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July 22, 1996 02:00 AM

HIGHER SPEED LIMITS IMPACT TRUCKS, TIRES: TIRES' LOAD-CARRYING CAPACITIES REDUCED

Peggy Fisher
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    You may have noticed that the speed limit signs in your city or state have changed recently. I was surprised to find when I returned to Columbus, Ohio, last week from an out-of-state trip, that the speed limit signs on I-270 had been changed from 55 mph to 65 mph for cars. I was delighted. Now the legal speed limit is a little closer to the speed I drive.

    Truck speed limits are still 55 mph here in Ohio, but in many states these limits have changed, too. Four states have raised truck speed limits to 70 mph, seven have raised them to 75 mph and still more are considering higher limits.

    These higher speed limits can have significant effects on these vehicles and their tires. Commercial dealers who are servicing fleets running in excess of 70 mph need to know what to expect.

    I believe most fleets are unaware of the consequences-including a drop in tires' load-carrying capacities.

    The motivation for operating trucks and tractor/trailers at higher speeds is increased productivity and driver retention. If a truck can haul freight faster, it follows that it can haul more loads.

    And past experience indicates that, if all else remains the same, drivers will switch companies if their trucks will go as little as 2 mph faster. In this day and age of driver shortages and high driver turnover, driver retention is a primary concern of truck fleets.

    However, as in almost everything in life, there's no such thing as a free lunch. Something has to be sacrificed for this increase in productivity and driver satisfaction.

    Naturally, the first thing to suffer is fuel economy. However, since 1975 the trucking industry has concentrated on technology that offsets the negatives associated with an increase in speed: aerodynamic aids, fuel-efficient tires and engines, electronic features such as cruise control and gear-down protection, and optimized gearing and driveline components.

    These components have made vehicles more productive at higher speeds, but fuel consumption still will increase an average of 0.1 mile per gallon for every 1 mph of increased speed.

    Maintenance on the engine will increase, too. Higher road speeds mean higher engine speeds, which translate into higher fuel and oil consumption rates. This means that oil change intervals will shorten and air and fuel filters probably will need to be changed more often as well.

    Since increased rpm quickens the degradation and contamination rate of oil, the durability of the turbo, engine bearings, air compressor and piston rings is affected. Bottom line: Maintenance costs may increase 10 to 15 percent, and engine durability may decrease 10 to 15 percent.

    Braking also will be affected. A tractor-trailer's stopping distance at 55 mph will increase by almost 200 feet at 75 mph. Because there probably will be more panic stops, as well as more brake snubbing to control the vehicle in traffic, brake temperatures will rise and brake lining wear will increase, which will hike brake cost per mile.

    Other areas of the vehicle will be affected as well. At 75 mph, driveline shaft torque increases 20 to 25 percent compared with 65 mph, and U-joint life decreases more than 30 percent. Road and wind noise increase some 3 to 4 decibels, and vehicle vibration becomes more noticeable, meaning shocks and bushings could wear faster.

    The startling thing is that most over-the-road tires, according to the Tire and Rim Association's 1996 Year Book, are not capable of carrying the loads we have always expected them to carry at these higher speed limits.

    According to this bible of the tire/wheel industry, 5 psi should be added for speeds over 65 mph. However, from 66 through 70 mph, load-carrying capacity drops 4 percent, and at speeds of 71 through 75 mph, it drops 12 percent.

    What does this mean? As the accompanying charts show, on a 12,000-pound steer axle, the only size tire that can haul this load at 71-75 mph is the 11R24.5 load range H. All others are overloaded. And none are rated to carry 13,000 or 14,000 pounds on a steer axle.

    With four tires on an axle, the charts indicate most drive and trailer tires have ample carrying capacity at 75 mph to handle 34,000 pounds on a tandem axle-except the 265/70R19.5, which has a capacity of only 4,114 pounds.

    However, the 11R22.5, 295/75R22.5, 11R24.5, and 285/75R24.5 load range G tires and the 255/70R22.5 load range H tire are unable at this speed to carry 20,000 pounds on a single axle. Fleets hauling 28-ft. trailers with these sizes of tires could have a problem.

    The consequences are easy to predict: Tires will run hotter and will develop internal separations; more tires will fail on the road; durability and retreadability will be reduced; rolling resistance will increase, so fuel economy will drop.

    Because the tire's footprint becomes more elongated at 75 mph and the middle of the tread area is what is primarily contacting the road, the shoulders will tend to scrub and irregular shoulder wear will result. The tread also will wear much faster overall. Traction will be decreased since there is less rubber contacting the road.

    ``How will a crisis be averted?'' you ask. Well, each tire maker probably will evaluate its product offerings and raise the load-carrying capacities on specific tires so that they can carry the weight at 75 mph on the axles for which they were designed.

    I don't think they will arbitrarily raise the weight limits on all tires of one size. This could theoretically result in drive tires, for example, being placed on steer axles and carrying much more weight than they were meant to.

    The key is to stay in touch with your suppliers. Remember: Even if tire makers say a tire can carry more weight, it may not perform as well as it did at lower speeds.

    Since the air pressure is what actually carries the load, good inflation pressure maintenance will be more critical than ever before.

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    Do you have an opinion about this story? Do you have some thoughts you'd like to share with our readers? Tire Business would love to hear from you. Email your letter to Editor Don Detore at [email protected].

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