When selecting new rims, car enthusiasts are often faced with a set of incomprehensible numbers and markings, among which the parameter occupies a special place ET. Many beginners mistakenly believe that this is an abbreviation for the English "Extra Terrestrial" or simply a factory code that has nothing to do with the physics of vehicle movement. In fact, this is a critically important geometric characteristic that determines the distance between the plane of attachment of the disk to the hub and its vertical axis of symmetry.
Ignoring value ET can lead to serious consequences: from simply touching the arch with a wheel when turning to accelerated destruction of the hub bearings. In this article, we will analyze in detail exactly how offset affects the car’s behavior on the road, why you can’t install wheels with arbitrary parameters, and how to correctly calculate the permissible deviations for your car.
Understanding the physics of the process will help you avoid costly suspension repairs and provide predictable handling in emergency situations. Let's figure out what's behind this marking and why engineers pay so much attention to it when designing the chassis.
Explanation of the concept of ET and wheel geometry
Abbreviation ET comes from the German word Einpresstiefe, which literally means “indentation depth”. In English-language literature, this same parameter is often referred to as Offset. The essence of the parameter is to measure the distance in millimeters between the mating plane of the disk (point of contact with the hub) and the plane passing through the middle of the rim width.
Depending on the location of the central plane relative to the fastening plane, the overhang can be of three types. It depends on how much the wheel will be “recessed” into the arch or, conversely, protrude beyond the dimensions of the body. Correct calculation of this value ensures the correct operation of all components of the chassis.
- 🔹 Zero departure — the disc mounting plane perfectly coincides with the central axis of symmetry of the rim.
- 🔹 Positive ET - the most common option, when the mount is shifted from the center to the outside, “hiding” the disk deeper into the arch.
- 🔹 Negative ET — the mounting plane is shifted to the inside, which is why the disc protrudes significantly outward, creating a “wide track” effect.
It is worth noting that most civilian passenger cars are characterized by a positive offset. Changing this parameter even by a few millimeters changes the vector of application of forces acting on the suspension when moving.
How to visually determine the type of departure?
Visually, without taking measurements, it is difficult to determine the type of departure. However, if you look at the spokes of the disk: if they go almost straight from the center to the edges, the offset is positive. If the spokes have a strong inward bend (toward the brake caliper), the offset may be negative or near zero. The exact data is always indicated on the label.
The physical effect of overhang on handling and stability
Parameter ET directly determines the vehicle's track width. When installing rims with a shorter offset (or negative offset), the wheels are spaced wider, which should theoretically improve cornering stability. However, this is a simplified understanding, since a change in geometry entails a displacement shoulder break-in — the distance between the point of contact of the tire with the road and the projection of the axis of rotation of the rack.
Factory engineers calculate the suspension design so that the break-in shoulder is minimal or negative. This ensures stability when braking and on slippery roads. If you change the disc offset, the break-in shoulder increases, and the car becomes more sensitive to road irregularities, and the steering loses its “zero” clarity.
⚠️ Attention: Installing rims with a very different ET changes the suspension kinematics. This can lead to a “steering” effect, when the car spontaneously changes its trajectory when hitting small bumps or ruts.
In addition, changes in reach affect the operation of stabilization systems ESP and anti-lock braking system ABS. The electronics are calibrated to standard wheel parameters, and a significant change in the lever arm (which is actually the disk) can introduce errors into the calculations of the on-board computer during emergency maneuvering.
Effect of incorrect ET on wear of suspension parts
The most critical consequence of ignoring a parameter ET is accelerated wear of suspension components. When the disc offset differs from the standard one, the load vector on the hub bearing shifts. Instead of being distributed evenly, the load ends up on the outer or inner row of rollers of the bearing, causing it to overheat and fail.
When installing a disk with an abnormal offset, the bearing life can be reduced significantly. This is especially true for the front suspension, where the loads are maximum. Also affected are the silent blocks of the levers and ball joints, which experience non-design bending and torsion loads.
The table below compares the effect of various offset deviations on the life of parts:
| Disk option | Effect on bearing | Impact on leverage | Risk of contact |
|---|---|---|---|
| Staff E.T. | Even wear | Design loads | Missing |
| ET less than normal (disk out) | Outer row overload | Increased bending moment | Arch, wing |
| ET greater than normal (disc inward) | Inner row overload | Resists on the shock absorber | Strut, brake hose |
It is also worth mentioning the braking system. If the offset changes significantly, the disc may begin to rub against the caliper or, conversely, move too far, which impairs the effectiveness of cooling the brake mechanisms by oncoming air flows.
When purchasing used discs, be sure to check the inner shelf for scuffs. If the disc rubs against the shock absorber strut, characteristic scratches will remain on the metal, and the geometry of the disc may be disrupted.
Tolerances and disc compatibility
Many car enthusiasts are wondering: is it possible to install wheels with a different ET? The short answer is yes, but within strictly limited limits. The engineering tolerance is usually ±5 mm from the standard value. That is, if the factory recommends ET 45, then the range from 40 to 50 mm is considered conditionally safe for short-term use.
However, it is worth considering that the permissible deviation depends on the class of the car and the type of suspension. For heavy SUVs or sports cars with rigid suspension, the requirements are stricter. For light city cars, small deviations may be less noticeable, but are still undesirable.
When selecting disks, it is important to take into account not only the offset, but also other parameters, which together give a picture of compatibility:
- 🔸 Rim diameter and width - affect the physical size of the wheel and its position in the arch.
- 🔸 PCD (bolt pattern) - must match perfectly, there are practically no tolerances here.
- 🔸 DIA (center hole) - must be equal to or greater than standard (using spacer rings).
If you plan to install wheels with an offset that differs by more than 5-7 mm, you must definitely consult with a suspension specialist or carry out a fitting and check the wheel angles and clearances.
☑️ Check before purchasing discs
Wheel spacers: a solution or a risk?
Often, to correct offset or install wider wheels, car enthusiasts resort to using spacers. These are metal or polyurethane discs that are installed between the hub and the wheel, effectively reducing the ET (taking the wheel out).
Using spacers up to 10-12mm thick is generally safe as long as they are made from high-strength aircraft-grade aluminum and are installed correctly. However, thicker spacers (20 mm and higher) are already a serious interference in the design, requiring the replacement of standard wheel studs with elongated ones.
⚠️ Attention: Cheap spacers made of silumin or low-quality steel can burst under load. This will lead to the wheel coming off while driving, with all the ensuing consequences. Saving on this item is strictly prohibited.
In addition, installing spacers increases the load on the wheel bearings, as the lever arm lengthens. This inevitably leads to a reduction in the life of the chassis. If your goal is purely aesthetic (to align the wheels with the arch), be prepared for the fact that the suspension will have to be serviced more often.
Spacers are a compromise between appearance and suspension life. For daily use, it's best to look for wheels with the correct factory offset and leave spacers for track cars or show projects.
Practical advice on selection and installation
When choosing wheels, always rely on the car manufacturer's recommendations, which can be found in the technical documentation or on a sticker in the doorway. If you want to change the appearance of the car, choose wheels within the acceptable tolerances.
Before final purchasing the set, be sure to try on at least one wheel. Check the gap between the inner disk flange and the suspension elements (shock absorber, brake hose, lever) with the steering wheel turned all the way.
It is also important to consider seasonality. In winter, when an ice crust builds up on the arches, the gap between the disc and the body decreases. A disc with a low offset may begin to rub against the fender liner even when the cabin is lightly loaded.
A correctly selected wheel is not only a beautiful appearance, but also a guarantee of the safety of you and your passengers on the road. Do not chase extreme parameters at the expense of reliability.
What happens if you install a disk with ET that is 20 mm smaller than the standard one?
The wheel will move outward by 2 cm. This will lead to an increase in the running-in shoulder, accelerated wear of the outer hub bearing, possible contact with the wheel of the arches when fully loaded or in turns, as well as a change in the speedometer readings (if the overall diameter of the wheel changes) and ESP operation.
Is it possible to install wheels with different offsets on the front and rear axles?
Strongly not recommended. Different offsets will result in different track widths front and rear, which will unbalance the car, cause unpredictable cornering behavior (oversteer or understeer) and uneven tire wear.
Does disc offset affect fuel consumption?
Yes, it does have an indirect effect. A disc with a smaller offset (protruding) worsens the aerodynamics of the car, creating additional air turbulence in the arches. This can increase fuel consumption by 1-3%, especially at high speeds. The load on the engine also increases due to changes in the transmission torque on the wheel.
How can I find out the standard disc offset if the markings have been erased?
It is necessary to remove the wheel and measure the distance from the mating plane to the inner edge of the disk, then to the outer one. The difference divided by half will give the desired value. Or you can find data on the Internet using the VIN code of the car or the model of standard wheels.