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I'm creating a thread for my Ninety air suspension build - if only as somewhere to put my notes, in front of the LR4x4 "peer review panel".

The car eats front prop UJs because it's lifted (and the engine is high, tilting the powertrain), and it doesn't fit in the garage. I also want to improve the crossaxle traction (more on that later), but first and foremost this is a condensing thought experiment and a chance to learn some coding, rather than a serious proposition for transport.

I've been amassing bits for a while, as these projects tend to do, and coronavirus has developed some time for me to play in lunchbreaks while working from home. I've got:

  • New mounts for the front shocks, to move them to the forward position
  • A P38 RR compressor, and a couple of valve blocks
  • A set of P38 front and rear springs
  • A pair of Disco rear springs
  • A set of RR height sensors
  • An Arduino, some peripherals and a very vague memory of programming in BASIC from twenty years ago

The Ninety went onto a new galv chassis three years ago so I will not be chopping bits off the spring cups. If I can't make it fit then it'll go back onto coils. If I can develop an open-source bolt-on air suspension system for other people to use, so much the better.

I anticipate updates will be sporadic and probably inconclusive. I'll try to cover my approach as well as the actual build.

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Posted (edited)

The coding was the biggest unknown so for the first step I stole bits of code off the internet and changed the words developed a programme which mocked up some control logic. 

Much more to do on interrupts, outputting via MOSFETs to drive the valve block and some PWM to stop the solenoids getting hot, but this was enough to make me believe it was possible. 

Video here: 

 

Edited by Turbocharger
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Then I set about looking at springs and things.

Three different springs (P38 front, P38 rear, Disco 2 rear) give different lengths, diameters and lower pedestals.

IMG_20200321_173231.thumb.jpg.c2bf75b35d05398e6226fb06d20059cb.jpg

Aside: an air spring is a clever bit of kit - if it was a parallel spring then the pressure inside would be the same at any ride height - it's a mistake to assume that they're stiffer at full height. In fact, though the *pressure* would be the same at full height (since pressure = force/area) the *effective spring rate* would be lower because the spring rate depends on the area of the lower piston and also on the enclosed volume - there's more air inside doing the work. This means it's more complex than just levering a spring into the wheelarch and piping it up.

It's not so simple to measure the 'spring rate' with an air spring - the coils were about 170lb/in when they were new (they were RR Classic 'red & white', originally giving a nominal 2" lift). I used a phone app to get accelerometer data and work out the natural frequency. Much easier without the shocks connected but the bounce still drops away to nothing within five or six cycles, just from the friction in the bushes etc. The coil spring (with a 300Tdi, winch on the front, 6 point cage etc) is 1.6Hz. That fits the expected 1 to 2Hz range, anyway.

Screenshot_20200324-151930.thumb.png.76705ff695fef1a58eec80bc66a57e7a.png

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Firstly I've played with the P38 front spring, because it's probably closest to fitting easily. For the sake of getting some numbers (and an easy, visual win which keeps my interest) I've bodged them in with bits of wood, it's not going anywhere on the road yet.

IMG_20200321_165609.thumb.jpg.50f9b904b41def2ad5f7b71b8d21f4d5.jpg

Surprisingly simple, some 6mm pipe and a couple of fittings and "car goes up, car goes down".

The natural frequency changes at different heights, I measured c.1.9Hz at normal ride height and 1.6Hz at +75%. If 1.6Hz is 'standard' then 1.9Hz for normal driving represents about +40% spring rate (since the natural frequency = sqrt [k / m] ).

However it would be relatively easy to reduce that rate by piping another volume (say, a 1 litre pressure bottle) somewhere near the spring and allow air to push back and forth. With a tap or (posh) a solenoid, it could even be selectable, but let's not run away with ourselves yet.

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Wow, I love air suspension but have a complex love/hate relationship with my P38,... subscribed, and look forward to seeing how you get on.

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Posted (edited)

The springs themselves are fairly simple units, the rubber bag rolls over the lower piston. That piston is shaped, to adjust the spring rate at different heights, which means there's some scope for tuning this by swapping parts about. The Disco 2 springs are a different diameter and the bags are crimped on but the P38 and RR Classic springs are built like the bus springs I know, and they're easily disassembled:

There's a small lip inside to retain the rubber bellows, but the spring is mostly held in place by air pressure - and by the constraint of the axle geometry so that it's never overstressed.

IMG_20200322_100318.jpg

 Putting that piston back in is rather harder at first glance. Vaseline/silicone lube is your friend, twisting as you go, and just a little airflow into the bag helps to keep it in shape. I've done a 'how to' video to save anyone else the hour I spent chasing it around the floor before figuring this out. Lying across it and using your hands and knees also helps - recruit a helper to squirt a little air in, but not enough to put you into orbit if it pops!

 

 

Edited by Turbocharger

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Interesting..

 

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Will follow with interest

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Very interesting, I like.

I get that you're building a controller out of interest and to learn (can only applaud that), but I do have to wonder why you're not just using the EAS system for a P38, it's pretty much stand-alone.

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1 hour ago, elbekko said:

but I do have to wonder why you're not just using the EAS system for a P38, it's pretty much stand-alone.

Noooo!,  standalone but way too complex surely?? and despite the fact of it being 'safe', I hate the way it dumps you on the ground for some small difficulty.

I have to say that if I were embarking on this project my natural instinct would be to start with any other  manufacturers equipment (and I have no idea if they are really any more reliable), but P38 does keep it LR and makes it of interest to me, particularly if we get to see a simplified controller.

On the spring front I have only had them pop out on the ramp, simply putting the weight back on the wheels sorts it out in that case. But the little vid is spot on, its good to know that it isn't a big problem if it happens off the vehicle.

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Oooh, very interesting. Subscribed :)

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The US has many kits for air suspension and controller to be retro fitted

There was a UK system that used to sell kits that worked very well.

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2 minutes ago, Arjan said:

The US has many kits for air suspension and controller to be retro fitted

There was a UK system that used to sell kits that worked very well.

Ogden Air springs to mind (pardon the pun)

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I think Ogden disappeared many years ago, though their legend lives on! 

I want my own controller rather than an aftermarket or P38 controller for several reasons - first is the learning (remember this is a hobby, not transport), and second is the control. Third is that they're all quite spendy, probably good value compared to buying new bits but I've got most of the ingredients for about £200 using second hand/scrap/begging etc.

It's certainly more complicated than cart springs or bits of metal, but we understand the failure modes after twenty years of P38 (and 25 years since RRC got airbags) - we should be able to tackle the common issues:

  • Airbag failure - they're fairly easy to change the bladders, off the car at least.
  • Air feed failure - override control of the individual solenoids, and/or manual inflation points.
  • Ride height sensor failure - some clever logic (see below), or back to manual solenoids for inflation/deflation
  • Compressor failure - could have an inflation point for an external airline, or carry a Halfords tyre compressor. 

If the P38 loses a ride height sensor, it freezes or drops you on the floor. If the L322 loses a height sensor (bong bong SUSPENSION FAILURE), it'll only do normal height.

Surely, if it's blind to the height at one corner (and assuming flat ground and no other leaks or faults) then it could work out one height from the other three, or cycle the car down to the bump stops and fill both bags on that axle equally until the one sensor shows a good ride height. It might need some manual trimming, and maybe adjustment every half hour, but that'll get you home.

It's more complex but springs break, become unseated, bushes wear out, shocks break etc etc etc.

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It all comes down to how complicated you want to make it, really..

4 airbags on 4 corners will require a certain pressure to keep the vehicle level,

Once they're level, not much regulating is needed as long as the weight does not change.

Can all be done manually - obviously some PLC / ECU will make it all look 2020 and all that..

We had a 4.500 kgs. trailer on air springs & shocks.

Just 1 tank and 2 valves - that was all.

We used a fixed compressor to fill the tank from time to time.

 

BTW :

 

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16 hours ago, Turbocharger said:

Now imagine air springs, but instead of controlling the ride height there's a pipe between the front left and front right. The car would settle down slightly as the weight transferred equally across the axle, and there would be more grip as quickly as the air was able to transfer. LandRover have done this since L322/Disco 3 with a cross-axle valve, but the pipe they use is small so a lot of that work is done in the software too, which means it all looks very stiff and unnatural as these trucks make adjustments, and offroad progress is jerky. I'm hoping to make something much more responsive, even if I have to use a larger bore pipe.

Professor Rafferty (SimonR) inspired a lot of this work, and he found an important point by actually going and trying this - by cross piping the axle the effect is to support the vehicle at the centreline - there would be almost no static roll stiffness if both axles were cross-connected. That would be exciting at any speed, but if climbing or descending a slope it could destabilise the vehicle. My hope will be to only open the crossaxle valve on the uphill axle, should be easy enough with an accelerometer, so that the vehicle climbs a slope like a Robin Reliant (supported on the two rear wheels and the middle of the front axle) and descends like a Morgan three-wheeler (two front wheels and the middle of the rear axle). The traction benefit will be more marginal on the uphill axle, but I'd prefer to stay the right way up). 

If you connect lh front to Lh rear and rh rear to rh front would have the same effect, but not the roll stiffness problem. On the road you can use the brake light signal to close the valve between front and rear. Also, if the axle height sensors realise you are not in a cross axle situation (so climbing a hill with the rear axle going up and the front axle coming down), you know this via the height sensors, and you can also close the valves. Or leave them closed all the time and only open when you sense cross axling.

 

Interesting project, keep the updates coming!

Daan

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Ooooh what's this - science and coding on the forum? :blink:

Subscribed!

 

BTW I have a dead air-ride controller here that someone gave to me to see if I could fix it, apparently they're hella money and have a specific failure quite commonly (ISTR the pressure monitoring for the tank pressure?), it's all a bit fiddly and monolithic but if the valve block etc. might be of use I'll pop it in the post if you like?

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5 hours ago, Turbocharger said:
  • Airbag failure - they're fairly easy to change the bladders, off the car at least.

And on it - I suspect even easier, but I've only done them in situ.

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Today's progress- one end of one rear shock absorber removed. Short lunch and it was being stubborn. 

7 hours ago, Arjan said:

We had a 4.500 kgs. trailer on air springs & shocks.

Just 1 tank and 2 valves - that was all.

We used a fixed compressor to fill the tank from time to time.

I'd seen the old thread, thanks. Some useful info in there!

The trailer system is a good challenge - if any parts are fitted, are they adding value? I'd say if it's just air springs then it's the same as coils (arguably more fragile). Air springs with mechanical levelling valves would be bombproof but would only adjust for height, so doesn't give different ride heights and/or crossaxle performance.

I did think about using mechanical valves with a worm gear and servo to change the link length, to 'fool' the mechanical valves into eg lowering the car as required. This has the benefit of benign failure modes, but I'd need to get small IP67 linear motors or similar and even then it would likely try to fight any cross-axle linking I put in. Solid farmer-proof idea though. 

5 hours ago, Daan said:

If you connect lh front to Lh rear and rh rear to rh front would have the same effect, but not the roll stiffness problem. On the road you can use the brake light signal to close the valve between front and rear. Also, if the axle height sensors realise you are not in a cross axle situation (so climbing a hill with the rear axle going up and the front axle coming down), you know this via the height sensors, and you can also close the valves. Or leave them closed all the time and only open when you sense cross axling.

I hadn't considered front to rear - I've no idea how it would handle on the road. It could be made to work if the springs were selected to recognise the extra weight of the engine over the front axle. By extension I could just pipe all four corners together and let nature do the work in equalising, bit I suspect that would handle like a bag of wet tits rolling down stairs. There's only one way to find out, and it's a small pipework tweak (or some temporary coding) once it's all working. 

3 hours ago, FridgeFreezer said:

Ooooh what's this - science and coding on the forum? :blink:

<snip> dead air-ride controller <snip>

I try... I have to balance out Nige's 2000A welding of 16mm shims 😉

I'll snatch off your hand for the air ride bits, if only to fiddle with and work out what's what - thank you! DM'd.

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Managed some fiddling today (cold and windy) - once the rear shocks and springs were stripped out I stuck two springs under the rear, for the visual reward of appearing to make progress.

It might look to the uninitiated like the rears are just wedged in with some bits of wood and a piece of 2x4 for a bump stop extension. This is clearly a highly technical trials environment and shouldn't be misunderstood.

IMG_20200329_184255.thumb.jpg.33ee26072cd7184e62ed849ae4248b4b.jpg

Manual inflation allows each end to be lifted or lowered - I've got P38 rears at the front and fronts at the back, because they fit the spring chairs better that way round, but the extra travel of the rear springs at my front end is clearly visible.

Here both axles just have the springs piped with a T-piece. It's very wobbly in roll even if I just hang off the cage. On the road it would be truly exciting. Separately I piped all four springs in parallel with 3 T-pieces, and the rear lifted to max height before the fronts moved much at all, the weight of the engine being much heavier. If I linked the wheels front/rear it could only be one side at a time and I think the handling would be very strange; that's one to try at an offroad site only I think.

Next stop, mounting height sensors.

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There was a thread on here where it was proposed to replace the 110 CSW Boge Strut with an air spring (from an air suspended cab) . It had occurred to me that would let you vary the pressure in the outer bags if you needed to adjust roll stiffness while maintaining the load capacity.

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12 minutes ago, cackshifter said:

There was a thread on here where it was proposed to replace the 110 CSW Boge Strut with an air spring (from an air suspended cab) . It had occurred to me that would let you vary the pressure in the outer bags if you needed to adjust roll stiffness while maintaining the load capacity.

I've wondered about it for my 90.... not done anything about it get mind :P 

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