When choosing new wheels, the car owner often encounters the abbreviation ET, which stands among the technical characteristics along with diameter, width and chatter. Many newcomers to tuning mistakenly believe that this parameter is secondary, and it can be ignored if the disc is visually sized. However, it is precisely disk-out determines how deep the wheel will "go" into the arch or, conversely, will bulge outside the body.
Understanding the physics of the process is necessary not only for the aesthetics of the appearance of the car, but also for the preservation of the chassis life. Incorrectly selected displacement parameter can lead to accelerated wear of the hub bearings, uneven abrasion of the tread and even touching the wheel elements of the suspension or body when turning the steering wheel. In this article, we will discuss in detail what is behind the ET marking, how it is calculated and why deviation from factory norms can be dangerous.
It is important to note right away that ET is not an abstract number, but a specific geometric size measured in millimeters. It is strictly tied to the plane of attachment of the disc to the hub and the central axis of rim symmetry. Ignoring this value when buying cast or forged wheels turns a car into a potentially uncontrollable vehicle, especially at high speeds or in extreme road conditions.
The physical essence of the ET parameter
The abbreviation ET comes from the German word Einpress-Tiefe, which literally translates as “depth of pressing”. In technical documentation in English, the same parameter is often referred to as Offset. The essence of the indicator is the distance between the plane, which the disc is adjacent to the hub of the car during installation, and the conditional plane, which passes strictly in the middle of the width of the rim of the disk. If you imagine the disc in a section, then ET shows how far the landfill is shifted relative to the center of the width of the rim.
The departure value can be positive, zero or negative. In most modern passenger cars used flight-backwhen the landing plane is closer to the outside of the disk, and the hub itself goes deeper inside the wheel. This design solution allows you to optimally distribute the load on the suspension and reduce the shoulder of the run-in. The negative departure, characteristic of SUVs and some retro cars, means that the hub protrudes outwards, and the disc itself is “drowned” deep inwards.
Mathematically, this parameter is calculated using the formula, taking into account the width of the rim and the distance from the landing plane to the inner edge of the disk. For the average user, it is important to understand one thing: the larger the ET number, the deeper the wheel sits in the arch. For example, a disc with an ET 45 will sit deeper than a disc with an ET 35, provided the same rim width. The difference of 10 mm is visually and technically very noticeable.
It's important to understand: The plane of the disk attachment must always coincide with the axis of rotation of the hub. If this does not happen, parasitic moments of forces arise that are not provided for by the suspension design. That is why car manufacturers so strictly regulate the permissible departure values for each particular model.
⚠️ Attention: Never focus only on the visual coincidence of holes under the bolts. Even if the disc is physically wound on the hub, an incorrect ET can lead to the fact that when the car is fully loaded or hit by unevenness, the wheel will destroy the suspension elements or the ranger.
The impact of departure on the handling and resource of the car
Changing the normal departure of the disc is an intervention in the geometry of the suspension, which entails a chain reaction of changes in the operation of the entire chassis complex. When installing discs with less ET (wheel extends outward), the shoulder of the run-in increases. This leads to the fact that when braking or hitting a pit, the load on the steering mechanism and the hub bearing increases many times over. Stack bearing In such conditions, it works in extreme mode and can fail several times faster.
On the other hand, if you install discs with a large ET (wheel goes deeper into the arch), there is a risk of contact with the inner part of the tire or the disc itself with suspension elements (shock absorber, levers) or brake caliper. This is especially critical when turning the wheels or working suspension for compression. At best, this will damage the paint on the disc and abrasion of the rubber, at worst - to jam the wheel on the go.
The behavior of the car on the road is also changing. Increasing the track (at a decrease in ET) can theoretically improve stability on straight lines, but at the same time the risk of aquaplaning increases and controllability in corners deteriorates due to changes in force vectors. The steering becomes more "heavy", returns more acutely reacts to roughness of the road, passing the blows directly to the steering wheel.
Special attention should be paid to the load on the body elements. With the wheel extended outwards, the suspension levers work with a large pressure lever, which can lead to fatigue deformations of the places where the levers and even spars are attached during prolonged operation. Manufacturers lay a margin of safety, but it is not unlimited and is designed for a certain range of departures.
Permissible deviations and disc compatibility
Often motorists wonder: can you put the wheels with a departure different from the factory? Engineering practice allows for small fluctuations, but they are strictly limited. It is believed that deviation This is ±5 mm for passenger cars. This means that if the standard setting ET 40, you can use the wheels with a departure from 35 to 45 mm without critical consequences for the chassis.
But there is a big difference: the width of the disk. If you change the width of the rim, then the ET value should be adjusted accordingly to keep the position of the inner shelf of the disk relative to the hub unchanged. For example, when the disk expands by 1 inch (25.4 mm), the center of the disk shifts by 12.7 mm. To compensate for this and leave the inside of the disk in place, you need to increase the ET by about 12-13 units.
For all-wheel drive cars, the requirements are even tougher. The difference in the departure of the front and rear wheels can disrupt the operation of the stability and all-wheel drive system, since the wheel rolling diameter (in fact, the shoulder) will change. In systems of type Quattro or xDrive This can lead to overheating of the center differential.
The impact of paddles on departure
The use of pads is an artificial change of departure. If you put a box 20 mm thick, you reduce the ET of the disc by those 20 mm. This is the most dangerous method of changing the geometry, as the spacers increase the load on the studs and often do not have a central hole that perfectly matches the hub, which leads to a beat at high speeds.
When buying non-original discs (replicas), a situation is often encountered when the same disc is sold with different ET values for different brands of cars. This is normal practice, but requires careful scrutiny. Universal drives "for all cars" with one value ET does not exist.
Comparison of staff and tuning parameters
For clarity, let’s look at how changing parameters affects the position of the wheel. The table below gives examples for a conventional car with a 7-inch width R17 standard disc.
| Type of disk | Width (J) | Departure (ET) | Wheel position | Risks. |
|---|---|---|---|---|
| Staffed | 7.0 J | 45 | Level with the arch | No. |
| Tuning (light) | 7.5 J | 40 | Slightly performing. | Minimum |
| Tuning (aggressive) | 8.0 J | 30 | He's a strong performer. | Wear of bearing |
| Wrong choice. | 7.0 J | 20 | Critically speaking | Suspension breakdown |
As you can see from the table, even a small change in width requires correction of departure. If you take a disc 8 inches wide (instead of 7) and leave the ET 45, then much more metal will go inside the arch, which can lead to a shock absorber. Therefore, in the selection wide-disc ET must necessarily be reduced.
Owners of crossovers and SUVs often want to put wheels with a negative departure to expand the track. This is permissible only under the condition of reinforced suspension and the use of wheel arches of large volume. On a standard city car, a negative departure is guaranteed to lead to rapid destruction of nodes and problems during the inspection.
When buying used disks, pay attention to the internal shelf. If there are traces of contact with a shock absorber or caliper (paint, chipped paint), then the drive departure is not suitable for the car on which it was previously standing.
How to calculate and check the flight independently
If the marking on the disk is erased or you want to recheck the parameters of the existing set, you can make measurements yourself. This will require a flat surface, ruler or roulette and a flat rake (for example, building level). It is important to take measurements on a disk without a tire, as rubber will distort the results.
Place the disc flat on a flat floor with the outside down. Throw the rack through the center of the disk (diametric section). Measure the distance from the floor to the rack – this will be the distance to the outer edge of the rim. Then turn the disk over and measure the distance to the inner edge. The difference will give you the full width of the rim. Next, you need to measure the distance from the landing plane (which is adjacent to the hub) to the edge of the disk.
The formula for the calculation is as follows: ET = A - (B/2), where A is the distance from the landing plane to the outer edge of the disk, and B is the full width of the rim. All measurements are made in millimeters. Measurement accuracy is critical, as a 5mm error is already considered significant.
☑️ Checking before buying discs
Do not forget that cast discs can have a complex geometry of spokes, and it can be difficult to find a flat surface for applying a ruler. In such cases, it is better to use a rod or contact specialists in the tire center, where measurements will be made with a professional tool.
Legal aspects and technical inspection
The issue of legality of installation of disks with modified departure is regulated by technical regulations. According to the current regulations, the installation of wheels not provided by the manufacturer may be regarded as a change in the design of the vehicle. If the drive is out of the acceptable range specified in the documentation of the car, you risk not to undergo a technical inspection.
Road inspectors may also pay attention to the heavily protruding wheels. If the disc extends beyond the arches (which often happens with a strong decrease in ET), it is a direct violation of traffic rules in many countries, as such wheels can spray other users and pose a danger. In this case, a fine is issued and a prescription is given for the elimination of the malfunction.
In the event of an accident, the insurance company may also conduct an examination. If it is established that the cause of the accident was a suspension failure caused by the installation of discs with an inappropriate departure, compensation may be refused. Insurance case may be invalidated due to a technical malfunction of the car, caused by the fault of the owner.
⚠️ Attention: Using the paddles to change the departure is often equated with making changes to the design. To legalize such changes, it is necessary to undergo certification and make changes to the documents for the car, which is almost impossible in the case of expository.
Therefore, before you buy “beautiful” discs with avito or on the fallout, think about the consequences three times. Savings on the right selection can result in expensive suspension repairs and problems with the law.
Frequently Asked Questions (FAQ)
Can I put ET 35 discs instead of the regular ET 40?
Yes, that's a tolerance of 5 mm. The wheel will extend outwards by half a centimeter, which is visually almost imperceptible and technically safe for most passenger cars. The load on the bearing will increase slightly.
What happens if you put a disc with ET 20 instead of ET 45?
The difference of 25 mm is critically high. The wheel will extend 2.5 cm outwards. This will lead to rapid wear of the hub bearing, possibly grazing the wheel of the arch or suspension elements when turning, as well as deterioration of handling. Such a replacement is prohibited.
Does the drive impact the fuel consumption?
Indirectly, yes. The outwardly extended wheel (less than ET) degrades the aerodynamics of the car, increasing air resistance. In addition, increased friction in the suspension units and the change in the shoulder of the run-in can slightly increase fuel consumption, especially at track speeds.
How does the flight affect winter operation?
In winter, when snow and ice accumulate on the arches, a reduced departure (wheel closer to the edge) increases the risk that the wheel will start rubbing against the ice crust on the arch. It could damage the tire. The regular departure provides the optimal clearance for winter conditions.
The golden rule of selection: the closer the ET parameter of your new drive to the factory value, the longer the suspension will last and the behavior of the car on the road will be more stable.