Hold It!

Folks spend serious time choosing big-ticket items like engines and axles when they buy a new truck. Even air dryers and U-joints draw attention. But the fifth wheel is often ignored-and, given the utterly critical job it performs, underestimated.

Granted, fifth wheels do seem to be rather simple things. Why not let the truck salesperson and the factory engineer apply the basic rules and take what you get? You can do exactly that and not go an inch wrong, but you have more options than you might think. Here’s where to start:

* Fifth-wheel height. Your fifth-wheel height is a function of your truck’s suspension, wheel and tire size, and the fifth wheel combined, so choosing the right fifth wheel type can get a bit complicated. It’s especially critical if you pull something other than standard trailers.

In general, the top of the fifth wheel is 47 inches off the ground, but that will change depending on the combination of components.

Spec a low-mount air-ride suspension with the wrong fifth wheel, and you’ll have too little clearance between tires and trailer. Add big 24.5-inch wheels to that mix and you may have no clearance at all. And forget about those chrome fenders you’re lusting after.

Now, those are extreme situations and unlikely ones, but you get the point: as with many other truck components, you can’t spec the fifth wheel in isolation.

The difference between the right height and the wrong height is only going to be an inch or two.

* Capacity. As well as the height of the total combination out back, there are two other matters of measurement to consider: the vertical capacity and drawbar capacity.

Vertical capacity is a function of the weight of a loaded trailer’s nose as it rests on the fifth wheel Drawbar capacity refers to the relative strength of the fifth wheel in terms of what it must pull. If you only make international runs and are never going to haul anything but 80,000-pound gross weights, that’s one thing, but if you switch to heavier domestic loads sometimes, you’d better think it through. You have no choice but to get the fifth wheel that can handle the heaviest weights you’re likely to pull.

Generally, building in a safety factor, an 80,000-pound gross weight demands a vertical capacity of about 40,000 pounds and drawbar capacity of 150,000 pounds or so.

If you’ve bought a used truck, make sure you know the capacities of its fifth wheel. You may have to scrape through the grime to find a serial number, but it’s worth doing so that you can call the fifth-wheel manufacturer to verify what you’ve got. It may be simpler to call the truck maker with your VIN number, but that assumes the truck still has the fifth wheel that was originally installed at the factory. Whatever you do, make the call.

FEATURES

There are obvious additional features to consider when choosing a fifth wheel. The more common choices involve features like a sliding or stationary mount.

Just about every driver would choose a sliding fifth wheel to gain flexibility in weight distribution by being able to pour more weight onto or off of a given axle set-or quite possibly to adjust the ride quality.

Probably the only case in which a stationary fifth wheel makes sense is when the trailer and load are consistent trip after trip. If that doesn’t describe your situation, go slider.

Other options are less obvious. For instance, which side of the truck do you want to be on when you release the jaws? In most cases, you can order the unlocking handle on either side.

Why should you care? While most buyers favor the driver’s side of the truck, some people want to be out of the line of traffic when they struggle to unlock the jaws, so they order the handle on the right side.

On that subject, do you want manual or air release? If you order the latter, be careful how the system is designed. The dashboard release has to be protected, either by location or by a guard, to avoid accidentally tripping it. And it has to be plumbed into the parking brake system so that it won’t release unless the brake is applied.

You can also have a fifth wheel that can oscillate a lot or a little. Oscillation refers to the side-to-side tilting movement of the fifth wheel that changes as the relationship between tractor and trailer changes while negotiating bumps and curves and other road-surface peculiarities. In most highway applications, semi-oscillating is the choice.

Most fifth wheels can also move fore and aft, but there are some cases-some dump trailers, for instance-in which a fifth wheel that stays rigid in that sense will be appropriate.

Another option may attract interest: at least one major manufacturer offers a greaseless fifth wheel, and several small companies offer plastic plates that do away with the need to lubricate the top plate.

But beware: some aftermarket products can alter the grip of the fifth wheel’s jaws on the trailer kingpin. They can add to the height of the fifth wheel and thus leave less of the pin for the jaws to grab.

MOUNTING MATTERS

By and large, you can leave mounting up to the factory or the dealer to do correctly. There are industry standards to adhere to in terms of fasteners and the angle-iron frame reinforcements required. These are covered in Recommended Practice 603B published by The Maintenance Council of the American Trucking Associations.

Among other things, RP 603B says the angle-iron length must be at least 36 inches unless the fifth-wheel maker says otherwise.

On over-the-frame installations, the angle-iron’s vertical leg must be at least 3-132 inches, says TMC, and its horizontal leg at least four inches.

Outboard installations of the angle-iron reinforcement demand a vertical leg of 3-132 inches and a minimum horizontal leg of three inches.

The fasteners that attach the angle iron to the frame are critical, of course. There must be at least five bolts used, and they must be spaced no more than eight inches apart, and at least one inch above the bottom of the angle iron. Bolts must be at least 538-inch in diameter, Grade 8, with Grade C lock nuts using hardened-steel washers under both bolt heads and lock nuts.

Another consideration is swing clearance, meaning the distance between the front of the trailer and the back of the cab when the unit is articulated.

Obviously, the fifth wheel must be positioned so that the corners of the trailer don’t interfere with exhaust stacks and whatever else may be mounted behind the cab and sleeper. Swing clearance is a function of fifth-wheel positioning but also of the kingpin setting on the trailer.

Considering how much is riding-literally and otherwise-on that single kingpin locked in those big jaws, we urge you to consider this article an introduction to the basic principles of fifth-wheel spec’ing and installation, not an exhaustive guide. For more complete technical advice, contact the manufacturers of your fifth wheel, your tractor, and your trailer.