Add 'integrated' and 'systems approach' to your buying vocabulary.
Axles & Suspensions
Tom Berg
Equipment Editor
'Integration" is becoming a buzzword in the truck business, not just for vehicles as a whole but also for components. Original equipment manufacturers are steering customers away from custom building and into trucks assembled from select components — a trend usually called "vertical integration." Suppliers, meanwhile, are developing integrated components that eliminate separate spec'ing of component parts. Prime examples are axles and suspensions.
In the not-so-old days, you picked the axle by capacity, make and model because your experience as a fleet operator or maintenance manager told you this worked best for your operation. You sometimes also chose an axle or suspension with a higher or lower capacity than the truck builder's standard. If the standard was a 12,000-pound XYZ steer axle with ABC brakes on 12,000-pound leaf springs, you spec'd a beefy 14,600-pound XYZ-brand axle (for greater durability) with wider DEF-brand brakes (for longer lining life) on 12,000-pound springs (to retain a decent ride).
You also chose to go with the standard wheel bearings and seals. Or perhaps you chose non-standard items because you had some and were pleased with them, or you knew that the OEM periodically changed his standard seals or brakes and you wanted to stay standard with what you had for the previous five years. Or maybe a salesman convinced you that his brakes or seals were better. Maybe the salesman was a buddy who took you to nightclubs for good dinners and drinks, so you favored him with some business. Over the years, a lot of trucks and components have been sold that way, for better or worse.
More recently, slim profits have all but squeezed out the wining and dining. Suppliers have put more emphasis on engineering advances, quality manufacturing and extended warranties. Improvements to individual parts, like wheel seals, have evolved into integrated components that promise some real advantages, even if they eliminate some of the fun (or work) of specifying. This is also called the "systems approach" to component design.
Integrated Hubs
For instance, in heavy duty truck and tractor steer axles, it began with integrated wheel hubs. Their bearings and seals are precision-installed by the supplier instead of individually by harried workers on a truck assembly line. This all but eliminates misadjusted bearings and damaged seals, which can be major headaches for truck owners. One major leasing fleet says its switch to integrated hubs almost entirely did away with leaking seals, which usually soaked brake linings with oil and ruined them.
The next step was axle-and-suspension combinations, which appeared first on semitrailers. Virtually all parts, from axle tubes, bearings, seals and sometimes brakes to the air-bag suspension, are assembled by one supplier and shipped to trailer manufacturers. All that's left to choose are the wheels and tires. Aside from maintenance advantages, the integrated trailer tandem weighs less, often carries a longer warranty and will cost less than individually spec'd parts.
Thanks partly to integrated tandems, the use of smooth-riding air-ride trailer suspensions has greatly increased, from about 12% of the market in 1988 to about 62% by late last year. In that dozen years, suppliers say they cut weight by 20% and the cost premium by 50%. Air-ride's popularity climbed steadily except during the recent downturn, when it slipped slightly as some cash-short fleets temporarily went back to cheaper steel-spring suspensions.
In vans, air-ride cushions the cargo — a strong selling point to a fleet's shipper-customers and to its own financial officers, who pay fewer cargo-damage claims. And drivers are spared the road shock that is otherwise transmitted from the trailer's tandem up through the fifth wheel and into the tractor. Air suspensions also add stability in some applications, like high-mounted tankers and dumps, and protect a vehicle's body and running gear from damage. Aluminum tankers, in particular, last longer and require fewer expensive repairs when they ride on air bags.
Integrated Steer
The next step in axle-suspension integration is again at the power unit's steer axle, and again it involves air-ride. Several builders have introduced air-suspended front axles designed expressly for individual models. This is necessary to accommodate designs of frame rails and engine mounts, as well as how the axle's configuration interacts with those pieces. Softer ride implied by air bags is not necessarily the big advantage here; rather, it's lateral stability and weight savings, suppliers say.
They can get better performance and lower weight by using all pieces of the axle-suspension package as a system instead of individually. For example, while a beefy I-beam axle is almost always found at the front of a Class 8 truck, a lighter tube-type axle can be strong enough because the rest of the package is designed around it.
In all types of suspensions, lighter fabricated pieces save weight over cast-metal items, and also have the strength to take weight and road shock. The entire truck can be lighter in weight and perform better if designers know that such an axle-suspension system will be used.
Rear Axles: Yes & No
Dumpers, mixers and block trucks often use lift axles to bolster weight-carrying capacity in many states, especially those that greatly limit axle loads. Lift axles have been integrated from the start, with suppliers making axles and suspensions themselves or at least acquiring the parts and assembling them before shipping them to where they're installed.
Whether installed as pushers (ahead of the tandem) or tags (behind it), lift axles have grown in variety and complexity as more demands are made on them. Bridge-formula states that are strict about axle loads often encourage use of more lift axles, leading suppliers to offer lighter-duty models. It makes more sense to use a 9,000-pound-capacity axle if that's all the state allows, instead of a heavier and more costly 12,000-pounder. And steerable lift axles are gaining wider use as some states and municipalities enforce weight limits even while the truck is turning or maneuvering.
Drive axles, however, are one place where total integration with suspensions has not occurred and is not likely to. That's because the axles themselves are necessarily hefty and fairly complex components built by only two major suppliers. Still, suspension makers and truck builders know which axles they'll be dealing with and design accordingly. You can specify drive-axle suspension products from truck OEMs and suppliers, and be confident of excellent service if your choice is correct for your application.
Over the years, designers of drive-axle suspensions — both single and tandem — have eliminated considerable weight while adding more durability and better performance. Fabricated brackets and hangers have replaced cast items, and computer-aided design has allowed engineers to determine where stress is and isn't in the various parts. This lets them put metal where it's needed and remove it from where it's not.
Driveline vibration has become an increasing problem as engines have become more powerful. High torque loads can produce vibes that wreak havoc on U-joints, driveshafts and axles, and sometimes make the ride miserable for drivers. Engineers have determined that suspension type and driveline angles can both be to blame.
A trailing-arm air-ride suspension can allow axle windup under high torque that can increase U-joint angles and cause damaging vibrations. As a result, one suspension maker devised special shock absorbers that control windup. Axle manufacturers, meanwhile, redesigned differentials so they face each other more squarely, reducing the basic U-joint angles to start with. Several tandem axles are advertised as having this advantage, and are worth your consideration.
Air-ride took over the highway tractor market about 10 years before it became popular on trailers. Lately, air-ride has begun appearing on vocational trucks, like dumpers and concrete mixers, and on medium-duty straight trucks. The advantages are the same as for trailers: smoother ride and less damage to vehicle and driver — even if the cargo is immune to bruising. Resale can also be better, depending on the application and locale.
Integrated axle-suspension components seem to have everything going for them except that they probably deviate from the way you've always done things. Also, they may not be your standard components, and that can complicate parts stockage. And you may have to hurt the feelings and income of your buddy the salesman, if he's still around. But that's the way the world is going.
