Parameter ET (EinpressTiefe) is always measured exclusively in millimeters, and any attempt to find its value in inches or centimeters will lead to a critical error when selecting rims. This is the distance from the plane of attachment of the disk to the hub to the vertical plane of symmetry of the rim, and its exact numerical expression determines whether the wheel will fit into the arch or touch the suspension elements. Ignoring the fact that ET - these are millimeters, often becomes the reason for purchasing incompatible disks, which threatens the destruction of the hub bearings or damage to the body when turning.
Understanding the physical essence of this parameter allows you to avoid costly mistakes when tuning or replacing standard wheels. When the driver wonders What is ET measured in?, he looks for the key to correct wheel geometry, which directly affects the handling and safety of the car. Incorrect offset changes the roll-in arm, which can make steering unpredictable or cause accelerated tire wear.
Physical essence of the ET parameter
Disc overhang, designated as ET, is the distance in millimeters between the mating plane of the disk and its central axis. The mating plane is the surface with which the disc is pressed against the vehicle's hub with bolts or nuts. If this plane is located exactly in the middle of the width of the disk, then the offset is considered zero, and if it is offset, it is considered positive or negative.
It is important to understand that ET is not a random variable, but a parameter strictly calculated by engineers for each car model. Changing this value even by 5 mm can significantly change the distribution of loads on the chassis elements. Therefore, when looking for an answer to the question “What is ET measured in,” you need to remember that we are talking about a highly accurate engineering quantity.
⚠️ Attention: Never rely on a visual assessment of the disc offset “by eye”. A difference of a few millimeters may not be noticeable visually, but will result in the disc hitting the brake caliper or shock absorber when the suspension is compressed.
There are three main types of overhang, which are determined relative to the center of the disk. Zero departure means that the mounting plane coincides with the axis of symmetry. Positive departure typical for most modern front-wheel drive and all-wheel drive cars, where the disc is “recessed” deeper into the arch. Negative departure occurs less frequently, usually on SUVs or in tuning, when the disk, on the contrary, protrudes outward.
Units of measurement and markings on discs
Basic unit of measurement of the parameter ET is millimeter (mm). On the disk itself, this value is usually marked with the letters ET, EinpressTiefe or just a number with a plus or minus. For example, marking ET 35 means that the offset is 35 millimeters in the positive direction. In rare cases, on American disks you can find a designation in inches, but this is an exception that requires recalculation.
- 📏 Millimeters (mm): Standard global unit of measurement for European, Asian and Russian wheel manufacturers.
- 🇺🇸 Inches: It is extremely rare on specific American tuning; it requires conversion to millimeters (1 inch = 25.4 mm).
- 🔢 Dimensionless coefficients: Does not exist for ET, but is often confused with PCD or drill diameter, which are also measured in mm.
When deciphering the markings on the back of the disk spoke, you will see a set of numbers, for example: 7.5J x 16 H2 ET 45 5/112. Here ET 45 - this is the desired offset of 45 millimeters. Number 7.5 denotes the width of the rim in inches, and 16 - diameter in inches. Confusion often arises precisely because of the mixing of the inch system (width and diameter) and the metric system (offset).
Always check the ET marking on the inside of the rim, even if you buy new wheels. Sometimes manufacturers make universal disks, and the actual value may differ from that stated in the catalog.
Effect of offset on handling and suspension
Changing the default value ET directly affects the dynamics of the car. Reducing the offset (the disc sticks out more) widens the track, which theoretically improves cornering stability, but at the same time increases the load on the wheel bearings and suspension arms. This happens because the wheel's center of gravity moves away from the axis of rotation, creating additional leverage.
If you install a disk with an offset significantly different from the factory one, you may encounter a number of problems. Suspension may begin to work incorrectly, the shock absorbers will experience overload, and the steering will become more sensitive to road irregularities, but less stable at high speeds.
| Type of change ET | Effect on the car | Risks |
|---|---|---|
| Decrease ET (disk out) | Wider track, improved stability | Bearing wear, arches hitting |
| Increase ET (disc inward) | Narrowing of the track, risk of touching the calipers | Damage to the brake system |
| Severe deviation from the norm | Steering instability | Destruction of suspension elements |
Compliance with the parameter is especially critical ET for vehicles with independent suspension. In such systems, the geometry of the levers is calculated with high precision, and a displacement of the point of application of forces can lead to rapid failure of silent blocks and ball joints.
Tolerances and compatibility
Many car enthusiasts wonder how strictly they need to adhere to factory values. Engineers allow slight fluctuations in the parameter ET, usually within ±5 mm of the standard value. However, this is only true for disks of the same width. If you change the width of the rim, the acceptable offset range also changes.
When installing wider rims, it is often necessary to reduce the offset so that the inner part of the rim does not interfere with the suspension components. In this case, the plus or minus 5 mm rule no longer works linearly. It is necessary to use special disc calculators or consult with specialized specialists who will calculate the change in the rolling shoulder.
⚠️ Attention: Using spacers to change the offset is a temporary and not always safe solution. Spacers increase the load on the wheel bolts and can become a source of vibration or an accident at high speeds.
The compatibility of disks is determined not only by diameter and drilling, but also by a set of parameters, where ET plays the role of a balancer. The wrong choice will lead to the fact that the wheel may simply not fit into place, resting against the shock absorber, or, on the contrary, it will strongly protrude beyond the dimensions of the body, violating aerodynamics and laws (in many countries protrusion beyond the arch is prohibited).
Diagnostics and checking disk parameters
If the markings on the disc are erased or questionable, the parameter ET you can measure it yourself. To do this, you will need a flat surface, a ruler or caliper and a level staff. Lay the disc flat on the floor with the inside facing up, lay the rack across the rim and measure the distance from the floor to the rack, and then the distance from the mating plane to the floor.
☑️ Check before purchasing discs
The calculation formula is as follows: ET = A - (B / 2), where A - the distance from the mating plane to the floor, and B — total width of the disk (distance from edge to edge, not seat width). The resulting value will show the actual offset in millimeters.
When diagnosing suspension problems, it is also worth paying attention to tire wear patterns. If the rubber wears unevenly along the inner or outer edge, this may be an indirect sign that the disc offset is incorrect and the wheel is at an abnormal angle relative to the arch.
Legal aspects and safety
The issue of legality of installing disks with modified ET regulated by technical regulations. In Russia and many CIS countries, making changes to the design of a vehicle, which includes installing wheels with parameters not provided by the manufacturer, formally requires certification. In practice, if the disc does not protrude beyond the dimensions of the body and does not touch elements, problems usually do not arise during technical inspection.
Legal nuances
In the event of an accident involving a car on non-standard wheels, the insurance company may initiate a technical examination. If it is proven that the cause of the accident was a violation of the suspension geometry due to incorrect ET, compensation may be denied.
Traffic safety is priority number one. Even if the disc fits into the arch and does not rub, this does not guarantee the safety of the chassis. Long-term operation of a vehicle with a critically changed offset leads to the accumulation of fatigue cracks in the levers and hubs, which can result in the wheel coming off while driving.
Therefore, answering the question “What is ET measured in,” we are talking not just about millimeters on a ruler, but about millimeters on which the lives of the driver and passengers depend. Accuracy is more important than aesthetics here.
Key takeaway: ET is always measured in millimeters. A deviation from the factory values of more than 5 mm without recalculating the entire suspension geometry is dangerous for the operation of the vehicle.
FAQ: Frequently asked questions
Is it possible to install a disk with ET 35 instead of the standard ET 45?
A difference of 10mm is significant. The disc will move out 1 centimeter. This can cause the arches to hit when the vehicle is fully loaded or the brakes are applied. It is recommended to consult a specialist and check the gaps.
Does the width of the rim affect the acceptable ET value?
Yes, directly. For a wider rim, the allowable offset value is usually less (the rim must be "recessed" less or have a negative offset) to prevent the inside of the rim from hitting the suspension components. The compatibility formula is complex and depends on the specific car model.
What happens if you confuse inches and millimeters in the ET calculation?
This is a fatal mistake. If you think that ET 35 is inches and try to find such a disk, you simply will not find it, since such disks do not exist. If you make a mistake in the calculations when making spacers, the wheel may not fit on the hub or destroy the suspension instantly.
Is there a difference between ET and Offset?
No, these are synonyms. Offset is the English name of the parameter, ET (EinpressTiefe) - German. Both parameters are measured in millimeters and mean the same thing - disc offset.