Dub Dilemma
GM Says No To Rolling Taller Than 20s: Legit Safety Issue Or Just Marketing?
When it comes to rolling stock today, no doubt, bigger is badder. And in the aftermarket world of hip-hop-culture-dominated automotive street fashions, the trend's gone from rolling 16s in the early '90s to pushing past 26s just to get noticed today.
The trouble in double-dub (22) land is that the larger the diameter and the wider the tire and wheel combos become, the more they affect your rig's handling, ride, stability control systems, and braking performance. Some dynamic qualities of your truck are enhanced, others degraded, and at some point, the wheel and tire package is so large that it sometimes degrades performance to the point of being dangerous. At least that's the pitch made by the GM Accessory division.
GM also tests wheel and tire...
GM also tests wheel and tire combinations with specific vehicles to ensure performance and safety levels.
We do a lot of tire testing at Sport Truck and we're very aware that the handling and ride quality of truck changes with plus-one and plus-two rolling stock upgrades. At that level, we've only experienced positive performance enhancement, sometimes with the trade-off of a slightly firmer ride. And we're aware of the performance changes that occur with seriously tall wheels, so we concede GM has a point, up to a point. Whether or not that point is 20-inch wheels remains to be seen.
But that is where GM is drawing the line for its line of factory-authorized wheel and tire packages. Part of the reason is federal safety standards car makers have to live up to.
The Wheel and Tire Performance Criteria (TPC) tire systems produced for GM's fullsize pickups and utilities are good examples of GM's approach to using the extensive development, testing, and validation process for its accessory wheels and tires that it uses to develop the standard factory equipment.
GM's says it puts its new...
GM's says it puts its new 20-inch wheel and tire accessories through the same extensive testing as the regular production parts go through.
"We use the same parameters as the group that engineered the trucks - the same mass, same offset, same width, same mounting flange design, same tire pressure monitoring sensor requirements, same brake clearance, same dimensional tolerances," said Todd Brechtelsbauer, GM Engineering Group manager, Powertrain and Chassis Accessories.
"Those critical elements were carried over to our accessory program," according to Brechtelsbauer. "Once we had the wheel designed, we ran through the finite element analysis (FEA) - a mathematical model that can help point out where your stresses are going to be and determine the problematic areas in the wheel design. We go through all that data to ensure that we have an optimal design. The wheel is then validated using the same type of tests that Tire-Wheel Systems runs every time it does a wheel, such as Fatigue and Corrosion Testing. We follow the same process."
Brechtelsbauer's group also taps into the body of engineering data drawn from GM's TPC specification system to hand-select the tires that are matched up to the accessory plus-size wheels. By specifying both parts of the system - the plus-size wheel and the tire that's specially selected and validated to go with it - GM can ensure that vehicles equipped with a GM Accessories wheel and TPC tire system designed for them continue to meet or exceed federal safety standards and GM's own safety and performance goals. TPC is an industry-leading methodology employed by General Motors to develop tires as an integrated part of the vehicle they're intended for, to provide specified tire performance, handling, and durability characteristics under a variety of load and driving conditions.
The wheels are manufactured...
The wheels are manufactured to OEM quality and specs and are even crash-tested during development.
The new tire and wheel combination is put through its paces "to make sure the vehicle operates as expected with the 20-inch wheels and TPC tires," Brechtelsbauer said. Those tests include straight-line vibration, quick lane changes, driving the trucks over potholes and bumpy roads, and driving on different traction conditions such as ice and snow, looking for issues such as wheel kick and ensuring the vehicle responds as expected to the various inputs.
"When there is a question whether a new wheel and TPC tire combination may cause non-compliance with Federal Motor Vehicle Safety Standards (FMVSS), GM will run tests - even crash tests - to ensure that the vehicle will continue to comply," according to Brechtelsbauer.
Crash-testing led to a redesigned and revalidated knee bolster bracket under the steering column for select fullsize pickup trucks. "When a customer purchases a GM Accessory wheel and TPC tire system for his '04 pickup, the dealership is instructed to retrofit the vehicle with the knee bolster brackets so that the vehicle can continue to meet federal crash requirements," said Brechtelsbauer.
The Powertrain and Chassis Accessories group also validated brakes and chassis controls. The team designed the 20-inch wheel in conjunction with a GM TPC-approved 20-inch tire to minimize the impact on the brake and chassis control system performance, helping them to maintain compliance with FMVSS requirements.
Modifications to accommodate the plus-size wheel and TPC tire systems are made at the point of sale by trained GM dealership technicians. Dealerships are instructed to recalibrate the speedometer and the ABS system, and install a new jounce bumper on two-wheel-drive models. Technicians also install the knee bolster bracket, if necessary, to meet crash-test requirements.
That's GM's pitch. It seems to us that given the regulatory and safety concerns the factories need to address, the company has made the call based on sound engineering and a healthy sense of caution.
On the other hand, aftermarket wheel makers have to contend with the style-conscious enthusiast, who considers rolling 20s and 22s as entry-level, and if you're not rollin' 24s, 25s, 26s, and even 28s, you'll get clowned. From our informal survey of several wheel marketers, we found they're doubtful about GM's insinuation that wheel sizes larger than 20 inches are basically unsafe. And for the most part we agree with the argument that as long as you stay with the proper load range and air pressure spec for your tires, your aftermarket wheel and tire combo will perform as designed.
We will say you have to pay attention as you shop, as all aftermarket wheels are not made the same. Some are more round than others, some stronger than others, and so on. Most important is to buy the combination that is high-quality and will handle the gross vehicle weight rating. Beyond that, you may need to consider tuning the suspension, brakes, and steering to compensate for your wicked-tall rolling stock, especially if you're rollin' larger than 24s.