Auto Service World
Feature   February 1, 2009   by Jim Anderton, Technical Editor

Riding On Air

A time tested retrofit technology that can breathe new life into light truck suspension dynamics

Anyone old enough to remember classic subcompacts, like the Datsun B-210 or early Toyota Corollas, will remember the ride. Empty, they rode like a buckboard in a Western movie, rear wheels bouncing over every bump in the road, with only anemic engines keeping them from swapping ends in anything like a quick turn. Loaded, it was the opposite: a hippopotamus wallow that challenged the bump stops. Heavy (meaning four adults) and the little subcompacts offered the door handle-scraping body roll of an early-Seventies Lincoln Town car, while still subjecting the occupants to a harsh ride.

The reason was a primitive solid axle rear suspension and leaf springs, the same system under millions of modern SUVs and light trucks. Canadian owners have used weight in the rear for years as a winter traction aid and many notice the smoother ride. Can a technician offer something more practical?

SSGM tested a technology that’s hardly new, but that can give owners tremendous flexibility in rear suspension dynamics: air shock absorbers. “Air shocks” have been around for decades and were once extremely popular with full size rear wheel drive cars, especially those pulling trailers where the tongue weight combined with soft suspension calibration created tail dragging rear sag. Unless you’re in a Crown Victoria or Town Car, trailering the boat or “sleds” today means light trucks, whose suspension systems are very much like the full size cars of thirty years ago.

SSGM’s test vehicle was a “Heritage” F-150, typical of body-on-frame SUVs and light trucks, with simple leaf springs, an 8.8-inch 31-spline solid axle with open differential and non-staggered tubular shocks. The test technology was Gabriel’s Hi-Jacker (clearly a pre-9/11 brand name) air shock kit in the classic stud top, eye bottom mounting configuration. The kit comes with two shocks, mounting hardware kits that include sleeves for different diameter lower cross bolts and an air line kit with a tire-type filling valve. One of the beauties of working on these simple rear suspensions is speed. The Hi-Jackers install the same way as conventional shock absorbers, with a small amount of additional time needed for routing the polymer air lines and filling valve.

As most techs find almost every day, getting the old components off can be the most time consuming part of the job. The test F-150 uses an inboard tank with vapour lines uncomfortably close to the upper stud mount on the left side, making the “hot wrench” an unsafe proposition. Unfortunately, corrosion of the stud’s exposed threads and the poor access to the upper mount made a stud breaker impractical too. The solution was a classic knuckle-busting, short-swing of a ratchet from outboard of the frame rail. Depending on the old shock design, it might be possible to hack the old shock’s upper body and turn it while holding the upper nut stationary. Either way it’s tight in this application.

Installation of the Hi-Jackers was straightforward. Like standard shocks, it’s important to remember that the mounting rubbers have a correct orientation, with the shoulder facing the chassis hole and the flat surface against the cupped washers. The kit included both upper mounting nuts and thin jam nuts for extra security. Correct tightening torque is easy to see as the rubbers compress to the outer diameter of the cupped washers. Lower eye mounts were also dead easy, although the Ford application required the supplied sleeves to bring the eye I. D. to a close fit with the cross bolt. Forget the sleeve and the lower mount will either rattle, or require the cross bolt to be overtorqued until the mounting eye contacts the yoke of the mounting bracket, causing noise and wear. The Hi-Jackers are almost twice the diameter of the admittedly undersized stock shocks, which presented no problem in the roomy Ford chassis.

In some applications, exhaust or brake components might have to be “tweaked” for comfortable clearance. The air fittings are mounted on the side of the shock body, requiring a little planning before torquing down the mountings. In the test vehicle, the nipples were mounted upward to keep them away from road debris and to make a cleaner air line installation. Air lines are surprisingly small relative to the wide Hi-Jackers and come with O-rings and fittings pre-assembled for speed. There’s no cutting for length, Gabriel suggests simply coiling the excess length and strapping it up and out of the way.

Like any chassis-mounted liquid line, standing the air lines off from chafing surfaces is essential. Gabriel achieves this with snap-in mounts that use -inch drilled holes and push in easily without tools. Similarly, the filling valve mounts through a -inch hole in any location where the owner can get an air chuck. It’s important to choose a location that’s convenient, but also protected from impact damage and water immersion. On light trucks and SUVs step bumpers are a common spot, but there’s lots of air line length, so there should be few limitations for the creative. Connections at the valve and shock body are plastic and there’s no specified torque setting in the instructions. Fingers proved to be more than adequate to get the compression nuts fully bottomed and the dual O-rings seated. Total installation time was a little over an hour. With judicious use of an air chisel, acetylene torch and skipping the photography should bring it to 45 minutes or less for solid axle trucks like the test F-150.

How do they work? Similar to Dad’s station wagon, with the rear ride height directly controlled by the 25-to 200- pound pressure range available with the Hi-Jacker system. Ride compliance is similarly affected, with the ride stiffening appreciably at pressures over 70 PSI for the half ton Ford when empty. For hauling and/or towing, the ability to maintain proper ride height and therefore front suspension geometry is obvious. What’s surprising is the amount that the truck’s ride and handling can be fine tuned empty by adjusting the pressure. Gabriel notes that the Hi-Jackers can be installed with individual filling valves to allow split pressures left to right, but for most applications this is too much adjustability for the average driver. Exceptions include vehicles that must be more heavily loaded on one side, like glaziers’ trucks with glass racks on one side only, as well as the classic setup under old-school muscle cars, where the split pressures help launch the car straighter “out of the hole” with better traction.

Why install products like Hi-Jackers? The most important reason is of course profitability, but almost as important is the ability to offer consumers options that are better than the OEM equipment and enhance safety. Keeping consumers away form the OE dealership and loyal to your shop requires a reputation for service that’s superior to the dealership. The Gabriel Hi-Jackers are an example of a performance-enhancing upgrade that’s easy for consumers to understand, simple and quick to install and returns good margins for installers. Like premium brake and tire service, however, upselling like this requires driving home the value proposition rather than “racing to the bottom” with marginal white box suspension components. It takes just as long to install cheap shocks and sometimes longer if the fit is poor … “short changing” both shop and consumer. In today’s economy, we can expect longer ownership between trades; upgrades like Hi-Jackers are very inexpensive relative to trade-in even with today’s low interest rates, a “win-win” for buyer and seller.

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