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scarecrow
Member Since: 14 Dec 2019
Location: Gloucestershire
Posts: 49
  
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Pressure equals force per unit area. The pressure in the system is the weight of the car divided by the cross sectional area of the air springs. Simple as that. Height is not a factor.
When you raise the suspension you pump more air in. There will be a momentary pressure spike while pumping and as the car raises, but then it will reach equilibrium at the same pressure as before. The job of the compressor is to raise external air to this pressure.
What varies with height is the mass and volume of air in the system. The compliance of the system (ie softness) is related to how it reacts to changes in that volume when the spring is momentarily compressed by going over a bump. For a given sized bump, say 1cm, a 1cm compression of the spring is a smaller percentage of the spring volume when the spring is larger (ie has more air in it).
Bottom line, there should be more compliance in the system when it’s raised, so it should be softer. But whether this is a significant change you can actually feel given all the other mechanicals between your seat and the road I couldn’t say!
Note that this is not the same situation regarding tyre pressures. With tyres you have stiff walls and essentially a fixed volume, so pumping more air in changes the pressure and the shape of the tyre to keep the road contact patch area consistent with pressure divided by weight. With tyres, lower pressure equals a softer ride, totally different situation. Not to mention sidewall stiffness. Except I just did.
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 28th Nov 2020 11:00 am |
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Hardware
Member Since: 28 Jun 2016
Location: Hiding under the M60
Posts: 12769
 
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Thinking logically here ...
To raise the car, air is pumped in to the bellows and wouldn't go in unless at a higher pressure than what is already there. HOWEVER, that only remains true whilst actually raising the car and might not be substantially greater anyway.
Once raised, the air is probably at same pressure as when it started.
There is now more air and more volume, but the extra volume is almost 100% extra height added to a cylinder.
When you then compress the air, it has some "give " before the compression increases the internal pressure to the point the weight of the car can't compress it further. For the sake of argument let's say that's 20%.
This is going to be true no matter the height setting so if you've added 100mm of height then the "give" allows the bellows to compress the original 20% PLUS 20% of the added 100mm = an extra 20mm of "give"
IMHO this would translate into slightly softer ride. .
Dean
====================================
2011 D4 XS - OBD port protection, RLD spare wheel protector, All LED interiors lights, Timed Climate enabled, iiD tool paired.
2011 D4 Landmark - Stolen from same dealer before I paid for it
2011 D4 GS - Stolen whilst at dealer  ... All LED interiors lights, DRLs, Spare Wheel protector.
1996 300Tdi - Eaten by tin worms
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| 28th Nov 2020 11:01 am |
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TigerRecovery
Member Since: 31 May 2017
Location: Long Stratton, Norwich, Norfolk
Posts: 901
 
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Having spent some time as a Land Rover Experience instructor demonstrating the differences between normal and off road height on some very bumpy tracks, I can tell you it's better when the suspension is in normal mode compared with off road height.
I've also travelled around Morocco in my D4 and I can safely say that in off road height, the ride is much harsher.
It could be simply due to the fact that in the extended setting the rubber in the spring bellows is thicker and less compliant, but conversely, as has been explained above pressure is force over area and by extending the springs to a longer setting, the area they are showing is also greater. When in normal or access height, some of the spring is folded over and isn't actually exposed to the air. 1972 Range Rover Classic 2 door V8
2013 Land Rover Discovery 4 HSE - SCRAPPED
2016 Land Rover Discovery 4 Landmark
2022 Volvo XC40 T5 PHEV 282HP FWD 3 cylinder!
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| 28th Nov 2020 12:14 pm |
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McAdam
Member Since: 21 Nov 2020
Location: Canary Islands
Posts: 129
  
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Wow, thanks for all the great replies.
@ Kelvo, RichardK, garrycol - I was actually looking at the LR Au forums as I know you guys have these corrugations as a daily occurrence. I will one day do a Aus tour, visit all the great places, including hopefully Fraiser Island 
@MotoLab yup the LR Passion one is the one I am planning to get, just need to fix the suspension issues first, which needs an IID, which I decided can wait till I get my nav screen in first, which will be done at the same time as my 360 3d camera system for offroading.... I have a loooong and growing list of cool things to do.
RE Engineering and pressure etc.... My understanding is:
Think of the air suspension like a tyre (it is a rubber bag in the same way).
Less pressure = less stress on the walls, more compliance. For air suspension, this means that the suspension is lower, and there is more room for expansion in the airbags... i.e. more capability for shock absorption.
More pressure (which is also = more air) makes it harder, in the case of a tyre more inflated, in the case of the airbags taller. but due to the increased pressure and volume, there is less room for the air to spread out and less capability for shock absorption.
BUT
What I was wondering is if the air valves on the Disco are dynamic, as in they are never sealed and always adjusting how much pressure there is in each bag, sending the excess volume to the tank when needed.
If they are dynamic, then the pressure vs stiffness debate is moot, as the excess air volume can be displaced to the tank, thus offering the same compliance in whichever height.
If it dosnt make sense to you, thats ok, im just wondering pre coffee Pre-Production
D3 V8 #264
LHD '04 HSE
Living its life as a 50/50 On/Off road car exploring the Canary Islands
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| 28th Nov 2020 1:07 pm |
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aja4x4
Member Since: 14 Apr 2019
Location: Westbury
Posts: 2459
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You also need to take into account air is compressable, the greater volume of air the more it can compress compared to a smaller amount at the same pressure
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| 28th Nov 2020 2:15 pm |
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