Maintenance Q&A
Side skirts and mileage: Truths about testing and real-world conditions

By Paul Abelson, senior technical editor

Q. I have to run in and out of California, so I invested in a set of trailer skirts. They were supposed to pay for themselves quickly by saving at least 5 percent in fuel.

I don’t know where they get that number, because all I’m getting is barely more than 1 percent. I took it back to the trailer dealer. They checked it over and said it was installed according to instructions and there’s nothing wrong with the skirts. Can you help?

A. When EPA’s SmartWay approves a product, they review the test procedures to make sure that testing was conducted according to SAE fuel economy testing procedure J1321 and that it was conducted by an approved, independent third-party testing organization. Tests are generally done under ideal conditions with the vehicle set up to maximize the performance of the device being tested.

For example, tests may be run with the trailer as far forward as possible or with fairings between tractor and trailer. If your gap is greater than under test conditions, mileage will suffer. The most common reason that real-world results don’t match SmartWay projections is California’s 40-foot kingpin-to-rear-axle rule.

Since skirts generally do not cover trailer tandems, they must end at the slider’s forward-most positions. When these trailers slide their bogies rearward for better ride and weight distribution when operating outside California, a gap is created between the rear of the skirt and the front of the wheels. This gap allows air to flow in under the trailer and, especially in crosswinds, to increase pressure on the undercarriage, increasing drag.

Not every fuel-saving device approved by SmartWay has that kind of falloff in performance. Some approved devices, like wide-base single tires are constant because they do not depend on the variability of other settings. Aerodynamic extensions behind the trailer called boat tails or taper tails will perform as tested because their conditions don’t change or, if they do, it is for the better. Since drag increases with the square of truck speed, driving faster than speeds used in tests could result in more drag reduction than realized in testing.

I am not in favor of going faster to make your devices more effective. On the contrary, going faster reduces fuel economy compared with slower speeds. But if a device showed a 5 percent improvement when tested at 60 mph and you operate at the speed limit in states that allow 70, the device should provide better than its nominal 5 percent improvement, but that faster speed is also decreasing your fuel economy at the same time.

There is at least one device that compensates for the varying gap as the slide is moved. In our May issue it was a “Paul’s Picks” from the TMC meeting. The Aerofficient Side Fairing is a two-piece skirt. One piece attaches to the trailer as any normal side skirt does. The other piece slides to fill the gap no matter where the tandem is. That way, the skirt will always fill the gap and mpg improvement will not fall off.

Q. The air conditioner on my 2004 Pete 379 didn’t exactly stop cooling, but the cold air is not as cold as it used to be. If I keep the rpm above 1500, the air stays colder, but splitting one gear down kills my fuel mileage. My dealer wants a lot of money just to check it, and said it could cost close to $1,000 to fix it. Can I just use a recharge can of R134 from my local parts store?

A. There are strict procedures dealers must follow when working on air conditioning. The system must be pressure tested, checked for leaks, and then evacuated to capture refrigerant. Failure to do so can result in stiff fines.

Your eight-year-old truck may have something as simple as a loose hose clamp or as problematic as a cracked fitting at one of the major components – the compressor, receiver-dryer or condenser. The hoses may have deteriorated. A dealer must check all possibilities, usually by adding dye and looking for leaks with an ultraviolet light. The entire refrigerant flushing process is described in TMC RP520, A/C System Refrigerant Flushing.

You, on the other hand, as an individual vehicle owner, are not bound by those regulations. You can just add refrigerant to restore cooling capability, but there are precautions to be taken. The first time you add, get a can that has a pressure gauge built into the hose. Do not exceed recommended pressures. Follow instructions precisely.

If you need to recharge again, you can get cans without pressure gauges. Just save the hose-gauge assembly and reuse it. Cans without the gauges or hoses are less expensive.

Avoid refrigerants containing sealers. They can gum up the valves and compressor and can turn minor problems into potentially major rebuilds. If your recharge only lasts a short time, you will need a professional to work on the system sooner rather than later. When you do the recharging yourself, chances are you are merely postponing the inevitable.

Q. In the May issue, we answered a question about how over-torquing wheel nuts results in stretched and broken studs. One of my TMC “Brain Trust” members, Tom Tahaney, wrote to add hints for maintainers.

A. Here’s Tom’s tip, which he got from Alcoa. Place a drop or two (no more) of motor oil on the surface between each nut and its flange. Rotate the flange to circulate and distribute the oil evenly. This reduces friction between the nut and its flange and provides a better torque reading.

Alcoa also advises that you wipe the threads of the studs so they’re clean and dry. They also suggest you place one drop of 30 weight motor oil on the clean, dry threads. But don’t be like some shade tree mechanics who feel that “if a drop is good, a pint is better.” According to Tahaney, this is where you want to follow directions precisely. LL

Senior Technical Editor Paul Abelson is a life member of OOIDA, holds an Illinois CDL, is active in the Technology & Maintenance Council, and is a recipient of the Lifetime Achievement Award from the Truck Writers of North America.