Round and black was once the acceptable way to describe a tire. Today, most are still black, but car owners demand that they must be a lot rounder.
What the consumer really wants is a tire that not only is round or concentric, but also perfectly balanced with uniform distribution of mass.
The evolving need for a flawlessly balanced tire runs parallel to technological advances in vehicle performance. While car makers continue to wrestle with developing alternative power systems and upcoming CAFE standard increases, they have been most successful introducing more fuel-efficient engines, removing excess materials and using lighter-weight components to offset higher horsepower engines and increased curb weight resulting from performance add-ons and roomier passenger compartments.
Consumers have done their part by helping grow compact and sub-compact car sales in the U.S. market from 21% in 1995 to 33% today, according to a J.D. Power & Associates report. Internationally, small car sales have grown at an even higher rate.
An outcome of higher performing engines, advanced transmissions, tighter assembly tolerances, sophisticated suspension systems and better-built small cars is a smoother, more comfortable ride for drivers and passengers.
The implication for tire manufacturers is that the “feel of the road” is more prevalent in today’s vehicles compared to 10 years ago, according to Mike Franklin, product quality manager for Goodyear Tire & Rubber Co.
“Tire and wheel assembly balance is much more noticeable in today’s vehicles,” said Franklin. “The slightest discrepancy is felt in the steering wheel and ride.”
It’s an ongoing debate, the constant face-off between advocates of smaller, lighter, more fuel-efficient cars and trucks and customers demanding more acceleration power and room. But a discussion about a smooth ride no longer takes place because it’s expected and immediately noticed.
Balance vs. Concentricity
Franklin explains that balance exists with the uniform distribution of mass around a tire’s circumference. Concentricity occurs when all components are centered around its axis, creating a mirror image if split in half at any point. He says balance and concentricity are independent attributes, but they can influence each other.
“Static imbalance is a condition when the shaft axis and principal axis are parallel, but do not intersect,” says Franklin. “If a perfectly round and balanced tire was offset at its center of rotation, which is non-concentricity, the high point would be the heavy spot of the rotation.”
Goodyear, Hankook and Michelin agree that imbalance typically doesn’t impact fuel efficiency on a passenger vehicle to a measurable degree and rarely affects handling or braking. But imbalance usually does create vibration, which impacts customer ride satisfaction. Responding to market demand, tire manufacturers have been building higher levels of balance and concentricity tolerances into their product.
Two decades ago, all tires were built entirely by hand. Talented, well-trained tire builders would apply beads, plies and other components on a rotating drum to form an uncured tire. But even a builder’s extensive experience couldn’t prevent deviations in splice widths and locations, which would create heavy and light spots in finished products.
Today, computer-controlled equipment applies a tire’s components and adjusts to constant feedback from monitoring equipment. Precise positioning and uniform splices produce near-perfectly balanced tires, even for tires that still see a bit of hand-crafting.
Hankook’s Ray Labuda, vice president of technology, said his company has developed equipment and implemented procedures during the past decade to closely control tire uniformity. “We have installed new tire component preparation facilities and tire building equipment, as well as implemented quality control procedures to consistently provide the highest level of OE tire uniformity specs,” he says.
Goodyear’s Franklin says new technologies in extrusion, fully-automated tire building equipment and curing press controls help ensure consistent tire balance and uniformity at the plant. He said programmable logic controls, or PLCs, are used at every level of the manufacturing process. As technology advances, tighter limits and specifications are set, based on evolving manufacturing capabilities.
“We control the quality of components, starting with our suppliers,” he says. “No matter its complexity or price, each and every tire is subject to identical quality and test systems to meet established performance standards.”
All tire companies were hesitant to specify their tolerance goals, but Franklin says every tire leaving a Goodyear plant is screened to stay well within auto manufacturers’ tolerance requirements. “Every tire is mounted on a highly accurate, true-balance, split-wheel system for inspection. If it doesn’t pass the test, it doesn’t leave the plant.”
Michelin said it uses standard industry uniformity machines to measure radial and lateral forces, as well as imbalance and runout of tires it produces. Concentricity is part of those measurements. The company sets tight limits for all measured parameters, based on actual vehicle testing, as well as consumer feedback.
The end result is that balancing really isn’t needed for a top-brand tire when it first arrives at a dealership.
Balancing With Weights
But, there are two pieces to the balance puzzle – the tire itself and the mounting process. A combination of tire mounting and bead seating procedures, bead lubricant application, dirt buildup and other variables usually generate discrepancies in a wheel assembly’s static and dynamic balance. Then there is the wheel itself, which no one can assume is perfectly balanced – or even as close to perfect as a top-end tire would be.
Enter the aesthetic impact of an imbalanced wheel assembly. The weight.
Often a cosmetic challenge, wheel weights are a necessary evil for solving static and dynamic imperfections or heavy spots in tire/wheel assemblies. But the ongoing phase-out of lead weights to eliminate environmental concerns of their disposal is creating a challenge for tire technicians, as substitute lighter materials such as zinc and steel require larger weights to do the same work.
Smaller diameter wheels and hidden “stick-on” weights also add to the challenge.
Balance is calculated as a force, and a specific amount of force is generated by an unbalanced wheel assembly. To balance it, the unbalanced force is measured and wheel weights are applied to generate an equal and opposing force. A technician enters a wheel diameter, width and offset for the weight locations into a balancer, which measures the unbalanced force and determines the correction weight size and location.
Wheels with no lip for the weight attachment on the outer flange are becoming more prevalent to improve appearance, necessitating a hidden weight applied inside the wheel. Unfortunately, the weight is also closer to the assembly’s axis. Its smaller radius location requires a proportionately larger weight to generate the same correctional force of a weight on a conventional wheel, according to Goodyear’s Gordon Brown, chief engineer.
Brown says a 20-inch tire and wheel assembly that could be corrected with a two-ounce weight at the rim flange might require a four-ounce stick-on weight inside the wheel.
Compounding the challenge of hidden weights and their accompanying smaller correctional forces are large-diameter tires mounted on specialty wheels, which have a greater tendency to generate unbalanced force, he says. “A 20-inch assembly is likely to generate a greater unbalanced force than a 13-inch assembly.”
So, while tire manufacturers and tire technicians team up to create optimally balanced tire/wheel assemblies that provide customers with the smoothest, most vibration-free ride available, pot holes await, panic stops with screeching tires may be just around the corner, and uneven tire wear surely adds to the question of balance.
The cold, hard fact is that once the vehicle leaves the service bay, things will never be the same. But the tire technician will have another chance to bring it close to perfect – in about 5,000 miles.
