When purchasing new rims, car enthusiasts often come across the abbreviation ET, which is on par with the diameter, width and number of holes. Many people ignore this parameter, believing that if the disk “fits” onto the hub, then there will be no problems. However, it is disc offset (this is how ET is translated from German Einpresstiefe) is a critical parameter that determines the geometry of the suspension.
An incorrectly selected offset can lead to accelerated wear of the hub bearings, changes in the turning radius, and even damage to body elements when driving over uneven surfaces. Understanding the physics of the process will help you avoid costly repairs and ensure safety on the road. In this article we will analyze in detail how this parameter is calculated and why you cannot deviate from it.
Physical meaning of the ET parameter
Disc offset - this is the distance between the vertical plane of symmetry of the wheel and the plane of application of the disk to the hub. It is measured in millimeters. It is important to understand that this parameter can be positive, zero or negative, and this directly affects how much the wheel will “stick out” from under the arch or, conversely, be recessed inside.
If we are talking about positive overhang, then the mounting plane is shifted outward relative to the central axis of the disk. This is the most common situation for front wheel drive vehicles. In the case of a negative value, the landing plane is closer to the inside of the disk, which is typical for SUVs and some rear-wheel drive models with wide wheels.
There is a common misconception that ET You can change it up or down at your discretion, as long as the disc fits on the arch. In fact, engineers calculate this parameter with millimeter precision so that the load on the suspension is distributed optimally. Changing the reach changes the roll-in arm, which causes the steering mechanism and suspension to work abnormally.
⚠️ Attention: Installing discs with an offset that differs from the factory offset by more than 5 mm can lead to the separation of the hub or destruction of suspension elements in an emergency.
The geometry of the wheel directly affects the stability of the car. When changing departure Not only the appearance of the car changes, but also its behavior on the track. Too much offset can cause the wheel to hit the internal suspension components or brake caliper.
Departure mathematics: formula and calculation
For those who love precision, the concept Einpresstiefe has a clear mathematical expression. The offset is not taken out of the blue; it is calculated based on the width of the rim. The formula looks like this: ET = (B / 2) - X, where B is the full width of the disk, and X — the distance from the inner plane of the disk mounting to its inner edge.
Let's look at an example. Let's say we have a disk that is 7 inches wide (which is approximately 177.8 mm). Half the width will be 88.9 mm. If the distance from the mating plane to the inner edge of the disk (X) is 60 mm, then the calculation will be as follows: 88.9 - 60 = 28.9 mm. We round and get ET 29.
Car enthusiasts often confuse the wheel width in inches with the tire width in millimeters. These are different things. The width of the rim is measured between the flanges of the rim, not the outer edges. That is why for an accurate calculation departure It’s better to use technical documentation or specialized software rather than a ruler.
Below is a table showing the dependence of the wheel position on the value of the ET parameter for a fixed rim width:
| ET value | Departure type | Wheel position | Effect on track |
|---|---|---|---|
| ET > 0 | Positive | Recessed into the arch | The track is already |
| ET = 0 | Zero | Hub center | Average track |
| ET < 0 | Negative | Sticks out | Wider track |
| ET +50 | High + | Heavily recessed | Risk of contact |
| ET -15 | Negative | Strongly outward | Risk of hitting arches |
Is it possible to calculate ET with a ruler?
Accurate calculation of ET using a conventional ruler is extremely difficult due to the complex geometry of the inner part of the disk. Trying to measure the distance from the mating plane to the edge of the rim without removing the tire and knowing the exact width of the flange (B) often leads to errors of 3-5 mm, which is already a critical deviation.
Effect of offset on suspension and handling
When you cheat ET disk, you are essentially changing the lever on which the car's mass acts. Reducing the offset (the wheel gets closer to the edge) increases the load on wheel bearing. Imagine holding a heavy object at arm's length - it's much harder on your muscles and joints than holding the object close to your body.
In addition, changing the reach affects the performance of the shock absorbers. The stroke of the rod may change, which will lead to a more rigid passage of irregularities or, conversely, to suspension breakdowns. The steering also reacts to changes in the rolling shoulder: the car may become more sensitive to ruts, and the steering force will increase.
Particular attention should be paid to stabilization systems and ABS. The car's electronics are programmed for certain driving parameters. If the wheel geometry is changed, the sensors may read incorrect wheel speed data, which will lead to incorrect operation of the safety systems.
Any deviation from the factory ET changes the load vector on the suspension, accelerating the wear of silent blocks, ball joints and wheel bearings.
Owners of cars with MacPherson struts should be especially careful. In this design, the shock absorber is a load-bearing element, and the displacement of the wheel support point critically affects the life of the entire strut. Moving the wheel outward also increases the risk of damage to the arches when the vehicle is fully loaded.
Tolerances and compatibility
Is there a “gold standard” that allows for a small margin of error? Wheel and car manufacturers often specify a range of acceptable values. Typically, deviation is allowed within ±5 mm from the factory setting. However, this rule does not work for all machines.
For cars with powerful engines and complex multi-link suspension, the tolerances may be even narrower. At the same time, older VAZ models or some SUVs may be more tolerant of changes. But even in this case departure It's better not to touch unless necessary.
- 🚗 For sedans and hatchbacks, a deviation of no more than 3-5 mm in any direction is allowed.
- 🚙 SUVs with a suspension lift often require wheels with reduced ET (closer to zero or negative).
- 🏎️ For sports cars with wide tires, it is critical to comply with factory specifications to avoid friction with the body.
If you plan to install disks with a different ET, be sure to consult a tire specialist. Sometimes spacers solve the problem, but their use also has its limitations and risks. Spacers increase the load on the hub bolts, which requires regular monitoring of tightening.
Comparison of ET, J and PCD: What's the difference?
Beginners often get confused ET with other markings on the disc such as J or PCD. Let's figure it out to avoid mistakes when buying. PCD Pitch Circle Diameter is the diameter of the circle on which the mounting holes are located. This is a compatibility parameter: if the PCD does not match, the disc simply will not fit onto the hub.
Parameter J (e.g. 7J) denotes the shape of the rim flange and, by implication, the width of the rim in inches. Width directly affects offset calculations, but the J itself does not indicate offset. You may find two rims with the same 7J width but different ET and they will sit differently.
Here are the main differences between the parameters:
- 🔩 PCD — is responsible for the physical possibility of attaching the disk to the hub (coincidence of holes).
- 📏 ET — is responsible for the seating depth of the disk relative to the arches and suspension elements.
- 📐 J (Width) — determines the width of the tire seat and affects the overall width of the wheel.
Confusion between these parameters can lead to the purchase of discs that either will not fit on the brakes or will rest against the shock absorber strut. Always check the full label, which usually looks like 7.5J x 17 H2 ET35 5x112 DIA66.6. Here ET35 and there is our desired departure.
When purchasing used wheels, be sure to check the markings on the inside of the spokes. Often sellers indicate only the diameter, hiding the real offset, which may not suit your car.
Problems with incorrect selection of ET
Ignoring a parameter departure is fraught with serious consequences. The most common problem is wheel friction on a body or suspension part. This may not happen all the time, but only when you turn the steering wheel all the way or when you hit a hole and the suspension is compressed.
Another hidden threat is uneven tire wear. A change in the angle of the wheel (camber) due to a shift in the fulcrum leads to the fact that the rubber wears out “fangs” or on one side faster. This not only wastes money on new tires, but also reduces traction.
⚠️ Attention: If, after installing new disks, you feel the steering wheel beating at high speeds or a rumble from the wheel, first check that the ET parameter meets the factory requirements.
In the worst case scenario, if the offset is greatly reduced (negative ET on a car where it should be positive), the wheel can hit internal components as the suspension compresses. This can lead to the wheel blocking while moving or damage to the brake hoses, which can lead to an emergency.
☑️ Check before purchasing discs
Frequently asked questions (FAQ)
Is it possible to install a disc with ET 45 instead of ET 35?
A difference of 10mm is significant. The wheel will go 1 cm inside the arch. This may cause the tire to start rubbing against the shock absorber strut or suspension elements when turning. The load on the inner wheel bearing will also increase. It is recommended to look for a disk closer to the factory value.
Does disc offset affect fuel consumption?
Yes, it does have an indirect effect. Increasing the offset (the wheel moves outward) improves the aerodynamics of the wheel arches, but increases rolling resistance and the load on the transmission. Reducing the offset (the wheel moving inward) can create turbulence in the arches. On average, the change in consumption is 1-3%, but safety is more important than savings.
What is DIA and how is it related to ET?
DIA (or D, CO) is the diameter of the central hole. It must exactly match the diameter of the car's hub. If the DIA is larger, centering rings are needed. ET (offset) and DIA are independent parameters, but both must meet vehicle specifications for proper wheel fit.
Why are SUVs often fitted with negative ET rims?
This is done to widen the track, which increases the stability of the car off-road and during lateral rolls. However, on conventional road cars, a negative offset will lead to accelerated wear of the suspension and contamination of the sills with dirt.
How can I find out the factory ET of my drive?
Departure information is located on the back of the disc (marking ETxx), in the technical documentation of the car or on an information sticker, most often located on the central pillar of the body on the driver's side or on the inside of the gas tank flap.