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Car Forum / Land Rover Cars / December 2006How does Air Suspension work ?
Good evening all, hope you had a good Christmas. Can anyone tell me how air suspension works, how the air lines connect the bags and what stops one side filling in to the other side and it all falling over, how are the bags filled to keep the vehicle level and at a constant height ???? " Not yet started the Toylander " Rich Rich <r3engineering@ntlworldspam.com> uttered summat worrerz funny about: > Good evening all, hope you had a good Christmas. Can anyone tell me > how air suspension works, how the air lines connect the bags and what [quoted text clipped - 4 lines] > > Rich Valves & sensors.... but for more... http://auto.howstuffworks.com/car-suspension.htm Lee D >Rich <r3engineering@ntlworldspam.com> uttered summat worrerz funny >about: [quoted text clipped - 8 lines] > >Valves & sensors.... I was going to say "cleverness", but I admit that your answer has more actual practical value. Basically, there are ride height sensors, I assume one on each corner, and a control box which maintains the ride height by adding or dumping air from the relevant air spring.  SignatureAustin Shackles. www.ddol-las.net my opinions are just that "The breezy call of incense-breathing Morn, The swallow twittering from the strawbuilt shed, The cock's shrill clarion, or the echoing horn, No more shall rouse them from their lowly bed." Thomas Gray, Elegy Written in a Country Churchyard. > Basically, there are ride height sensors, I assume one on each corner, and a > control box which maintains the ride height by adding or dumping air from > the relevant air spring. No, the air pressure is adjusted to compensate for the weight of the vehicle, the levelling itself is carried out by the hydraulics. Many vehicles (Citroens, classic Rangies) have purely hydraulic self levellers. Some (Citroen) use pumps, most (classic Rangies, many larger estates) use a self-contained system in what looks like a shock absorber. You don't even need much in the way of "sensors" or "valves". Obviously the two systems interact. If the vehicle was stationary, then adjusting the air pressure alone would self level it adequately. The vehicle weight is estimated from the spring position (i.e. ride height) anyway. If we want a level ride over bumps though, you need some involvement with the hydraulics. In the simplest case (classic Rangie) then there's just a shock absorber lookalike with a strongly non-linear damping characteristic. They're gas-filled to give a bit more rebound below this than a simple oil dashpot alone. In effect it's another booster gas spring that only comes into play gradually, when the suspension is compressed beyond the ideal ride height. These self-contained units have limited capacity for varying weight though (why the classic Rangie unit needs to be so big and heavy). If an adjustable air spring is added, then this balances the gross changes due to vehicle weight and allows the hydraulic system an easier life. Citroens are a purely hydraulic levelling system with an external ride-height measuring valve and active control of the hydraulics through a hydraulic servo arrangement, rather than just a simple non-linear damping rate. This requires an engine driven pump. The gas pressure spheres take little part in this - they're just a fixed gas spring which is coupled to the unsprung suspension by hydraulic pressure, rather than a mechanical lever. When the spheres lose pressure the ride doesn't suffer much because the hydraulics take over and the car is still fairly comfortable. The hydraulics are now doing a lot more work though, work they weren't designed for, and if the spheres aren't refilled promptly, then there will be hydraulic wear failures. To see a simple air spring in action, look closely at the back end of a tipper truck. These have short rear bodies to keep the raised tipper weight near to the axle and are equipped with air springs to cope with rough ground. Watch for what looks like a pair of rubber doughnuts with a metal flowerpot inside each one. Rather than being a simple squashy balloon, an air spring is two concentric rubber tubes rolled over at one end. When they move, the tubes roll over each other, allowing a change in volume and long distance movement, without needing the elasticity that a simpel balloon would. > > Basically, there are ride height sensors, I assume one on each corner, and a > > control box which maintains the ride height by adding or dumping air from [quoted text clipped - 12 lines] > anyway. If we want a level ride over bumps though, you need some > involvement with the hydraulics. The Range Rover Classic/Discovery I/110 Station Wagon have "simple" self-levellers that compensate for loads in the rear of the vehicle by pumping themselves up (or down, after a load has been removed) to the optimum ride height for the vehicle. Essentially it works in the fore-aft plane only. The presence, or not, of other suspension systems has no effect on the unit as such (though a 2" lift is going to confuse the hell out of it as it's always trying to let itself down!). The system used by Land Rover on the Classics, 38a, Discovery II etc is purely based on compressed air and height sensors as Lord Austin says, anyone who's had an air spring puncture will have found this out! The self-leveller was only fitted to air suspension Classics as removing it would have necessited re-designing the axle, which was pointless as the 38a was well on the way (it might even have been launched) when this was added, so the car was doomed. <snip> Richard Signaturewww.beamends-lrspares.co.uk sales@beamends-lrspares.co.uk www.radioparadise.com - Good Music, No Vine Lib Dems - Townies keeping comedy alive >The Range Rover Classic/Discovery I/110 Station Wagon have "simple" Did you have one too many over Christmas Richard ?????? Discovery 1 was never fitted with a self leveller as standard, and was much better for it. Adjustable head lamps as an option helped keep them pointing in the right direction when loaded. Keep up the good work ;-) SignatureMarc Draper > >The Range Rover Classic/Discovery I/110 Station Wagon have "simple" > > Did you have one too many over Christmas Richard ?????? Not enough it seems! > Discovery 1 was never fitted with a self leveller as standard, and was > much better for it. Adjustable head lamps as an option helped keep them > pointing in the right direction when loaded. I'm blaming the weather. For no immediately apparent reason. > Keep up the good work ;-) Richard Signaturewww.beamends-lrspares.co.uk sales@beamends-lrspares.co.uk www.radioparadise.com - Good Music, No Vine Lib Dems - Townies keeping comedy alive >> Basically, there are ride height sensors, I assume one on each corner, and a >> control box which maintains the ride height by adding or dumping air from >> the relevant air spring. > >No, the air pressure is adjusted to compensate for the weight of the >vehicle, the levelling itself is carried out by the hydraulics. Many Wrong !!! The air suspension on modern Landrovers is the only source of maintaining a level ride. Disco II and P38 use different systems, the Disco's being much simpler in operation in that it is only there as a levelling device hence the reason it is called SLS. The OP was totally correct to assume that the levelling was achieved by adjusting pressure in the air bags and monitoring the height using sensors. SignatureMarc Draper > Wrong !!! The air suspension on modern Landrovers is the only source of > maintaining a level ride. Take the dampers off and see how it handles bumps. The recent production (well, post-Classic anyway, I've no interest in the Transit vans) didn't tout the dampers as "self levelling" but they're still a progressively damped shock absorber and thus have some influence on ride height. If you fit constant-rate shock absorbers, even good ones, to a vehicle with controllable pressure air suspension (fairly common on "repairing" the early-90s Jags too cheaply) then it gets horribly confused and starts to bounce up and down like something from South Central LA. You _must_ have some progressive increase in damping (effectively a PD system) or else the air pressure system will start to hunt. <dingbat@codesmiths.com> wrote in message news:1167321770.017947.168870@h40g2000cwb.googlegroups.com... >> Wrong !!! The air suspension on modern Landrovers is the only source of >> maintaining a level ride. [quoted text clipped - 11 lines] > _must_ have some progressive increase in damping (effectively a PD > system) or else the air pressure system will start to hunt. Sorry to say I am well aware how dampers effect the ride of a vehicle, and how the single LARGE self levelling unit on the A frame of most classic Range Rovers works, these I am not interested in, only how the air suspension is connected to the air supply and how it maintains a set height and controls it, but thanks for the insight. So they have height sensors on each air spring and the controller tries to keep the vehicle at this height all the time ? Rich > only how the air >suspension is connected to the air supply and how it maintains a set height [quoted text clipped - 3 lines] > >Rich Landrover use height sensors (P38 and Disco II) that are mounted between the radius arms and the chassis. These keep the ECU informed of the ride height of each corner (Disco II rear only ) The P38 is much more complicated (and troublesome) in that it has more components spread down the length of the vehicle (air pump, valve block, receiver drier, air tank etc.) The P38 can perform more "tricks" than the Disco II, it will lower it's ride height at motorway speeds and can raise up on all four corners for extra clearance when off road. A friend of mine has just built a 6x4 landcruiser VX and used air suspension on the rear two axles. By use of air solenoids he can change the way the air bags are linked together for on and off road use. Levelling can be achieved using a dash mounted switch to raise or lower dependant on load (saves on ECU) . SignatureMarc Draper 2005 110 CSW XS 2000 DISCOVERY II XS 1998 P38 4.6 VOGUE SE LPG 1998 300TDI DEFENDER 90 CAMPER !! AUTO !! 1998 300TDI DEFENDER 90 CSW 2005 MINI ONE CONVERTIBLE >> only how the air >>suspension is connected to the air supply and how it maintains a set [quoted text clipped - 24 lines] > Levelling can be achieved using a dash mounted switch to raise or lower > dependant on load (saves on ECU) . Thanks Mark, right, this is more up my street, what happens when a wheel goes over a hump and compresses the suspension on just one corner does the ecu then reduce the air in that spring to bring the ride height back to its setting ?? Does your friend have an e-mail address I could talk to him about what he has done ? Rich > Thanks Mark, right, this is more up my street, what happens when a > wheel goes over a hump and compresses the suspension on just one > corner does the ecu then reduce the air in that spring to bring the > ride height back to its setting ?? If it's anything like the standard ECU it will only be running on a lagged version of height so it ignores bumps and adjusts to loads. The air-springs are actually quite good springs and the ECU only adjusts the ride height. I've been playing with mine as I seem to have the fault on my Rangie that drives EAS ECUs mad. http://www.nigelhewitt.co.uk/stuff/EAS/ This is an unfinished project but the EAS side works, it's the bit about talking to the BeCM that still needs work. nigelH >Thanks Mark, right, this is more up my street, what happens when a wheel >goes over a hump and compresses the suspension on just one corner does the [quoted text clipped - 4 lines] > >Rich Hi Rich The air suspension on a landrover is not that dynamic. When a wheel rides over a bump it simply compresses that air in the bag which acts as a spring, job done. The front of the vehicle will still dive under braking and the rear will dip on acceleration. It does not give the same "magic carpet" ride that Citroen and Rolls Royce aim for. We are talking about and "off road" vehicle with live axles after all. The SLS system is to compensate for load more than anything and that does not really change that much between pulling away and stopping. Whilst underway it will be dealing with average figures and as such wont be making any sudden adjustments to ride height as that would be very bad. It does depend what you are aiming to achieve from your project. If you want to just improve the ride of your vehicle I would say don't bother as the ride on air is not that much better than coils. But if you want it for the SLS and the ability to have a 2" suspension lift only when you need it then I would say go for it. Drop me a mail for more info and I will pass it on. Use the contact option on the website (the only bit that works) SignatureMarc Draper www.mdeng.co.uk 2005 110 CSW XS 2000 DISCOVERY II XS 1998 P38 4.6 VOGUE SE LPG 1998 300TDI DEFENDER 90 CAMPER !! AUTO !! 1998 300TDI DEFENDER 90 CSW 2005 MINI ONE CONVERTIBLE >> Wrong !!! The air suspension on modern Landrovers is the only source of >> maintaining a level ride. > >Take the dampers off and see how it handles bumps. Take the dampers off a coil sprung vehicle and see what happens over bumps !! The dampers are their to "damp" and not to set ride height. Increase the load in the vehicle and the air suspension will compensate buy putting more air into the system and therefore self levelling. SignatureMarc Draper >>> Wrong !!! The air suspension on modern Landrovers is the only source of >>> maintaining a level ride. [quoted text clipped - 3 lines] >Take the dampers off a coil sprung vehicle and see what happens over >bumps !! I did it once on the back of a sherpa - I think it was necessary to move it patr way through replacing them or something. Most enlightening; considering my opinion that the dampers I was taking off were shagged, how much damping they still supplied. SignatureAustin Shackles. www.ddol-las.net my opinions are just that "My centre is giving way, my right is in retreat; situation excellent. I shall attack. - Marshal Foch (1851 - 1929) >>>>Wrong !!! The air suspension on modern Landrovers is the only source of >>>>maintaining a level ride. [quoted text clipped - 8 lines] > considering my opinion that the dampers I was taking off were shagged, how > much damping they still supplied. Rode a Yamaha Super Tenere for an unreasonable amount of time with a completely shot monoshock. I knew no better as it was my first bike. The effect of the replacement was quite startling! Richard >> Wrong !!! The air suspension on modern Landrovers is the only source of >> maintaining a level ride. [quoted text clipped - 4 lines] >progressively damped shock absorber and thus have some influence on >ride height. The ride height is still done by the airsus AFAIK. The dampers may or may not be variable-rate, I don't know. But taking even shot dampers of anything renders it almost undriveable. >If you fit constant-rate shock absorbers, even good ones, to a vehicle >with controllable pressure air suspension (fairly common on "repairing" >the early-90s Jags too cheaply) then it gets horribly confused and >starts to bounce up and down like something from South Central LA. You >_must_ have some progressive increase in damping (effectively a PD >system) or else the air pressure system will start to hunt. and I'm quite prepared to believe that fitting the wrong dampers will louse the system up - it's designed to work with the OE spec dampers and putting something with markedly different characteristics will get it confused. Nevertheless, the dampers aren't (except in respect of letting the airsus work correctly) controlling the ride height, that's done (as on the citroen) by adjusting the effcetive spring rate: on the rangie, it adds pressure to the air spring, on the citroen, it adds oil which compresses the gas in the suspension sphere. Same effect though, more pressure in a gas spring makes it carry more weight, and incidentally, I assume, makes it stiffer at the same time. SignatureAustin Shackles. www.ddol-las.net my opinions are just that Soon shall thy arm, unconquered steam! afar Drag the slow barge, or drive the rapid car; Or on wide-waving wings expanded bear the flying chariot through the field of air.- Erasmus Darwin (1731-1802) > >> Wrong !!! The air suspension on modern Landrovers is the only source of > >> maintaining a level ride. [quoted text clipped - 26 lines] > it carry more weight, and incidentally, I assume, makes it stiffer at the > same time. As you say, shock absorbers (unless bottomed out or over stretched) have nothing to do with ride height - all they do is damp the oscillations that a spring (of any type - the construction of the spring is irrelevant - it could be solid ruber as per early Mini's) naturally has. Fit the wrong ones and they will over-damp (giving a harsh ride), or under-damp (which will make the "bounce" a lot, to the point of becoming undrivable) as they encourage the spring to return to its natural, at-rest, state. The only practical difference between an air spring and a coil spring (or indeed a leaf spring) is that air springs have the potential have an "infinite" rating (and thus height), whereas coils (and leafs) have to rely on the charteristics of the material they are made of and their "construction" to achieve different rates, which (due to cost I'd imagine) is why they normally are resticted to singe or dual rates, which in turn are usually related to loaded and unloaded (in LR land any way, so-called performance cars use dual rate springs to control body roll as well when cornering etc). The air suspension system uses software to emulate the currently desired spring charateristics, it does not (specifically) attempt to do the shock abosrbers job as well, not least as controlling air pressure (at an affordable price) quickly enough is not yet practical - shock absorbers just work too well to bother replacing with todays technology. I'm sure it will happen though - it wasn't long ago that ACE was "impossible". Richard Signaturewww.beamends-lrspares.co.uk sales@beamends-lrspares.co.uk www.radioparadise.com - Good Music, No Vine Lib Dems - Townies keeping comedy alive >>>>> "Rich" == Rich <r3engineering@ntlworldspam.com> writes: Rich> Good evening all, hope you had a good Christmas. Can anyone Rich> tell me how air suspension works, how the air lines connect Rich> the bags and what stops one side filling in to the other Rich> side and it all falling over, how are the bags filled to Rich> keep the vehicle level and at a constant height ???? Magic, if you believe the answer from the Series vehicle owners :-) Seriously, check out the information on www.p38a.co.uk for more of an explanation... SignatureAndy Cunningham -- www.cunningham.me.uk >>>>>> "Rich" == Rich <r3engineering@ntlworldspam.com> writes: > [quoted text clipped - 8 lines] > Seriously, check out the information on www.p38a.co.uk for more of an > explanation... Thanks for the link, describes the layout well will read more later.. P.S. You might want to look at the flow diagram as it looks to me as though all the sensors and springs are on the left !!!!! Rich Thanks for all the advice, so do all vehicles have a compressor on board to pump up the air springs ? if so they all must have height sensors otherwise you could have the same pressure in the air springs but with different heights and different volumes, ( am I right with this ) pressure is pressure but it is the volume in the air springs which give height in relation to the same weight ? Do the ECU's control each air spring separatly all the time or once the desired height is obtained is that air spring then sealed off from the rest of the system.. Tooo much time off and not enough to do ( nooooot ) Rich > Thanks for all the advice, so do all vehicles have a compressor on > board to pump up the air springs ? Yep. > if so they all must have height sensors otherwise you could have the > same pressure in the air springs but with different heights and > different volumes, ( am I right with this ) pressure is pressure > but it is the volume in the air springs which give height in relation > to the same weight ? I don't think the volume of an air spring changes that much for a given height, they aren't just a simple bag. They are more toroidal with the lower connection to a curved cone that is inside the center of the toroid. Take a look at the big air springs under some trucks when the truck is empty to see this. > Do the ECU's control each air spring separatly all the time or once the > desired height is obtained is that air spring then sealed off from > the rest of the system.. Each bag is controlled separately and continuously (with a DII even after the car is switched off to some extent). The control is very slugged so bumps and humps don't get air being released or pumped in. The control is for average ride height over a minute or so. Increase the load (say load six bags of cement into the back) and the bags will compress the ride height drop and the ECU fires the pump (if engine on) to increase the pressure and restore the height. Take out the cement, the springs expand with the lightened load, ride height increases, ECU opens exhaust valve(s) to lower the pressure and thus height. A DII will do this several times as you off load the back end. If you raise or lower the pressure without changing the load you can adjust the height of the vehicle. On the DII the "off road" switch lifts the back end a couple of inches from normal and you can get an optional key fob that will allow you to lower the back a couple of inches below normal. SignatureCheers new5pam@howhill.com Dave. pam is missing e-mail On Sunday, in article <nyyfbegfubjuvyypbz.jb4it24.pminews@srv1.howhill.com> > > if so they all must have height sensors otherwise you could have the > > same pressure in the air springs but with different heights and [quoted text clipped - 7 lines] > toroid. Take a look at the big air springs under some trucks when the > truck is empty to see this. Pressure, temperature, and volume are related. It's basic physics (though the temperature has to be measured on an absolute scale, with zero at about -273.16 Celsius). Part of the reason for the more complicated bag shape is to get the total volume big enough, compared to the changes arising from wheel/axle movement. Get too small a total volume, and things get hot too quickly. But the details of that are not quite so basic physics. SignatureDavid G. Bell -- SF Fan, Filker, and Punslinger. "I am Number Two," said Penfold. "You are Number Six." > On Sunday, in article > <nyyfbegfubjuvyypbz.jb4it24.pminews@srv1.howhill.com> [quoted text clipped - 18 lines] > volume, and things get hot too quickly. But the details of that are not > quite so basic physics. I'd hazard a guess that the shape of the air springs is related allowing flexibility without the "rubber" (let's not start that one again!) splitting. I also can't see the variables above being of great concern - the system (allowing for hysteresis) is far more simple - the sensor says the corner is lower than the target so more air is added until it isn't - and air is let out if over height. Richard > -- > David G. Bell -- SF Fan, Filker, and Punslinger. > > "I am Number Two," said Penfold. "You are Number Six." >> I don't think the volume of an air spring changes that much for a given >> height, they aren't just a simple bag. They are more toroidal with the [quoted text clipped - 9 lines] >volume, and things get hot too quickly. But the details of that are not >quite so basic physics. yeah, but for a given ride height unless the bag itself expands and contracts (which I assume it does to a small extent) then the pressure is the thing that varies most. Presumably, adding air raises the temperature, and this increases the pressure as well. Contrariwise, venting air from it should lower the temperature. when it's actually working, due to bumps in the road, then compressing it will reduce the volume and thus increase the pressure and temperature, while decompressing will decrease the volume and reduce the pressure and temperature, so on the average I'd not expect it to heat up much. SignatureAustin Shackles. www.ddol-las.net my opinions are just that "The breezy call of incense-breathing Morn, The swallow twittering from the strawbuilt shed, The cock's shrill clarion, or the echoing horn, No more shall rouse them from their lowly bed." Thomas Gray, Elegy Written in a Country Churchyard. > Pressure, temperature, and volume are related. It's basic physics This is true but the volume and temp are not changing a great deal in an air spring. As the ride height is being maintained at the same level the distance between top and bottom plates is, in the longer term, constant. Therefore the spring has the same shape and thus volume. The temp will probably rise from ambient under the repeated compression/expansion bumps/humps but I'd expect it to become fairly stable after a while, unless road conditions change (more or less bumps/humps). This just leaves the pressure to be adjusted to affect the ride height. SignatureCheers new5pam@howhill.com Dave. pam is missing e-mail |
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