November 01, 2008
Extreme plus sizing
Bigger may look cool, but you need to consider how the increase in unsprung weight and mass may affect vehicle durability and performance
By: Mike Mavrigian
Moving up to a Plus-Seven change can double the wheel/tire package weight. Depending on the vehicle, a brake upgrade may be a good idea.
OK, so let’s say a customer rolls in with a large SUV that’s OE-equipped with 19-inch wheels. Since he or she feels the need to be a sheep and blindly follow the “bling” crowd, the customer informs you that he or she wants to max-out the wheel/tire package simply for the sake of appearance. The new target: 26-inch chromed alloys mated to massive section-width performance rubber.
Such a severe plus move, in this case, Plus-Seven, may provide the visual statement that the customer desires, but there’s more to the picture that the customer needs to see and consider. Transitioning to such an extreme plus move is going to result in a heavier unsprung weight factor, which, in turn, may adversely affect fuel economy and steering, suspension and brake system performance and durability.
Unsprung weight
Gross vehicle weight can be separated into two broad categories: sprung and unsprung.
Sprung weight refers to the weight of, well, everything that’s supported by the vehicle springs. That would include the frame, body, engine, drivetrain, fuel, passengers, etc.
Unsprung weight refers to the weight of components that are not supported by the springs, which would include tires, wheels, brake rotors, brake pads and calipers, brake drums, hubs, axles and control arms.
Actually, depending on suspension design, a percentage of control arm, half-shaft or CV shaft weight would account for unsprung weight, since a portion of component weight is supported by the vehicle springs. If the weight of the item places load on the springs, it’s sprung weight. If not, it’s unsprung weight.
Keep in mind that even using an alloy wheel can result in greater unsprung weight if the wheel size increases. For instance, moving from a 19-inch wheel to a 26-inch wheel results in a heavier wheel, and because of a wider section width, potentially a heavier tire as well. As an example, a typical 16-inch or 17-inch wheel/tire package will likely weigh around 40 to 50 pounds or more. For the sake of comparison, moving to a 26-inch wheel and tire package can easily more than double that weight.
Braking system
The lighter the rotating mass (wheel/tire package), the more efficient the braking system is likely to become. On the opposite side of the coin, as you increase rotating mass, the brake system must work harder to overcome the heavier load. And there’s the rub. Many original equipment brake systems simply are not designed to accommodate these heavier masses in terms of rotor diameter, rotor design, caliper design, friction pad size and friction pad composition.
Under heavy braking conditions, heat buildup will contribute to loss of braking efficiency as pad grip begins to fade. Keeping the brakes ventilated will help maintain braking efficiency.
While a larger wheel and plus-sized tire package may be heavier than its OE counterpart, a larger wheel diameter conveniently provides a larger cavity that would accept a larger-diameter brake rotor and larger brake caliper. In short, moving to an extreme plus size should be accompanied by at least the consideration of moving to a more robust aftermarket performance braking package.
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