Every car owner planning to replace the regular wheels with cast or forged wheels inevitably encounters the mysterious letters ET on the inner surface of the metal. This parameter is known as disk-outIt is one of the key factors determining not only the appearance of the car, but also the safety of traffic. Incorrect selection of this value can lead to the fact that the wheel simply does not stand in place or will start to touch the suspension elements when turning.
Many motorists mistakenly believe that the main thing is the diameter and number of holes for bolts, ignoring the geometry of the departure. However, it is the distance from the plane of the disk attachment to its vertical axis of symmetry that dictates how the car will behave on the road. Design engineers calculate this parameter with high accuracy, taking into account the kinematics of the suspension and the load on the hub bearings.
Understanding how this setting is defined and what it affects will help you avoid costly mistakes when buying new shoes for your vehicle. In this article we will discuss the physical meaning of the departure, methods of its calculation and the consequences of deviations from factory norms.
Physical sense of the ET parameter and wheel geometry
Disk departure, often indicated by marking ET (from German Einpresstiefe), is the distance in millimeters between the vertical plane of wheel symmetry and the plane of application of the disc to the hub. It is important to understand that the symmetry plane is an imaginary line dividing the disk in half across the width, not the rim edge. It is this geometric feature that determines how deep the wheel is "sunk" into the arch or, conversely, stands for the dimensions of the body.
⚠️ Attention: Do not confuse the departure (ET) with the width of the disk (J). The width is measured in inches and indicates the distance between the edges of the rim, whereas the departure is a strictly linear size in millimeters, determining the position of the disc relative to the hub.
There are three main types of departures that you can find when labeling products: positive, negative and zero. With a positive departure, the mounting plane is shifted outward relative to the center of the disk, which is typical for most modern passenger cars. A negative departure means that the mounting plane is closer to the inside of the wheel, which is often found on SUVs or retro cars. A zero departure indicates that the attachment plane coincides with the central axis.
The accuracy of this parameter is critical, as even a small deviation changes the shoulder of the run-in - the distance between the point of contact of the tire with the road and the projection of the axis of rotation of the fist. Changes in the shoulder of the run-in directly affect steering effort, stability in cornering and wear of rubber. Therefore tire-maker Always indicate the permissible range of deviations.
Why are the Germans using ET?
The term Einpresstiefe literally translates as “deepness of indentation”. This reflects the essence of the process: how much the disk is "pressed" into the wheel arch when mounted on the hub. In English-language literature, the term Offset is sometimes used, which is a complete synonym.
Calculation method: formula and necessary measurements
If the labeling on the disk is erased or you want to double-check the data before buying, the departure can be determined by yourself using a simple mathematical formula. You will need a ruler or a bar, as well as a flat surface. The measurement process requires precision, as a few millimeters of error can distort the result.
First, you need to measure the total width of the disk (denote it as W). Then the disc is placed face down on a flat surface, and the distance from the landing plane (the place of contact with the hub) to the lower edge of the rim is measured (meaning as A). The formula for calculation is as follows: ET = A - (W / 2). The resulting value is the desired departure.
☑️ Checking the geometry of the disk before calculation
It is important to note that some manufacturers may use non-standard marking or rounding methods. For example, meaning ET 35 may actually be 34.5 mm or 35.2 mm. For most civilian vehicles, the tolerance is ±5 mm, but for sports models with a wide gauge, the requirements may be more stringent.
When self-measuring, place a wooden bar across the disc over the ruler to accurately determine the level of the landfill plane if it is recessed deep inside the spokes.
The impact of departure on the controllability and resource of nodes
Changing the normal departure of the disc is not just a matter of aesthetics, when the wheel is put “level” with the arch. Increased departure (disc sits deeper into the arch) or its reduction (disc protrudes outward) changes the distribution of loads on the suspension elements. With a decrease in the departure (the disc becomes "wider" relative to the stupa), the shoulder of the run-in decreases, which theoretically can improve stability, but the load on the hub bearings increases sharply.
Excessive protrusion of the wheel outside the wheel arch leads to accelerated wear of the hub bearings and steering elements. In addition, dirt from the wheels is more actively thrown on the doorsteps and doors of the car. On the other hand, too deep landing of the disc (big positive departure) can cause the inner part of the wheel or caliper will start to touch the suspension elements or the brake system when compressed.
- 🚗 Increased load on the hub bearing when the departure deviation is more than 10-15 mm.
- 🛑 Risk of hitting the caliper of the internal elements of the suspension when turning.
- 🌧️ Changes in aerodynamics and the nature of the mud splashing of the body.
- 📉 Deterioration of course stability with a sharp change in the track of the front wheels.
Especially critical is the observance of departure parameters for cars with independent suspension. Owners. SUV And crossovers should be extremely careful when installing slots or discs with non-standard geometry, as this affects the operation of all-wheel drive and wheel angles.
Safe is considered to be a deviation of departure from the standard value of not more than ± 5 mm for passenger cars and up to ± 10 mm for heavy SUVs, provided that the disc does not touch the brake mechanisms.
Table of conformity and tolerances
When selecting drives, it is important to focus not only on general recommendations, but also on specific tolerances for your car class. The following table shows the standard departure values for different categories of transport and the tolerance limits, which do not usually require major suspension modification.
| Class of car | Typical ET range (mm) | Permissible deviation (mm) | Risk of excess |
|---|---|---|---|
| Small class (A, B) | 35 – 45 | ± 3 | Grazing arches, wear of bearings |
| Middle class (C, D) | 30 – 40 | ± 5 | Instability on the track |
| Business class (E, F) | 25 – 35 | ± 5 | Discomfort on taxiing |
| Off-road vehicles (SUV) | 0 – 25 | ± 7 | Suspension damage |
It is worth noting that for some models of cars, especially German production (see the article).BMW, Mercedes, Audi), tolerances may be extremely stringent. Japanese and Korean manufacturers sometimes allow a wider range of parameters. Always check the technical documentation of the vehicle.
The use of pares for the correction of departure is a popular, but ambiguous method. It allows you to install disks with a large departure, reducing the actual distance to the hub. However, this adds an additional backlash point and increases the load on the fasteners.
Compatibility and legal issues
Installation of discs with parameters significantly different from factory ones can entail not only technical but also legal consequences. In many countries, including the Russian Federation, the introduction of changes in the design of the vehicle, which includes the change in the track of the car, requires official registration and certification. If the drive departure is changed so much that the wheel stands out from the body dimensions, this is guaranteed to lead to problems during the technical inspection.
⚠️ Attention: The legislation on road safety may change. Before installing extremely wide wheels or spacers, it is recommended to consult the current requirements of GOST or local regulations regarding vehicle modifications.
In addition, car manufacturers often withdraw the warranty on chassis components if it is proved that the cause of the failure was the installation of uncertified wheels. Dealership centers have the right to refuse warranty repair of the suspension, finding discs with an inappropriate departure.
Particular attention should be paid to stabilization systems (ESP) and anti-lock braking system (ABS). Changing the run-in shoulder and rolling diameter (when installing wider rubber on discs with a different flight) can make adjustments to the work of electronic assistants, causing them to work incorrectly or give errors.
Practical tips for selection and installation
When choosing new wheels, first of all, find the marking on the old, standard wheels. It is usually stamped on the inside of the spokes or rim. Look for the sign. ET35 or Offset 35. If this is not possible, use the online catalogs of disk manufacturers, where the parameters are specifically selected for your car model by VIN code.
If you plan to install wider discs to improve the appearance, remember the rule: the wider the disk, the less it should be its departure, so that the interior does not touch the suspension, but the outer part does not go beyond the arch. Often, wide drives require negative or near-zero relegation, which is typical of Jeeper culture.
- 🔍 Always try on the drive before your final purchase, especially if the settings are different from the standard ones.
- 🔧 Use the dynamometer key to tighten the bolts, observing the moment specified by the manufacturer.
- 📏 After installing the new wheels, be sure to check the angles of the wheel installation (fall-down).
Don't forget that forged-leaf Often have different geometric parameters compared to cast analogues of the same size. The design of the spokes and the shape of the rim can affect the actual position of the disc relative to the hub, so visual evaluation and fitting are more important than dry digits.
Can I use a car from another car brand?
Theoretically yes, if all the parameters are the same: drill (PCD), diameter of the central hole (DIA), departure (ET) and dimensions. However, finding the perfect match is difficult, and transition rings or hub ducts are often required, which is not always safe.
Frequently Asked Questions (FAQ)
What happens if you put a disk with a departure less than the standard one (for example, ET 20 instead of ET 35)?
The wheel will move outwards, increasing the track of the car. This can cause the wheel to hit the arches when fully loaded or turned, and also accelerate the wear of the hub bearings due to the increased load shoulder.
Does the disk flight affect the speedometer readings?
The departure (ET) itself does not affect the circumference of the wheel, so it does not affect the speedometer readings. However, if the change in departure is accompanied by the installation of a rubber profile, then the readings may change.
Can I correct the disk mechanically?
Theoretically, the flight can be changed by digesting the disk (displacement of the landing pad), but it is a complex and expensive process that violates the integrity of the structure. It is easier and safer to buy discs with the desired setting or use the spacers (with caution).
Why are some disks not marked with ET?
Lack of labeling may occur on old discs, replicas of poor quality or products manufactured for the domestic market of countries with other labeling standards. In such cases, the parameter can only be measured manually.