spring constant (of the sphere + of the bump stop + of the diaphragm) is only one, though quite important, of the several things determining quality of the ride. Damping force (also very non-linear) is, in fact, more important
you are correct with observation about doubling the weight. If we somehow put 1600 kg in the middle of the car (= additional 400 kg per wheel), there won't be enough pressure on the regulator to raise the car from the bump stops to the normal height
But there is nothing wrong with the math. We can calculate with 20% greater weight and get 44% greater spring constant. To get precise result, we should also calculate with very small suspension travel, like 0.01 mm or so (because hydropneumatic spring is progressive, unlike ordinary steel spring), or use different mathematic approach
Most of the useful load in the car goes to the rear suspension, so there is higher change of resonant frequency in rear suspension. Therefore, resonant frequency of unladen car (with new spheres) is tuned to be lower on the rear than on the front (on the car with conventional suspension it is higher). Also, damping plays significant role here, so we have smaller central damper holes on rear spheres than on the front, resulting in higher damping force for movements near resonant frequency. And, in the case of C5, rear suspension arms are almost in contact with rear progressive bump stops when the car is on the normal height (on Xantia mkII, there is about 2cm clearance). This element plays very active part during the drive (like in many other cars) and its spring constant doesn't depend on the load
About second question: when you change the pressure of the gas, its temperature also changes. If you compress it, it will get hot. That's why turbo diesel engines are usually fitted with intercoolers. So, p V = const is valid only when the nitrogen in the sphere has enough time to transmit the heat to/from the surrounding steel. On quick suspension movements during the drive, there will be very little heat exchange, so p V^k = const should be used
Of course, you won't notice spheres becoming hot when you load the car with passengers - mass of the steel in the sphere is much higher than mass of the nitrogen
Speed of suspension movements is somewhere in the range 0-2 m/s and resonant frequency of the wheel is about 10-15 Hz as you mentioned (effects of this resonant frequency can clearly be seen on many steepy tarmac roads by the way)
About the dnyeys question - diaphragm shape has almost no effect in hydropneumatic suspension. If you move the wheel 1 cm up and piston diameter is 2.2 cm, it will displace 3.8 ccm of LHM into the sphere and compress the gas for the same amount, regardless the shape of diaphragm. You need some force to deform it, and this force depends on the diaphragm geometry, but it is much lower than force produced by the gas pressure. It's different story in pneumatic suspension, where geometry of the air spring does have significant influence to its behavior, because there is no hydraulic fluid in between
Anyway, spring constant (of the sphere + of the bump stop + of the diaphragm) is only one, though quite important, of the several things determining quality of the ride. Damping force (also very non-linear) is, in fact, more important
Xantia mkII 1.8 16v '98
previous: BX16 '89, Visa 1.1 '86 and father's GSA '81
previous: BX16 '89, Visa 1.1 '86 and father's GSA '81