Thread: Defender Airbags v coils. Final verdict

Originally Posted by BilboBoggles

I want to do this, but I'm worried about roadworthiness rules and insurance. In Victoria would you need engineers certificates?

I'd say yes, but contact VicRoads to be sure. If they say no, get it in writing and keep it with you in the car incase plod disagree.

Originally Posted by BilboBoggles

I want to do this, but I'm worried about roadworthiness rules and insurance. In Victoria would you need engineers certificates?

Yes engineering is needed. See section 13.6 in this document on the Vicroads website.

http://www.vicroads.vic.gov.au/NR/rd...orvehicles.pdf

Tony

Last time I looked into an Engineer certificate was for a single bolt for a child restraint, and that was could to cost several hundred dollars. I would guess that such an extensive mod is going to cost a fortune to certify. Has anyone done this?

Are there any companies offering a certified conversion kit in Victoria?

Originally Posted by BilboBoggles

Last time I looked into an Engineer certificate was for a single bolt for a child restraint, and that was could to cost several hundred dollars. I would guess that such an extensive mod is going to cost a fortune to certify. Has anyone done this?

Are there any companies offering a certified conversion kit in Victoria?

Give Air bag man a ring. I asked them about this when I bought mine. I assume they are certifide in Vic as well, but they will soon set you strait.
Fitted my Air On Board pump today, much more confident in the system now. still a few bits to fix up but that can wait till I get back.

Originally Posted by Albert

Give Air bag man a ring. I asked them about this when I bought mine. I assume they are certifide in Vic as well, but they will soon set you strait.
Fitted my Air On Board pump today, much more confident in the system now. still a few bits to fix up but that can wait till I get back.

So, what's the story re: engineering certificate for QLD?

So getting back to this cross-linking. It appears the classics and P38's do it on the front bags.

According to the official literature, When initially moving off and then every time the speed drops to nothing, the front bags valves are opened to allow the pressure to equalize between the two front bags.

So. In an offroad situation, continuing to move will make them keep operating as independent springs. But stop (for example being cross-axled) and the valves will open to allow the pressures to equalise and the bags to cross-link. Letting the high side compress and the low side extend more than they would.
This makes perfect sense, because otherwise the combination of the large front sway bar and highly progressive air springs would prevent these vehicles ever getting through a marginal cross-axle situation.

I am not sure at what or any point the self-levelling then takes back over. The side-side levelling of classics and P38's is done solely on the rear bags, but the height of the fronts is read for levelling purposes where the lowest height is used. If the front bit counts are set wrong, the ECU on mine spends a bit of time shuffling air in and out to find a suitable height. There is I think a 2 bit window for the dead-zone where it deems the height to be correct.
Setting the front height sensor bits unevenly results in more valve activity, but it still levels out.

IIRC the air springs used in RRC and P38's were of a different style to the Firestone 1T style air springs. I can only guess why Firestone caution against carrying load over when the 1T air springs are near full stroke.

Firestone provide a design manual for using their air springs for suspension, which they refer to as 'airide' and a separate manual for actuators and vibration isolators ('airstroke' and 'airmount'). The data sheet I posted earlier was from the airstroke and airmount manual, but a good deal of it is relevant to the same air spring when used for suspension. Unfortunately the airide manual only includes one data sheet for illustration. Airide data sheets have more information that is useful for suspension design, but much of that can be calculated from the static data provided in airstroke/airmount data sheets (see my spreadsheet for springs that I have posted elsewhere on aulro).

When the 1T style air spring extends beyond the height where the bellows begins to roll off the piston, the effective area reduces at a rapid rate. This can be seen on the constant pressure curves for load vs length, where even at 100 psi, the load is small at maximum extension.

This, I guess, is the crux of why Firestone say not to carry load at those lengths - as the sustained periods of the high pressure required, combined with a stretched bellows, leads to failure of the bellows near the bead where the end closure is crimped on. Another issue would be the additional increase in pressure of the trapped air as the air spring is compressed from near it's maximum height.

However it may also result in the bellows not rolling back onto the piston properly as it is compressed from maximum extension under load.

With a manual control system for ride height, unless limit straps or shockies were set-up to limit maximum extension, it would be possible to inadvertently raise the air spring too far under high load - user beware!

Edit: this pic is the air springs I fitted to my rangie many years ago. They had a useable height range from 7" to 20". From the one on the right you can see how the effective area reduces when the bellows is no longer rolling on the piston.

Here are some pics from my RRC with factory EAS.
It only has ~165mm of stroke. From the bumpstops low ride height is +60mm, std height is +80mm, hi position is +120 and max height is ~165mm.

Old bags vs new bags:
The old bag is a front which is supposed to be 350mm long when new. The replacement is a rear bag which is 360mm long as shown.


Old bag at Hi Position Left Front. This one was leaking.
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Other side at Max extension (this is ~45mm higher than the offroad hi position should be. But at this point my heights were a bit screwy).


Close up same position:


New bag installed and on the bumpstops, this is a 360mm rear bag installed on the front:


Same bag at the offroad high position.