Suspension Set ups

[postalWagon]

In an attempt to keep my light weight rig from having to much suspension, making it hard to drive without a heavy load or to soft to have a load. I have been kicking around an idea to fix my problem. the idea is to use both leaf springs an air shocks to manage my load and dampen the ride. The hope is that I can run a light to medium duty leaf pack with a ~2 inch diameter air shock. That would give me a better margine of load control and handling while still keeping my overall weight down.
Any pros or cons?
(I am looking at King, Radflo and Fox air shocks in the ~8 inch of travel size)

[JR Greenhorn]

Using air over a minimal rate spring is a great way to achieve a better ride without giving up load-carrying capacity. Various OEMs have been doing this for years, especially GM. Lots of large GM sedans over the last 20 years have had rear coil springs that barely hold up the rear of the vehicle when it's empty, and then used air shocks with a small on-board compressor and a simple electronic leveling device. Both of the cars in our garage came this way from the factory, and neither has failed in nearly (our Oldsmobile) or well over (my Cadillac) 200k miles of service.

My wife's car (Oldsmobile Aurora) is all stock and her leveling system has developed a small leak (the leak hasn't seemed to get any worse in over 6 years now). After her car has been sitting a while (like overnight), I can bounce on my hands and knees on the back seat inside the car enough to bottom the rear suspension out. However, start the car, pile the trunk full of luggage and five people in the cabin, and you drive down the road pretty much the same as if the car was empty. You can feel the extra weight if you make aggressive turns or hit particularly nasty bumps, but for most normal driving, the air shocks just carry the weight and you don't notice any difference from when the car is unladen. I don't recall ever bottoming the car's suspension out.




If you want to go this route (and I strongly recommend it), I would recommend finding the parts off a '91-96 Buick Roadmaster (sedan or wagon), or similar year Cadillac Fleetwood (what I have). These RWD cars have the "Auto Level Control" installed on GM's long-lived 4-link live axle rear suspension:



For the auto-level system, there is a ride height sensor and controller (#6) mounted directly over the top of the differential pumpkin. The sensor has a small adjustable link (#15) connecting the axle to a swinging arm coming out the side of the sensor's enclosure (#6). Inside the enclosure is a circuit board that is permanently encased in potting.

The controller has several functions:

• 35-40 second delay after the ignition key is turned on to when the compressor is allowed to run (so that it doesn't run until the engine is running)
• 4 second compressor purge after initial startup delay to ensure that the air shocks always have a minimum of 8-14 psi (this can be modified)
• Opens the exhaust valve for 1.5 seconds when the compressor is started, to relieve pressure on the compressor and lower its start-up demand
• 8-15 second delay between sensing an out-of-spec ride height and commanding up (compressor) or down (exhaust) to correct ride height to spec (+/-1")
• Compressor and exhaust (solenoid valve) circuits are limited to 6 minutes of continuous on time. After the 6 minute maximum is exceeded, the system disables these functions until reset by cycling the ignition key
• Short circuit protection: after detecting an overcurrent condition, the controller will retry the function/circuit 32 times with a 10-20 second timeout between tries. After 32 retries, that particular circuit will be disabled until an ignition key cycle resets the system
• System remains active for 6 minutes after the ignition key is switched off (so that suspension can still adjust if weight is added or removed shortly after the vehicle is stopped and shut off)

The sensor harness has fused battery power, ground, and "chime" (ignition key on) wires coming in, plus adjust up (compressor) and adjust down (exhaust) leads heading back out. The compressor harness has three fused 12V inputs (exhaust solenoid, relay control circuit, and relay load [compressor motor]), ground, and the adjust up/adjust down leads coming from the controller. The compressor harness is all powered through a single fuse, so you can easily disable the whole system by simply pulling its fuse. A 1/16" diameter plastic air line leads back to the shocks from the compressor.

You can see in the parts diagram how the compressor assembly (compressor, compressor relay, exhaust solenoid, air filter and dryer) simply mount to a frame rail on a bracket, and similarly, the sensor/controller simply mounts to the chassis on a bracket. After unbolting these two brackets from a donor car, it'd be pretty simple to mount them on another vehicle. The sensor link mounts through a small hole in one of the control arms, but you could weld a small tab or bolt a small bracket on just about any rear axle to make this work on another vehicle.



My Cadillac has this exact system. It's really very simple but works very well. A previous owner installed tow package rear springs on my car, so the rear suspension always rides firm, but never harsh. I've thrown 800lbs of softener salt into my trunk with no visible sag; the gas station attendant asked me if I was sure my car was going to be all right, but didn't say anything after the compressor ran for a while and I drove away straight and level. I do a fair amount of towing with my car (up to 3000lbs), and the auto level control works great for that. The one drawback is that sometimes it's difficult to get a feel for tongue weight while loading stuff onto the trailer, because the car compensates. I really should pull the fuse for the level control while I'm loading up the trailer (then replace it before driving away), but I never do.






On air shocks, however, I don't have much good to say. My first car ('74 Camaro) had wrinkles in its body sheetmetal from some previous owner running air shocks to "jack up" the car's rear end. Even in my current Cadillac, I hear some unusual creaks in the back when towing a trailer with a lot of tongue weight and the trunk full. This is one of the problems with air shocks: they force the chassis to carry a lot of load through the shock mounts, which are almost never designed for the amount of load an air shock can put on them.

Another problem with air shocks is that their damping is almost never the top priority in their design and construction. Because of this, most owners of the above-mentioned Buick Roadmasters and Cadillac Fleetwoods ditch the factory air shocks and run Airlift 1000 air bags that install inside the rear coil springs. This works much better for load carrying anyway, and frees up the option to use shocks with much better damping (Bilsteins are the norm on these cars).





In your case, because you're talking leaf springs, I would recommend looking into the air bag kits that install between brackets on the frame and the axle/leaf springs, like this:



My dad used to have a set like this on his 1/2-ton pickup to help when towing a smaller 5th wheel camper. Like many people do, he just went with the manual-fill valves and aired them up when towing the camper. However, no one ever aired them up when towing trailers smaller than that (even though they could've helped), and even with the minimum pressure in the bags, that much over the stock rear leaf springs made the rear suspension ride quite a bit firmer than it should have (or needed to) when the truck was empty.

Going with lower rate rear springs in combination with air assist for load capacity is certainly the way to go, as is an automatic ride height controller.

[sasfrontier]

I would not recommend using an air shock from the companies you listed. THose are designed to carry the entire weight of the vehicle and running them at a low enough pressure to not make the ride stiff will ruin their dampening properties. I would run a factory spring rate leaf spring with a good set of shocks. Then add a standard style airbag to help when carrying a load. The larger diameter the airbag the less pressure you will need to carry the load and it will also keep the ride soft, a small diameter air bag will require more pressure to help with the load and make the ride very harsh. There are two ways to do this in my opinion. you can run a bag between both leaf springs and the frame or you can build a crossmember and run one on top of the diff. Running the bags between the springs and frame will keep the load more stable but you will need to limit articulation so that you do not try and tear the bag apart if the rear end needs to flex. If you run one large diameter airbag on top of the diff to a crossember you won't have the need to limit the articulation of the axle, however it won't be as stable as an airbag on each spring. If you are carrying tall loads it could be an issue but if you are just carrying a lot of gear in the bed space then i don't think it would be an issue.

[Key-Lock]

I don't know what vehicle you're planning this for, but the ones in your signature aren't really high payload vehicles. I suggest getting one of the custom spring companies (Deaver, Alcan, National, etc.) to calculate a spring pack for both weight conditions: loaded and unloaded. Then compare the spring rates for both setups. You might find that the spring rates are close enough to each other that a spring built for the middle of the two extremes is a good compromise. If nothing else, it gives you a target spring rate to achieve for loaded and unloaded conditions. A custom spring composed of many thin leafs will also move easier than the fewer thick leaves from the factory.

Good Luck,
Dennis

[Hawkz]

I have a set of the Firestone airbags on my 1/2 dodge ram. I leave them at 7PSI when the truck is empty. (It does have a flippac, so there is some weight.) And I air up to about 20PSI when towing a trailer with the Jeep on it.

The bags max out at 100PSI, but I can't imagine ever running more than 30-40. I've only had mine a couple of years, but I am happy with them.

[Scoutn79]

I have friends who have used the typical air shocks Gabriel, Monroe type not the high end racing coilover styles) for years and liked them. One thing you will need to do is reinforce your shock mounts, both upper and lower, these were designed to control the oscillation of the suspension not to help carry the load. If you don't you may very well rip the mount off.
Darrell

[IdaSHO]

Going with lower rate rear springs in combination with air assist for load capacity is certainly the way to go, as is an automatic ride height controller.

Careful, if you put light enough springs on leaf spring setup with air bags, you will increase the possibility of hyper-extending the bags when the suspension is unloaded to its (new) max when articulating.