When choosing new wheels, most motorists first of all pay attention to the diameter, number of holes for fasteners (PCD) and the width of the rim. However, there is a parameter that is often overlooked, although ignoring it can lead to serious technical consequences. It's about fly-outwhich is marked by letters in the marking ET. Incorrect choice of this parameter can spoil the handling of the machine, accelerate the wear of the bearings of the hub and even cause an accident.
Disk departure is the distance between the vertical plane of wheel symmetry and the plane of application of the disc to the hub. This size is measured in millimeters and is a critically important geometric parameter. If you plan to install wheels that don’t meet your car’s factory specifications, you need to clearly understand the physics of the process of changing the track and shoulder of the run-in. In this article, we will discuss how much size ET It affects the behavior of the car on the road and why deviations from the norm are not always safe.
The physical meaning of the ET parameter and its measurement
abbreviation ET derived from the German word Einpress TiefeWhich literally translates as “deepness of pressure.” This distance shows how far the disk "lands" on the hub relative to its central axis. The measurement is made in millimeters, and it is this number that stands after the letter. ET on the inside of the spoke. Understanding the physics of the process helps to understand why shifting the wheel’s fulcrum relative to the suspension changes load vectors.
There are three main types of departure, the knowledge of which is necessary when selecting wheels. Positive departure means that the disk mounting plane is displaced outward from the central line of symmetry of the wheel. This is the most common option for modern passenger cars. Zero departure indicates that the attachment plane coincides with the center line. Negative departure characteristic of SUVs and means that the plane of attachment is shifted inward, to the brake mechanisms, making the wheel more "blown" outwards.
Always check the labeling on the inside of the old drive before buying a new one – often that’s where the exact ET settings are listed that fit your car.
It is important to note that the flight is directly related to the width of the disk. If you change the width of the rim, for example, from 7J to 8J, then to maintain the previous position of the wheel relative to the arch and suspension elements, the parameter ET It also has to change. Otherwise, the wheel will either rest in the shock absorber rack, or it will be too much to protrude beyond the dimensions of the body, soiling the sidewalls of the car with dirt.
The impact of departure on the handling and life of the suspension
Change of size ET causes the center of gravity of the wheel to shift relative to the axis of rotation of the hub. This, in turn, changes the so-called “shoulder run” – the distance between the point of contact of the tire with the road and the projection of the axis of rotation of the fist. Increased run-in shoulder (when reducing the departure) makes steering more sensitive, but at the same time transfers to the steering wheel all the irregularities of the road and increases the load on the steering mechanism.
Moreover, shifting the load application point from the hub bearing creates an additional lever that increases the torque acting on the bearing. If the factory calculation assumed a certain departure, then its significant reduction (going into the minus) can reduce the resource of the hub bearing many times. The bearing begins to experience loads for which it was not designed by engineers, which leads to its premature destruction and the appearance of a characteristic hum.
Optimal drive departure ensures uniform distribution of load on the bearings of the hub and guarantees stability of the course stability of the car at high speeds.
The suspension itself is also suffering. Levers, Silentblocks and ball supports experience increased tension when changing the geometry of the suspension levers. A car with an incorrectly selected departure can become "nervous" on the track, it is worse to keep a straight line when braking and have an increased turning radius. This is especially critical for front-suspension vehicles. McPherson.where geometry is rigidly tied to the parameters of the disk.
Pros and cons of deviation from factory parameters
Many tuning projects involve the installation of disks with a modified departure to expand the track and improve the appearance of the car. However, this approach has both advantages and serious disadvantages that should not be forgotten. Before you decide on experiments, weigh all the pros and cons.
- ✅ Improved stability in cornering by increasing the width of the track and reducing the rolls of the body.
- ✅ More aggressive and sporty appearance of the car, the wheels are not "sunk" in the arches.
- ❌ Accelerated wear of hub bearings, Silentblocks and steering tips due to increased load shoulder.
- ❌ Deterioration of acceleration and braking dynamics due to changes in the moment of inertia and the shoulder of the run-in.
- ❌ The risk of damage to the body elements (arches, splashers) with a full load of the car or the passage of irregularities.
It is worth noting that a slight deviation from the factory value ET (usually within ±5 mm) most wheel and car manufacturers consider acceptable. However, going beyond this framework requires a professional assessment of the suspension condition and regular monitoring of the technical condition of the units. Ignoring this rule can lead to expensive repairs of the chassis after several thousand kilometers of run.
Table of conformity and calculation of allowable values
To understand how the change in the width of the disk affects the desired departure, you can use a simple calculation logic. As the disk width increases by 1 inch (25.4 mm), the center of the disk is shifted by half of this value, that is, by 12.7 mm. To compensate for this displacement and leave the outer part of the wheel in the same place, it is necessary to reduce the value of the wheel. ET about 6-7 mm (since 1 inch in an inch system is often rounded, and the actual width pitch of the discs is 0.5 inch or 12.7 mm).
Consider an approximate match table for a popular size to illustrate the dependence. Let’s say the factory value for your car is ET 45 It's 7 inches wide.
| Disc width (inches) | Factory ET (example) | Permissible ET (min) | Permissible ET (max) | Commentary |
|---|---|---|---|---|
| 7.0 | 45 | 40 | 50 | Basic size |
| 7.5 | 42 | 37 | 47 | Wheel wider, ET less |
| 8.0 | 39 | 34 | 44 | Notable widening of the track |
| 8.5 | 36 | 31 | 41 | Requires arch inspection |
It is important to understand that the data in the table are reference. For each specific car there are tolerances, which are prescribed in the technical documentation. Critical is considered to be a deviation of the departure by more than 10 mm from the factory value without finalizing the suspension. In such cases, the load on the nodes increases exponentially, not linearly.
How to calculate the change of departure on your own?
The formula is simple: New ET = Old ET - (New width - Old width) * 12.7. For example, switch from 7J ET45 to 8J. 1 inch difference. 45 - 12.7 - 32. But it is to preserve the external border. To maintain the internal, you need a different meaning. It is better to use online disk calculators.
Practical recommendations for the selection of discs
The process of selecting discs with the correct departure requires care and compliance with a certain sequence of actions. Don’t rely on the words of the retailers in the stores, as their primary purpose is often to sell the product, not to save the life of your car. The best way is to consult the vehicle’s technical passport or find a sticker with manufacturer’s recommendations (often on the gas tank hatch or door rack).
If you still decide to experiment with the appearance of the car, installing wheels with a smaller departure, be prepared for the fact that you may need to install pads or finalize the arches. However, the use of slots is an additional risk, as they increase the load on the studs or bolts of the wheel mounting, which can lead to their break when driving dynamically.
☑️ Checking before buying discs
Particular attention should be paid to gaps. With a decrease ET (the disc becomes "outside") the inner part of the disc can touch the elements of the suspension or brake caliper, if it is not sufficiently deep. Increase ET (the disc "goes" inwards) the outer part of the disk or tire can rub against the inside of the arch or body elements when turning the steering wheel. Visual fitting of the disc before painting and tire mounting is a mandatory procedure.
⚠️ Attention: Installing discs with departures that are significantly different from factory ones can lead to legal problems during the technical inspection. In some countries, changing the suspension and chassis without appropriate coordination is a traffic violation.
Frequently Asked Questions (FAQ)
Can I put discs with ET less than the factory 10 mm?
A 10 mm reduction in departure is considered a boundary value. For most modern cars with a margin of suspension strength, this is acceptable, but the life of the hub bearings can be reduced by 15-20%. It is recommended to regularly check the presence of backlash in the hubs.
Does the ET parameter affect fuel consumption?
Yes, it does, but it's a little bit. Increased track (reduced ET) worsens the aerodynamics of the car and increases rolling resistance, which can lead to an increase in fuel consumption of 0.5-1 liters per 100 km, especially at track speeds.
What happens if you put a disk with ET more than recommended?
If the flight is too big, the disk will go deep into the arch. This can cause the inside of the tire or the disc itself to start touching the suspension elements, shock absorber or brake hoses when driving on irregularities or in corners, which is fraught with instant breakage.
Does the flight depend on the diameter of the disk?
No, the diameter of the disk (R16, R17, etc.) does not directly affect the value of the departure. The departure depends on the design of the spokes and the place of welding of the hub site. However, with increasing diameter, the requirements for disk width often change, which indirectly requires correction of ET.
⚠️ Attention: Never use discs with different departures on the front and rear axles of the car, unless it is provided by the design (for example, boiling). This will lead to a violation of the course stability and unpredictable behavior of the car on the road.