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Many car owners can change to a larger wheel size. Increasing the size of your wheels can help to improve the look and impact of your wheels - making them even more attractive on your car.

Changing within the same rolling radius of your current wheel

“Up stepping” as it is known, is acheived by increasing your wheel size while reducing the profile of your tire. This ensures that the “rolling radius” of the new wheel and tire remains the same as the original.

Benefits – Up stepping should improve the handling of your car. Each step will reduce the proportion of flexible tire 'sidewall' to rigid alloy. This will improve response, will help keep the tire tread square to the road and will improve your car's 'feedback'.

Disadvantages - In the majority of situations, tire inches are lighter than wheel inches. Plus-sizing can make your overall wheel/tire package heavier. Reducing the profile of your tyres will also reduce your car's damping deflection under compression ie the comfort of the ride could deteriorate. Think of increasing rotational mass as adding another person's weight to the car.

If the rolling radius is larger, you will need to get your speedometer recalibrated . Bigger wheels are often heavier so you should compare the weights of alloy wheels with the standard wheels you have. A bigger wheel can resist a change of direction so handling can be slightly affected.

It is commonly recognized that eliminating weight from a vehicle will aid it's acceleration. Even greater gains in acceleration, or wheel horsepower, can be made by eliminating weight from rotating parts of the drivetrain and chassis. To even further complicate matters, there are more gains to be made in reducing certain areas of rotating mass than others.

For example: Reducing 10 lbs of weight from parts that rotate at engine speed (crankshaft, flywheel, etc.) will have more benefits than reducing 10 lbs of weight from parts that rotate at axle speed (hub, rotor, wheel/tire, etc.) This somewhat relates to drivetrain loss, which most people are familiar with.

However, even reducing the weight of parts that rotate at axle speed, still far outweighs the benefits of reducing weight from non-rotating parts.

There are other benefits to reducing unsprung rotational mass (the part of the car not supported by the springs: tires/wheels). Reducing unsprung mass, specifically from rotating parts, will not only improve acceleration, but it can greatly improve handling and braking too.

Rotational Inertia (or Momentum)

Rotational inertia is a concept a bit more difficult to deal with than unsprung weight. Inertia can be thought of as why a car wants to keep rolling once moving, or remain in place once stopped (unless you forget to set the parking brake on that hill). I believe the terms momentum and inertia are interchangeable. The term “flywheel effect” also refers to these concepts. In a car, there are a number of rotating masses which require energy to accelerate. Up front, ignoring the internal engine components like the crankshaft, we have to worry about the flywheel, clutch assembly, gears, axles, brake rotors and wheel/tire. Out back its a little simpler (for FWD) with just the brakes and wheel/tire contributing most of the mass.

The more mass an object has, the more energy it takes to accelerate it. To accelerate a rolling object such as a wheel, you must both accelerate its mass plus overcome its rotational inertia. As for braking, you must overcome its rotational inertia plus decelerate its mass. By reducing the weight of the vehicle's rotational mass, lightweight wheels provide more responsive acceleration and braking.

The effect of rotating mass can be calculated using Moment of Inertia (MOI). MoI is related to not only the mass of the rotating object, but the distribution of that mass around the rotational center. The further from the center, the higher the MoI. The higher the MoI, the more torque required to accelerate the object. The higher the acceleration, the higher the torque required.

Because of this, the weight of rotating mass such as wheels and tires on a car have a bigger effect on acceleration than static weight such as on the chassis on a car. When purchasing new wheels and tires for a performance car, it can be useful to compare the effects of different wheel and tire combinations. This is especially true when considering upgrading to larger wheels or tires on a car.

The use of light-weight alloys in wheels reduces rotational mass. This means that less energy will be required to accelerate the wheel. Given that each pound of rotational mass lost provides an equivalent performance gain as a 10 pound reduction in vehicle weight, the benefits of light alloy wheels on vehicle performance cannot be overlooked.

The effect of rotating mass can be calculated using Moment of Inertia (MOI). MoI is related to not only the mass of the rotating object, but the distribution of that mass around the rotational center. The further from the center, the higher the MoI. The higher the MoI, the more torque required to accelerate the object. The higher the acceleration, the higher the torque required.

Because of this, the weight of rotating mass such as wheels and tires on a car have a bigger effect on acceleration than static weight such as on the chassis on a car. When purchasing new wheels and tires for a performance car, it can be useful to compare the effects of different wheel and tire combinations. This is especially true when considering upgrading to larger wheels or tires on a car.

Note: The use of light-weight alloys in wheels reduces rotational mass. This means that less energy will be required to accelerate the wheel. Given that each pound of rotational mass lost provides an equivalent performance gain as a 10 pound reduction in vehicle weight, the benefits of light alloy wheels on vehicle performance cannot be overlooked.

So a couple pounds here and there on wheels and tires will make a difference, but that difference is magnified because that weight is placed further from the axis of rotation than any other mentioned. All these masses must be accelerated, so any reduction is a good thing. Now you know why we always say don't get those 18" rims for your car. Not only are the heavier, they have a larger overall diameter. Even with lower profile tires, most plus sizing leaves us with a slightly larger wheel.

Think of it this way; would a concrete wheel that weighs 200 pounds take more hp to turn than a wheel weighing 22 lbs. I think you know the answer!

REDUCTION IN ROTATING MASS WILL BENEFIT ACCELERATION. GOT IT!!!

Width ...... Offset

6.5" ....... +53 to +55
7.0" ....... +50 to +53
7.5" ....... +47 to +50
8.0" ....... +43 to +47

Note: offset is only a variable of width not height.

Offset Diagram:

Visual representation of different offsets:

Rotating Mass, Available Horsepower, and Acceleration