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Posted: August 1, 2014

Spec your fifth wheel correctly and it’ll pull its weight.

Fifth wheels don’t attract a lot of attention. They just sort of sit there behind the cab, black and greasy. There’s often no mention of the things at spec’ing time, and seldom any thought about their condition until something goes wrong. Fortunately, fifth-wheel failures are rare, though they can be spectacular. The problems are mostly just annoying, like when they won’t release. At least when one won’t lock, you can bobtail away to have it looked at. But stuck, and pinned-up to a trailer the receiver plans to unload over the weekend, is a different matter.

They should be given more consideration during the spec’ing process, particularly if there’s some specialty application in your future, such as tanks, end dumps, or some severe-service off-road application such as logging, for example. If the product matches the weight-bearing and draw-weight requirements, it’s on the truck, and that means the model and brand often become the sales rep’s choice, not yours. But if you look at what’s available, there really are quite a few options. Under various circumstances, one product may be more suitable than another. Do a little research.

As a starting point, there are three minimum considerations in spec’ing the fifth wheel: vertical load capacity – the maximum, recommended vertical downward force than can be safely applied; drawbar capacity – the maximum, horizontal pulling force that can be safely applied; and fifth-wheel height – the distance from the ground to the top of the fifth wheel when it’s level and parallel with the ground.

Fifth wheels are rated anywhere from 50,000 to 70,000 lb vertical load capacity, and from 70,000 to 200,000 lb drawbar capacity. Fifth-wheel manufacturers tend to rate their products in terms of service environment, given that weight capacities are similar under most conditions.

A moderate or standard-duty product will suit most on-highway applications (given the right weight and drawbar capacities), while severe-duty units are required for applications where the fifth wheel is subject to abnormal stresses, such as an off-road environment or situations where very frequent hook/unhook cycles are anticipated. A heavy load pulled over paved roads is easier service than a lighter load pulled in severe off-highway conditions. In other words, the term ‘heavy duty’ should not be understood to mean ‘severe service’.

Maintenance and life-cycle factors should be considered as well. Most owner-operators won’t have access to the facilities to rebuild a fifth wheel (though it’s not a difficult job), so they should probably be more concerned with ease of maintenance or cost of replacement when making their choice. Most of these products are designed to outlive the truck they’re attached to, assuming regular and proper maintenance.

Ease of adjustment, however, should be a consideration when you’re spec’ing a fifth wheel. Over the life of the truck, the fifth wheel may require adjustment as often as once a year, maybe more. This depends largely on the condition of the trailer kingpins and how hard you’re inclined to ‘hit the pin.’

According to Chuck Jeary, Holland’s warranty and technical services manager, hooking up is the most damaging process as far as the fifth wheel is concerned.

“The hammering on the U-shaped throat caused by hitting it too hard will eventually cause a step that’s very hard to adjust for,” he says. “Hitting it harder won’t make much difference to the function of the locking mechanism. It just wrecks the top plate.”

Aside from perhaps the frame itself, the fifth wheel is one of the least complex components of the truck, yet they’re called upon to hold the unit together under some unimaginable circumstances. The purchase cost should always be weighed against life-cycle and repair costs that might arise from an improperly spec’d fifth wheel.

The Options

Most brands now offer ‘preference’ options such as right- or left-hand release handles, low-pull-pressure release mechanisms, and even in-cab air release interlocked with the parking brake system.

ConMet, Fontaine and Jost all offer a locking-bar locking mechanism, where a hardened steel bar slips in behind the kingpin to lock it into place. Some come with a wedge device to tighten the locking bar against the pin, nearly eliminating the need to perform adjustments. Holland’s locking mechanism features a split jaw that surrounds the pin from right and left. When the jaws are around the kingpin, a U-shaped clamp slips down around the jaws, locking them closed around the pin.

Two other considerations may play into your spec: sliding or stationary, and the height of the top plate. A slider adds flexibility in terms of weight distribution and maneuverability. There’s a driver comfort dimension to the sliding fifth wheel as well, and of course resale value. Unless you’re pulling the same trailer with the same load all the time, a sliding fifth wheel is practically a given, especially for the owner-operator. But the slider can add height to the unit, overall. Make sure this fifth-wheel dimension is compatible with the tractor frame height in terms of the overall height allowance of 4.1 meters, or 13 ft, 6 in.

And bear in mind that the lower the fifth-wheel height, the less articulation is available, especially in off-road applications. Operating conditions must be considered to ensure the tractor-trailer will not exceed the available articulation, resulting in damage to the fifth wheel, tractor frame, or trailer.

An owner-operator needs to consider the likelihood of doing all sorts of different work with his truck, so flexibility is key to the spec. Build in more than you think you’ll need and you’ll be safe. And if you’re planning to buy a used truck, ensure that the fifth wheel on the truck suits your needs. No point in putting a light-spec fifth wheel under a B-train. It sure won’t last very long.

Signs of Wear

Like anything else mechanical, fifth wheels will wear over time, but you can be fooled into thinking that the jaws are worn excessively if you happen to have a trailer with a badly worn kingpin. The symptoms will appear the same. You can do a simple tug test to determine how much slack exists, by backing against the pin, marking a point under the trailer then pulling forward and observing how the mark may have moved. In most cases, adjustments can be performed without any special tools, by following the adjustment guidelines that came with the product. But Jeary cautions against adjusting the lock with a worn kingpin.

“If you tighten the jaws up to fit a worn pin, they will be too tight on a normal pin, causing accelerated wear on the jaws and the pin,” he says. “For that reason, Holland recommends using a proper kingpin gauge.”

As far as the pivot bushings are concerned, most fifth wheels are designed with rubber or urethane sleeves around the pivot tube, but the actual load-bearing surface is steel on steel. The sleeve compresses as load is applied, so about 1/4 of an inch of up-and-down movement is normal on some models and brands.

Jost designs its fifth wheel with an easy-to-replace rubber insert located at the front end of the opening to absorb the impact of hitting the pin.

Inspecting the fifth wheel isn’t too difficult, either. When you clean it, inspect the top plate for cracks or warping. Examine the locking mechanism for excessive wear, and ensure the safety catch or the secondary lock is functioning properly. Jim Murphy, Fontaine’s Canadian sales manager says pay attention to the way it feels and sounds when you lock or release it.

“If it sounds or feels different, check it out.”

Even a full rebuild is a job most mechanically inclined individuals could do at home if they wanted to. Lock and jaw kits are available, and the process isn’t terribly demanding. The fifth wheel is one of the few components remaining on a truck that still has some user-serviceable parts inside. Spec it properly, and look after it, and it will likely be working long after you’ve dealt the truck.


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