With advances in technology, materials and manufacturing, today?ˉs bus tires are tougher than ever before. Bus tire manufacturers design their tires for multiple lives, meaning the tires are designed to be retreaded. This article explains how through retreading tires, bus fleets can add thousands of dollars annually to their bottom line, and at the same time, help reduce the scrap tire problem and conserve oil.
Bus fleet operators have an inherent tire operating problem: the drive tires on buses often face hot operating conditions that can shorten their tires?ˉ tread life expectancies.
The problem is caused by the traditional configuration of bus bodies. Usually, drive position tires are shrouded by sheet metal, which has been formed to create a rear wheel well that is almost completely enclosed. The design restricts the airflow around the tire and actually encourages heat build up. Heat is a tire?ˉs single greatest enemy.
The higher operating temperatures found in many bus tire applications require special compounding and casing constructions to improve the usefulness of these tires.
Most tire manufacturers design and manufacture bus tires using the same advanced technology that goes into truck tires for linehaul service. When fitted to trucks and trailers, linehaul tires are generally in the open, operating with plenty of air circulation to help moderate temperatures. For tires built for bus operation, construction changes are made to afford better heat dissipation from the tread area and to improve sidewall resistance to scuff and curb damage.
Retreading of bus tires can improve the value of these tires in any operation by reducing overall tire costs.
A properly constructed and well-maintained retreaded bus tire will provide at least as many miles of use as it did when new. The more expensive the original new tire, the greater the retread savings.
All bus tire manufacturers design their tires for multiple lives, meaning the tires are designed to be retreaded. Therefore, to discard a worn tire without retreading is to lose much of the tire?ˉs value. This is a wasteful and unnecessary expense.
To ensure that tires can be retreaded for at least one more lifecycle, and perhaps even a second, bus operators need to do several things:
- Follow good bus maintenance practices.
- Closely watch tire air pressure by checking air pressure regularly using a properly calibrated tire gauge. It is important to break bus drivers of the habit of thumping a tire to determine air pressure. Doing so is as effective as trying to determine if a bus needs oil by thumping its hood.
- Load vehicles properly and do not overload.
- Teach bus drivers to make turns carefully to avoid excessive scrubbing of the tire tread surface on the pavement. Because of the longer chassis of most buses compared to conventional trucks, tread scrubbing is almost inevitable, but cautious turns will reduce the wear rate.
Vehicle handling can be improved by maintaining correct air pressure in tires. Avoid running tires at low pressures.
When a tire is underinflated, most of a vehicle?ˉs weight is concentrated on the tread located just under the sidewalls, rather than being spread out evenly across the full width of the tire. As such, as the tire rolls, the sidewall gets continually overflexed and heats up, creating destructive temperatures and faster tread wear. This affects both performance and safety.
Problems also result from too much air pressure. Overinflation causes tires to wear excessively at the center of the tread because it will bear the majority of a vehicle?ˉs weight, with little wear on the outer edges of the tire. This causes reduced traction, handling problems, a harsher ride, and increased and uneven wear.
Not only is tire retreading an economic positive, it is also environmentally responsible. Retreading helps reduce the scrap tire problem and saves oil.
Bus tires are basically petrochemical products. It takes approximately 22 gallons of oil to manufacture one new bus tire. Since most of that oil is used in the tire casing, which is reused in the retreading process, only 7 gallons of oil is required to retread that same tire. Consequently, each time a tire is retreaded, approximately 15 gallons of oil are saved. Considering that the overall tire market for transit and commercial buses is about 430,000 to 470,000 units annually, retreading conserves millions of gallons of oil every year.
The approximately 500,000 private commercial truck owner- operators and commercial truck fleets in the United States have discovered that truck tire retreading makes them good environmental citizens and smart business operators. Bus fleet operators are making the same discovery.
Increasing numbers of highway users, concerned citizens, government agencies and drivers recognize that a retreaded truck or bus tire is a tire that is not part of the solid waste stream.
The basic difference between a retreaded tire and a new one is that a retread has new tread rubber added to a previously used casing that has been designed for a multiple life. Retreads enable the bus operator to keep costs down, improve profit margins and increase the value received from tires.
The foundation of a successful retreading program starts with good tire casings. Structurally, new and retreaded tires are virtually the same. With proper care, commercial bus tires can be retreaded two or three times, providing hundreds of thousands of miles of additional tire life.
The tire casing will eventually wear beyond retreadability, but collecting that many miles takes a long time, and during this time, the tire being kept in use as a retread reduces the scrap tire problem and saves the bus fleet operator money.
Although tires are highly complex structures ¨C among the most durable objects the rubber industry has ever created ¨C bus operators are not overly fascinated by tire complexity. They simply demand that a tire support the bus and carry its ?°cargo?± over the road surface with relative smoothness, and do those things for a long time with no trouble. However, when thinking retreading, it helps to know what?ˉs in a tire and how they?ˉre made.
A new bus tire begins life as a mixture of natural and synthetic rubbers, oils, carbon black, pigments and other additives, each contributing certain properties to a rubber compound.
The tire?ˉs components ¨C sidewalls, tread rubber, chafer and apex strips, innerliner and reinforcing body fabric and steel plies ¨C are all assembled in a shape that resembles a doughnut. The tire industry refers to this as a ?°green?± tire.
This assembly is placed in a tire curing press where, under extreme heat and pressure for specific time periods, the doughnut is molded into a conventional tire shape. These same forces are used in retreading previously used tires.
After the new tire is removed from its mold, it should have a routine, original life expectancy of hundreds of thousands of miles, given proper attention to inflation pressures and avoiding collisions with curbs, potholes and other hazards.
The time to retread, tire experts say, is when a tread measurement shows at least 4/32?± of tread remaining on the tire casing. They strongly advise against waiting until the tread wear indicators show in the tire. This delay could cause the casing to be wasted from a retread standpoint. Some of the old tread rubber base is needed for good adhesion of the new tread, which is why it is a good idea to pull tires early. It can be a false economy to wait until the last minute.
If the casing is in good condition, retreading can extend the useful life of the tire by another hundred-plus thousand miles. And often, the process can be repeated a second and third time.
During the retread process, only the tread rubber on a tire is replaced. The tire casing is usually untouched except for buffing, unless there is some needed section or nail-hole repair required. Buffing is the process where a tire has its old tread mechanically removed by a specifically designed lathe-type machine.
The first step in retreading is a thorough visual inspection of the tire to be retreaded. Inspectors typically also use various high-tech, non-destructive inspection equipment to uncover damage invisible to the naked eye. These inspections look for manufacturing defects, signs of impact that might have broken the casing, repairable damage, non-repairable damage and excessive aging.
Once a tire casing passes the inspections, it has most of its remaining tread rubber buffed off with a specifically designed buffing machine. Using template guides for accuracy and consistency, this machine removes the worn tread to the correct shape, size and texture, preparing the casing surface to receive a new tread. At this point, highly trained and skilled professionals repair any injuries remaining in the tire casing.
The next step in the retreading process is the application of the new tread. Because proper alignment is critical for tire life and performance, a device known as a tread builder centers and aligns the new tread rubber on the buffed casing. The tire is then ready for tread curing (also called vulcanizing) ¨C the step that bonds new tread rubber to a previously used tire casing.
Two types of processes are used in bonding tread rubber to the tire casing: mold cure and precure. Simply put, with the mold cure process, uncured tread rubber is applied to the tire casing after which it is placed into a rigid mold, which contains the tread design. The mold is heated and the tread rubber takes on the tread design.
In the precure process, the tread rubber that is applied to the tire casing has already been molded with a tread design. A thin layer of cushion gum bonding is applied around the tire casing after which the precured tread is applied. The tire is then placed in a chamber where pressure and temperature adheres the tread to the tire.
Following the curing comes the last step in the retreading process: final inspection. All retreaded tires are rigorously inspected to ensure a quality, safe and attractive product. Once inspectors approve the tire it is then painted and labeled for a like-new appearance, ready to return to full service and a second (or third) life as an economical alternative to higher-priced new tires.
A single principle governs the selection of a retread process: the correct tread design and compound must be selected for the bus operators?ˉ intended use.
Retreaded tires are a viable alternative to new tires. The more times a tire casing is retreaded, the lower the cost-per-mile of operation. The result is an improved financial picture for bus fleets, and a tire that is kept out of the solid waste stream for a longer period of time.
NOTE: Retreaded tires may not be used on the steer wheel positions of buses.
This article is originally published by the Tire Retread Information Bureau (TRIB) ¨C a non-profit, member-supported industry association dedicated to the recycling of tires through retreading and repairing.