By Paul Williams

Milford, Michigan – The engineers at General Motors’ Tire-Wheel Systems testing facility are self-described, “tire and wheel nerds.” They may easily remind visitors of NASA engineers running the Apollo space missions with their short-sleeved white shirts buttoned up to the neck, and jokes about Society of Automotive Engineer (SAE) protocols and mathematical formulae.

20″ tire/wheel combination from GM for its Sierra/Silverado trucks. Click image to enlarge

But surely these are just the kind of people you want setting standards for tires and wheels. “They’re so important because they’re the only part of the vehicle that actually is in contact with the road,” states James Gutting, director of GM’s Milford Proving Ground Tire-Wheel Systems in Michigan. It’s an obvious but crucial point.

The increasingly popular practice of putting larger, sometimes much larger, wheels and tires on cars, a 3.2-billion dollar (U.S.) industry in 2002, is the reason for my visit to the Tire-Wheel facility here. In a major undertaking, GM plans to introduce approximately 200 wheel and tire packages over the full range of its vehicles (in addition to its current wheel/tire options) in the next 18-months. For the full-size pickup truck line, 10 sets are available now. Choosing such a package could increase the cost of a vehicle by $3,000 – $4,000.

“If you can’t beat ’em, join ’em,” is the old adage, but in the case of GM, the company will ensure that their so-called “accessory” wheels (rims) and tires conform to stringent internal standards called Tire Performance Critera (TPC), a system GM pioneered in 1973.

Frank Taverna, a GM vehicle dynamics engineer, explains that for each GM vehicle the tire and wheel assembly is part of a fully functioning and cohesive system. A vehicle’s dynamics can be customized, or “tuned” to create a particular set of handling and performance characteristics. The characteristics will be predictable in various road conditions, which are simulated (both electronically and on the test grounds) by GM’s wheel and tire specialists.

“If you change one part of the system,” he points out, “it affects all the others, including acceleration, braking, steering, fuel economy, durability, ride and vibration. Using TPC, we work with manufacturers to engineer tires and wheels to work exclusively with each other, and with the specific vehicle on which they’re installed.”

Dave Cowger, Engineering Group Manager at the Milford Proving Ground  Tire-Wheel centre, discusses tire wear
Dave Cowger, Engineering Group Manager at the Milford Proving Ground Tire-Wheel centre, discusses tire wear. Click image to enlarge

Already, all suppliers of wheels and tires to GM conform to TPC requirements, and the TPC symbol is moulded in the tire’s sidewall to identify it.

“We understand that wheels and tires are the ultimate “jewellery” for your car and truck, and that there’s a desire to personalize your vehicle,” says Mr. Gutting, “But in our view, the question for the consumer has to be, ‘How do I know what I’m getting?'”

The benefits of purchasing a factory-backed accessory wheel/tire combination will be conformity to warranty requirements and the knowledge that the wheels and tires have been tested to the same parameters as the standard tires and wheels for your particular vehicle, explained Mr. Gutting.

Testing equipment at GM's Tire-Wheel facility simulates extreme road conditions
Testing equipment at GM’s Tire-Wheel facility simulates extreme road conditions. Click image to enlarge

Some of these combinations will provide wheel diameters up to four-inches larger than a base wheel, and three-inches larger than an existing optional wheel/tire package.

Of course, entering this market is not motivated simply a safety/performance concerns. “An accessorized vehicle sells more quickly; we know this,” explains Nancy Phillipart, executive director of GM Accessories. “Now you’ll be able to customize your GM vehicle with GM-approved equipment designed specifically for your car or truck.”

Testing equipment at the GM's Tire-Wheel facility, Milford Proving Ground
Testing equipment at the GM’s Tire-Wheel facility, Milford Proving Ground. Click image to enlarge

In a demonstration on some of the Milford Proving Ground test tracks, auto writers were given the opportunity to drive double-cab Chevrolet Silverado pickup trucks fitted with three different wheel/tire combinations.

The base Silverado truck comes with 16″ steel wheels, but our testers used optional 17″ alloy wheels (a popular choice), newly available 20″ TPC accessory wheels with GM-approved Goodyear Eagle LS 275/55-20 tires, and name-brand 20″ wheels of the same physical specification fitted with a competitive brand tire (in order to maintain fairness, the price of each non-TPC tire exceeded the Goodyear TPC tire by $47.00).

“In other words,” said engineer Jay Pistana, “We didn’t just select some cheap wheels and tires and compare them with our approved brands.”

The driving course enabled drivers to compare balance and vibration, rapid lane changes and potholes with each wheel/tire combination.

Our experience was both helped and hindered by localized storms and torrential downpours, but the effect of the different wheels and tires was still evident.

17″ optional wheels from GM

20″ tire/wheel combination from GM for its Sierra/Silverado trucks.
Click image to enlarge

The smallest wheel provided a benchmark for the test, giving a smooth and quiet ride throughout the course. Rapid lane changes did not upset the truck, but body roll was evident as it responded to quick steering inputs.

The 20″ TPC wheels and tires did not sacrifice ride quality, although you could hear the effect of road irregularities on them (due to the shorter sidewall). Rapid lane changes were accomplished with a sharper steering response. Overall, the truck felt more nimble with these performance-oriented accessory wheels and tires, but still provided a comfortable ride.

The effect of the non-TPC wheels and tires was not dramatic, but was definitely evident. Road irregularities could easily be felt in the seat and the steering column (rather than just heard); the truck seemed prone to dart in response to aggressive steering, and felt less balanced, or “planted” on the road at speed.

While this was a seat-of-the-pants driving test, engineers assured us that these manoeuvres were scientifically based and measured, consistent with the TPC testing carried out at the facility. Evidently, braking, traction, wear and fuel economy will also be affected by the use of a wheel/tire combination not optimized for the vehicle. In some cases, TPC wheels will survive pothole impacts, while non-TPC wheels will deform, causing a flat tire.

“When you change components on a vehicle, especially chassis components, you can change the driving characteristics of the vehicle,” says Mr. Gutting. “The TPC standard ensures the wheels and tires meet all regulations and exceed expectations.”

Expect additional accessory wheel/tire combinations from GM over the next eighteen months.

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