When choosing new wheels, motorists often encounter mysterious markings embossed on the inside of the rim and wonder: What is the ET40 on the drives? This acronym stands for a critically important geometric parameter known as disk outlet. The number 40 indicates the distance in millimeters between the vertical plane of symmetry of the wheel and the plane of application of the disk to the hub. Incorrect selection of this parameter can lead not only to the friction of the wheel against the body, but also to accelerated wear of the suspension elements.
Ignoring factory departure recommendations is a common mistake made by car owners who want to make the appearance of the car more aggressive or simply do not have enough information. Disc launch directly affects the width of the track of the car, changing the vector of loads acting on the bearings of the hub and suspension levers. If you plan to install ET40-labeled discs, but the manufacturer recommends ET35 or ET45, you need to carefully calculate the tolerances.
In this article, we will analyze in detail the physical meaning of the ET parameter, consider the consequences of installing discs with inappropriate departure and answer the most frequent questions. Understanding these nuances will help you avoid costly chassis repairs and ensure a stable vehicle behavior on the road. Without a deep analysis of the geometry of the wheel, tuning can turn into a headache.
Physical sense of the ET parameter and wheel geometry
The abbreviation ET comes from the German word "Einpresstiefe", which literally translates as "depth of indentation". This is technically more accurate description of the process than just "flight out" because the parameter characterizes how deep the disk "lands" on the hub relative to its central axis. Marking ET40 means that the landing plane (the place of contact with the hub) is displaced from the center of the width of the rim by 40 millimeters towards the street. If the flight was zero, the mounting plane would be exactly in the middle of the disk width.
It is important to understand that the departure does not depend on the width of the disk, although they are interrelated when calculating the final position of the wheel in the arch. Rim width (e.g., J7.0) determines how wide a tire can be worn, and ET determines how far the wheel will extend outside the wheel niche or, conversely, will go inward. For visualization, imagine a disk 200 mm wide: at ET40, the center of the disk will be 100 mm, and the mounting plane 140 mm from the inner edge. This displacement dictates how the wheel will stand relative to the shock absorber and splasher.
⚠️ Note: The ET parameter is the calculated value of design engineers. Changing this value changes the shoulder of the run-in and the kinematics of the suspension, which is not always obvious on visual inspection.
There are three main types of departures that can be found in catalogs: positive, zero and negative. In most modern passenger cars, it is a positive departure that is used, to which our company also applies. ET40. This means that the disc is not mounted in the center, but is shifted to the outside, which allows you to place brake mechanisms and suspension elements inside the wheel arch, optimizing the space.
Positive, zero and negative flight: what is the difference?
The differences between the types of departure are enormous in terms of handling and compatibility. ET40-marked drives are in the positive departure category, which is the standard for front-wheel drive and many all-wheel drive vehicles. When the flight is positive, the wheel is "sunk" deeper into the arch. This provides better stability on the straights and reduces the load on the steering mechanism, since the force vector is shifted closer to the center of gravity of the machine.
Zero departure (ET0) means that the mounting plane coincides with the wheel symmetry axis. Such drives are more common on SUVs or retro cars. They provide a wide track, which increases off-road stability, but impairs handling at high speeds and increases wear of bearings. Negative departure (for example, ET-15) is characteristic of jeeps and tuning cars: the wheel significantly protrudes beyond the body, creating an aggressive appearance, but making the car more rolling in corners.
- 🚗 Positive ET (ET40): Standard for passenger cars, optimal weighting, minimal wear suspension.
- 🚙 Zero ET (ET0): Used to expand the track, popular in off road, changes the dynamics of steering.
- 🏎️ Negative ET (ET-XX): Extreme tuning, risk of friction on the arch, high load on the hubs.
If your car’s factory specification requires an ET40, installing a zero-rebound drive will cause the wheel to shift outward by 40mm. This may seem insignificant, but in the sum of the width of the tire, such a displacement often leads to contact of the rubber with the arch when the car is fully loaded or the suspension is working on the break. Suspension geometry It is designed for specific parameters, and changing them always entails compromises.
The impact of the ET40 departure on the handling and wear of the suspension
Installing a disk with an ET40 parameter instead of a regular one, for example, ET45, shifts the center of the wheel outward by 5 millimeters. It seems a little, but in dynamics it changes. shoulder - the distance between the point of contact of the tire with the road and the projection of the axis of rotation of the rack. Increased run-in shoulder (when reducing the departure) makes steering more sensitive, but at the same time transmits more shocks from road irregularities to the steering wheel. The car can become more twitchy and demanding on the quality of the coating.
A more serious consequence is the change in load on the hub bearings. Designers lay a certain margin of safety, calculating force vectors for a regular departure. When a disc with a smaller departure (for example, ET35 instead of ET40) is installed, the lever of force increases, which leads to accelerated wear of the bearings and silent blocks of the levers. This is especially critical for the front suspension, where the load is maximum when braking and cornering.
⚠️ Attention: Reducing the drive departure (e.g., switching from ET45 to ET38) increases the load on the hub bearings. This can reduce their resources by two or more times.
On the other hand, the installation of a large-drive disc (e.g., ET50 instead of ET40) can cause the inner part of the disc or the caliper of the brake system to start touching the suspension elements. Brake calipers They are often complex, and even millimeter-sized displacement of the disc inwards can lead to fatal contact. Therefore, when selecting disks, it is important to take into account not only external geometry, but also internal gap.
When installing disks with departure, differing from the standard by more than 5 mm, it is recommended to check the puff of hub nuts and the condition of bearings more often.
Permissible deviations and compatibility table
Many motorists wonder: can you put the wheels with a departure other than the factory? There is an unspoken rule that allows deviations within ±5 mm for cast discs and up to ±10 mm for stamped discs (due to their plasticity). However, this rule only works if the changed parameter does not create physical conflicts with the body and suspension. For an ET40, the acceptable range is often considered to be ET35-ET45, but each car is different.
Below is a table showing how the wheel position changes relative to the splasher when the ET parameter changes for the disc standard width. This will help to understand the scale of displacement when choosing alternative sizes.
| Staffing parameter | New disc parameter | Difference (mm) | Direction of displacement | The risk of conflict |
|---|---|---|---|---|
| ET40 | ET35 | 5 mm | Outside (outside) | Friction on the arch/spray |
| ET40 | ET30 | 10 mm | Outside (strong) | High risk, need a bodlift |
| ET40 | ET45 | 5 mm | Inside (arch) | Friction on the rack/support |
| ET40 | ET50 | 10 mm | Inside (strong) | High risk of grazing |
When choosing disks, you also need to consider their width. If you change the width of the wheel, even if you keep the ET40 marking, the outer edge of the wheel will shift. The formula for calculating the change in the external boundary is as follows: (New Width - Old Width) / 2+ (Old ET - New ET). If the result is positive, the wheel will come out, if the negative one will go inside. This calculation is mandatory for those who plan to install wide-profile rubber.
How to find out the regular departure and check compatibility
To find out the manufacturer's recommended drive departure for your car, you do not need to guess. The most reliable information is contained in the technical documentation for the car or on a special sticker, which is often located on the inside of the hatch of the gas tank or on the central pillar of the body on the driver's side. It specifies the size of the tires and the parameters of the disks, including the departure. Look for a line where the parameter is listed after the size (e.g. 7Jx17) ET or Offset.
If the sticker is erased or the documentation is lost, you can use online catalogs of disk manufacturers or specialized forums dedicated to your car model. Also, the physical parameter is always struck on the inside of the disk itself. Find a label that may look like “ET 40”, “ET40” or “Einpresstiefe 40”. Important: if you buy used drives, be sure to check this figure, as it is impossible to visually determine the flight with an accuracy of up to a millimeter.
When ordering disks through online stores, filters are often used by parameters. Make sure you enter the data correctly. Some manufacturers may use the designation Offset instead of ET. The meaning remains the same. If your model catalogues the ET38-ET42 range, the ET40 disc is the perfect replacement. Don’t rely on “universal” wheels that are sold with the statement “fits for many cars” – for wheels this is a dangerous approach.
Frequent errors in the selection and installation of disks
One of the most common mistakes is trying to install a disk with an inappropriate departure using the spacers. Drivers buy large-relegation drives (e.g. ET50) that are deeply recessed in the arch and try to fix this with steel sheds. Although technically this puts the wheel back in the desired plane, this design creates an additional lever for the mount bolts, which increases the risk of them being cut off or the studs breaking under load.
Another mistake is ignoring the disk width when changing the departure. If you put a wider disc, it may require a smaller departure for compensation, but in total it can still lead to the wheel protruding beyond the body dimensions. In some countries (for example, in Germany according to the TÜV standards), the tyre's performance outside the arches is strictly prohibited and entails a ban on the operation of cars. In other regions, this can be a problem when passing a technical inspection.
⚠️ Attention: The use of slots thicker than 20 mm for the correction of departure requires replacement of the regular bolts with elongated or studs. The standard bolts may not catch on the thread of the hub, which will lead to the wheel breaking off on the move.
It is also worth remembering that discs with different departures can be cooled in different ways. If the ET40 disc sits tightly in the arch, the airflow to cool the brakes may be limited compared to the ET30 disc, which protrudes outward. This is rarely a problem for civilian cars, but for sports cars with intense braking, thermal modes are important.
Final selection recommendations
The choice of ET40 drives is a solution for those who want to maintain the factory handling and reliability of their car. This is the “golden mean” for most modern sedans and hatchbacks. When buying, pay attention not only to the design of spokes, but also to the technical marking. A beautiful disc with an inappropriate departure is money thrown away and a risk to safety.
If you want to change the appearance of the car, making the track wider, try not to go beyond the permissible ± 5 mm without serious preparation (suspension elevator, arches). Remember that safety It is always more important than visual effect. It is better to leave the standard parameters than to get unstable behavior of the car on a wet road or in an emergency situation.
In conclusion, the competent selection of wheels is a balance between aesthetics and technical feasibility. Knowing what the ET40 stands for and how this parameter affects your car will help you make an informed decision. Do not hesitate to consult with specialists in tire centers and check the data with official manuals.
Frequently Asked Questions (FAQ)
Can I use an ET35 instead of an ET40?
Technically, this is possible and the wheel will stand on the hub, as the number of bolts and the circumference diameter (PCD) are the same. However, the wheel will move outwards by 5 mm. It is necessary to check whether it will not touch the splasher or arch at full loading and suspension. The load on the bearings of the hub will also change slightly.
What happens if the drive is larger than the normal drive (e.g., ET45 instead of ET40)?
The wheel will go deeper into the arch by 5 mm. The main risk here is internal: the disc may begin to rub against the shock absorber rack, spring or steering elements. Also, the brake caliper can rest on the inner surface of the spokes of the disc. Trying on is required before installation.
Does the ET40 parameter affect speedometer speed?
No, the departure (ET) itself does not affect the speedometer readings, as it does not change the outer diameter of the wheel. The speedometer only lies if you change the size of the tire (for example, put a higher profile tire), which changes the circumference of the wheel.
How is the flight marked on the disks of different manufacturers?
The most commonly used designation ET (e.g. ET40). European and American manufacturers may use the term Offset reduction OFF. In Japan, sometimes there is a designation. InsertBut it's rare. The digital value remains the same standard in millimeters.
Do I need to do a break-down after replacing the disks with another ET?
Yes, it's highly recommended. Even a small change in departure changes the geometry of the suspension and the position of the wheel relative to the roadway. To avoid uneven wear of rubber and drive the car aside, the procedure for adjusting the angles of the wheel installation is better to carry out immediately after replacing the discs.