Selection of wheels is not just a matter of aesthetics, but a critical task for the safety and resource of the chassis of the car. Many drivers are faced with a situation where a beautiful cast-plate It stands in the arch visually perfectly, but on closer examination it turns out that its geometric parameters do not meet the requirements of the machine manufacturer. The most insidious parameter here is precisely the departure, often referred to as ET or Offset.
Wrongly selected Offset It can lead to accelerated wear of hub bearings, tire friction against suspension or even to the destruction of wheel mounting elements. Unlike the diameter or number of bolts that catch your eye right away, a flight requires careful examination of the markings on the inside of the product. Understanding how to decrypt this data will help you avoid costly mistakes when buying and installing.
In this article, we will discuss in detail where to look for the right numbers, what they mean and whether it is possible to deviate from factory values. We will look at physical measurement methods and mathematical calculations that will allow you to determine with confidence the suitability of the disc for your car.
⚠️ Attention: Installing discs with a departure different from the factory by more than 5 mm can lead to a change in the shoulder of the run-in and unpredictable steering behavior.
What is a Disk Departure and Why It Matters
Disk departure is the distance between the vertical plane of wheel symmetry and the plane of application of the disc to the hub. In simple words, it is how much the wheel is "sunk" in the arch or, conversely, protrudes outward. This parameter directly affects the load on hub-bearing and suspension elements. If Offset too large, the wheel goes deep into the arch, which can cause friction on the shock absorber rack or brake elements.
On the other hand, too small a flight (or negative) pushes the wheel outwards, beyond the dimensions of the body. This not only violates the aesthetics and rules of the road, but also creates a huge load on the hub, working as a lever. Stamped disk Often has a smaller range of tolerances compared to cast counterparts, as their design is less flexible in terms of engineering compromises.
Ignoring this parameter when tuning or seasonal replacement of rubber is one of the most common reasons for the failure of the chassis. Owners often think that if the disc climbed on the hub and twisted bolts, then everything is fine. However, the physics of the process dictates its conditions: the force vectors are shifted, and the parts designed for a certain load begin to work in extreme mode.
- 🚗 Positive departure: The mounting plane is located outside the center of the disk (the wheel is deeper in the arch).
- 🚙 Zero departure: The mounting plane coincides with the central axis of the disc.
- 🏎️ Negative departure: The mounting plane is located inside from the center (the wheel strongly protrudes outward).
Where to look for ET marking on disk
Manufacturers are required to apply all technical information to the inner surface of the product. To find this data, you do not need to dismantle the wheel completely, just turn the steering wheel and look behind the brake disc. Usually the marking is stamped on the spoke or on the inside of the rim. Look for an acronym ETfollowed by numbers, for example, ET 35 or ET 45.
Sometimes. ET (German Einpress-Tiefe standard) can be found designation Offset plainly OFF. It doesn't change the point. The number following these letters is the desired departure in millimeters. It is important not to confuse this parameter with the diameter of the disk (for example, R16) or the width of the rim (for example, the size of the rim). 7.0J). Often these numbers are side by side, which can be confusing for an inexperienced driver.
If the marking has erased from time and corrosion or was painted over during repair, it will not be possible to determine the parameter visually. In such cases, you have to resort to measurements or search for information by the catalog number of the disk model. Some manufacturers cast-disc hide the marking under decorative caps or engravings, so inspect the surface must be carefully.
Use a flashlight and a wet wipe to wipe the inner surface of the disc before looking for markings. Dirt and dust often hide important figures.
Formula for calculating the departure of the disk with their own hands
If there is no marking, the flight can be calculated independently using a linear measuring tool. To do this, you will need to remove the wheel from the car and put it flat with the outside down on a flat surface. You will need a barbell or, in extreme cases, a construction roulette and a flat rake. The accuracy of the measurements is critical, as a few millimeters error changes the result.
The essence of the method is to measure two values: the total width of the disk and the distance from the landfill plane to the edge of the rim. First, measure the full width of the disk (not the tires!) from edge to edge, let it be the value. A. Then we put the rack across the disk and measure the distance from the landfill (where the disk is pinned by bolts to the hub) to the lower edge of the rim - this is the value B.
The formula for calculation is as follows: ET = B - (A / 2). The value you get is your departure. If the result is positive, then the mounting is shifted outward. If it's negative, it's inside. This method is universal and suitable for stampedand for complex composite structures.
Example of calculation:Full width of disk (A) = 200 mm
Distance from site to edge (B) = 135 mm
Calculation: 135 - (200/2) = 135 - 100 = 35 mm
Bottom line: ET = 35
⚠️ Attention: When measuring the width of the disc, do not take into account the bulges and decorative elements of the rim. Measure the strictly flat part where the side of the tire sits.
Permissible deviations and compatibility
Many drivers wonder: can you put a drive with a departure different from the factory? The short answer is: you can, but with caution. A deviation within ± 5 mm of the normal value is considered acceptable. In this case, the change in the load on bearing It will be minimal and will not affect the resource of the site. However, if you plan to install drives with a departure different from 10-15 mm or more, the consequences can be serious.
With a decrease in the departure (the wheel protrudes outward), the load on the hub bearing and suspension elements increases. This is due to a change in the shoulder of the application of force. In addition, the protruding wheel can touch the arch when turning or compressing the suspension, especially if the car is equipped with a car. lift-pack Or it has big wheels. With an increase in the departure (the wheel goes inward), there is a risk of friction of the caliper of the brakes against the inner part of the disc.
The table below shows the approximate consequences of departure deviation from the norm:
| ET's rejection | Impact on the car | Risk of breakdown |
|---|---|---|
| ± 0-5 mm | It doesn't feel. | Absent. |
| ± 10 mm | Possible change in steering behaviour | Low (accelerated wear) |
| ± 20 mm | Noticeable load on the suspension | Medium (bearings, levers) |
| More than ± 25 mm | Critical change in geometry | High (knot breakage) |
☑️ Checking before installing non-standard disks
The impact of departure on handling and wear
The geometry of the wheelbase directly determines the nature of the car's handling. Changing. disk-outYou are actually changing the track of the car. Increasing the track (decreasing ET) theoretically increases the stability in cornering, making the car less prone to rolling. That is why on sports cars you can often see discs with minimal departure, almost in line with the arch.
However, this medal has a downside. The increase in the running-in shoulder leads to the fact that the steering becomes more sensitive, but also more "nervous". On rough roads, the steering wheel can begin to twitch in the hands, and the load on the steering rack will increase many times. For everyday use and comfort, such changes are rarely warranted.
The wear of rubber should also be taken into account. In case of improper departure, the angle of inclination of the wheel rotation plane relative to the road changes. This can lead to uneven tread wear, even if the collapse-adjoining is performed perfectly. The tire will wear off either the inner or outer shoulder faster, which will shorten its lifespan.
Why do SUVs often change departures?
Owners of SUVs often reduce the departure (put wheels with less ET) to expand the track. This increases the stability of the car on the slopes and reduces the likelihood of overturning. However, for urban crossovers, this is unnecessary and harmful to suspension.
Specificity of composite and forged discs
Special attention deserves compositeThey are made up of several parts (center and shelf). Their uniqueness is that the flight can be changed mechanically, replacing the spacers or using bolts of different lengths. This gives you a lot of flexibility in setting up, but requires high qualification in assembly. An error in the length of the fastener can lead to the fact that the bolt will rest in the brake caliper or, conversely, will not hook the hub.
Forged wheels, thanks to production technology, often have unique strength characteristics at lower weight. However, their geometry is also strictly regulated. Attempt to "scatter" or change the landfill plane of the forged disk at home is strictly prohibited - this violates the structure of the metal and leads to instant destruction during loading.
When buying used composite discs, be sure to check the presence of all original slots and bolts. Often, sellers lose these small but important elements, and finding a replacement for a specific pitch of thread and length can be extremely difficult. Always check the labeling on the components of the composite disc, as the entangled parts may have different thicknesses.
Composite disks allow you to flexibly adjust the departure, but require professional assembly and regular check of tightening of the bolts.
Frequently Asked Questions (FAQ)
Can I use the sleeves to change the flight?
Yes, installing wheel pads is a common way to change the drive’s departure without replacing the discs themselves. The spacer is set between the hub and the disc, effectively reducing the ET value (pushing the wheel outward). It is important to use high-strength alloys (e.g. aviation aluminum or steel) and elongated bolts or studs of appropriate length. It is not recommended to put the stretches thicker than 20-25 mm without digesting the hub, as the standard length of the thread may not be enough for reliable fastening.
Does the disk's flight affect the speedometer reading?
The departure parameter (ET) does not affect the circumference of the wheel, so it does not directly affect the readings of the speedometer. The speedometer responds to the number of wheel turns. However, if the change in departure is accompanied by the installation of tires of larger diameter or width, which is often done in conjunction with new discs, the speedometer readings may change. Purely geometrical displacement of the disc to the side does not change its rolling radius.
What happens if you put an ET 35 instead of an ET 45?
The difference of 10 mm is borderline. The wheel will extend outwards by 1 centimeter. Visually, this may be almost imperceptible, but the load on the hub bearing will increase. For most passenger cars, this deviation is considered acceptable in the short term or provided that good roads are driven carefully. However, for heavy SUVs or with an aggressive driving style, this is already a risk zone.
How to distinguish the disc for front-wheel drive car from rear-wheel drive on departure?
Formally, there is no division into "front" and "back" wheels on departure, there are specific parameters for a particular car model. However, on all-wheel drive cars or cars with different axles, the parameters of the disks in the front and rear may differ. For example, on some BMW or Mercedes models, the wheels on the rear axle have a larger relegation to fit into narrow arches. Always check the manual for each axis separately.
Is it true that cast discs break on impact, and stamped discs bend?
This is a common but simplistic view. Indeed, die-plate soft steel with a strong impact are more often deformed (bent), which allows in some cases to straighten them and continue operation. Cast discs Aluminum alloys are more hard and fragile: at a critical impact, they tend to crack or split, making them impossible or dangerous to repair. However, high-quality cast discs are designed for high loads and during normal operation serve longer.