Throughout your life I am sure you have been troubled by many imponderables as I have such as: Why do clocks run clockwise? Why do ketchup bottles have necks so narrow that a spoon won't fit inside? What happens to the tread that wears off tires? When is a tire too old to be used?
For most of these tough questions there are answers. Others still remain a mystery, including this question: What is a simple, fair and accurate way for state inspectors to determine if truck tires are underinflated? This is a question that the Commercial Vehicle Safety Alliance (CVSA)—the state enforcement officers who operate truck scales and perform commercial vehicle inspections—asked the Technology & Maintenance Council (TMC) to ponder since they were struggling with the enforcement of this aspect of safety regulations.
I know you're thinking right now, “How tough can this be?” All you need to know are the axle weights on the vehicle and the tire size and you can just look up what the proper inflation pressure should be to carry the load in the Load and Inflation tables provided by the Tire & Rim Association as well as any tire manufacturer.
But what if you don't have access to Load and Inflation tables? Ha! The safety regulations outlined under the Federal Motor Carrier Safety Administration (FMCSA) Parts 393.75 that address tires state that: (1) No motor vehicle shall be operated on a tire which has a cold inflation pressure less than that specified for the load being carried. (2)
If the inflation pressure of the tire has been increased by heat because of the recent operation of the vehicle, the cold inflation pressure shall be estimated by subtracting the inflation buildup factor (15 psi for tires with over 4,000 pounds load rating) from the measured inflation pressure. However, nowhere do these regulations tell inspectors how to determine what the proper cold inflation pressure should be for the load being carried, nor does regulation 393.75 specify a minimum inflation pressure for tires.
In addition, there are no tools specified to measure inflation pressure. Some inspectors use gauges (but who knows how accurate they are) while others use hammers or other tire clubs. So each inspector is on his or her own to determine what constitutes underinflation, since there is no established standard procedure.
The only thing that is standardized is that they must subtract 15 psi from the inflation pressure to account for heat buildup. The problem with this number is that heat buildup depends on load, speed, ambient and road surface temperature.
As a result, it is normal for tires on trucks in line-haul operations to experience an increase in pressures from heat buildup from about 5 to as much as 20 psi over “cold” inflation conditions which, in the Tire & Rim Load and Inflation tables, is defined as approximately 65-70 F.
It is very possible that tires on a tractor-trailer rig fully loaded, grossing out at 80,000 pounds and running 75 mph on a hot day in July across Arizona, could experience temperatures greater than 200 F and inflation pressure increases of 20 psi. Currently CVSA inspectors don't routinely use tire pressure gauges to determine if a tire is flat or underinflated.
They only check inflation pressure if they have a reason to believe a tire has a problem, such as if they see a nail in it, hear a leak or the tire has a more deflected sidewall than the other tires on the vehicle. They don't have time to check all 18 tires on a tractor-trailer rig with a gauge—which in effect is doing the fleet's or at least the driver's job.
They also don't have access to tire load and inflation tables. If they did, they would spend an inordinate amount of time trying to figure out what the proper pressures should be in tires. So why does anyone care? Well, according to a study conducted by the TMC for the FMCSA, only 44 percent of all truck tires are within +/-5 psi of their target inflation. That means a whole lot of tires are running around underinflated. While that is not an out-of-service condition as is running “flat,” more than 30,000 citations were issued for underinflated tires by state inspectors in 2011 and 2012 which, according to the CVSA, is a lot.
This is a large number for a condition which results in a 3-point citation that goes against a fleet's and the driver's CSA safety rating. Many fleets feel that they have been wrongly cited for underinflated tires due to the loads they were carrying and their operating conditions, and their CSA scores are being erroneously impacted.
Several years ago the CVSA asked the TMC's Tire & Wheel (S.2) Study Group to determine a quick and easy way for state inspectors to identify when a tire is flat. This group considered the tools and the time available to these enforcement officers and came up with the recommendation that, if a tire's gauged pressure is 50 percent of the maximum load ratings at a specific inflation pressure that is molded into the tire sidewall—after deducting the 15 psi for heat buildup—then something was seriously wrong with the tire.
Meaning it should be considered “flat,” an imminent hazard, and the vehicle should be put out-of-service. The inflation pressure for the maximum load on the sidewall was chosen because it is the only tire-specific inflation pressure number inspectors have to work with.
The Tire & Wheel Study Group, whose members include some of the best minds in the truck tire industry representing several tire manufacturers, fleets and other tire-related organizations and supplier companies, chose this method because it fit the CVSA's requirements, and they could think of no case in which this determination would not be true.
However, underinflation is quite another matter. The S.2 Study Group grappled with this question for over two years. It looked at various methods for determining underinflation that included using other percentages of the inflation pressure at maximum load on the sidewall. The group separated steer tires from all other tires since steer tires require higher inflation pressures than duals and their margin of safety, if you will, is much tighter.
It even considered creating a list of the most common tires found in over-the-road operations and developing a chart that provided underinflated pressures of tires after 15 psi was deducted. No matter what it came up with, practical scenarios could still be envisioned that negated their use. Some of the issues that prevented adoption of any of these methods were:
- Different tires build up operational heat differently depending upon the tires' construction, duration of operation, the ambient temperature, running speed, etc. Therefore, the mandatory deduction of 15 psi routinely was determined to be a bad and an unfair practice.
- In addition, inspectors are not necessarily aware of the length of time a vehicle has been operating. Deducting 15 psi from the tires of vehicles that have only been running a short amount of time unfairly penalizes these vehicles since they have not built up that level of operating heat.
- Vehicles frequently are operated lightly loaded or empty and therefore do not require high pressures to carry their loads as when heavily loaded. In order to be fair, the load has to be known and Load and Inflation Tables must be used. For example, a tire with 75 psi—60 psi after deducting 15 psi due to heat buildup—may still be perfectly fine to run if the vehicle is unloaded, even though the maximum load/inflation pressure on the sidewall is 110 psi.
- Steer axles are commonly rated now for 13,200 pounds instead of 12,000 pounds that were the norm for many years. That's due to the new diesel particulate filters, exhaust coolers and DEF tanks and all their associated plumbing and hardware needed to meet the Environmental Protection Agency emission regulations established over the past decade. As a result, load-range H tires are commonly spec'ed for steer axles and require that higher pressures such as 130 psi be molded on their sidewalls. However, when these tires are retreaded and moved to drive or trailer dual positions, they carry much less load—load range G tires are sufficient—and they require much less inflation pressure. During an inspection, if the inspector uses the high maximum load/inflation pressure written on the sidewall in his underinflation calculation, this would be totally wrong.
- Truck operators who routinely inflate their dual drive and/or trailer tires to 75-80 psi due to the light loads they carry would also be more apt to be unfairly penalized for underinflated tire conditions when the maximum load/inflation pressure on the sidewall of their tires is 110-120 psi and 15 psi must be deducted from their gauged pressures.
Given the technologies and inspection conditions available today, and since the determination of tire underinflation at CVSA inspection sites is too complex and imprecise because it involves so many variables, the TMC (S.2) Tire & Wheel Study Group decided that it is impossible to come up with an accurate, simple and quick way for enforcement officers to assess underinflation.
Therefore it asked the American Trucking Association (ATA) to petition the FMCSA to remove paragraph 393.75 (H) dealing with underinflated tires from the 49 CFR 393.75 (Tires) regulations.
The CVSA also petitioned the FMCSA to remove this section of the regulations in support of the ATA's recommendation. It will probably take at least five years for the wheels of government to roll on this issue, but at least for the time being, CVSA has decided that its inspectors will no longer issue citations for underinflation since they cannot properly determine it.
While the question of what is a simple, fair and accurate way for state inspectors to determine if truck tires are underinflated is still unknown, I can tell you why clocks run clockwise. Before the advent of clocks, people used sundials to measure the passing of time. In the northern hemisphere the shadows rotate in the direction we now call “clockwise.”
Clock hands were built to mimic the natural movement of the sun. If clocks had been invented in the southern hemisphere, “clockwise” would probably be the opposite direction.
Perhaps in the not too distant future, technology will advance and a mechanism will be developed that CVSA inspectors can use to quickly and accurately determine underinflation—just as the clock was an advancement over the sundial. Until then, assessing tire underinflation will have to remain a mystery.
Now, as far as that question about the spoon in the ketchup bottle goes....