The Air Carries the Load
What carries the cargo on a trailer in terms of load-bearing? Although the overall design of the trailer determines how much cargo capacity the trailer has, the air inside the tire carries the load. For this and several other reasons, trailer tires must be properly inflated for safe operation. Underinflated tires wear prematurely and cause excessive heat buildup in the tire, which can lead to catastrophic tire failure. Overinflated trailer tires can also be problematic, causing the trailer to “ride hard” due to excessive bouncing over every imperfection on the road. Overly inflated trailer tires may also cause premature tire wear or tire failure.
What is the value of tire pressure monitoring systems (TPMS)? Many trailer owners have reported that tire pressure monitoring systems (TPMS) and tire heat monitoring systems (THMS) pay for themselves by preventing just one tire failure. Tire failures at a high rate of speed can destroy fenders, mud flaps, and body panels when large pieces of the tire fly separately from the rim. Not only does the operator incur repair costs, but the downtime to make the repairs can be costly. Even if extra damage to the body of the trailer does not occur, a tire replacement alone will result in downtime, a service call, and a tow fee. In certain instances, the cargo inside the trailer could be damaged, and, if hauling food products, could spoil. Similarly, the driver may not receive the same amount of compensation if their delivery misses a tight deadline.
Other than physical damage to the trailer and costly downtime, tire failure can lead to loss of control; in the worst cases, the loss of control can cause traffic accidents and even fatalities. Many serious trailer accidents can be traced back to a tire failure as the root cause. One of the most common reasons for tire failure is excessive heat buildup in the tire because the tire was improperly inflated. When trailer tires blow out at high speeds, the trailer can sway or fishtail, causing the driver to lose control of the vehicle/ trailer combination resulting in a serious crash. In less severe incidents like the case study outlined below, trailer tires can still wear out prematurely, resulting in more frequent tire replacement and greater expense for customers.
A Case Study on Over Inflation
Under-inflated tires are the most common cause of tire failure. Regarding under inflation, many, if not most, towers may not be aware that at 10 percent below the proper inflation level, tires run substantially hotter. Tire tread temperature must be factored, using the formula: ambient air temperature + roadway temperature divided by two. For example, on a 100-degree Fahrenheit day, the roadway temperature will be approximately 140 degrees F. So, 100 degrees F + 140 degrees F = 240 degrees F/ 2, which = 120 degrees F. The reason under-inflation is so dangerous is that a loaded tire that is only 10 percent underinflated will run 80 degrees F hotter than it would if the tire were properly inflated. This would cause the tire tread temperature to reach 200 degrees F. This is dangerously close to the vulcanization temperature of the tire, which would be about 210 degrees F.
However, TPMS systems keep towers informed so they can correct underinflation issues as well as over inflation issues. The hypothetical scenario below contains an analysis of numerous elements that may come into play with over-inflated tires. The Technology Committee would like to thank Dan Shavers and Joe Ostrowski of the Kenda Americana Technical Center for their assistance in developing this educational scenario and providing their tire inflation chart below.
Hypothetical: The customer has a boat trailer with three axles and six tires, size 235/85x16F. Many of the tires were wearing excessively in the middle of the tire tread. This particular boat trailer gets hauled on long-distance trips. During these long trips, the boat is on the trailer for half of the trip, and the trailer is empty for the other half of the trip. Upon visual inspection, the trailer owner believes the tires look as though they may be wearing prematurely on the center of the tread, most likely due to over inflation. This individual is towing his/her trailer with the tires aired up to 110 PSI both when the trailer is loaded with the boat, and when the trailer is empty. The individual wants to know if the PSI should be lowered when the trailer is empty, and if so, what the correct PSI should be?
To answer these questions, more information is required. For example, what is the shipping or curb weight of the trailer unloaded? (3,500 lbs.). What is the maximum gross vehicle weight rating (GVWR) of the trailer when the trailer is fully loaded? (22,000 lbs.). How many axles does the trailer have? (3). How many tires? (6).
The Metric ST Tire Load Calculations Chart below provides a visual to illustrate this problem. Getting into the numbers, assume the make and model of the tire have been identified, and the max rated load for each tire is 3,960 lbs at 110 PSI. This is the inflation pressure required to carry the stated loaded trailer weight, listed at 22,000 lbs. With 3,960 lbs x 6 tires = 23,760 lbs of overall tire capacity, this tire makeup is more than sufficient for a trailer that should never weigh over 22,000 lbs when fully loaded with the boat. At this loaded condition at 110 psi, each tire is carrying approximately 3,667 lbs (22,000 lbs / 6 tires = 3,667 lbs), assuming the load is evenly distributed among the six tires. At this condition, each tire would be loaded to approximately 93 percent of its max rated load capacity at 110 PSI, because 3,667 lbs / 3,960 lbs = 93 percent.
However, when the trailer is unloaded, it only weighs roughly 3,500 lbs. That means each tire is carrying a mere 582 lbs (3,500 lbs / 6 tires = 582 lbs, if each of the six tires are equally loaded. This is only 15 percent of the tire’s maximum rated load capacity (582 lbs). At this lighter, unloaded condition and the same tire inflation pressure of 110 PSI, each tire would barely be in contact with the road. Although difficult to estimate, the actual surface area making contact with the road may be as small as the size of a half-dollar coin. The customer was correct, the heavy wear on the centerline of their tire is due to keeping the inflation pressure at 110 PSI even when the trailer is unloaded.
The difference in the loaded (3,667 lbs) and unloaded (582 lbs) loads actually exerted on each tire would be considered unusually large compared to what is commonplace in the industry for the load each tire must bear in a trailer’s loaded versus unloaded condition. On a typical dual axle utility trailer with two 3,500 lbs axles for example, those two figures would be significantly closer together. Instead, in the example provided here, the load exerted on each tire fluctuates greatly between 15 percent and 93 percent of the tire’s maximum capacity during the unloaded and loaded conditions, respectively, when the inflation pressure remains at a constant 110 PSI.
The most desirable solution recommended in an over-inflation scenario like this is for the owner to decrease the loaded tire pressure to 25 PSI to minimize this excessive centerline wear. This may also help keep the trailer from bouncing all over the road while the trailer is unloaded. As you can see on the tire chart below, if the customer reduced the inflation pressure down to 25 PSI for each tire in the unloaded condition, each tire would only be loaded to approximately 32 percent of its maximum load-carrying capacity of 583 lbs. On the tire capacity chart, if these tires are inflated to 25 PSI, their load capacity would be 1,830 lbs. So, if the owner reduces the inflation pressure to 25 PSI, this adjustment would significantly assist in slowing down the rapid centerline wear on these tires. This is just one example showcasing how tire load capacities can change based on the air pressure in the tire. The main thing a customer can do in an
over-inflation scenario to reduce centerline wear when the trailer is empty is to reduce the tire inflation pressure down to what the tire manufacturer suggests for the curb weight of the trailer. Tire manufacturers publish charts for reference that tell the towers how their tire load capacities change based on changes in the PSI of the tire. The principles in this scenario are universally applicable. All other criteria being equal, the issues presented would be similar no matter what company manufactured the tires or the trailer.
The Potential Expenses of a Tire Blowout
How expensive can one simple trailer tire blowout become? Depending on how the actual blowout occurs, including what trailer components are damaged, a tire blowout could easily cost up to $2,000 or $3,000 by the time the trailer is back on the road. Are these potential expenses enough to justify spending the extra money on an aftermarket tire pressure monitoring system (TPMS) or tire heat monitoring system (THMS)? Many feel this answer is an easy, “yes.” Some rough estimates of possible expenses a customer could incur due to a tire blowout while in transit include:
The tire that blew out must be replaced ($200).
If the fender was damaged, it must be replaced ($200).
Body damage to the trailer ($300).
The lost time whether working in a commercial capacity or on a family vacation ($300 worth of lost time at, for example $100 per hour x 3 hours).
Tow bill ($250 wrecker fee).
Shop labor ($75).
Insurance claim for possible accident/collision (deductible, increase in premiums).
Potential damage to rims, hubs, bearings, etc. ($400).
Fuel expenses could be anywhere from 2 percent to 7 percent higher per trip if the tires were underinflated prior to the blowout.
Correcting the Issue
Once under inflation or over inflation problems have been identified, the next step is to examine the best way to get them corrected as soon as possible. Most new tow vehicle manufacturers offer TPMS as a feature from the factory for monitoring tire PSI on the tow vehicle. This makes it easy enough to see when a tire is getting low. Once detected, the tower knows to pull over, and either air up the tire or have the tire replaced if necessary. Regarding the trailer, some trailer manufacturers also offer TPMS as a feature from the factory, and there are several aftermarket systems available that trailer owners can have installed on their trailers so they can receive alerts and be made aware of underinflated or overinflated trailer tires, as well as their tow vehicle tires.
Today, some of the typical components included with a TPMS or THMS are displays and valve stem cap sensors. Some systems can be paired with apps that allow the use of smartphones as display monitors. On the commercial side of the trailering business on large semi-trailers where air brakes are common, some systems can not only identify when the air pressure in a tire is low; they are also equipped to air up the underinflated tire as the combination drives down the road. The task is much easier when a source of compressed air is already available aboard the tow vehicle and trailer.
However, products could eventually be available in light- and medium-duty trailering applications that similarly air up tires in electric, hydraulic, or electric/hydraulic brake configurations. Look for new developments in this area in the next few years. The NATM Technology Committee will also revisit this topic in upcoming meetings. All of these advancements provide the tower with more information which can be utilized to help the tower save money and make everyone safer on the road.
The contact information for NATM Associate Members that sell tires, tire pressure monitoring systems, and tire heat monitoring systems can be found on the NATM Buyer’s Guide at www.NATM.com.