P38 air bags on a Defender chassis?

[TD5toV8110]

So, on my very slow project, I was planning Range Rover Classic front springs (soft) and air bag rears (soft unloaded, then pump up for a load)

Here in Oz aftermarket air bag setups cost heaps.

I am thinking now get a set of P38 bags and modify/mount them to my Defender chassis.

No ride height control or any electrics. Just pump them up and go.

What could go wrong?

Anyone got decent pix of the chassis and axle mounts for bP38 bags?

I know I'll need external shock mount for the front, rear should be easy Ish?

Cheers

Dave

[elbekko]

Shouldn't be too difficult, no. There was a thread on here a few months back of someone doing exactly that.

The top mount needs a cutout for the airline to stick through. Top and bottom are held in place with a pin.

I'd go for some Arnott (or knock-off) bags that have the bag crimped onto the pedestal.

[Escape]

You'll want to use the front P38 airbags, not the rears, as those are easier to mount in standard spring cradles. If you don't want/need ride height control, plumbing can be pretty straightforward.

[FridgeFreezer]

Didn't @simonr  do this on his 90 a hundred years ago?

[TD5toV8110]

9 hours ago, Escape said:

You'll want to use the front P38 airbags, not the rears, as those are easier to mount in standard spring cradles. If you don't want/need ride height control, plumbing can be pretty straightforward.

Do you mean use P38 fronts in the rear and front?

Or just not use rears at all?

[Escape]

Indeed, I would try to use the P38 fronts in both the front and the rear. They do take up a bit more room, but should be easier to fit to the Defender spring mounts, with both the fasteners and air connection at the top instead of sideways.

[simonr]

On 7/9/2023 at 9:00 PM, FridgeFreezer said:

Didn't @simonr  do this on his 90 a hundred years ago?

Kind of!  I used Firstone Air Bags.

While it all worked quite well, they were very prone to punctures off road.  The smallest stick seemed able to push through the wall of the bag.

If I were doing it again, I'd use airbags from a Discovery 3 / 4 / RRS for the main reason that the airbag itself is protected inside a metal can - and they seem largely puncture-proof.

The negative (maybe not so much of a negative?) is that the damper forms a part of the spring assembly & lives in the middle of the air-bag.

[TD5toV8110]

On 7/10/2023 at 4:00 PM, Escape said:

Indeed, I would try to use the P38 fronts in both the front and the rear. They do take up a bit more room, but should be easier to fit to the Defender spring mounts, with both the fasteners and air connection at the top instead of sideways.

So,  are the front bags a different rate/load etc to allow for the extra weight up front? 🤔

[mickeyw]

ISTR that @Turbocharger was working on an air suspension setup on his 90 during lockdown. I don't recall how that finished up.

Edit: here you go 

 

 

[Escape]

As far as I can tell, the bottom cones are identical front and rear. And it's their shape/surface area that determines the relation between pressure and force for any given height.

Aftermarket brands like Arnott do have a different shape and corresponding different characteristics.

[Chicken Drumstick]

12 hours ago, TD5toV8110 said:

So,  are the front bags a different rate/load etc to allow for the extra weight up front? 🤔

Don't think so. As the air pressure sets the springs rate and ride height. The front shocks are longer than the rears. So the front bags may well be a different length.

The main issue with the p38 setup is, raised ride heights mean inflated air springs, thus a high and hard spring rate. When lowered there is less pressure and a softer spring. So it sort of works the wrong way round. Soft and squishy in motorway/high speed mode and stiff and bouncy in off road mode.

Arnott offer some Gen 3 springs which use a different cone system and in theory they should offer a stiffer spring rate on a lowered ride height and a softer spring rate on a raised spring height. But you need take out a mortgage to buy them

[TD5toV8110]

Forgot to say, this is a mega budget build.....

 

So far the build has me in profit!

Buying and selling, keeping the bits I want re-selling the bits I don't!

 

So, Iam going back to suffix A steel front springs and, P38 front bags in the rear! ($175 Aussie dollars for a new pair!)

Thanks for the advice.

 

I'll start a build thread one day.....

 

[Carloz]

Why not use Discovery rear airbags? They are about a straight swap on Defender 90 right? 

[TD5toV8110]

Dunno,   I already bought a pair of P38 fronts.....🤪

[TD5toV8110]

Ok guys, I got some P38 fronts! (for the rear)

Thinking just link the pair and have a T off to a schrader valve.

Safety wise if a line or fitting lets go both deflate to bump stops.

Anyone know the pipe size required, and fittings etc?

Don't care about original LR stuff, just whatever works!

 

Cheers

Dave

 

 

 

[elbekko]

6 hours ago, TD5toV8110 said:

Thinking just link the pair and have a T off to a schrader valve.

That could give some "interesting" on-road handling...

You'll want 4mm internal, 6mm external diameter nylon line. Use olive fittings if you don't want it to leak.

Edited July 25, 2023 by elbekko

[Escape]

You do not want to link the bags side to side for a car that doesn't anything above a crawl. Not permanently at least. It will have little resistance to rolling, unless you add a beefy roll bar. Think of it as a tractor with a pendulum axle and no dampers!

[simonr]

1 hour ago, Escape said:

You do not want to link the bags side to side

I tried this, with solenoid valves - but found it made the vehicle too unstable to be worth engaging.

 

[TD5toV8110]

Cheers guys, so individual lines with two Schrader valves! 👌

 

Thinking about linked bags.... Is it like if one bag hits a bump it them squishes air across to the other bag essentially lifting that (opposite) side for a moment, and then back and forth? Wibbly wobbly style! 🤪

(and other weird stuff!) 😆

[Escape]

Yes, exactly that. With the only dampening caused by the resistance in the airline between the bags.

The L320 Range Rover and later use this to mimic a live axle off road. But it's only at low speeds and probably dependent on the terrain response setting that the valves open to link both bags. I have done similar on my P38, switchable from the dash. I made sure it is only available in low gear. It's an attempt to get more flex from the front axle and still keep the stabiliser. Not tested it yet though.

[TD5toV8110]

Hey guys, I'm not near my bags right now....

 

What is the size thread in the bags? Is it male or female?

 

Need fittings to go in the bags to then go to 6mm nylon lines.

 

Cheers

Dave

[elbekko]

P38 bags don't have thread, just a 6mm push-fit fitting.

Unless you went for Arnott Gen 2 bags?

[TD5toV8110]

Thanks!

 

Std bags! 👍

[Escape]

On 7/25/2023 at 3:58 PM, simonr said:

I tried this, with solenoid valves - but found it made the vehicle too unstable to be worth engaging.

 

I plan to keep the front stabiliser bar, so hope it will be doable at low speeds. I'll report back when I get to test it. 😉