ET on disks: what is it, how to calculate it and can it be changed

Parameter ET on a stamped or light alloy wheel it indicates the distance from the vertical plane of symmetry of the wheel to the mating plane of attachment to the hub, and it is this figure that determines how deeply the wheel is “recessed” into the arch or, conversely, sticks out. Incorrect calculation of this value when purchasing new wheels often leads to the fact that the tire begins to rub against the suspension spar when turning the steering wheel or, even worse, the inner part of the tire touches the elements of the brake system and shock absorber. In manufacturers' technical documentation, this parameter may also be abbreviated Einpressung (German origin of the term) or simply the word Offset, but the essence remains the same: this is a critical geometric characteristic, ignoring which jeopardizes traffic safety and the life of the vehicle's chassis.

If you notice that after replacing the wheels, the car has become worse at keeping straight, and vibrations have appeared on the steering wheel at high speeds, the cause is often precisely the shift in the center of gravity of the wheel relative to the hub. Disc offset directly affects the load on the hub bearings: the greater the difference between the standard value and the installed value, the faster the wear of the metal suspension elements occurs. Understanding the physics of the process allows you to avoid expensive repairs of levers, steering rods and the discs themselves, which may not withstand the increased shoulder of force application.

Physical meaning and geometry of the ET parameter

To figure it out ET in disks - what exactly is the size, it is necessary to imagine the wheel in cross-section. An imaginary line dividing the disk exactly in the middle of its width is a plane of symmetry. The distance from this line to the surface with which the disc is pressed against the car hub is the desired offset. If the mating plane is closer to the outside of the disk (from the street side) than the plane of symmetry, then the offset is considered positive. In the case when the mounting location is shifted to the inside (towards the brakes), the offset becomes negative, although positive values ​​are typical for most civilian passenger cars.

Changing this parameter even by 5 millimeters changes the distribution of force vectors acting on the suspension. When installing a disk with a smaller offset (the wheel protrudes outward), the load on the suspension elements increases in pull-out, and with a larger offset (the wheel is recessed deeper) - in compression. Automotive engineers calculate these loads with a reserve, but systematically exceeding the permissible values leads to metal fatigue and the appearance of microcracks in suspension arms and shock absorber mounting locations.

It is important to understand that the disk width (parameter J or JJ) and departure ET interconnected. When changing the rim width, you often have to adjust the offset to maintain the correct position of the wheel in the arch. However, this must be done with extreme caution, since a simple arithmetic difference does not always take into account the design features of a particular car model, such as shock absorber travel and wheel steering angle.

Mathematical calculation and determination formula

For those who are accustomed to relying on exact numbers, there is a formula by which the flight is calculated: ET = a - b/2. In this equation a is the distance from the mating plane of the disk to its inner edge, and b — total width of the disk. Manufacturers usually do not require the car owner to carry out these measurements themselves, since the markings are applied to the inside of the disc. However, knowledge of the formula helps to understand the principle: any change in the width of the disk requires recalculation of the offset to maintain the rolling geometry.

Let's look at a practical example. If you have a 7" wide disk (which is approximately 177.8mm), then half the width is 88.9mm. If the distance from the mating plane to the inner edge is 120 mm, then the offset will be equal to 120 - 88.9 = 31.1 mm, which is marked as ET31. An error in measurements even by a couple of millimeters can lead to incorrect selection of components, so when measuring old disks yourself, use an accurate caliper and a straight ruler.

⚠️ Warning: Never rely solely on visual assessment or eyeball measurements. Marking ET often stamped into the spokes or inside of the disc, sometimes under a layer of paint or dirt. Using incorrect calculation data may result in the purchase of a kit that is physically impossible to install without compromising security.

There is a common misconception that you can compensate for incorrect offset using spacers. Although this is technically possible, installing spacers changes the rolling arm and increases the load on the hub bolt, which is not always provided for by the design of the unit. If you plan to use spacers for correction disc ejection, make sure that the thread length of the standard bolts is sufficient for reliable fastening, or purchase extended fasteners of the appropriate strength class.

Tolerances and size compatibility

The question is whether it is possible to install disks with other ET, worries many car enthusiasts who want to improve the appearance of their car or install wider tires. A deviation within ±5 mm from the standard value for most passenger cars is considered acceptable. In this range, changes in suspension geometry are minimal and do not lead to a critical reduction in the service life of the components. However, going beyond these limits is already considered interference with the design of the vehicle.

If you install a disk with an offset less than standard (the wheel protrudes more), you increase the vehicle's track. On the one hand, this can improve cornering stability, but on the other hand, the risk of damage to the wheels on curbs and contamination of the sills with dirt increases sharply. In addition, the protruding wheel may touch the arch when the vehicle is fully loaded or when going over uneven surfaces, which will lead to destruction. wing or bumper.

When installing a disk with a large offset (the wheel is recessed deeper), the situation is often even more dangerous. The inner part of the tire or the wheel itself may begin to touch the suspension components, brake hoses or shock absorber housing. This is especially critical for cars with multi-link suspension, where the space in the wheel arch is limited by many levers and rods. Even slight friction while driving can cause a brake hose to rupture or a wheel to seize.

It is worth noting that for SUVs operated in difficult conditions, the requirements for compliance with geometry are even stricter. An increase in the force application arm when overcoming fords or diagonal hanging can instantly break the hub or bend the axle shaft if the disk parameters do not correspond to the engineering calculations of the manufacturer.

The effect of incorrect offset on the suspension

Ignoring a parameter ET leads to a chain reaction of wear of parts. The first to suffer are the wheel bearings, which begin to hum and play due to the increased radial load. Next, the silent blocks of the levers come into play, which, if the wheel is installed at an incorrect angle, experience constant twisting, which is not provided for by the design of the rubber-metal hinge.

The steering also responds to changes in reach. Increasing the reach (negative offset) makes the steering wheel more sensitive, but at the same time increases the risk of receiving impacts through the steering rack from road irregularities. This can lead to accelerated wear of the rack teeth and the appearance of knocking noises in the front of the car. The driver may notice that the car has begun to “scour” along the track, and maintaining a straight trajectory requires constant steering.

⚠️ Attention: If after installing new discs you feel the steering wheel beating at speeds above 80 km/h, do not rush to do balancing. Check the parameter matches ET and the quality of fit of the disc to the hub. Runout can be caused by deformation of the disc due to improper loading or contact with internal elements.

In the long term, incorrect overhang affects the performance of active safety systems such as ESP and ABS. Wheel speed sensors transmit distorted data due to changes in the effective rolling radius and the dynamics of wheel behavior, which can lead to incorrect operation of stabilization systems at a critical moment.

Visual impact and style

Many car owners deliberately change the offset of the discs for the sake of aesthetics. Installing disks with minimal ET (the so-called “in the heat”) allows you to make the car visually wider and more aggressive. The wheels become flush with the arches or even protrude slightly beyond them, creating the effect of a powerful sports car. This style is popular in tuning communities, where appearance often takes precedence over practicality.

However, it is worth considering the other side of the coin. Protruding wheels not only spoil the appearance of a clean car with flying dirt, but can also cause problems during technical inspection. In some jurisdictions, protruding a tire beyond the body without installing fender flares is a direct violation of traffic rules and grounds for a fine or prohibition of operation.

To achieve a balance between style and safety, experts recommend using the method flush (level with the arch). This is a condition where the outer edge of the disc is in the same plane as the edge of the wing. This can be achieved by choosing wheels with a slightly smaller offset, but remaining within reason so as not to affect the suspension. This approach keeps the car's handling close to factory, giving it the desired appearance.

Diagnosing departure-related problems

Determine that problems with the car are caused by incorrect ET, it is possible based on a number of indirect signs. First of all, pay attention to the wear pattern of your tires. If the inner or outer part of the tread wears off much faster than the rest of the surface (the so-called “hernia” or uneven wear of the shoulder area), this is a signal of a violation of the wheel alignment angles caused by abnormal disc geometry.

It is also worth listening to the sounds coming from the area of the wheel arches. A characteristic rustling sound when turning the steering wheel to its extreme position indicates that the tire is touching the wheel arch liner or suspension elements. A visual inspection of the inner surface of the arch after driving on a wet road may show fresh abrasions or the absence of traces of dirt in the friction areas, which is also a diagnostic sign.

Checking hub play is another diagnostic method. Rock the wheel by grasping the top and bottom points. The presence of free play, which was not there before, may indicate accelerated wear of the bearing due to overload by incorrect overhang. In this case, immediate replacement of the bearing and review of the parameters of the installed disks are required.

Frequently asked questions (FAQ)