FISHER: Pros, cons of aerodynamic wheel covers

Fisher

In the quest for better and better fuel economy, fleets are installing all kinds of aerodynamic devices. After improvements in engine technology and fuel efficient tires, aerodynamics is the next technology that offers significant reductions in fuel consumption.

You most likely have seen tractors with side and bumper fairings and air shields that sit on top of the cab to direct air flow smoothly over and around the tractor. Trailers have skirts that direct air flow around the vehicle and boat tails attached to the backs of trailers to reduce the low-pressure vacuum that creates rear drag. Savings can be significant with these devices — trailer boat tails alone are reported to improve fuel efficiency by 5.5 percent.

• This column appeared in the May 22 print edition of Tire Business.

Another aerodynamic device that you will encounter are aerodynamic wheel covers. The purpose of these devices is to reduce drag and turbulence around the wheels. Manufacturers of these devices claim they will save 1 to 2 percent in fuel. That can be huge when fuel is expensive.

While fuel savings may result from using aerodynamic wheel covers, are there other considerations of which users should be aware — some that might not be so beneficial?

This was the question posed to the Tire & Wheel Study Group of the Technology & Maintenance Council (TMC), and perhaps you have fleets asking you the same thing.

To be honest, when TMC initially started looking at aerodynamic wheel covers, the tire and wheel experts who make up this group had some preconceived notions that these devices could be detrimental to the wheel end by trapping in heat, which, if built up enough, could damage tires and wheels.

Devices that were not transparent could hide wheel-end problems and result in unexpected increases in maintenance and operational costs. Fortunately, these men and women are quite smart. They set aside their biases and researched the true effects of wheel covers on commercial vehicles.

What they found was that there are several important operational, inspection and maintenance considerations associated with the use of aerodynamic wheel covers in line-haul operations that fleets, of which inspectors and commercial tire service providers should be aware.

Since your technicians are going to be servicing vehicles with aerodynamic wheel covers from time to time, they should know what they are dealing with, too.

Inspection impact

The first area of concern is whether drivers and technicians could inspect the vehicle's wheel ends properly with aerodynamic wheel covers installed.

A wide spectrum of aerodynamic wheel covers was examined during this study, and it was found that some wheel covers are clear and enable inspectors (drivers, technicians and inspectors) to see through them to check for axle-end anomalies. However, it was found that some designs may not remain clear after being placed in service, which inhibits a good inspection.

Other wheel covers are not transparent and do not enable maintenance personnel to do a proper wheel-end inspection, which should include looking for loose and missing wheel nuts, missing or damaged studs, loose, cracked or corroded wheels, hub cap leakage and tire inflation pressure.

In addition, the wheel cover attachment hardware should be inspected to ensure it has not been damaged and will keep the cover on the wheel.

While the group was concerned that Commercial Vehicle Safety Alliance (CVSA) inspectors would have a problem with inspecting axle ends that were hidden by aerodynamic wheel covers, it found that this enforcement arm has not stayed abreast of technological advances.

While the concern was that the inability to inspect axle ends with wheel covers might affect fleet CVSA scores, the group discovered that CVSA's policy is to not inspect any truck part or component that inspectors cannot access by visual inspection without tools.

So the CVSA does not require that wheel covers be removed for the inspector.

The inspector must assume that the components are in working order. However, it is expected that CVSA will update its position on this issue and require that wheel ends are able to be inspected easily.

Maintenance issues

The fact that wheel covers hide the wheel hand holes and prevent access to tire valve stems is a real obstacle to good tire maintenance. In order to check and adjust inflation pressure, wheel covers have to be removed. Fleets that are shopping for wheel covers should select those that are removed and reinstalled easily and quickly .

The attaching hardware should be reusable and able to withstand many removals and installations over their lives. If it isn't, the wheel covers can come off while the vehicle is in motion. Another consideration is the wheel cover's impact on balance. It should have no effect on wheel-end balance.

Mounting concerns

There are a variety of ways aerodynamic wheel cover products are mounted on the axle end. Depending upon how they are mounted, they may cause issues with wheels, tires and hubcaps.

Therefore, the manner in which wheel covers are mounted is of great importance when selecting a product.

These products use a variety of attachment systems that include mounting brackets, systems that attach to the wheel hand holes or wheel flanges and some that snap in the drop-well of the wheel. Be aware that some wheel covers are not designed to fit all wheels.

As a general rule, wheel covers should not touch and rub the wheel, since this may rub the wheel coating off, degrade the metal and cause premature corrosion. Remember, if two dissimilar bare metals come in contact, galvanic corrosion will develop that can freeze the wheel cover to the wheel.

There are four types of mounting bracket designs:

1. Installed under the wheel nuts;

2. Installed under the axle nuts;

3. Installed under the axle-end cap bolts; and

4. Installed on the studs after the wheel nuts are attached.

When brackets are installed under the nuts of any attaching system, another joint is added to the system.

This joint has to be kept clean and free of rust, paint and other debris, or the component "sandwich" will lose clamping force and you'll get loose wheels and wheel-offs if the bracket is under the wheel nuts; axle flange or hub cap rotation, which will cause damage to the studs and the axle shaft if the bracket is under the axle nuts; and oil leaks, bearing failures and axle-end detachments if the bracket is installed under the axle end cap bolts.

In all of these systems, it is vital to ensure that the bracket is made of a material that does not compress since that can result in a loose joint, and all of these bad things can happen.

For brackets that are installed on the stud standout after wheel nuts are attached, technicians must ensure that there is full thread engagement between the bracket and the bracket attaching nut; otherwise the bracket may slip off along with the wheel cover while the vehicle is rolling down the road.

Another type of bracket attaches to the wheel hand hole. It's important to ensure the anchor does not damage the hand hole and that it is secured tightly to withstand impacts of road debris that may damage the mounting system.

Other wheel covers attach to the wheel flange. As you know, wheel flanges were not designed specifically for holding wheel covers on.

The flanges of some wheels may not curl completely over the cover, and this mounting system may not be the best for these types of wheels. If the flange does not completely curl over the cover, the wheel cover will not be securely fastened to the wheel — and you know what happens next.

The last type of attachment system uses rim-flange clamps, similar to balance weights, to attach the wheel cover to the wheel.

Inflation pressure must be let out of the tire to enable the clamp to slide between the wheel and the tire without damaging either one, just like balance weights.

And just like balance weights, they can only be used once. If they are removed and reattached using the same clamps, the metal may weaken, and the covers can detach from the wheel en route.

So every time you remove the wheel cover to check pressure, inspect the wheel end or perform any other tire, wheel, brake or axle-end maintenance, new flange clamps must be installed.

Entering this study, the Tire & Wheel Study Group surmised that wheel covers would reduce air flow and lock operating heat into the axle end and damage axle-end components.

Members thought it would be possible that the effects of reduced air flow could increase brake lining wear and fade on drum brakes, reduce bearing life, distort brake rotors and drums, and affect the lubrication of the axle-end bearing.

However, it was discovered that according to fleets that had experience running aerodynamic wheel covers and wheel manufacturers that had tested them, wheel covers have no adverse effect on these components.

Having said this, though, if there is another heat-generating component problem, wheel covers may mask and aggravate these problems.

If you have commercial accounts that are considering using aerodynamic wheel covers, encourage them to run a fleet evaluation of the products they are considering.

The evaluation should uncover any unexpected increases in maintenance and operational costs that could offset the potential savings they provide in fuel economy.

If you have fleet clients that are running aerodynamic wheel covers, make sure your technicians know how to remove and reinstall them correctly, have the tools they need to do this work and know to recognize any irregularities in them.

Also, you should work out with your customers the procedures that should be followed if the wheel cover and/or mounting hardware are damaged. The more you know, the better equipped you'll be to reduce drag and turbulence in your tire service.

For more information about aerodynamic wheel covers, see TMC RP261 Considerations for Aerodynamic Wheel Covers.

Peggy can be reached via e-mail at [email protected].

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