When purchasing rims on their own or when trying to install wider tires, car enthusiasts often come across the abbreviation ET. This is not just a set of letters, but a critical parameter that determines whether the disc will fit on the car and whether it will not touch the suspension or body elements. 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. Incorrect calculation of this value can lead to serious consequences, including the destruction of the wheel bearing or the wheel coming off while driving.
Many drivers mistakenly believe that if the bolts match and the disc does not rub against the caliper, then everything is in order. However, the wheel geometry affects the distribution of loads in the suspension units. In this article, we will analyze in detail how exactly offset is indicated on disks, how to calculate it correctly, and what nuances of the markings often go unnoticed when purchasing.
Basic definition and physical meaning of the parameter
To understand how disc offset is indicated, you need to visualize the wheel geometry. Imagine a cross-section of a disk. It has a central axis. Overhang is the offset of the mating plane (the one that presses against the hub) from this central axis. If the mating plane is shifted outward, the overhang is positive, if inward, it is negative. It is this parameter that determines how deep the wheel will “go” into the arch or, conversely, will stick out.
In technical documentation and on the stamping of the disks themselves, this parameter is almost always marked with the letters ET. This abbreviation comes from the German word "Einpresstiefe", which literally translates as "depth of indentation". Despite the German origins of the term, the ET designation has become an international standard and is used by manufacturers around the world, including Chinese and American brands. Sometimes you can find the designation Offset, but this is more often typical for English-language documentation.
⚠️ Attention: Never rely solely on the appearance of the disc. Two wheels with the same design and diameter may have different ET offsets, making one of them unsuitable for your vehicle.
The physical meaning of the parameter is to ensure the correct break-in shoulder and distribution of the vehicle’s weight. Engineers calculate the offset so that the load on the wheel bearing is optimal. If you change this parameter by changing the force application arm, you will artificially create a lever that will constantly “rock” the hub, accelerating wear of the parts.
Basic symbols on the inside of the disc
You don't need to be a professional auto mechanic to find departure information. All the necessary information is stamped on the inside of the disc, often on the spokes or in the center. The marking usually looks like a long string of numbers and letters, for example: 7.5Jx16 H2 ET45 5x112. Let's look at this abbreviation in parts, since they are all interconnected.
The first number (in example 7.5) is the width of the rim in inches. Letter J indicates the shape of the rim flange (hump profile), which is important for tubeless tires. Next comes the diameter (16), the number of holes (5) and the diameter of the circle of their location (112 mm, or PCD). And only after these parameters comes our desired ET45. The number 45 means that the offset is 45 millimeters.
- 🚗 ET - standard departure designation (Einpresstiefe).
- 📏 Offset - an alternative name, often found in catalogs.
- 🇩🇪 Einpresstiefe - full German name, rare, but can be found in full.
It is important to understand that the offset can be positive, zero and negative. Positive (for example, ET45) means that the mounting plane is shifted from the center of the disk towards the outer side. Negative (for example, ET-15) is typical for SUVs and means that the disc is “recessed” deeper and the wheel will protrude more outward. Zero offset means that the mounting plane coincides with the central axis of the disk.
Detailed explanation of markings and related parameters
Let's look at the full marking line in more detail, since disc offset cannot be considered in isolation from other parameters. Often car enthusiasts focus only on the diameter and offset, forgetting about PCD and DIA, which leads to the impossibility of installing the wheel. String 8.0Jx18 ET35 5x120 DIA 72.6 reads as follows: width 8 inches, diameter 18, offset 35 mm, 5 holes on a diameter of 120 mm, center hole diameter 72.6 mm.
Parameter PCD (Pitch Circle Diameter) must match perfectly. A difference of even 2 mm will result in the disc not fitting into place or the bolts being tightened askew, which is dangerous. Center hole diameter DIA (or D) is also critical. If the hole on the disc is larger than the hub, adapter rings (spacers) will be required, otherwise the wheel will vibrate. If it’s less, the disc simply won’t fit.
| Parameter | Designation | Units of measurement | Impact on the car |
|---|---|---|---|
| Departure | ET / Offset | Millimeters (mm) | Wheel position in the arch, suspension load |
| Width | J (inches) | Inches | Tire width, stability |
| Bolt pattern | PCD | Millimeters (mm) | Ability to physically install the disk |
| CO | DIA / D | Millimeters (mm) | Wheel alignment, no vibrations |
There is also an option Max Load, which indicates the maximum load on the disk. When installing long offset rims (or using spacers), leverage on the rim increases, so safety margins become even more important. Always select wheels with a load greater than half the curb weight of the vehicle.
What is Hump (H) in markings?
Hump (H) are ring-shaped protrusions on the rim flanges that prevent the tubeless tire from jumping off the rim when the pressure is suddenly reduced (for example, when turning). Designated as H, H2 (two humps) or FH (flat humps). For ordinary tires this is not critical, but for safety it is desirable to have at least one hump.
Departure mathematics: calculation formula and tolerances
For those who want to understand the essence of the process or calculate the parameters for a non-standard disk, there is a simple formula. Overhang (ET) is calculated as the difference between the distance from the mating plane to the outer edge of the disc and half the overall width of the disc. The formula looks like this: ET = a - b/2, where a is the distance from the mating plane to the edge of the disk, and b — total width of the disk.
Car manufacturers allow slight deviations from the standard offset. Typically the acceptable spread is ±5 mm for passenger cars. This means that if the standard offset is ET40, then wheels with ET35 or ET45 will most likely fit without problems. However, with a deviation of more than 5-7 mm, problems begin with the suspension geometry and bearing life.
⚠️ Attention: Using spacers to change the offset by more than 20 mm is strictly not recommended without strengthening the suspension, as this changes the kinematics of the levers and can lead to breakdown.
When calculating, it is also worth considering that the width of the rim (in inches) does not always correspond to the width of the tire. An inch is equal to 25.4 mm, but when converting rim width, the rounded value of 25 mm is often used to simplify calculations in engineering tables. However, to accurately calculate gaps, it is better to use an exact inch value.
☑️ Check before purchasing discs
Consequences of installing wheels with the wrong offset
Ignoring the ET parameter is not just an aesthetic issue. Installing a disc with a smaller offset (the wheel protrudes outward) increases the load on the hub bearing. The force arm lengthens, and when moving over uneven surfaces, the bearing experiences overloads, which can reduce its service life significantly. In the worst case, the hub may simply “break” and the wheel will come off.
If the offset is too large (the wheel is recessed deep inside), the disc may begin to touch the suspension elements, brake pipes or shock absorber during the compression stroke. It is also possible for the inner part of the disk or tire to rub against the side member or body elements when turning the steering wheel. This leads to damage to the braking system and loss of control.
- 📉 Accelerated wear of wheel bearings due to increased load shoulder.
- 🛑 Friction of the disc against the caliper or suspension elements when moving.
- 🚗 Deterioration of directional stability and change in turning radius.
In addition to mechanical damage, incorrect offset changes the vehicle's track. This affects the operation of the exchange rate stability system (ESP) and anti-lock braking system (ABS), since the sensors record incorrect wheel speed data. Electronics may malfunction, falsely activating security systems.
If you can't find rims with the right offset, it's safer to get a rim with a slightly higher ET (sink it deeper) than one with a smaller one. You can use washers to compensate, but never use washers if the disc is already protruding beyond the arch.
Specifics for SUVs and tuning
In the world of SUVs and SUVs, the laws of physics work the same way, but the goals are often opposite. They often try to increase the track to improve off-road stability. For this purpose, discs with negative offset are used (for example, ET-20, ET-40). Such discs significantly widen the track, moving the wheel beyond the arches. This requires the mandatory installation of arch extensions and a suspension lift.
Tuning wheels often have a complex design and non-standard parameters. When purchasing replicas (copies) of expensive disks, it is important to check not only the visual similarity, but also the actual dimensions. Chinese manufacturers sometimes provide approximate values. Therefore, before purchasing tuning wheels, it is highly recommended try on at least one wheel, ideally on the front axle, where there is least room for maneuver due to the shock absorber and steering linkages.
For heavy SUVs, not only geometry is critical, but also strength. Forged wheels allow complex shapes with long offsets to be made while maintaining strength, while cast wheels can be brittle with a strong negative offset. When tuning, always take into account the total weight of the vehicle with equipment.
The “Golden Mean” rule: the safest way is to buy wheels whose parameters completely coincide with the recommendations of the car manufacturer. Any deviations are always a compromise between appearance and suspension life.
Frequently asked questions (FAQ)
Is it possible to install a disc with an offset of ET35 instead of the standard ET40?
Yes, a difference of 5 mm is within the permissible technical tolerance for most passenger cars. The wheel will move outward by only 5 mm, which is visually almost imperceptible and will not have a critical effect on the life of the bearings. However, if the difference is 10 mm or more, this is already considered a significant change.
What happens if you confuse positive and negative offset?
A disc with a negative offset (for example, ET-10) simply will not fit into the place of a positive one (ET40) - its inside will rest against the suspension elements or brake caliper. If you physically managed to screw the disk on (which is unlikely without spacers), then the wheel will protrude far beyond the arch, which can lead to damage to it on the mudguards or body when loading the car.
Does disc offset affect speedometer readings?
The offset (ET) itself does not affect the wheel circumference, so it does not directly affect the speedometer readings. The speedometer reacts to the tire diameter. However, if the offset change is accompanied by the installation of a wider tire with a larger profile, the speedometer reading may change. Also, changing the offset can affect ABS performance if the effective rolling radius changes.
Do I need to do a wheel alignment after changing discs?
If you replaced the wheels with similar parameters (the same width and offset), no adjustment is required. But if you change the offset (ET) or rim width, the wheel alignment (camber) may change due to the suspension geometry. In this case, checking and adjusting the wheel alignment at the stand is mandatory to maintain directional stability and even tire wear.
How to find out the standard disc offset if the tag is lost?
Information on standard wheel parameters (size, offset, PCD) can be found in the vehicle's owner's manual (usually in the "Technical Data" or "Wheels and Tires" section). Also, this data is often duplicated on a sticker on the body pillar (near the driver’s door) or on the gas tank flap. As a last resort, the parameters can be found on the Internet using the VIN code of the car or model.