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1999 — A look into the past: Matching tires on 4WDS

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Matching tires on 4WDs

(Editor's Note: This story is part of our #TireBiz30 in which we feature one archived story every day of September to celebrate Tire Business' 30th anniversary. Each story represents one of the most relevant news story published in our pages for that year.)

PITTSBURGH — For Dean Albert, owner of Albert's Tire & Service in Pittsburgh, the episode began with a situation many independent tire dealers face almost daily.

Last December, a customer brought in his 1989 Jeep Cherokee for its state safety inspection. Subsequent examination showed that besides brake work, the four-wheel-drive, sport-utility vehicle also needed at least two replacement tires.

The Cherokee's dollar-conscious owner wanted to keep using the two Michelin XH4s on the front wheels since the tires still had usable tread. So Mr. Albert installed a pair of General Ameri*Way tires on the rear in the same P205/75R15 size as the front-mounted Michelins—or so he thought.

Two months later, the same customer was back, this time asking Mr. Albert to pay the $1,571.49 cost of repairing the Cherokee's transfer case—damaged by what the service department at a local Chrysler dealership contended were improperly matched tires on the four-wheel-drive vehicle's front and back axles.

The auto dealership said the circumference of the General and Michelin brand tires varied by "almost two inches." This caused the front tires to turn more rapidly than the rear, causing stress to the Cherokee's speed-sensitive drivetrain and ultimately cooking the fluid in its transfer case.

According to the car dealership's invoice, the owner had complained of a "severe grinding noise" when the vehicle was moving forward in gear.

The auto dealership's invoice went on to state that proper engaging and disengaging of the four-wheel drive "relies greatly on the equal circumference of the tires." A difference in tire circumference of more than 1/4-inch, the invoice said, "will slow the process of going in and out of four-wheel-drive" and a variance of more than 1/2-inch will "cause damage to the drivetrain."

Mr. Albert said he asked the customer: "Why are you coming back to me? I put on the same sized tire that was on the vehicle." The customer replied that the auto dealership told him all four tires have to be the same height and should be the same brand.

Mr. Albert said he turned the invoice over to his insurance company which promptly reimbursed the customer and assured him his tire department had done nothing wrong. But from now on, Mr. Albert said, he's going to insist on installing only four identical tires on four-wheel- and all-wheel-drive vehicles.

The problem Mr. Albert faced is not new. But it's become increasingly prevalent due to the popularity of four-wheel- and all-wheel-drive vehicles with speed-sensitive drivetrains.

Tires may bear the same size markings and meet industry design guidelines, yet have slight differences in circumference that can affect rolling speeds enough to damage vehicle drivetrains, industry sources said.

Elsewhere in Pittsburgh, Whitehall Tire Service Inc. faced a similar situation with a newer Cherokee still under factory warranty.

Whitehall Tire, following the vehicle owner's instructions, installed a pair of BFGoodrich tires on the Cherokee's front wheels, leaving two half-worn, original equipment Goodyears on the back. Although all four tires bore the same size designation, damage to the vehicle's transfer case also occurred.

In this instance, the servicing auto dealership refused to repair the damage under warranty, citing a portion of the Cherokee owner's manual that states that the vehicle "must be equipped with the same size and type tires of equal circumference on all four wheels."

Certain combinations of special aftermarket tires and wheels, the manual continued, "may change tread measurement, resulting in difficult transfer case shifting, changes of steering and suspension geometry, and inaccurate speedometer readings. This can cause unpredictable handling and stress to steering and suspension components."

John Stickley, Whitehall Tire's service adviser, said the auto dealership, in attempting to explain Chrysler's policy, said the tires needed to be "exactly alike" to prevent the occurrence of such problems.

In reply, Mr. Stickley said he asked what "exactly alike" meant. Does it mean all four tires must be identical? Or can they simply be equal in circumference?

Moreover, he wondered, if dealers can't assume interchangeability of different brands of tires bearing the same size designation, must they install tires only in sets of four on four-wheel-drive vehicles? And if they don't, is it necessary for them to use a measuring tape to make certain all four tires are exactly alike in circumference? What tolerances in regard to tire circumference are allowable to avoid potential problems?

Mr. Stickley said the vehicle's owner took the matter to court, where a judge ordered Chrysler to cover the repair cost. However, Mr. Stickley said he's still looking for answers to his questions regarding tire size uniformity.

He suggested that Tire Business look into the issue and offered the names of several dealers known to have had similar problems.

One of those dealers, Terry Evans of Five Star Tire in Pittsburgh, said he ran into difficulty fitting different brand tires on an all-wheel-drive Chevrolet Astro.

If anything, he believes, uniformity in tire circumference is even more critical in the case of all-wheel-drive vehicles, such as the Astro, than with their four-wheel-drive counterparts. All-wheel-drive vehicles, unlike 424s, run constantly in four-wheel drive, rather than occasionally on demand.

In Mr. Evan's case, the vehicle's owner initially purchased a pair of Sigma-brand touring tires then returned at a later date to replace the vehicle's remaining two.

By that time, the dealership no longer had Sigma brand tires in stock, so Mr. Evans installed a pair of Cooper Classics bearing the same size designation.

About four months later, he said, the Astro's owner telephoned to say the vehicle was at a Chevrolet dealership, where a considerable amount of work had been done to the vehicle's drivetrain in an unsuccessful attempt to cure an "extreme handling problem."

Finally, the auto dealership's technicians took the wheels off another vehicle and installed them on the customer's Astro. The handling problem disappeared. Closer examination of the Astro's tires disclosed a difference of 1.5 inches in the diameters of the two brands of tires.

Mr. Evans said he was fortunate that most of the expensive drivetrain work was done under warranty. But Mr. Evans said he, too, will be doubly careful when fitting tires to such vehicles from now on.

Skeptics might dismiss these examples as mere coincidence or "hysteria" on the part of a few dealers located close enough to talk with each other. But dealers in other areas also have reported having similar experiences.

Matt Matlock, training manager for the Tirecraft Auto Centers chain in Sherwood Park, Alberta, said some auto dealerships in his area have refused warranty coverage on vehicles found to have new tires on one axle and worn tires on the other.

The auto dealerships, he said, have told customers the damage to their vehicle's drivetrain resulted from a difference in circumference between the older and newer tires and therefore is not eligible for repair under terms of the warranty.

"We've got customers who, for years, have been putting new tires on the front wheels and keeping their old ones on the back. Now, suddenly, it's a liability issue," said Mr. Matlock, whose job calls for him to keep Tirecraft's 128-store dealer network up to date on such matters.

What, then, does Mr. Matlock advise his company's dealers?

"That they better match the tires," he told Tire Business. "If (the customer) has Uniroyals on the vehicle and wants to buy only two replacement tires, then they'd better be Uniroyals. Don't mismatch manufacturers," he warned.

Mr. Matlock added that Tirecraft also advises dealers to inform customers about these issues and provide them with printed material on the subject.

Not all tire-related drivetrain problems have resulted from variations in the circumference of highway-type tire designs. Some have stemmed from size differences in other types of tires, such as temporary spares or deep-tread winter designs.

Chrysler Corp., in 1998, recalled approximately 65,000 Jeep Grand Cherokees in order to replace their temporary spares with full-sized tires.

In announcing the recall, the auto maker said the mini-spares, which were smaller than the vehicle's conventional P225/70R16 or P245/ 70R15 OE tires, could result in the build-up of excessive heat in its QuadraTrac transfer case, forcing fluid out of the unit's seals and creating a "potential fire hazard."

Meanwhile, in similar although apparently unrelated incidents in Tokyo earlier this year, two unidentified four-wheel-drive cars actually did catch fire due to transmission malfunctions blamed on the use of winter tires in combination with conventional highway designs.

According to an article published by the Mainichi Daily News, the difference in rolling speed between the rear-mounted winter tires and the conventional highway tires on the front was perceived as slippage by the vehicles' differential systems, causing the viscous coupling in their transfer cases to divert increased engine power to the vehicle's front wheels.

Under the stress of repeated intermittent cycling in this fashion, the viscous fluid used in the transfer case became increasingly overheated until it ultimately ignited, fire department officials said. Both fires quickly were extinguished and no injuries were reported, the newspaper said.

Asked how this could occur, Randy Yost, a technical integration engineer for drive quality at General Motors Corp., explained that the transfer case "transfers" power from one axle to the other similar to the way a limited-slip differential transfers power from one wheel to another on the same axle of a two-wheel-drive vehicle.

He said some transfer cases accomplish this using a "viscous coupler," a sealed unit containing a heat-sensitive fluid that stiffens when heated, thereby locking the differential and applying torque to the other axle.

Mr. Yost said General Motors has taken steps to eliminate potential problems in its vehicles.

Can passenger and light truck tires bearing the same size designation vary in diameter by an inch or more? Yes, particularly when comparing new and worn tires. But appearances can be deceiving.

Take, for instance, the General Ameri*Ways and Michelin XH4 tires cited in Mr. Albert's example. Place one of the General Ameri*Ways alongside one of Michelin XH4s and the two tires would appear to vary in diameter by as much as an inch and a half—the Ameri*Way being the taller and narrower of the two.

Yet when inflated and expertly measured under the supervision of Jon Gerhardt, vice president and technical director of Standards Testing Labs in Massillon, Ohio, the two tires' diameters turned out to be only 0.62 inch different—not 1.5 inches as everyone, including Mr. Albert, had believed.

The newer General Ameri*Way was 27.11 inches in diameter, whereas the partially worn Michelin XH4 measured 26.49 inches. Both tires, however, were within industry size guidelines. (See story, page 17.)

Yet even this small difference in diameter obviously was enough to create a serious problem for the four-wheel-drive vehicle's transfer case.

Tire Business put Mr. Stickley's question about how much difference in tire/wheel circumference is allowable to Dick Gratz, service engineer in the tire/wheel group at GM's proving ground in Milford, Mich. Mr. Gratz serves as the auto maker's service and quality liaison to GM servicing dealers.

He said the maximum difference in wheel circumference between front- and rear-mounted wheels that most four-wheel-drive systems can tolerate without difficulty is "plus or minus four revolutions per kilometer" or 6.4 revolutions per mile.

In the case of the General Ameri*Way and Michelin XH4 mentioned earlier, the smaller of the two tires will complete more than 17 more revolutions per mile than its larger counterpart—considerably beyond the 6.4-revolutions-per-mile threshold Mr. Gratz described.

Without going deeply into the mathematics involved, this threshold of difference for these two tires—the difference in diameter that could result in drivetrain damage--amounts to a scant 0.22 to 0.23 inch, or just less than a quarter of an inch.

Some readers may view this as similar to the rule of thumb many use when selecting truck tires for dual applications—namely, that there should be no more than a quarter-inch difference in the diameter of the paired tires.

Mr. Gratz believes most vehicle owners are alerted by the increased noise and consult a dealership or other service outlet before damage to the transfer case actually occurs.

He pointed out that all OE tires used by General Motors have a Tire Performance Criteria (TPC) number molded on their sidewalls. Dimensional uniformity is one of the criteria taken into account when assigning a TPC number to the tire. That's one reason why, he said, "we (at GM) recommend customers replace (their OE tires) with those having the same TPC number."

Following this recommendation would, of course, limit the consumer's choices to major brand tires--and only the tires within those brands carrying the appropriate TPC number. It would exclude the majority of replacement tires available to consumers and dealers alike, Mr. Gratz acknowledged.

In such cases, the tire dealer is faced with two alternatives:

1) Installing only four new tires on four-wheel- and all-wheel-drive vehicles; or,

2) Matching, or coming as close as possible to matching, all four tires in terms of circumference.

No doubt the surest method of maintaining uniformity in circumference is to measure each tire. Revolutions per mile are calculated by dividing the tire's circumference in inches into 63,360 (the number of inches in a mile).

Comparing tires on the basis of diameter is done by dividing the circumference in inches by p (pi, a mathematical symbol whose value is approximately 3.14159). Some dealers may wish to purchase a measuring tape designed for this purpose. Such tapes, available from most tire shop jobbers, automatically take the p factor into account and provide a diameter measurement rather than circumference when wrapped around the tire's center rib.

The bottom line, said one tire engineer, is that dealers are being held to a higher standard by the increased number of vehicles having sophisticated, speed-sensitive drivetrains.

"As a consumer, I don't think tire sizing has changed dramatically over the years. But I do believe there are a lot more vehicles that are sensitive to it than there used to be," he said.

In order to properly meet the tire needs of four-wheel- and all-wheel-drive vehicles, he said, dealers need to insist on installing tires in sets of four whenever possible and become sophisticated in the use of a p measuring tape to assure tire uniformity when that's impractical.

Unfortunately, dealers are going to get a lot of flack from customers when trying to convince them to replace all four tires instead of just one or two, the same engineer added.

"The customer is going to say: 'I don't want to spend the money. You're trying to rook me.' Then later, after the vehicle's powertrain breaks, that same customer will be back complaining the dealer should have done something to prevent it. It's a no-win situation.''

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