Rated tires properly sized ensure a safer ride
By Larry Yohe
A front tire failure can cause the motorcoach to be either extremely difficult to control or uncontrollable entirely. There is always the possibility of a catastrophic accident. A careful examination of the variables and practices that affect the control of a motorcoach can reduce the incidence of front tire failures.
Those variables include the types of failure such as a delamination, blowout or slow loss of air, and the manner in which the tire comes apart. The power steering system and steering wheel size may come into play, as well as human factors that include alertness, skill and the physical strength of the driver.
Steering wheel size is a factor
Power steering force is a major factor in coach control, but the size of the steering wheel should not go overlooked. A small steering wheel is fine as long as everything is working properly, but extra leverage can help during the case of a failed tire, engine stall, or power steering failure. To see coach manufacturers transitioning to smaller diameter steering wheels causes some concern
A new take on braking
For years the practice for drivers has been to not apply the brakes during a tire failure. However in tests where NTSB and Greyhound dynamically failed 14 tires, braking did not appear to adversely affect handling. In fact, in a couple of cases, applying the brakes actually helped control the motorcoach. Information recorded from the instrumented load cell steering wheel substantiated this conclusion, which was consistent with what I felt at the wheel as the test driver.
On a couple of hard braking attempts the outboard camera showed the left wheel locking and the flat tire rotating around the wheel rim, while the right front tire benefited from full braking. This provided more usable brake force to the right side, which actually assisted in bringing the vehicle back to the right and under control.
Where a tire has not yet gone flat, such as in some delaminations, the force applied to both front wheels is still equal provided the braking system is reasonably balanced and brake forces are normal. Prior to these tests technicians at the test preparation facility in Louisville, KY took measurements of the brake pushrods and found them all in good adjustment. Greyhound recorded these measurements in its maintenance records.
Other variables such as independent front suspension may show different results of braking during a tire failure than what NTSB discovered in its tests. Nonetheless, in testing this 45-foot MCIDL3, we found that light, moderate and heavy braking did not negatively impact coach handling — and even assisted handling in two of the tests.
The driver is always a factor
A tire failure always demands immediate response, making the physical strength of the driver a variable impossible to eliminate. It must be included in the evaluation of front tire failures. It is a definite plus when a capable and alert driver with two hands on the steering wheel can keep it between the white lines.
Properly size rated tires = prevention
Obviously it is best a front tire never fail in the first place. But it can happen for any number of reasons and it is important to know the causes beforehand.
The primary preventive factor is a top quality, properly sized rated tire. This cannot be emphasized enough. Most 45-foot coaches have a front axle rating of about 4,000 to 6,000 pounds heavier than a standard 3-axle truck tractor. A common axle rating for a 45-foot motorcoach is 16,500 lbs, with a few older ones at 14,400 lbs and some new ones over 18,000 lbs. Most standard three-axle truck tractors have front axle ratings between 12,000 and 12,500 lbs. With the exception of some concrete mixers and other special equipment, the front axle on a 45-foot motorcoach is one of the heaviest rated front axles operating on the highways.
When the 45-foot coach increased in popularity in the 1990s, the primary tire in use was the 315/80R22.5 with a “J” load rating (8270 lbs @ 120 psi for a single tire) and an “L” (75 mph) speed rating — still a popular tire in the industry. However, a 16,500-lb front axle and a combined front tire load rating of 16,540 lbs inflated to 120 psi allows only an extra 40 load pounds before the tire is technically overloaded. All front axles do not carry the same weight rating, and all carriers are not running heavy, especially on daily commuter runs. But it is a critical point to consider in the purchase of tires.
According to one forensic tire expert, a tire subjected to a heavy-duty cycle may weaken over time and become more vulnerable to a failure. Essentially there is no insurance margin for an occasional overload on a fully loaded 45-foot motorcoach with a 16,500 lb axle and a tire with a J load rating. The tire may run most of its life on a heavier than normal duty cycle.
Over the past few years more carriers have gone to an L-rated tire (9,090 lbs. at 130 psi). This larger tire has an “L” load rating and “L” speed rating. This tire was not available in the early years of 45-foot motorcoaches. One large U.S. carrier that has kept tire incident records has reported a dramatic decrease in front tire problems since using an L-load rated tire.
While an L -load rated tire is more expensive, it is still prudent for fleet managers to purchase tires with axle and tire ratings sufficient to do support the actual weight loads. It is especially important to have a heavy tire on the steer axle. The use of an L-load rated tire on the steer axle only is usually sufficient for safety concerns.
Inflation pressure is important
The standard “J” rated tire requires 120 psi for the maximum load limit of 8,270 lbs, or a total tire load carrying capacity of 16,540 pounds for a single axle. If that pressure decreases even by 5 psi, it reduces the single axle load limit to 15,840 psi, or 660 lbs less than what is required for a 16,500 front axle. Correct inflation is critical, especially to a fully loaded motorcoach.
Tire pressure monitoring systems (TPMS) are becoming more popular. The NTSB recently recommended their use as the result of the Sherman, TX accident, which killed 17 passengers. Currently not all coaches have this system. Even if they did, it is no substitute for a manual air pressure check using a quality gauge — especially on the front tires.
Unfortunately in one accident I investigated while at the NTSB, the bus had a TPMS, but we could not determine if a fault code was present or just not recognized by the driver. In either case, the tire still delaminated resulting in a loss of control and an overturned bus with numerous injuries.
Because a front tire 40 or 50 psi low is not easily discernable to a driver during a visual inspection, the best safety practice is for the driver always to carry a quality tire gauge. It takes less than two minutes to check both front tires — a small inconvenience considering all it may prevent. Additionally, the driver needs to conduct a visual check for low tread, uneven wear or other tire damage — something a TPMS cannot do.
To prevent the failure in the first place the driver can take a few initial precautions to lessen the chance of a front tire failure. The driver can ensure the heaviest baggage is loaded in the rear baggage bay to take unnecessary weight off the front tires, especially if the coach is fully loaded. There is more for everyone in the bus and tire industries to learn on this topic, but I trust someone is alive today, or that lives will be saved in the future, because of the contributions from those who worked so tirelessly on these investigations and tests.
The opinions and analysis of issues addressed in this article are solely those of Larry Yohe and do not necessarily represent the shared views or official endorsement of the NTSB.
Larry Yohe served as an NTSB investigator for nearly 25 years with the majority of his time spent on truck and bus issues. Presently he is a motorcoach consultant and drives motorcoaches professionally. Contact Yohe at email@example.com.