What Is Wheel Offset?
Technically, Wheel Offset is the measurement of the distance from the center line of your wheel to the mounting face of the hub that is touching the rotor. It is measured in units of millimeters and can either be positive, negative or zero. When you are using stock wheels, wheel offset is not important. However, when you want to change into a new set of wheels, it becomes a very important factor.
Many people purchase new wheels based on design and stud pattern, not carefully considering the technicality of the installation, i.e. how the wheels sit in the wells when mounted. Proper wheel offset ensures that your new set of wheels has enough clearance so it does not have any friction point with the brakes, suspensions, bumpers, fenders, or mud flaps. Any miscalculation impairs safety in your drive and can cause problems like hampered braking or decreased vehicular stability.
Types of Wheel Offset
Positive Offset is also known as mounting outboard of the center line. In layman’s terms, the wheels during positive offset are set too far into the car body. Too much positive offset may increase the wheel’s likelihood of rubbing into the components of the inner brakes or suspension. This makes a car unstable during high- speed driving and may cause tire ruptures.
Negative Offset happens when the mounting of the wheel occurs on the inboard side of the center line. This is when the wheel sits too far out from the car body. Too little negative offset may cause the tires to rub on the fenders.
Zero Offset happens when the mounting pad position is aligned with the position of the center line.
What is a Wheel Backspace?
Although quite often this measurement is confused with wheel offset, they are different. While wheel offset is the distance from the center line of the wheel to the mounting hub, backspacing is the distance between the inner flange of the wheel and the mounting plate. In other words, backspacing is defined as "wheel width plus wheel offset".
Backspacing becomes a consideration when changing your wheels when the new acquisitions are wider than the old ones. This will entail the adjustment of the offset to compensate for the wider girth of the new wheel.
Where Is the Offset Stamped on the Wheel?
You can find the offset engraved on the wheel as part of the wheel markings with the measurement in millimeters. It is typically located at the back of the spokes, stamped on the barrel, or cast into the face or outside the barrel.
- Always remember the common name for wheel offset, which is ET. This is an abbreviation for a German word which means "press depth." Most stamps on the wheels use the indicator ET but some manufacturers use it as a prefix. An example would be “IS. ET50 = IS5.”
- Mislabeling is a common problem for wheel offsets. Often, misinterpretation also occurs when people do not know what the difference is between negative and positive offsets. When in doubt, always doublecheck with the sales specialist or ask to be shown the stamp or picture of the markings.
- Check your vehicle's manual for its “Tire Guide” for the car's particular year of manufacture, which will yield the default offset of the stock tires.
- If there is no indication of the offset on the wheel nor can you source it out anywhere else, it is best to measure the offset yourself (see below).
How to Measure and Calculate Wheel Offset
Follow these steps to properly measure and calculate wheel offset:
- Prepare a tape measure, straight-edged ruler, and the wheel, preferably without the tire.
- Measure with the straight-edged ruler the overall width of the wheel. Always make sure that your ruler has millimeter measurements indicated on it. You can also use a standard tape measure and convert the inches to millimeters (1 inch = 25.4 mm).
- Measure width by measuring from inside lip to inside lip of the wheel. Subtract 1 inch from the measurement of the width.
- Divide the width measurement above by 2 to get the true center line.
- Place wheel on a flat surface, making sure it is not scratched or raised. Place a long straight-edged ruler across the wheel width, making sure that it is a perfectly straight line across. Place a second ruler on the surface of the wheel hub and record the precise numerical result in millimeters. This is also known as wheel backspacing. If this measurement is less than the center line, the offset is considered negative.
Tips to Consider on Wheel Offset
- Ideally, the offset of the new wheels must not be more than 5 millimeters away in either direction of the old measurement. Too much negative offset may push the wheel too far out and cause an odd car aesthetic. Too much positive offset will push the wheel too much into the suspension. This poses a danger as it compromises the performance and life of the wheel as well as the adjacent machinery.
- Offset spacers are an option to compensate for a positive offset that is too high. They are metal shims that have bolt patterns drilled into them and are installed between the wheel and the rotor. The spacers push the wheel further away from the suspensions. Installation of wheels with the correct size and offset is the best recommendation. However, should you really need to use spacers, make sure that they fit exactly over the axle hub and the wheel hub. These are weight-bearing connections so it is vital that you use them correctly.
- Test wheels before installation if you are unsure of the proper offset or backspace measurements.Always try the front axle and the rear axle separately since they differ in offset measurements. This way, you can be more confident of the fit of the tires before they are mounted. Mounted tires are, more often than not, non-returnable unless they have a factory defect.