Profits: Hanging in the Balance

| December 12, 2008

Doug Woolverton of Hunter Engineering uses an electrically driven rubber roller to accelerate this Peterbilt front axle wheel to highway speed, at which imbalance can easily be measured. Then the roller is pulled away so that it won’t interfere with what the wheel is doing.

Take an imbalanced 22.5-inch tire, put it in motion, and you’ve got a powerful force. “At 60 mph, a 6-ounce dynamic imbalance becomes 60 pounds of side force per revolution,” says Nick Powers, director of heavy-duty sales at Hennessy Industries, maker of Coats truck alignment and balancing equipment.

Each blow chips away at tire life, fuel economy and steering and suspension components.
That’s why it pays bigger than expected dividends to make sure your tire balance program is routine and thorough.

Imbalance gives shocks an especially hard time, says Doug Woolverton, a division manager at Hunter Engineering, a manufacturer of wheel alignment and tire balancing equipment for cars and trucks. “Shock absorbers are designed to dampen movement in the suspension from occasional bumps, not a constant vibration,” he says. Imbalance creates heat that shock fluids and seals are not designed for. It also puts tremendous stress on spring shackles, kingpins and the tiny ball bearings in tie rods and other steering components.

“Imbalance is there, even when the driver does not feel it,” Woolverton says. “When the driver doesn’t notice it at some speeds, it’s only because it’s not transmitted into the cab under those conditions.”

When you consider the costs of suspension and steering repairs commonly caused by imbalance, it’s easy to justify the cost of wheel balancing.

A set of shock absorbers for a three-axle tractor would cost you about $500, says Bill Valenti, a service adviser at the TravelCenters of America in Paulsboro, N.J. The labor alone for a set of kingpins would typically be about $200 and, for a pitman arm, $350. Including the cost of parts, a couple of front-end repairs like these could easily set you back $750 or more.

Consider the cost of dynamically balancing the 10 wheels of a three-axle tractor – about $250. If doing so just twice were to delay installation of shock absorbers from two years to four years, you save the cost of balancing on shocks alone. Delaying those $750 front-end repairs from three years to six would pay for balancing three times.

Woolverton points out that deteriorating front-end components affect alignment and, ultimately, both tire wear and fuel economy. Consider a trucker who runs 100,000 miles a year, gets 6 mpg, and buys fuel at $2. If expenditures of $1,650 on alignment and balancing (two alignments at $200 and five balancing operations at $250) net him a fuel savings of just 1 percent over a period of 5 years, he comes out slightly ahead on costs, saving $1,666 on fuel alone. The savings in tire, suspension and steering parts wear are gravy.

There are two different balancing processes: static and dynamic. Static balance gets rid of only up and down motion, but dynamic balancing also gets rid of wobble. This is because the static process balances the overall mass around the center of the wheel, while the dynamic process balances both sides of the wheel independently.

Static balancing can be done with the wheel on the vehicle. It typically involves installing a weight only on one side. Dynamic balancing, on the other hand, recognizes that the weight of tire and wheel won’t actually be even from side to side, Powers says. The assembly is removed from the truck and spun on a device that is sensitive to both these kinds of motion, and then weights of different sizes are installed on either side. This eliminates any wobble in the wheel that could be annoying to the driver or destructive to the vehicle, especially on the front axle.

Which of these processes should you use? Powers and Guy Walenga of Bridgestone/Firestone recommend dynamic balancing because it addresses two problems, not just one. “There is both wheel hop and a lateral moment,” Walenga says.

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