Arnott Generation III P38 Air Springs
Technical Design Details
Author testing extreme articulation of the new Gen III springs in the
The standard Range Rover P38 spring piston widens towards the top, effectively stiffening the spring rates as vehicle height increases. This gives a harder ride off road and a softer one at freeway speeds -- the opposite of what is needed. On the new Mk III, spring piston design has sensibly been modified to reverse this situation, giving softer spring rates in high profile and stiffer rates in low. This makes a lot more sense.
Adam Arnott (owner of Arnott Industries) decided to give P38 owners the same benefits by designing a new Generation III spring for the 1995-2002 models . Adam is a fellow Range Rover owner, with a Mk III as his daily driver and several P38s used as test mules. Working in consultation with RangeRovers.net, he came up with a totally new design (see pictures at right and below) with a two-stage piston that is wide at the bottom, where the bellows folds over it when in low profile (highway mode), but reduces to a narrower diameter for most of its length, where the bellows sits at standard and high profile (low speed and off road). To cap it off, a longer piston provides 2-3 inches more travel than the stock design.
The result is a noticeably smoother ride in high mode, noticeably more control in highway mode, and more usable wheel travel.
Image at right: Designer's illustration of the new Generation III spring (rear shown), with two stage lower piston.
Technical Design Details
rear springs showing OEM and Gen 3 lower piston design. Whereas OEM
piston cross-section gets larger higher up, the Gen 3 piston
cross-section is a maximum at the bottom and is reduced at standard and
High heights. (The diagram of the Gen 3 spring shows the bellows
position at standard height setting).
The secret of the new Generation III air
springs lies in the
custom-made lower piston which is machined out of a solid aluminum
billet, specially shaped to vary the spring rates according to
conditions (see diagram above right). A smaller piston cross-section
displaces less air for a given perturbation, giving a softer spring
rate. Conversely, when the bellows are operating on a larger piston
cross-section, a bump in the road produces a larger displacement of
air, effectively increasing the spring rate or stiffness.
For example, in the Gen 3 rear spring, the narrower top zone reduces the spring rate of about 145 lb/in in high mode, greatly softening the ride. In standard mode, the bellows are near the bottom of this zone and yield a spring rate of about 155 lb/in. The fatter bottom section gives a spring rate of 225 lb/in in low or highway mode.
Another feature built into the new spring is significantly longer travel -- with two inches more extension in the front and three in the rear, achieved via the longer piston and special bellows. This capability, illustrated in the photos above, is especially useful for those who want some lift or have re calibrated their EAS settings for higher ride height in "high" mode. Adding longer shocks and recalibrating the "hi" height setting could provide an extra 2 inch "lift" using these springs, while maintaining full articulation ability. In my own case, the extension at full articulation of my front springs is greater than normal anyway, as I have removed the sway bar from the vehicle. The longer extension available from the Gen 3 spring allows this extra articulation to be fully utilized (photos above right and at top of page).
RangeRovers.net was privileged to assist in the on and off-road testing of the new Gen III springs. (Since Arnott is based in Florida, it can be difficult to find much mountainous terrain where extreme articulation is needed!!).
I installed the new springs prior to an expedition to the Nevada desert. Installation was easy and exactly the same as for stock air springs (see the air spring replacement page or the instructions on the Arnott Industries website for details). The first thing I noticed on the trip was the improvement in the highway ride and handling -- the higher effective spring rate in the "lo" or highway setting was very noticeable. In rapid high speed manoevers the vehicle felt considerably more stable. As mentioned above, I long ago removed the sway bar from my 4.0 to improve off road traction, and the resulting ride on the highway was always a bit on the loose side. The new springs tightened up the handling and made the ride more stable without introducing any harshness. On vehicles with the sway bar still in place, the highway handling would be even better.
Once out in the terrain, I found the ride at standard height nice and soft, gliding over the bumps. In "high" mode the improvement over the stock springs was nothing less than dramatic. I had become so used to the harsh ride in high profile that I had generally avoided using it except when necessary to clear an obstacle. The Gen 3 springs transformed the situation so that I found myself using high profile almost any time I was off pavement in order to get a nice soft ride. The benefits were particularly noticeable when accidentally encountering dips, washes and "whoop de do's" at a speed that was a bit higher than ideal; in hi profile the new springs absorbed the resulting compression extremely comfortably, and without bottoming out.
Articulation in hi profile seemed to be improved also; the lower effective spring rate allowed the axles to flex more freely and made negotiating cross-axle situations a piece of cake (see picture at top of page). Without a sway bar the front axle has greater potential to flex, and the new longer springs freed it up so it could deliver the maximum available travel.
Overall, I was very impressed with the new design, and the improved ride it provides. Having more control and better handling at highway speeds is a major plus, but even more of an advantage is the softer ride off road. The resulting improvement in ground contact pressure on a drooping wheel also gives greater traction on even mildly uneven surfaces, as explained on the Range Rover Suspension page.
The new springs also work great in Extended Profile using my Manual Extended Profile Selector to gain an extra 1.2 inches of lift. With stock springs I was afraid to use this setting for any length of time in case it strained the airbags, but with the extra extension of the Gen III's, Extended Profile can be used with confidence for indefinite periods. Longer shocks would take even better advantage of the new springs' performance characteristics, allowing them to droop to their maximum extent. However even with stock shocks limiting downward travel, I was sold on the new design enough to keep the Gen 3's permanently on the vehicle.
The new Generation III springs cement the position of Arnott as the premier innovator and supplier of air spring products for Range Rovers. The new design further expands their range which already includes two other alternatives for 4.0/4.6 owners (the standard low cost stock look-alike replacements and their strengthened Generation II design), as well as coil conversion kits. The Generation III allows P38 owners to enjoy many of the design benefits afforded by the newer Mk III Range Rover suspension, combining improved handling on the highway with greater comfort and traction off road. In addition, the greater available travel provides the potential to experiment with moderate lifts and/or longer shocks. I for one plan to keep these springs on my vehicle! For one thing, they come with a lifetime warranty.
More information on the design, specifications and technical details of the new springs are available at this link on the Arnott Industries website, which also includes ordering information and instructions for installation.
Spring Replacement Page
Air Suspension Operation and Diagnosis page
Air Suspension Field Recovery Page
Arnott's illustrated instructions for air spring replacement
Low cost and generic parts sources page including air suspension parts
Manual Extended Profile Selector page
Manual Pump-up of Air Suspension
Shock Replacement, P38
Range Rover Suspension Information
Arnott Industries was founded in 1989 by Adam Arnott and his father to develop more reliable replacement parts for air suspensions that made their appearance on vehicles like the 1984-87 Lincoln Continental and Mark VII. With the adoption of air suspension by Range Rovers, Arnott began developing less expensive and improved air springs for them. At times their springs were the only ones available in the US when a severe shortage of the more expensive Genuine parts occurred. Some owners have even informed me that other brands of air springs and bellows they have purchased elsewhere were actually marked with the Arnott logo! Adam is a Range Rover owner himself, and takes a very personal interest in their range of air springs, bellows, and coil conversions for both the Classic and 4.0/4.6 Range Rovers. He is still developing more products to aid long-suffering Range Rover owners with their EAS problems! Amazingly, all Arnott air springs come with a lifetime warranty, so you don't have to worry about expensive replacements from the dealer any more
Arnott Industries has kindly agreed to help sponsor this website, so if you do purchase from them please mention RangeRovers.net.