When choosing new wheels, most motorists first pay attention to the diameter, number of bolts and design. However, there is a parameter that is often ignored, relying on the “maybe” or assurances of the seller – it is the drive’s departure. This characteristic determines how deep the wheel will sit in the arch or, conversely, will stick outwards. The wrong choice can lead not only to friction of the tire against the body, but also to critical breakdowns of the chassis.
Disc departure denoted by marking ET (Einpress Tief) or OFFSET, is the distance between the vertical plane of symmetry of the wheel and the plane of adjoining the disk to the hub of the car. This parameter is measured in millimeters and can be positive, negative or zero. Understanding the physics of the process is essential for anyone who plans to change the standard size of the wheels on their car, whether it is a car or not. Volkswagen, BMW or Lada.
In this article, we will analyze in detail the geometry of the wheel, consider the consequences of installing discs with an inappropriate departure and provide a method for independently calculating the permissible values. Ignoring this data often leads to financial losses and reduced road safety.
Geometry and Physics: What's Behind the ET Marking
To understand the parameter ETIt is necessary to visualize the structure of the disk. Imagine a disc lying flat. If we cut it in half vertically, the cut line will pass through the symmetry axis. The distance from this central axis to the plane by which the disc is screwed to the hub is the departure. If the mounting plane is closer to the outer side of the disk (front), the departure is considered positive. If the hub is “sunk” deeper than the central axis, the departure will be negative.
In the technical documentation and on the labeling of the disk itself you can find the designations ET35, ET45 or OFFSET -10. The number indicates the distance in millimeters. It is important to understand that flight-back This is the standard for most modern cars. It allows the wheel to be closer to the center of the car, which improves stability. Negative departure is more common on SUVs or sports cars with a wide track, where you need to expand the base for better terrain or aerodynamics.
⚠️ Attention: The ET marking has nothing to do with the width of the disk (parameter). J or JJ). Two discs of the same width can have a completely different departure, which will radically change their seat on the car.
The physical meaning of this parameter lies in the distribution of loads. The wheel is not just a rotating element, it is the arm of the lever, transferring the forces from the engine and brakes to the body. Changing the departure changes the length of this lever, which directly affects the operation of the hub bearings and suspension levers.
The impact of departure on the chassis and controllability
Many drivers believe that if the disc is physically stuck on the hub and the bolts are twisted, then everything will work normally. That's a dangerous misconception. Engineers of automakers calculate the geometry of the suspension taking into account the specific shoulder of the application of forces. Changing the departure even by 5-10 mm can significantly change the nature of the behavior of the car on the road.
When installing discs with a smaller departure (the wheel moves outward), the load on the suspension elements increases. Stack bearing It begins to experience increased radial pressure, which reduces its resource. Levers and Silentblocks also wear out faster as the force vector shifts away from the optimal point. In addition, the kinematics of the suspension changes, which can lead to the car being pulled aside during braking or uneven wear of the tires.
On the other hand, the increase in departure (the wheel goes deeper into the arch) also carries risks. The tire can begin to rub against the internal elements of the suspension, shock absorber or brake caliper. This is especially true when installing wide tires or using spacers. In cornering, when the suspension is compressed, the tire’s contact with the internal parts can cause instant rubber damage or even wheel jamming.
Here are the main consequences of ignoring the parameter ET:
- 🚗 Accelerated wear of hub bearings and steering components.
- 🛑 Deterioration of exchange rate stability and increase in the braking distance.
- 🔧 Possibility of damage to arches, splashers or elements of the brake system.
- 📉 Change in the shoulder run-in, which affects steering effort and steering wheel self-return.
Optimal drive departure provides maximum suspension life and predictable behavior of the car in emergency situations.
How to calculate the permissible departure: formulas and methods
If you want to install discs that are different from the regular ones, or just check the current wheels, you need to know how the departure is calculated. You don’t have to be a mechanical engineer, you just have to know the basic formula. Departure is defined as half the width of the disk minus the distance from the landing plane to the inner edge of the disk.
The formula is as follows:
ET = (a - b) / 2Where:
- a - the width of the disc (measured between the inner edges of the shelves, not including the sides).
- b - the distance from the landing plane (the place of contact with the hub) to the inner edge of the disk.
In practice, measuring these parameters with a ruler is quite difficult due to the complex geometry of spokes and sides. Therefore, it is easier to use the reverse method or find a marking on the inside of the disk. It's usually stamped in form. ET45 or OFFSET 35. If you are planning to install pads, the final departure is calculated as ET disc - thickness surplus. For example, a disk with ET40 And the 10 mm space will give the final flight. ET30.
⚠️ Attention: When using the paddles, be sure to check the length of the regular bolts or nuts. The staff fastener may be too short, which will lead to the failure of the thread and the tearing of the wheel on the go.
For an accurate calculation of the permissible departure range for your car, it is best to use online calculators or consult the catalogs of disk manufacturers, such as: K&K, SKAD or Replica. They often indicate not only the standard values, but also the permissible deviations.
Departure compliance table for popular car classes
Although each car is unique, there are averaged standards for different classes of vehicles. Understanding these ranges will help to quickly weed out inappropriate options when searching for used drives or choosing new sets. Below is a table with indicative departure values.
| Class of car | Typical ET range (mm) | Features | Examples of models |
|---|---|---|---|
| Small class (B, C) | 35 – 48 | High positive departure for compactness | Ford Focus, VW Polo, Hyundai Solaris |
| Middle class (D, E) | 35 – 50 | Standard departure for comfort and stability | Toyota Camry, BMW 3-series, Mazda 6 |
| Off-road vehicles (SUV) | 0 – 35 | Often smaller departure to widen the rut | Toyota Land Cruiser, UAZ Patriot, Jeep Wrangler |
| Sports cars | 20 – 45 | Depends on the width of the disk, often negative ET | Porsche 911, Audi RS |
It is important to note that even within the same model, values can vary depending on the year of release and configuration. For example, on BMW The different flight series may vary from ET20 before ET50. Always check the technical documentation of a particular vehicle.
Why is there such a small departure on a BMW?
BMW cars traditionally have a small drive departure (often ET20-ET35), which is due to the design features of the front suspension McPherson and rear multi-link. This allows you to compensate for heavy loads and ensure the necessary kinematics of the levers. Installation of disks with a large departure (for example, ET45) on such cars is strictly not recommended, as this can lead to breakage of the hub.
Permissible deviations: can I put a disk with another ET?
The issue of permissible departure deviations is one of the most discussed in car forums. It is widely believed that a deviation of ±5 mm is safe. Indeed, for many vehicles with a safety margin, this value is a critical threshold beyond which problems begin. However, this figure cannot be relied on blindly.
Safety depends on a combination of factors: suspension condition, driving style and road quality. If you operate the car in gentle mode on good roads, a deviation of 5 mm may go unnoticed. But if you enjoy active riding or get into pits often, even a minimal change in the lever shoulder can be the "last straw" for a worn bearing.
Recommendations on tolerances:
- 🟢 ± 5 mmIt is considered conditionally safe for most cars, but requires monitoring of the state of the running gear.
- 🟡 ± 10 mm: It is permissible only when installing pads with mandatory check of absence of contact of the tire with the body at full load.
- 🔴 More than ± 10 mmRequires serious alteration of the suspension (lift, pruning of springs, replacement of levers) and is not recommended for everyday use.
When buying used discs, be sure to measure their departure with a rod. Marking on old discs can be erased with rust or paint, and the real parameters may differ from those claimed by the seller.
Practical tips for installation and inspection
The process of installing disks with modified departure requires careful preparation. You can't just screw up the wheels and leave. A number of checks must be carried out to ensure the safety of operation. This is especially true when you change the width of the disk or the diameter.
The first step should always be to try it on. Install one wheel (usually the front one, as there is less free space due to the swivel fist) and rock the suspension. Check the gaps between the tire and shock absorber, spring, and body elements (wings, splashers). The steering wheel should be turned to the point in both directions.
Checklist before installing non-standard disks:
- 🔍 Check the gap between the tyre and shock absorber (minimum 3-5 mm).
- 🔧 Make sure that the inner shelf of the disc does not touch the brake caliper.
- 🚗 Check the absence of contact with the arch when the car is fully loaded.
- 📏 Make sure that the bolts/nuts fly out to the depth of the disc holes.
☑️ Checking wheel geometry
It is also worth considering that the change in departure may require adjustment of the angles of installation of wheels (fall-down). After installing any new discs, visiting the collapse-descend stand is a mandatory procedure, even if the visual wheels are flat.
⚠️ Attention: The use of disks with a central hole (DIA / DC) more than necessary is permissible only with the use of center rings. The absence of a ring will cause the wheel to beat at high speeds, regardless of the correct departure.
Frequently Asked Questions (FAQ)
What happens if you put a disk with a departure more than a regular one (for example, ET40 instead of ET35)?
The disc will go deeper into the arch of the car. This can lead to the fact that the tire will start to touch the internal elements of the suspension (shock absorber, levers) or brake caliper. Also, the track of the car will decrease, which can negatively affect the stability in cornering.
Can I correct the disk departure by welding or forging?
Theoretically, it is possible to change the flight, but in practice it is highly recommended. Any interference with the geometry of a cast or forged disc violates its integrity and the distribution of internal stresses. Such a disk can burst when it enters a hole. It is safer to use the spacers or buy discs with the desired parameter.
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 tyres of a different profile or diameter, then the speedometer readings will change.
Why do some cars have different drives in front and behind?
This is typical for cars with different track widths in the front and rear or for models where the factory has multi-wide wheels (for example, some sports wheels). Porsche or BMW M-series). In such cases, the rear discs have a smaller outlet (wider outwards) to improve grip and stabilization.