When choosing new wheels, motorists often encounter a set of incomprehensible numbers and abbreviations on the inside of the product. One of the most important characteristics to pay attention to is the parameter, denoted as ETH or ET. It depends on this value whether the disc will stand on the hub of your car, whether it will touch the suspension elements or body arches.
Incorrectly selected departure can lead to serious consequences, including accelerated wear of the hub bearings, changing the width of the track and even destroying the suspension elements at high speed. In this article, we will discuss in detail the physical meaning of this value, the ways it is measured and the tolerances that your car can withstand without risk to safety.
Understanding disc labeling is not just theoretical knowledge, but a practical skill that will help you avoid buying inappropriate parts. Many car owners make the mistake of buying beautiful, but technically incompatible wheels, which eventually translates into additional costs for the repair of the chassis.
Physical sense of the ET parameter and disk geometry
abbreviation ETH It comes from the German word "Einpresstiefe", which literally translates as "depth of pressing". In technical documentation in English, this parameter is often referred to as Offset. The essence of the parameter is the distance between the plane of attachment of the disc to the hub and the vertical plane passing through the center of the width of the rim.
If you think of the disc in a section, you see two important lines: one where the disc is screwed to the car, and the second, which divides the width of the rim exactly in half. The distance between these planes is the departure. It can be positive, negative or zero, which determines the position of the wheel relative to the arch of the car.
Visually, this influence manifests itself as follows:
- π Positive ET: The mounting plane is shifted outward from the center of the disk, the wheel is "recessed" deeper into the arch (standard for most passenger cars).
- π Negative ET: The plane of attachment is closer to the inner side of the disk, the wheel strongly bulges outward (typical for SUVs and jeeps).
- ποΈ Zero ET: The mounting plane coincides with the central axis of the disc.
It is important to understand that suspension-geometry It was originally designed by engineers for a specific flight. Changing this parameter changes the run-in shoulder - the distance between the tyre contact point with the road and the projection of the wheel axis. This directly affects steering effort and vehicle stability.
How to visually determine the flight without measurements?
If you take two discs of the same width and put them on the outside side to each other, the distance between the inner planes of the attachment will be equal to twice the departure value (provided the same ET). If the discs are different, the difference in depth of landing immediately catches your eye.
Why you can not ignore factory departure standards
Many motorists mistakenly believe that if the disk sat on the hub and the bolts twisted, then everything is fine. However, ignoring the manufacturer's recommended ET It can be fatal for the undercarriage. The main problem lies in the redistribution of load vectors on the suspension.
When installing a disk with a smaller departure (the wheel bulges more strongly), the shoulder of the run-in increases. This leads to the fact that when turning the steering wheel and passing irregularities on the bearing of the hub and ball supports, much greater forces are acting than was calculated during the design. The resource of these details can be reduced significantly.
β οΈ Attention: Installing discs with a departure that differs from the standard by more than 5 mm creates a critical load on the suspension elements, which was not taken into account by the engineers of the manufacturer.
On the other hand, too large a flight (wheel recessed deeper) can lead to the fact that the inner part of the disk or the bus itself will start to touch the shock absorber or elements of the suspension levers during compression. This is especially true for cars with low ground clearance or sports suspension.
In addition, the change in the width of the track of the car affects its course-stability. Too wide a track can improve stability on the straight, but impair maneuverability and increase the load on the steering mechanism. Too narrow - will make the car more rolling and prone to drifts.
Permissible deviations and compatibility table
The question of whether it is possible to put discs with a different departure worries many. Strictly speaking, any deviation from the norm is undesirable. However, in real life, finding discs with millimeter accuracy can be difficult. There is a concept of "permissible spread", which depends on the design of the suspension of a particular car.
For most passenger cars with suspension type McPherson. A deviation within Β± 5 mm of the standard value is considered acceptable. For multi-link suspensions, the requirements can be tougher, since there the geometry of the levers is more complex and sensitive to changes. For SUVs with dependent suspension, tolerances may be wider, but it is important to take into account the dimensions of the arches.
Below is a table illustrating the effect of the departure change on the wheel position (with a disk width of 7 inches):
| Staff E.T. | Installed EET | Difference | The result |
|---|---|---|---|
| 45 mm | 45 mm | 0 mm | Perfect matching |
| 45 mm | 40 mm | -5 mm | The wheel extends 5 mm outwards |
| 45 mm | 35 mm | -10 mm | Critical performance, risk for suspension |
| 45 mm | 50 mm | +5 mm | Wheel sunk 5mm inwards |
It is worth noting that the width of the disk also plays a role. If you change the width of the disk, even if you keep the number. ETThe physical position of the wheel will change. The formula for calculating the change in the position of the center of the disk looks like this: Ξ = (Breadth new - Width old) * 12.7/2 + (ET old - ET new).
When selecting drives, always use online calculators or specialized suitability guides for your particular car model. This will avoid errors associated with kompleksnym izmeneniem parametrov.
Safe is considered to be a departure deviation of no more than Β± 5 mm for passenger cars, but only if the disc does not touch the body elements and suspension.
The impact of departure on the handling and wear of tires
Change of parameter ET It directly affects the behavior of the car on the road. With a decrease in the departure (the wheel becomes wider along the track), the car becomes more stable in corners, as the shoulder of resistance to lateral forces increases. However, this has a downside: the steering becomes heavier, and the feedback on the steering wheel can become less informative.
In addition, the bulging wheels quickly stain the sidewalls of the car with dirt and snow in winter. But the bigger problem is the tire wear. When changing the departure angle changes the angle of inclination of the plane of rotation of the wheel relative to the road, which can lead to uneven wear of the tread, especially if the wheel collapse was not corrected after installing new discs.
- π Increased load: on the bearings of the hub in negative departure.
- π Changes in kinematics: suspension, which affects the operation of stabilizers and silentblocks.
- π Risk of hitting: internal elements of the disc of brake calipers or shock absorbers.
Owners of cars with a system ESP (Electronic stability) should be especially careful. Changing the width of the track and the shoulder of the run-in can make adjustments to the algorithms of the system, since the angular velocity sensors will read information under changed conditions.
After installing discs with a different departure from the regular one, be sure to visit the stand of the collapse-descendence. This will prolong the life of your tires and return the sharpness of the steering.
How to measure the diskβs departure by yourself
If the labeling on the disc is erased or in doubt, the parameter ET You can calculate it yourself using a ruler and simple mathematics. For this purpose, complex tools are not needed, it is enough to know the width of the disk and the distance from the landfill plane to the rim edge.
The measurement process is as follows:
- Place the disc flat on a flat surface with the outside up.
- Place a straight rack or ruler across the disk through the center.
- Measure the distance from the landing plane (the place of contact with the hub) to the inner edge of the rim. Let's define this meaning as
A. - Measure the total width of the rim of the disc (not including the edges). Let's mark it
B. - Calculate the departure by the formula:
ET = A - (B / 2).
4 mm), although inch values can be used for approximate calculations, if you are careful.
The resulting value may differ from the factory by 1-2 mm due to measurement error or features (casting), but if the difference is 5-10 mm or more, then most likely the disc does not fit your car or has a non-standard geometry.
βοΈ Checking before buying used discs
Specificity of departure for different types of cars
Parameter requirements ETH It varies greatly depending on the class of the car. For urban sedans and hatchbacks, a large positive departure is characteristic, usually in the range from +35 to +55 mm. This is due to the compactness of the engine compartment and the need to place the wheels inside the arches, without touching the shock absorbers.
For SUVs and crossovers, the situation is different. There are often discs with a departure from +10 to +30 mm, and for prepared jeeps - even a negative departure. This is necessary to increase the track, which increases the cross-country and stability on the road. However, on the track, such wheels can adversely affect handling.
β οΈ Attention: The use of wheel arch expanders (spacers) for the correction of departure is permissible only as an exception. Spacers put additional stress on the studs of the wheels and can become a source of vibrations.
Sports cars require an individual approach. Here, the departure is selected in such a way as to maximize the track for better stability in cornering, but the wheel should fit perfectly into the extended arches. Often for such cars are used disks with forgingwhere the geometry can be changed to order.
When tuning a car, always remember the balance. The desire to make the car wider and more aggressive should not outweigh the safety and resource requirements of the units. If you are planning a major departure change, consult with specialists who will be able to assess the condition of your suspension and give accurate recommendations.
Frequently Asked Questions (FAQ)
Can I put discs with departures less than the regular 10 mm?
Technically, the disc will stand on the hub, but this is considered a critical deviation for most passenger cars. This will lead to increased wear of the hub bearings, a change in the behavior of the steering and possible touching of the arches. The permissible limit is generally considered to be Β± 5 mm.
Does the ET parameter affect fuel consumption?
Yeah, it's indirectly. An increase in departure (wheel inward) or decrease (gauge extension) changes the aerodynamics of the wheel arches and rolling resistance. In addition, the increased load on bearings and transmissions can also slightly increase fuel consumption.
What happens if you confuse the discs with different departures on the front and rear axles?
It's strictly forbidden. The car will become unstable, it will be taken aside, the work of the ABS and ESP systems will be disrupted. Different track widths in front and rear will lead to unpredictable behavior of the car in corners and emergency braking.
Does the flight depend on the diameter of the disk (R15, R16, R17)?
Diameter alone does not dictate departure, but for different diameters manufacturers often provide different ET values. When selecting a disk, it is important to look not only at the radius, but also at a set of parameters: width (J), departure (ET) and diameter of the central hole (DIA).