Wheels: Performance and “Plus Sizes”

Whether tuning or personalizing or modifying, changing or replacing the basic operating equipment or just the aesthetics of your ride–from the audio system to the engine itself-is a way to show off your engineering or design skills, and make a statement about yourself and what your car means to you. Indeed, the market for aftermarket wheels, tires and suspension components represents multi-billions of dollars annually.


In the case of tires and wheels, aftermarket modifications very often involve replacing the original equipment with larger wheels and wider tires.  But ‘plus-sizing’ isn’t just a matter of ‘what fits.’  Not when critical issues of performance, handling, tire life and most important, safety, are involved.

WheelsPlusSizing_   “Plus-sizing” refers to, for example, changing a standard 16” diameter wheel for an 18” diameter wheel, in this case entailing a “Plus-2 Fitment.”  But beyond pricing and aesthetics, other issues are faced, such as compatibility of the wheel with the load factor, suitable offset range, matching the wheel to the tire, and determining optimal tire inflation pressure, all of which affect a car’s dynamics, tire/wheel life, and more. Even if all is done right, plus-sizing can affect cornering, ride smoothness, hydroplaning and other performance aspects, and possibly even basic vehicle systems reliability (like antilock brakes, electronic stability control, brake pad and steering gear wear).Plus-sizing

What to Consider

The size of the upgraded tires and wheels must match, they must fit and be mounted without interference within the wheel well, and be able to sustain the vehicle load. Tire load capacity varies with inflation pressure, which will differ tires-106391_1with a larger than Original Equipment wheel/tire set. General industry recommendations say that replacement tires should have a load rating equal to or larger than the OE tires, and tires must be at a pressure level to provide the best performance possible. Driving at higher than optimal pressure means a harsher or stiffer ride and higher potential for road hazard damage to the tire. Lower than optimal pressure means less precise handling and possible tire fatigue. Either higher or lower than optimal pressure means more abnormal tire wear.


Wheel Width and Wheel Offset.  Wheel Width and Wheel Offset.  Customers should avoid the common problem of trying to fit a rim that’s narrower than recommended while staying with a given standard tire. are-bigger-wheels-better-wheels_0Wider wheels do tend to be more expensive; but a rim that is too narrow will cause the tire beads to be run too close together, leading to higher operating stresses in the tire shoulder.

An OEM wheel that came with the car has a specific offset (the distance measured from the wheel-mounting surface to the wheel’s centerline, referred to as “ET”), designed to place the wheel center where it would optimize the car’s steering forces. But aftermarket wheels come in a wide range of offset variations (to account for situations like fender clearance, and room for wider tires). Changing the wheel offset changes track width and scrub radius (this important parameter, also called “steering offset,” is the lateral offset between the intersection of the steering axis with the ground and the center of the tire; it directly affects handling and stability, especially in the event of a blowout, and under certain road quality conditions (like bumps and potholes).


What to Choose

As wheel diameter increases, the mass of the tire and wheel for equal load capacity –and the inertia of the rotating system–increase as well. This means a change in unsprung weight and vertical stiffness that could affect your suspension system’s response, as well as systems like antilock braking, electronic stability control, and tire pressure monitoring based on ABS sensors.  A larger wheel dictates a shorter tire sidewall and wider tread to maintain the same outside tire diameter and prevent a reduced load-carrying capacity. But a shorter and wider tire can also deliver better handling and cornering grip.

So it’s important to select the right size, the right quality, and the right installer who’s experienced with plus-sizing.  Before purchasing, careful consideration is in order–of all the pro and con impacts bigger wheels and wider tires will have on the vehicle’s functionality, its performance, and the driver’s safety.


BMW m3 forged wheels
Photo credit: SMW Wheels

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