When purchasing new rims, many car enthusiasts pay attention to the diameter and number of bolt holes, but often overlook a parameter that is critical for safety. We are talking about departure, or ET. This is the distance between the vertical plane of symmetry of the wheel and the plane of application of the disk to the hub. Incorrectly selected disc offset can lead to accelerated wear of the wheel bearings, friction of the tire against body elements, and even destruction of the suspension at high speeds.
Ignoring factory specifications when selecting wheels is a common mistake that can be costly. Many drivers believe that if the bolts match, then the disc will fit like a glove. However, the geometry of the car's suspension is calculated by engineers with millimeter accuracy, and a wheel displacement of even 5 mm changes the vector of loads on the transmission components. In this article we will analyze in detail the physics of the process, consider permissible deviations and answer frequently asked questions.
⚠️ Attention: Installing discs with an offset that differs from the factory offset by more than 5 mm can lead to loss of warranty on the chassis and the occurrence of emergency situations when maneuvering.
Physical essence of the ET parameter
The term "offset" is often confused with other wheel geometry parameters such as bolt pattern or center hole diameter. Actually ET parameter (Einpresstiefe) is a strictly defined value, measured in millimeters. It shows how much the disk's seating plane is offset relative to its central axis. If the mounting plane is strictly in the center of the width of the disk, the offset is considered zero.
There are three main types of disks depending on the ET value. Positive offset means that the mounting plane is offset outward from the central axis of the disk. This is the most common option for modern passenger cars, where the wheel is “recessed” into the arch. Negative offset is typical for SUVs and tuned cars: here the mounting plane is shifted inward, and the disk and tire itself protrude noticeably beyond the dimensions of the body, creating the effect of a wide track.
Understanding this physics is necessary for proper tuning. When you change rim width, the ET value often has to be recalculated to maintain the correct wheel position relative to the hub. If you simply take a wider disc with the same ET, its inner part may rest against the suspension elements or brake caliper, and the outer part may begin to rub against the arch.
Impact of departure on dynamics and safety
Changing the offset directly affects the running-in arm and the load on the suspension components. As the positive offset increases (the wheel sits deeper in the arch), the load on the hub bearing decreases, but the risk of internal disc elements touching the brake calipers or suspension arms increases. This is especially true when installing oversized brake systems.
On the other hand, reducing the overhang (the wheel extending outwards) widens the vehicle's track, which theoretically improves cornering stability. However, this creates excess load on the wheel bearing. Wheel bearing begins to experience moment loads for which it was not designed, which leads to its premature failure. In addition, a protruding wheel collects more dirt on the sides of the car.
Don't forget about the work of shock absorbers. Shifting the point of application of forces changes the kinematics of the suspension. In some cases this can result in a softer ride, but more often than not the balance is thrown off and the car becomes more prone to roll or yaw at high speeds. Correct wheel offset ensures predictable behavior of the machine in emergency situations.
Permissible deviations and correspondence table
Car manufacturers indicate in technical documentation the exact ET value for each model. However, in reality, finding discs with a perfect match is not always possible, especially when purchasing non-original replicas. There is a concept of permissible run-up. For passenger cars with single-wheel drive, a deviation within ±5 mm from the standard value is considered safe.
For all-wheel drive vehicles and heavy SUVs, the requirements are stricter. Here the load on the transmission is higher, and even a small change in the shoulder can affect the performance of the differentials. In such cases, it is better to stick to the factory settings or consult a tuning engineer. Below is a table illustrating the dependence of the disc position on its width and offset.
| Disc width (J) | Staff E.T. | ET + 5 mm | ET - 5 mm | Consequence of change |
|---|---|---|---|---|
| 7.0 J | 45 | 50 (deeper) | 40 (outside) | Minimal risk, within tolerance |
| 8.0 J | 35 | 40 (deeper) | 30 (outside) | Possible interference with the arch when loading |
| 9.0 J | 25 | 30 (deeper) | 20 (outside) | High risk for wheel bearing |
| 10.0 J | 15 | 20 (deeper) | 10 (outside) | Critical change in load arm |
It is important to understand that when changing the width of the rim, the ET value must be adjusted. If you are moving from a 7J ET45 rim to an 8J rim, then to maintain the same position of the outer edge of the tire relative to the body, the new offset should be around ET50-52. This compensates for the increase in disk flange width.
Problems with incorrect selection of disks
Ignoring the departure parameters can lead to a number of technical problems that will not appear immediately, but after several thousand kilometers. Usually the first one to give up wheel bearing. Due to the shift in the center of gravity of the wheel, the load is distributed unevenly, causing play and hum, which is initially only heard at certain speeds.
The second common problem is friction between the tire and body parts. If the offset is negative or the rim is too wide, the inside of the tread may hit the side members or suspension components when the shock absorber is compressed. This results in immediate tire failure and potential loss of control. It is also possible for the brake calipers to touch the inside of the disc.
The third problem concerns security systems. In modern cars, ABS and ESP sensors are calibrated for a specific rolling diameter and wheel position. A strong change in the offset can introduce errors in the sensor readings, which will lead to incorrect operation of the stabilization systems at a critical moment.
⚠️ Attention: If, after installing new discs, you feel the steering wheel beating at speed or an extraneous hum, immediately check the tightness of the bolts and the compliance of the offset parameters.
How to calculate and check departure yourself
If the marking on the inside of the disk is erased or you doubt its accuracy, you can calculate the offset yourself. To do this you will need a ruler, a straight edge and a caliper. First, measure the overall width of the rim (not the tire!) from edge to edge. Divide the resulting value in half to find the geometric center.
Then place the rack across the disk through the center and measure the distance from the mating plane (point of contact with the hub) to this rack. This distance is the departure. If the mounting plane is closer to the front side, the offset is positive. The calculation formula is simple: ET = a - b/2, where a - distance from the mating plane to the inner edge, b — total width of the disk.
For measurement accuracy, it is recommended to take measurements at several points, since alloy wheels may have geometry defects. It is also important to measure the metal part of the rim, without taking into account the decorative trims or the width of the tire, which is always wider than the rim.
Departure specifics for different types of cars
Reach requirements vary greatly depending on the class of vehicle. Small city cars are characterized by wheels with a large positive offset (ET40-ET50), which makes it possible to compactly place the wheels in narrow arches and reduce the load on weak suspension components. Here deviations are extremely undesirable.
In the segment of SUVs and crossovers the situation is different. To improve cross-country ability and stability, manufacturers often allow a smaller offset or even negative values. Installing spacers under wheels is a popular but controversial tuning method. A spacer technically reduces offset by moving the wheel outward. This is acceptable for off-road driving, but dangerous for highway driving at high speeds.
Sports cars require an individual approach. Here the offset is selected in such a way that the wheel, with the maximum possible width, does not touch the arch when the suspension is operating, but also does not go deep inside, interfering with the operation of the brakes. Often in motorsport, wheels with variable flange geometry are used, where the offset can be adjusted.
Is it possible to install wheels with a different offset if I use spacers?
The use of spacers allows you to adjust the offset, but this introduces additional load points. The spacers must be made of a high-strength alloy (for example, aircraft aluminum) and have perfect balance. The fastening studs must be elongated and correspond to the strength class of the original ones. It is considered safe to use spacers up to 20 mm thick, provided that the tightening is regularly checked.
Does offset affect fuel consumption?
Yes, it does have an indirect effect. Reducing the overhang (the wheel coming out) increases the aerodynamic drag of the car, since the wheel protrudes from the streamlined contour of the body. In addition, changing the load arm may require more effort to turn the wheels, which is minimal, but increases fuel consumption, especially in the urban cycle.
What to do if purchased discs rub against the caliper?
If the disc hits the inside of the caliper, the overhang is too large (the disc is too deep). In this case, you can use a spacer to move the disc away from the hub. If the disc rubs against the caliper from the outside (rarely, but happens on very wide discs), then it is impossible to correct the situation without replacing the disc.
Is it necessary to do a wheel alignment after changing discs with a different offset?
Yes, this is a mandatory procedure. Changing the offset changes the run-in arm and the position of the wheels relative to the road surface. Even if the wheels are visually level, the installation angles may have changed. Adjusting the wheel alignment will help avoid uneven tire wear and return the car to directional stability.
Is negative offset dangerous for a front-wheel drive car?
For a front-wheel drive car, a negative offset is extremely undesirable. In such machines, torque is transmitted to the front wheels, and a change in the shoulder of the force application creates parasitic moments that destroy CV joints and wheel bearings. For front-wheel drive, it's best to aim for factory positive ET values.