The selection of wheel rims is not only a matter of aesthetics, but also a critically important technical procedure on which the safety of driving depends. Many car enthusiasts, wanting to install wider or more stylish wheels, often ignore geometric parameters, relying only on the diameter and number of bolts. However, it is disc offset (denoted as ET) is the parameter that determines whether the wheel will fit into the arch, whether it will touch the suspension elements and how the car will behave on the road.
An incorrectly selected offset can lead to accelerated wear of wheel bearings, destruction of suspension elements, and even sudden wheel separation while driving. In this article we will analyze the physics of the process, learn to distinguish between positive and negative overhang, and also provide a step-by-step algorithm for measuring an old disk if the markings on it have been erased. Understanding these nuances will save you from expensive repairs and ensure stable operation.
Before you take up a measuring tool, you need to clearly understand what overhang is in a geometric sense. This is the distance between the vertical plane of symmetry of the wheel and the plane of application of the disk to the hub. It is measured in millimeters. The disc offset can be positive, negative or zero, and this parameter determines how much the wheel will be recessed into the arch or, conversely, protrude outward. An error of even 5 millimeters can be fatal to the suspension geometry of a particular car.
Departure theory: positive, negative and zero
To correctly interpret the measurements obtained, you need to understand the types of departure. Positive departure means that the mating plane (point of contact with the hub) is shifted relative to the central axis of the disk towards the outer side. This is the most common option for front-wheel drive passenger cars. The greater the positive offset value, the deeper the disk is “recessed” into the wheel niche.
In the case of negative offset the situation is mirrored: the mating plane is closer to the inner side of the disk, behind the central axis. Such wheels are typical for SUVs and some sports cars with a wide track. They visually make the car wider and more aggressive, but require modification of the arches or suspension lift, since the wheel protrudes significantly beyond the dimensions of the body.
There is also a concept zero departure, when the mounting plane coincides with the central axis of the disk. This is an intermediate option that is less common. It is important to understand that when installing disks with an offset different from the factory one, the break-in arm and the load on the components change. For example, reducing positive offset (moving the wheel outward) increases the load on the wheel bearing, which can lead to premature failure.
Always check the ET marking on the inside of the disc before purchasing a used set. If the markings are worn off, use the measurement method described below.
Necessary tools for accurate measurements
You don't need sophisticated laboratory equipment to make quality measurements, but the (accuracy of) the instruments directly affects the results. The main tool will be ruler or, better yet, caliper with sufficient jaw length. A regular school ruler may not be suitable if the disc has a complex spoke shape that makes it difficult to fit the measuring device tightly.
You will also need a flat surface on which to lay the disc flat. This could be a smooth concrete garage floor or a smooth plank of wood. To control perpendicularity and convenience of measurements, they often use straight metal profile or a long construction ruler, which is placed across the disk. Without a level base, it will be impossible to obtain reliable data on the depth of the mating plane.
Additionally, prepare a marker or chalk to mark measurement points, and a calculator to calculate the formula. If the disc has decorative covers or caps, they must be removed. The measurement is carried out on metal, since plastic may have a different geometry and distort real data on the design of the disk.
Step-by-step instructions: how to measure the offset with your own hands
The measurement process consists of several sequential actions, violation of which will lead to an error. First, place the disc face down on a completely flat surface. Make sure that the edges of the disc (rim) are flush with the floor all the way around. If the disc wobbles, place something under the edges, but not under the center, to level it.
Then take a straight strip or metal profile and lay it across the disk, strictly through the center. Now you need to measure the distance from this rack to the mating plane (the place where the disc is bolted to the hub). We denote this distance as A. Be careful: you need to measure exactly to the plane of contact, and not to the heads of the bolts or the recesses between them.
After this, flip the rim over or simply measure the overall width of the rim. We are interested in the distance from the same mounting plane to the inner edge of the rim (which is now facing upwards). We denote this distance as B. In effect, we are measuring two components of the rim width relative to the mounting point. The sum of these two values will give us the total width of the disk at the measurement location.
☑️ Algorithm for measuring departure
Now let's apply the formula. Departure (ET) is calculated using the formula: ET = (A + B) / 2 - A. Where (A + B) / 2 is half the total width of the disk (center of symmetry), and A — distance from the edge to the mating plane. If the result is positive, the departure is positive, if negative, the departure is accordingly negative.
The accuracy of the measurement is critical: an error of 2-3 mm can lead to the wheel touching the shock absorber or spar when turning the steering wheel.
Conformity table and permissible deviations
After receiving the digital value, you need to compare it with the factory parameters of your car. Car manufacturers allow small deviations, but they are strictly regulated. Below is a table with approximate data for different classes of cars, but always check the manual of your specific car.
| Vehicle type | Standard Reach (ET), mm | Permissible deviation, mm | Consequences of strong deviation |
|---|---|---|---|
| Subcompact hatchback | 35 - 45 | ± 5 | Impact of the shock absorber strut |
| D-class sedan | 40 - 50 | ± 5 | Accelerated bearing wear |
| SUV | 15 - 30 | ± 10 | Wheels sticking out of the arches, dirt on the body |
| Sports coupe | 45 - 55 | ± 3 | Violation of suspension kinematics |
It is important to note that the permissible deviation depends on the suspension design. On multi-link suspensions, the requirements are stricter than on MacPherson struts. If your estimated overhang falls within the acceptable range, the installation can be considered safe. Otherwise, spacers or adapter rings will be required, the use of which also has its own limitations and risks.
The influence of offset on handling and suspension life
Changing the disc offset is an intervention in the suspension kinematics designed by the engineers. As the overhang decreases (the wheel moves outward), the running shoulder increases. This results in the steering becoming more sensitive, but at the same time the load on the steering knuckles and steering ends increases. The vehicle may become more prone to yaw at high speeds.
On the other hand, increasing the offset (the wheel goes deeper into the arch) can cause the inside of the tire or wheel to contact the suspension components, brake calipers or shock absorbers. This is especially true when the steering wheel is fully turned or when the car is loaded with passengers, when the suspension is compressed. The friction of a tire against a suspension part can instantly destroy the sidewall of the tire, which can lead to an accident at speed.
⚠️ Attention: Installing wheels with an offset that differs from the standard one by more than 5-7 mm may lead to a refusal of warranty service for the chassis from an authorized dealer. In addition, this is a change in the design of the vehicle, which theoretically could raise questions when passing a technical inspection.
It is also worth mentioning the load on the wheel bearing. As the wheel moves outward, the leverage of the vehicle's weight on the bearing increases. If a standard bearing is designed for a certain load vector, its displacement can reduce the life of the unit by two or more times. Therefore, “unwelding” with a huge negative offset requires a reinforced chassis.
Can spacers be used to change the offset?
Yes, spacers are a common way to change the offset of a wheel. However, they must be made from high-strength aircraft-grade aluminum or steel. The use of cheap plastic or silumin spacers is strictly prohibited, as they can burst under load, which will lead to the wheel coming off.
Common mistakes when making your own selection and measurements
One of the most common mistakes is measuring the offset along the outer edge of the disc. Many people try to measure the distance from the edge of the disk to the hub with a ruler, forgetting that the outer edge can be rolled or have a decorative bend. The measurement must be taken strictly along the inner plane of the rim, which is in contact with the tire, or use the method of halving the total width.
Another mistake is ignoring the shape of the spokes. On cast and forged wheels, the spokes may extend from the center at an angle. If you place a ruler on the ends of the spokes, it will not pass through the geometric center of the disk, and the measurement will be incorrect. In such cases, it is better to use a long caliper or special gauges, taking measurements through the central hole, if the design allows.
⚠️ Warning: Never rely on visual judgment or “eyeballing”. The difference between ET35 and ET45 is visually almost invisible on a lying disk, but technically this is a different geometry that may not suit your car.
Tire width is also something that is often forgotten to consider. Even if the disc offset is ideal, tires that are too wide may begin to touch the arch or suspension elements. Therefore, when selecting wheels, always consider the “wheel + tire” combination. Sometimes, to install the desired wheels, you have to change the tire size to a narrower or lower profile.
When purchasing used wheels, be sure to check them for runout and cracks. A crack in the spokes or center of the disc makes it dangerous to use, regardless of the correct offset.
Final Safety Recommendations
To summarize, we can say that measuring the disc offset is a simple procedure, but it requires care and precision. A correctly selected ET parameter ensures not only a beautiful appearance of the car, but also predictable behavior on the road, as well as a long service life of expensive suspension components. Do not neglect checking the parameters, even if the seller assures that the disk will “definitely fit.”
Always test-fit a wheel before making a final purchase or installing all four rims. Spin the wheel on a suspended car, ask an assistant to turn the steering wheel all the way in both directions and rock the suspension. Only after making sure that there are no contacts or touches can you hit the road. Take care of your car and do not experiment with the geometry of the chassis unless necessary.
What happens if you install a disc with a smaller offset (wheel outward)?
The wheel will move closer to the edge of the arch. This may improve cornering stability by widening the track, but will increase the load on the hub bearings and steering ends. It is also possible for the wheel to hit the arches when the car is fully loaded or on an uneven road.
Is it possible to measure the offset without removing the wheel from the car?
It is impossible to accurately measure the offset on an installed wheel, since the mating plane and the inner part of the disk are closed. You can only visually assess how much the wheel protrudes relative to the arches by comparing it with a standard stock wheel, but this will only give an approximate idea, and not exact ET numbers.
Does wheel width affect offset calculations?
Wheel width itself does not change the offset value (ET), which is a fixed geometric characteristic. However, if you change the width of the rim (for example, from J7 to J8) while maintaining the same offset, the wheel will become wider both towards the street and towards the suspension. Therefore, when selecting wide disks, you need to double-check the gaps with the pillars and arches.
Why do the disks say ET, and not just crash?
ET is an abbreviation for the German word Einpresstiefe, which literally translates as “indentation depth.” Since many leading wheel and car manufacturers are based in Germany, this marking has become an international standard. In English-language literature you can also find the designation Offset.