When choosing new wheels, motorists often come across the mysterious acronym ET, which sellers call "departure." Many beginners believe that this parameter affects only the appearance of the car, determining how much the wheel will “stick” out of the arch or, conversely, be recessed inward. However, disk-out This is a critically important geometric parameter, on which the suspension life, handling and even safety of movement directly depends.
Incorrectly selected value can lead to accelerated wear of the hub bearings, destruction of suspension elements and even to touching the brake calipers or body elements. Understanding the physics of the process will help you avoid costly mistakes when buying and tuning.
In this article, we will discuss in detail how this size is calculated, why manufacturers set strict tolerances and in which cases deviation from the factory specifications of your company is permissible. Volkswagen, BMW or Toyota.
Physical essence of the departure parameter
Geometrically disk-out (Denoted ET, Einpress-Tiefe) is the distance between the vertical plane of wheel symmetry and the plane of application of the disc to the hub. In simple terms, it is how far the wheel is shifted relative to the center of its width. If the plane of attachment is exactly in the middle of the width of the rim, then the departure is zero.
It is important to understand that this parameter is strictly tied to the design of a particular car model. Engineers calculate the kinematics of the suspension so that the axis of rotation of the wheel passes through the center of the spot of contact with the road. The displacement of this axis violates the shoulder of the run-in, which changes the behavior of the car on the road. For different brands, whether it's Mercedes or LadaThese values can be radically different.
There are three main types of departures that you can find in the catalogs:
- 📏 Positive departure - the mounting plane is displaced outward from the central axis of the disk (the most common option for modern passenger cars).
- 📏 Zero departure The mounting plane coincides with the central axis of the disc.
- 📏 Negative departure - the mounting plane is shifted inward, to the brake mechanisms (often found on SUVs and retro cars).
⚠️ Attention: Changing the departure even by 5-7 mm changes the load on the suspension. Installing a disc with an ET that is very different from the factory one can cause the wheel to start touching the arch when the suspension is compressed or damage the brake caliper.
When manufacturing a disc, this parameter is laid constructively and cannot be changed at home without violating the integrity of the product. That's why. disk-parameter You need to select strictly according to the catalogs or technical passport of the vehicle.
Calculation formula and decoding of marking
On the inside of each certified disc, you can find markings, for example: 7.5J x 16 H2 ET35. In this line, the "ET" is exactly what the desired value is in millimeters. For those who want to understand how engineers arrive at these numbers, there is a simple calculation formula.
The departure is calculated as half the width of the disk (in mm) minus the distance from the landing plane to the inner edge of the rim. The formula looks like this: ET = 0.5 * B - Awhere B is the total width of the disc and A is the distance from the attachment plane to the inner edge.
How to find the width of the disk without tools?
If the marking is erased, the width of the disc (B) can be found by measuring the distance between the inner edges of the rim and multiplying the resulting value by 1.06 (the transition coefficient from inches to millimeters, taking into account the thickness of the walls). However, for accuracy, it is better to use a barbecue.
Let's take an example. Let’s say you have a disk that is 7 inches wide. Translated into millimeters: 7 * 25.4 = 177.8 mm. Half the width will be 88.9 mm. If the distance from the landing plane to the inner edge (A) was 60 mm, the departure will be: 88.9 - 60 = 28.9 mm. Round it to a whole number, we get it. ET29.
Knowing this logic, it becomes clear why you can not just “squander” the disk to change the flight. Any mechanical action on the landing plane or the inside of the rim changes the balancing and strength of the structure. For premium vehicles, such as Audi or BMWThe requirements for the accuracy of these parameters are particularly high due to the complex multi-link suspensions.
Disk departure (ET) is a constructive constant laid down in production. You can change it only by replacing the disk with a model with a different parameter, but not by mechanical processing of the current one.
The impact of departure on suspension and handling
Many motorists tend to put wheels with a smaller departure, so that the wheel rises “to level” with the arch and the car looks more aggressive. However, this decision has a downside. Reducing the ET value (shifting the wheel outwards) increases the shoulder of the run-in. This leads to the fact that the steering becomes more sensitive, but at the same time increases the load on the steering tips and levers.
When moving on irregularities, the impact load is transmitted not only to shock absorbers, but also to the elements of attachment of the suspension to the body. In the long term, this can cause backlashes, creaks and accelerated wear of Silentblocks. This is especially important for heavy duty vehicles such as Toyota Land Cruiser or Land Rover.
On the other hand, increased departure (drowning the wheel deeper into the arch) also carries risks. The wheel may start to touch the brake system elements or suspension details when turning the steering wheel. In addition, the nature of handling changes: the car can become more “sluggish” in corners, and the load on the hub bearing will be redistributed unevenly.
Let’s look at the main consequences of improper selection:
- ⚙️ Wear of bearings - the shift of the center of gravity of the wheel creates an additional moment of force, destroying the bearing of the hub.
- ⚙️ Problems with ABS and ESP Changes in the shoulder of the run-in may incorrectly affect the operation of electronic stabilization systems.
- ⚙️ Grinding wheels - with full loading of the car or passing steep turns, the tire can rub against the underwings or body elements.
⚠️ Attention: If after installing new wheels with a modified departure you notice the car sideways or the steering wheel beating at high speeds, immediately contact the service to check the geometry of the suspension.
Modern comfort and safety systems are sharpened to factory parameters. Ignoring this fact turns tuning into a lottery with a car resource.
Permissible deviations and compatibility tables
There is a popular opinion that the drive departure can be changed within ± 5 mm without consequences for the car. Indeed, for many budget cars with a simple suspension design (for example, the classic one). VAZ old-model Ford) the deviation may pass unnoticed. However, for modern machines with complex geometry, this rule does not always work.
Disc manufacturers often specify the range of permissible values, but it is better to focus on the recommendations of the car manufacturer. If the normal departure is ET45, then the installation of the ET40 disk already creates a noticeable lever of pressure on the suspension. For heavy crossovers and sports sedans, the tolerance is usually even lower.
Below is a table of approximate tolerances for different classes of cars (provided that the disc does not touch the body and calipers):
| Class of car | Staff departure (example) | Permissible deviation | Risks in excess |
|---|---|---|---|
| Small class (B, C) | ET 35–45 | ± 5 mm | Wearing of bearings, grazing of arches |
| Middle class (D, E) | ET 45–55 | ± 3-4 mm | ABS malfunction, backlash steering |
| Off-road vehicles (SUV) | ET 10–30 | ± 5-7 mm | Lever failure, hub failure. |
| Sports cars | ET 30–50 | 0 mm (strictly) | Critical reduction of controllability |
When selecting disks from other ranges (for example, installing disks from Mercedes on Volkswagen) there are often problems not only with the departure, but also with the diameter of the central hole (DIA) and the drill. Using the sled rings to change the departure is possible, but it adds extra points of vibration and potential danger.
Always check the physical fitting. Even if the numbers all agree, the shape of the inner part of the disk may differ, and it will rest in the brake caliper or shock absorber.
When buying used disks, be sure to measure the actual departure with a rod. Sometimes the discs are broken or recovered, and their actual geometry may differ from the claimed marking.
Sparks for changing departures: pros and cons
If you still decided to change the appearance of the car, making the track wider, the most common method is wheeled spacers. These are metal plates that are mounted between the hub and the disc, effectively reducing the value of the departure (shifting the wheel outward).
The stands are of different thicknesses: from 3 mm to 50 mm or more. Thin stretchers (up to 10-12 mm) are usually put without changing the fastener, using standard bolts. The thicker versions require replacement of the regular bolts with longer ones or have their own studs, which turns the wheel into a separate knot.
Arguments for the use of pads:
- 🚗 Aesthetics The car looks wider and more stable, the wheels stand level with the arches.
- 🚗 Installation of chains - allows the use of snow chains on cars with minimal clearance.
- 🚗 Correction Helps to correct the situation if the purchased disks were too large a flight.
However, there are serious disadvantages. The exhibition is an additional element that takes on huge loads. Cheap silumin sheds can burst at speed. In addition, the load on the hub bearing increases, as we discussed earlier. For daily use on bad roads, this method is not recommended.
⚠️ Attention: Never use a stretcher thicker than 20 mm without digesting the hub or using special elongated studs. The standard length of the bolt thread may not be sufficient for reliable mounting of the wheel through the slot.
If you use the spacers, (check regularly) tighten the bolts after the first 100 km run, as the metal may "sit down."
Checklist of the correct selection of discs
To avoid installation and operation problems, a thorough check of all parameters is necessary before buying new wheels. Don’t rely on the words of the seller or the “eye gauge.” Use the following algorithm of actions.
☑️ Checking before buying discs
First, find information about factory parameters. It can be glued on the gas tank hatch, on the body rack or in the service book. If it's written ET48Try to find a disk with that value. We have already discussed the permissible deviations, but they should be minimal.
Pay attention to the design of the internal spokes of the disk. Even with the correct departure and diameter, the spokes can rest on the caliper of the brake. This is a common problem when switching from stamped discs to cast ones or when installing larger brakes (tuning brakes). In such cases, mandatory fitting is required.
It is also important to consider seasonality. In winter, when the roads are mud and snow porridge, wheels with a reduced departure (bulging out) will stain the doorsteps of the car much faster. Summer drift style in winter can turn into a daily washing of the body.
Can I put discs with different departures in front and behind?
On front-wheel drive cars with the same wheel width in the front and rear - categorically not, this will violate handling. On rear-wheel drive sports cars (for example, BMW, Porsche) often used a scheme with wider wheels at the rear, where the departure may differ from the front, but this is provided by the design.
Frequently Asked Questions (FAQ)
Can I put an ET40 disk instead of a regular ET45?
A difference of 5 mm is acceptable for most cars. The wheel will be pushed outwards by 5 mm, which is visually almost imperceptible and should not critically affect the life of the suspension, unless the car is used in extreme conditions of racing or heavy offroads.
What happens if you put a negative drive on a positive car?
The wheel will move outwards considerably. Most likely, it will touch the arch at the slightest load or turn of the steering wheel. The load on the hub bearings and steering elements will increase dramatically, which can lead to their rapid destruction and emergency.
Does the disk flight affect the speedometer readings?
No, the departure parameter (ET) does not affect the circumference of the wheel, so the speedometer readings remain the same. Speed is affected only by the change in the diameter of the wheel or the profile of the tire. However, changing track widths can indirectly affect track stability.
How to find out the departure of the disk if the marking is erased?
The exact flight without marking is impossible to know, as it depends on the internal geometry of the disk. You can only approximately measure the width and distance to the plane of attachment, but the error will be large. It is better to use catalogs or try on a disk on the car, knowing its standard parameters.
Are the squats dangerous for daily driving?
High-quality forged fork-posts up to 10-12 mm thick from well-known brands are generally safe for urban use. However, any stretches increase the load on the nodes. For aggressive driving or bad roads, it is better to choose discs with a correct departure without the use of additional elements.