When purchasing new wheels or tires, car enthusiasts often come across the abbreviation ET followed by numbers, such as ET50. This is not just a set of symbols, but a critical parameter that determines the geometry of the wheel installation in the car arch. Understanding what it is ET50 on disks, is necessary to maintain controllability, suspension life and traffic safety.
If you see the ET50 marking, this means that the disc offset is 50 millimeters. Overhang is the distance between the plane of attachment of the disc to the hub and the vertical plane passing through the center of symmetry of the rim. Ignoring this parameter may result in the wheel touching the suspension elements or, conversely, protruding outward too much, disrupting aerodynamics and kinematics.
In this article we will analyze in detail the physical meaning of the offset, the consequences of installing disks with the inappropriate ET50 parameter and methods for calculating permissible deviations. You'll find out why positive offset is standard for most modern passenger cars and how to choose the right replacement for standard wheels.
Physical meaning of the ET parameter
The ET parameter (from the German EinpressTiefe) indicates the disc overhang. The number 50 indicates the magnitude of this offset in millimeters. It is important to understand that the offset can be positive, zero or negative. In the case of ET50 We are talking about positive offset, which means: the disk mounting plane is 50 mm closer to the outside of the wheel than its central axis.
Geometrically, it looks like this: if you look at the disk from the side, then an imaginary line running exactly in the middle of the rim width will be shifted inside the car by 50 mm relative to the pad that presses against the hub. This is why most factory disks have exactly positive offset, as this allows the wheel to be placed deeper in the arch, freeing up space for steering and braking components.
โ ๏ธ Attention: The ET50 parameter is not universal for all brands. For one car model, ET50 may be normal, but for another it can be critically dangerous. Always check the manual or table of acceptable sizes.
There is a common misconception that offset can be measured from the edge of the rim with a ruler. This is not true. The reference point is the geometric center of the rim width. If the width of the disk (parameter J) changes, the position of the mounting plane relative to the edges also changes, even if the ET value remains the same. Therefore, when replacing disks with wider or narrower ones, compatibility must be double-checked.
Effect of offset on suspension and handling
Installing a disc with an offset of ET50, if it is not provided for by the design, or replacing the standard ET50 with another parameter (for example, ET35 or ET60) radically changes the break-in period. The break-in shoulder is the distance between the point where the wheel's steering axis intersects the road and the center of the tire's contact patch. Changing this setting directly affects controllability and car behavior when cornering.
When the offset is reduced (for example, moving from ET50 to ET35), the wheel moves outward. This increases the break-in arm, making the steering sharper, but at the same time increases the load on the steering gear and wheel bearings. The car may become more sensitive to ruts and uneven roads, and the steering wheel will begin to โyawโ when accelerating and braking.
On the other hand, increasing the offset (eg ET65 instead of ET50) hides the wheel deeper into the arch. This can cause the inside of the tire or the wheel itself to touch the suspension components, shock absorber or brake pipes when the vehicle is fully loaded or the brake pedal is pressed. Mechanical damage braking system in motion is a direct threat to life.
It is also worth noting the effect on wear of parts. Incorrect offset creates abnormal load vectors on the levers and silent blocks. Instead of working in the designed mode, the suspension units experience constant bending or twisting, which reduces their service life significantly. Owners often wonder why, after installing โbeautifulโ disks, their wheel bearings quickly failed - the reason often lies precisely in the discrepancy between ET.
How to calculate the permissible offset deviation
Many car enthusiasts are wondering: is it possible to install a disc with an offset different from the standard one? The engineering tolerance is usually ยฑ5 mm from the nominal value. That is, if you had an ET50 disk, then installing a disk with an offset from ET45 to ET55 is considered safe and does not require additional calculations.
However, if you are planning a more radical change (for example, installing ET35 drives instead of ET50), you need to do the calculations. A difference of 15 mm is a significant shift in the center of gravity of the wheel. In such cases, it is important to consider the width of the disk. The formula for calculating the change in the position of the edge of the disk relative to the hub looks like this: ฮ = (J_new - J_old) / 2 + (ET_old - ET_new), where J is the wheel width in inches (converted to mm) and ET is the offset.
To simplify the task, you can use the following verification algorithm:
- ๐ Measure the distance from the inside of the arch to the hub in the most compressed state of the suspension.
- ๐ Find out the exact width of the new rim and tire in millimeters.
- ๐งฎ Subtract half the width of the rim and tire plus the new offset from the resulting distance.
- โ If the result is positive and is at least 5-10 mm of margin, installation is possible.
Calculation example for wide rims
If you install a disk with a width of 8J (203 mm) instead of the standard 7J (178 mm) with an offset of ET50, and the new disk has an ET35. Inner edge offset = (203-178)/2 + (50-35) = 12.5 + 15 = 27.5 mm inward. This is almost guaranteed to cause the shock absorber strut to hit.
Do not forget that when calculating, you need to take into account not only the static position, but also the dynamic one. When the body rolls during a turn or the suspension compresses over a bump, the wheel moves. Therefore, there should be plenty of space to spare. Safety clearance - this is a guarantee that the wheel will not destroy the arch or suspension in an emergency.
Consequences of setting an abnormal departure
Ignoring the manufacturer's recommendations regarding the ET50 parameter (or other standard value) can lead to a number of negative consequences. They range from mild discomfort to critical breakdowns. Below is a table systematizing the main risks.
| Deviation type | Possible consequences | Risk level |
|---|---|---|
| ET is significantly less than standard (wheel out) | Accelerated wear of bearings, steering wheel beating, arches hitting, tire wear | High |
| ET is significantly larger than standard (wheel inward) | Touching shock absorber struts, brake hoses, steering rod elements | Critical |
| Different offset on axles | Vehicle instability, pulling to one side, uneven brake wear | High |
Touching the brake hoses is especially dangerous. When driving on an uneven road, the wheel may bounce and rub the rubber hose of the brake system. This leads to an immediate drop in pressure in the circuit and brake failure. Loss of brakes at speed due to incorrectly selected disc offset is one of the common causes of serious accidents.
In addition to mechanical damage, the behavior of the car also changes. An increased running-in shoulder (with a decrease in ET) creates the effect of โself-tighteningโ of the wheel when braking on a slippery road, which can destabilize directional stability. The car becomes more nervous and requires constant steering correction.
Methods for correcting disc offset
If you have already purchased wheels with the wrong offset, there is a technical solution - spacers for the wheels. A spacer is a metal plate of a certain thickness that is installed between the hub and the disk. It allows you to change the effective offset of the wheel without replacing the discs themselves.
For example, if you have an ET50 wheel but your vehicle requires an ET35, you will need a 15mm thick spacer. However, the use of spacers has its own nuances. Spacers increase the load on the studs or mounting bolts because the leverage of the forces increases. Therefore it is important to only use forged spacers made of high-strength alloys (for example, aviation aluminum D16T or steel) and be sure to use extended bolts or studs.
โ๏ธ Check before installing spacers
There is also a method of โunweldingโ disks, when a strip of metal is welded to the disk, increasing its width. This is a complex technological process that requires professional equipment and subsequent balancing and leak testing. It is not recommended to carry out such work on your own, since a violation of the welding technology can lead to destruction of the disk while moving.
โ ๏ธ Attention: When using spacers with a thickness of more than 20 mm, the standard wheel bolts become short. The use of short bolts with spacers is strictly prohibited - this will lead to the wheel coming off.
Legal aspects and TรV
In the Russian Federation and the countries of the Customs Union, making changes to the design of a vehicle, which includes the installation of wheel rims with parameters not provided by the manufacturer, formally requires registration. If you install wheels with an offset that is very different from the ET50 (if the stock one was different), and this changes the car's track, you are formally making changes to the design.
In practice, small deviations (within 5 mm) rarely raise questions from traffic police inspectors during a visual inspection, if the wheels do not protrude beyond the dimensions of the body. However, during a technical inspection or in the event of an accident, an expert may identify a discrepancy. If it is proven that the cause of the accident was a violation of the suspension geometry due to non-standard disks, the insurance company may refuse to pay, and the driver may be charged with violating traffic rules.
In Europe, there is a TรV standard that strictly regulates permissible deviations. Certified discs always have documentation indicating which models they are suitable for. In Russia, there is no such strict certification for each specific car-wheel combination, so responsibility for safety lies entirely with the car owner.
When purchasing used wheels, always check the markings on the inside. Often sellers indicate only the diameter and bolt pattern, forgetting about ET, which is critically important.
Frequently asked questions (FAQ)
Is it possible to install an ET45 disk instead of an ET50?
Yes, a 5mm difference is within engineering tolerance for most vehicles. The wheel will move outward by only 5 mm, which is visually almost imperceptible and will not have a significant effect on the suspension. However, always check the clearance to the shock absorber strut if the disc is wider than stock.
What happens if you put ET30 instead of ET50?
The wheel will move outward by 20 mm. This can lead to the fact that when the car is fully loaded, the tires begin to touch the arches. The load on the wheel bearing and steering rack will also increase. This change is considered significant and requires careful compatibility testing.
Does ET50 offset affect fuel consumption?
Indirectly - yes. If the offset differs greatly from the standard one, the aerodynamics of the car deteriorate (the wheels protrude beyond the body fairings), which increases drag. In addition, increased friction in the suspension components due to abnormal loads can also slightly increase fuel consumption.
How can I find out exactly what my scheduled departure time is?
The standard offset is located on the back of your current disc. Look for markings in the format "ETxx" or "EinpressTiefe xx". This information can also be found in the technical documentation for the car or on specialized catalog websites, selecting wheels according to the car model.
ET50 is a positive offset, meaning the mounting plane is shifted 50 mm inward from the center of the disk. Compliance with this parameter is critical to the safety and durability of the suspension.