A Guide To Wheel Offset
All You Need To Know About Wheel Offset
There is no faster and effective way to customize your car's look than to replace the wheels. Whether you want to upsize to the sporty 18-inch high-performance wheels, or you want to downsize to a smaller set of wheels for snow tires, there are factors that you need to keep in mind. Besides the correct tire and wheel size, proper handling of the wheel offset is essential.
Wheel Offset can also be one of the toughest aspects to get correct when changing tires. It is a term that is often very hard for consumers to grasp. However, it is vital to get it right, because getting the offset wrong risks to damage badly the new wheels and tires and the suspension components and can put the driver at serious risk of a blowout
Table Of Contents
3.1. Wheel Offset Chart
1. What Is Wheel Offset?
While wheels look simple from the outside, many components go into the decisions for wheel replacement. We are here to help you understand what a wheel offset is, the various types available, and some other relevant information that you will find helpful.
The Wheel Offset is basically the measurement of the distance between the centerline and the hub mounting surface of the wheel. This distance explains how much the wheel gets stuck out or sucked in from the wheel mounting surface of the hub. Traditionally, this measurement is also performed by using the difference between the centerline and the backspacing of the wheel.
The wheel offset is measured in inches or millimeters and explains the overall wheel dynamics. This is because different wheel offset results in different performances and security aspects. Thus, it is always important to research well and choose the correct wheel with the required offset, especially when you are looking for customizations and aftermarket purchases, ignoring the company fitted standard wheel measurements.
1.1. Types Of Wheel Offset
Depending on the distance between the centerline and the mounting hub surface, the offset can be of three types, i.e., Zero, Positive, and Negative. Below are descriptions of each of the three:
- Zero Offset: Zero or neutral offset means that the mounting face of the wheel is exactly in line with the centerline of the wheel. This means that there is zero difference between the wheel arches and the wheel. Zero Offset can be represented by “ET0”.
- Positive Offset: Positive offset means that the mounting hub surface is in front of the wheel’s centerline, at some positive distance. This means that the mounting surface sits further away from the centerline, towards the outwards direction, increasing the backspacing in the wheel. Positive Offset is represented by “ET+(number in millimeter)”, for example, ET+30. It can also be written as ET30.
- Negative Offset: Negative offset means that the wheel mounting surface sits inwards, at the back of the centerline of the wheel, thus reducing the backspacing. These types of wheels give a much more aggressive look and fit with a lot of concave and extreme dishes. Negative Offset is represented by “ET-(number in millimeter)”.
1.2. Problems From Increased Positive Offset
2. How To Find And Read Wheel Offset
Wheel offset affects the vehicle’s handling, stability, safety and appearance directly. Hence, it is important to know how to find and read the offset of your vehicle. Manufacturers may keep different offsets in the different models of the same vehicle, this is not uncommon and hence when venturing out to replace the stock wheels and tires with new ones, you should be careful. Your vehicles ‘tire guide’ would be the best place to know more, make sure you match the year, make and model accurately for this. In some vehicles, you may also note that the offsets differ among the front and rear axles too.
One way to find out the offset is to read it from the wheel itself, it will be inscribed in a format which will include three main identifiers, which are explained below:
- ET: ET is a German abbreviation for "Einpresstiefe" or "insertion depth". This abbreviation is usually stamped on the rear spoke or the mounting face of a wheel. The ET is the representation of the wheel offset and is measured in millimeters for the distance between the mounting face and the centerline.
- Positive AAnd Negative Offset: The positive offset usually doesn’t use any sign or can use a plus sign (+), whereas the negative offset is represented using a minus sign (-).
- Numeric Value: The final identifier is the numeric value of the offset, which is a number in millimeters (mm).
For example, an ET25 measurement has a positive offset of 25 millimeters. Similarly, ET-12 means that the wheel has a negative offset of 12 millimeters.
3. How To Calculate Wheel Offset
The other way to calculate the wheel offset is to do it manually, which is not that complicated but requires precision, and following these simple steps:
Step 1: Calculate The Wheel Backspacing
Backspacing is the distance between the wheel's inner lip and the mounting surface. To calculate the backspacing, first lay the wheel down with its backside facing up. Take a straight edge, laying it diagonally along the inner flange. Next, measure the distance between this edge and the hub mounting pad of the wheel.
Step 2: Calculate Wheel Width And Centerline
The wheel width for the offset is the overall wheel width, which includes the rated width and the wheel lips. Thus, the width of the wheel from its outside edges is usually an inch longer than the stated readings.
For example, a rated wheel width of 8 inches (203 mm) will have an overall width of approximately 9 inches (228 mm), thus having the centerline at 4.5 inches (114 mm) and not 4 inches (103 mm). Also, if the wheel has Zero Offset, the backspacing will be at 4.5 inches (114 mm).
The centerline is always half of the overall wheel width.
Step 3: Calculate The Offset
Convert the centerline and backspace measurements into millimeters by multiplying by 25.4. Subtract the Wheel Centerline from the Wheel Backspace to get the offset. If the backspace is more than the centerline, the offset is positive. If the backspace is less than the centerline, the offset is negative.
Centerline = 4.5 in. = 114.3 mm
Backspacing = 4.75 in. = 120.65 mm
Wheel Offset = Backspacing-Centerline = 120.65-114.3 = 6.35 mm
The wheel has a positive offset of 6.35mm.
3.1. Reading The Wheel Offset Chart
You can also determine the wheel offset from a chart depicting the offset based on the rim width and the backspacing. Conversely, it can also be a confirmation table to check your calculations for wheel offset, overall wheel width, centerline, and backspacing.
The first column represents the Rim Width. Note that the Overall Width is approximately 1 inch (25 mm) more than the Rim Width. So, a rim width of 8 inches (203 mm) will be 9 inches (228 mm) in overall wheel width.
The first row represents Backspacing. Note that, for the rim width of 8 inches (203 mm), the centerline will be 4.5 inches (114 mm) as the overall width is 9 inches (228 mm). That’s why the backspacing at 4.5 inches (114 mm) will have Zero Offset for the rim width of 8 inches (203 mm), thus shown by Green colour.
The Positive Offset wheels are represented by Yellow, showing that the backspacing is more than the centerline.
Blue shows the Negative Offset wheels, showing that the backspacing is less than the centerline.
4. What Are Wheel Spacers? How Do Spacers Affect Wheel Offset?
If you’re looking to change the offset of a wheel without having to get a new one, then wheel spacers are the answer. These are specifically manufactured to alter the wheel offset by reducing the distance between the wheel hub and the centerline, reducing the positive offset. This pushes the wheel outwards, towards and sometimes beyond the fenders. It is important to note that since the wheel is pushed out, it may not get properly mounted using stock bolts, and you will require longer bolts for proper installation and safety.
4.1. Advantages Of Wheel Spacers
Here are some noteworthy advantages of wheel spacers:
- Improved Aesthetics: Wheel spacers push the wheel outwards, allowing it to align perfectly with the fender and sometimes even go beyond the fenders. This gives a wider stance and an aggressive look to the vehicle.
- Improved Incorrect Offset: Wheel spacers help to reconfigure the incorrect offset, especially due to increased positive offset, because of which the tire may not fit properly and the wheel may hamper various brake and suspension parts.
- Improved Handling: Wheel spacers improve the stability and handling of the car, especially during aggressive cornering and turning. Since the wheels get pushed further away from the vehicle body, it creates a wider stance and enhanced center of gravity.
4.2. Disadvantages Of Wheel Spacers
Some disadvantages of wheel spacers to be aware of are:
- Steering And Suspension Changes: Since the wheel gets pushed further away from the wheel hub, steering can become stiffer or heavier because of increased fender wall clearance, and various components of the suspension system like the shocks and springs cannot function properly.
- Increased Wear And Tear: Using wheel spacers put additional strain on various parts in the hub assembly and can lead to premature wear and tear. This is especially true for parts like a wheel bearing, which loose contact with the wheel and can easily wear off prematurely.
- Increased Risk Of Accidents: If the wheel spacers are not installed properly or after careful examination of the wheel and tire, it can hinder the proper bending of the wheel stud due to reduced circling of torque and can cause major accidents on the road.
5. Wheel Offset Related Terms
Now that we understand what wheel offset is, it will also be helpful to familiarize ourselves with some terms related to it.
Declared/Rated Wheel Width: It is the distance between the inboard and outboard bead seat of the wheel where the tire sits inside the wheel. It is important to note that this width doesn’t include the lip or flange present on each side of the wheel.
Overall Wheel Width: The sum of rated wheel width and the wheel lip width is known as the overall wheel width. The lip or flange width in the wall's thickness on the sides of the rim. For example, if a wheel has a rated thickness of 6 inches (152 mm) and a lip width of 0.5 inches (13 mm), then the overall wheel width will be; 6+(0.5x2) =7 inches (177 mm).
The Centerline Of The Wheel: The centerline vertically divides the wheel into two halves, in the wheel's direction of movement.
Mounting Pad (Plate, Hub Face, Hub Mounting Face/Surface): The mounting pad is the flat surface on the back of the wheel plate and contains holes enabling bolts to tighten the wheel onto the vehicle axle pad.
Backspacing: Backspacing refers to the distance between the wheel backside and the mounting surface of the wheel. The backspacing is important for accommodating the offset and the width of the wheel properly. This directly affects the handling and performance of the vehicle.
6. Importance of Wheel Offset and Backspacing
The reason wheel offset and backspacing are important is that it helps in selecting the right wheels for the vehicle as per its needs. An improper wheel offset or backspacing can affect the axle, drivetrain, suspension and other related parts of the vehicle. Here are some significant aspects which one needs to know about concerning wheel offset and backspacing
- An optimal wheel offset allows the wheel to sit properly inside the wheel well, without hampering the suspension and brake components.
- It avoids damage to the inner flange and the tire from rubbing against the suspension, brake, and body parts like fenders, bumpers, and mud flaps.
- Nicely chosen wheels with proper offset will enhance the look of the car and also provide increased stability and handling, without interfering with braking.
- Optimal backspacing is vital, especially when installing new wheels that are wider than the older or company fitter wheel.
- The right wheel offset and wheel and tire package that fits perfectly, according to the load capacity, is also vital for stability.
6.1. Backspacing VS Offset
When we discuss wheels, both terms; ‘wheel offset’ and ‘backspacing’ are used a lot, and hence can cause some confusion. An important fact to remember is that both are not the same. The wheel offset is the distance between the mounting surface and the center of the wheel, whereas wheel spacing is the remaining distance from the mounting surface to the back of the wheel.
One may not realize the importance of wheel offsets when looking to change wheels or to simply upgrade them, it is hence helpful to gather not just information about the wheels you want, but also the current wheels installed in your vehicle. This will help in enhancing the performance of the vehicle while maintaining safety too. In case you’re not sure, it is always helpful to consult a trained technician when looking to replace or upgrade wheels so they suit your requirement and don’t compromise on your vehicle's drivability.