Once again it seems the media have stumbled over a mole hill and are making a mountain out of it.
No doubt you have read or heard about the Associated Press report headlined “Big Rigs Often Go Faster Than Tires Can Handle.” Or perhaps you saw the recent CBS news coverage of this issue. Apparently it was a slow news week in early April and the media were in need of some sensationalized overreaction to any issue large or small that could incite hysteria in the general public.
In case you missed it, the article alerted readers/motorists to the fact that tractor-trailers on U.S. highways are driven faster than the 75 mph limit that their tires are designed to handle. The result, according to the reports: This practice causes blowouts and accidents. In addition, state officials are blissfully ignorant that truck tires have these operational limits.
What it glibly glossed over is the critical role that inflation pressure plays in tire health and operation and that tires are a factor in an incredibly small percentage of truck accidents.
The article cited statistics from the National Highway Traffic Safety Administration (NHT-SA) that from 2009 through 2013, there were just over 14,000 fatal crashes in the U.S. involving heavy trucks and buses that killed almost 16,000 people. Tires were a factor—not the cause—in 198 of those crashes.
If you care to put a pencil to it, that's a whopping 1.4 percent.
Talk about your mole hill, which gets even smaller when you consider that inflation maintenance is most likely a bigger factor in tire problems than speed is.
What started the journalist to research this issue was NHTSA's investigation into 16 blowouts that occurred on tractor steer axles in a car hauling fleet, resulting in three accidents but no injuries. NHTSA concluded that truck operators, not the tires, were at fault.
An investigator wrote that exceeding the 75 mph rating was the most likely cause of all 16 complaints examined. James Perham, president of Extreme Transportation Corp., who filed the complaint with regulators after seven blowouts caused between $20,000 and $30,000 damage to his rigs, exclaimed to the reporter: “It's a recipe for disaster!”
And with that quote, you readers could imagine a tractor-trailer rig with flames streaming out of its tires careening out of control at 85 mph and crashing into a petroleum refinery with an eardrum blowing KABOOM and a magnificent, fiery explosion. Well, none of that happened.
In the last decade the speed limit of almost all over-the-road truck tires was raised to 75 mph from 65 mph. However, in the last few years 16 states, all west of the Mississippi, have raised their speed limits to 75 and over.
Four of these states—South Dakota, Texas, Utah and Wyoming—allow 80 mph, and there is a 41-mile stretch of Texas Highway 130 that has an 85 mph speed limit for all vehicles including commercial trucks.
Three more states—Missouri, Nevada and Washington—are considering raising their speed limits to 75 or higher as well.
Of course there is some legitimate concern that high speeds can create safety issues for truck tires. Nearly all line-haul tires used in North America are rated for 75 mph of sustained operation. This is a maximum continuous speed, not an absolute upper limit. (When tires run over the 75 mph mark, they don't immediately explode and die.)
Speed limits are not required to be marked on the sidewalls of trucks tires in North America. Tires that are rated for speeds less than 75 mph have “Speed Restricted” marked on the sidewall and are usually designed for vocational operations. If these tires are run faster than their limits, they can be subject to casing fatigue and irregular wear if they are not specifically spec'ed for the loads they are carrying and the speeds at which they are operating.
If we go back and look at the investigation of the auto hauler's tires by NHTSA, the tires involved were 295/60R22.5s with a maximum load of 7,390 pounds and a speed rating of 65 mph when inflated to 130 psi on a 9-inch rim. The maximum speed can be adjusted to 75 mph when the load is limited to 7,150 pounds according to the manufacturer.
By either not maintaining the pressure at 130 psi, which is most probable, and not reducing the load (by one car) which is also probable, operating the rig over 65 mph was certainly “a recipe for disaster!” That's one thing Mr. Perham got right.
We also know from studies done by the Federal Motor Carrier Safety Administration (FMCSA) that inflation maintenance is poor on commercial vehicles. One study found that only 44 percent of commercial truck tires in a sample of more than 35,000 tires were inflated to within +/- 5 psi of their target pressure. It is my opinion that lack of proper inflation to carry the load is a much greater issue than speeding over 75 mph.
The author of the “Big Rigs Often Go Faster Than Tires Can Handle” article proposed that tire manufacturers should make tires that can handle high speeds. He noted that some companies already produce a small number of tires that are rated at 81 mph and that they cost about the same as those built for 75 mph. Yet if there is really no market or only a small number of buyers for these tires, why should tire makers go to the trouble and expense of doing this? I really doubt an 85 mph tire will cost the same as a 75 mph tire.
NHTSA is considering a requirement that maximum speeds be listed on the sidewalls of all truck tires.
That is a requirement for truck tires in Europe to have their speed ratings molded on their sidewalls. Europe uses ISO load indexes and speed symbols. If you look at some tire manufacturers' data books and the Tire & Rim Association Yearbook, along with the weight ratings in the load and inflation tables, you will also find the ISO Load Index in small font (three numbers). For example a 275/80R22.5 load range G, single tire at 110 psi can carry 6,175 lbs. and the Load Index is 144. The ISO Speed Symbol would be L for most North American tires.
ISO weight and speed ratings are coded so they are not easy to understand at first glance.
So what do you tell your commercial truck tire customers when they start to worry if their tires are safe to operate in these states with high speed limits?
Well, speed is not a critical consideration in tire selection unless they plan on driving fast. Since almost all over-the-road, long-haul tires are rated for 75 mph, if they choose the right cold inflation pressure to carry the maximum load they haul, and maintain that pressure religiously, they won't have any issues with speed. Even if they travel on that 41-mile stretch of Texas Highway 130 at 85 mph, they should still be OK since the 29 minutes it will take to drive it at that speed is not considered sustained operation.
But the key here is that the tires must have the proper load range and inflation pressure to carry the load. As speeds increase, the load carrying capacity of tires actually can decrease. Tires may require additional inflation pressure and/or reduced loads to operate at very high speeds. Maintenance people should be aware of this factor should the fleet find that tire failures are increasing along with vehicle speed.
Why is that? Well, when tires are speeding down the road, they generate excessive heat from the additional internal friction that results from the sidewall flexing faster as it rolls through the road contact patch, which can hurt the tire. The higher rotational speed also can change the shape of the tire as the tread is thrown outward by centrifugal force. This can reduce the size of the contact patch, and therefore traction, as well as cause irregular wear.
This phenomenon may also be more of an issue with wide-base tires that have a wider tread area and lower aspect ratio than conventional, dual truck tires. If tires are underinflated or overloaded while speeding down the road, this situation is gravely aggravated.
Back when most truck tires were still rated for 65 mph and speed limits were 70 or below all across the nation, there was a chart in the Tire & Rim Association Yearbook, which you could also find in many tire manufacturer data books, that advised adding 5 psi to the recommended minimum inflation pressure for the indicated load for speeds between 66 and 70 mph while reducing the load by 4 percent. For speeds between 71 and 75 mph, an additional 5 psi was required and the load had to be dropped by 12 percent.
There were, however, a few limitations to this chart:
1. These load and inflation changes were required only when exceeding the tire manufacturer's rated speed for the tire;
2. They only applied to dual loads and inflation pressures; and
3. The maximum load and inflation capacity of the rim must not be exceeded.
It probably would be a good idea for fleets to consider adding inflation pressure to their tires and reducing load if they are going to run at continuous and sustained speeds over 75 mph. Keep in mind that prolonged high-speed, high-temperature operations will cause irregular wear and less than expected tire life. And there are also a whole lot of other reasons why running this fast is probably not a great idea.
Truckers have to be mindful of the maximum load and inflation pressure that is stamped on the wheel and not exceed it. But today most over-the-road fleets are very fuel conscious. They are spec'ing their vehicles with aerodynamic fairings, wheel covers, trailer skirts and wide-base or fuel-efficient dual tires.
They don't want their vehicles operated at high speed since speed is the largest single variable affecting fuel economy. Rolling resistance increases with speed and every mile per hour over 55 mph cuts fuel economy by a little over 2 percent.
An increase in speed from 65 mph to 75 mph results in a whopping mpg loss of 22 percent. As a result, the majority of fleets spec speed limiters on their vehicles, too.
The American Trucking Associations (ATA) opposes speed limits over 65 mph for commercial trucks and has petitioned the Feds to require speed-limiting devices on commercial vehicles. In a survey done by the ATA in 2007, it found 69 percent of trucking companies already had these devices on at least some of their vehicles—with an average limit of 69 mph. I am sure the number of trucks with these devices is much larger today.
While this inciting article may have made a mountain out of a mole hill, it may just have been the impetus needed to get things moving in Washington, D.C. And that would be a major accomplishment.
In 2013 NHTSA and the FMCSA proposed a rule to require speed-limiting devices on heavy-duty trucks. This initiative has been stalled like almost everything else in the nation's Capitol. As a result of the Associated Press report, NHTSA Administrator Mark Rosekind said he would push for quick action on the proposed regulation.
So only time will tell if that actually happens.