Among motorists who are passionate about tuning or just looking for a replacement for regular drives, you can often hear the phrase: “the smaller the departure, the wider the wheel is.” This statement may seem paradoxical, because the physical size of the tire remains unchanged. It is not about the size of the tire, but about its spatiality relative to the plane of attachment to the hub. Understanding this principle is critical for those who plan to install discs with non-standard parameters to avoid problems with arches and suspension elements.
Disc departure denoted by marking ET, is the distance from the vertical plane of wheel symmetry to the landing plane of the disk. This figure determines how deep the wheel is “sunk” into the arch or, conversely, how much it protrudes beyond the body. When we talk about a smaller departure value, we mean that the landing plane shifts closer to the inner side of the disk, which physically causes the entire wheel design to shift outward.
In this article, we will examine in detail the geometry of this process, consider the mathematical dependence and evaluate the consequences of such changes for the chassis of your car. Engineering accuracy This is a key factor, as even a seemingly small deviation can lead to serious mechanical problems or disturbances in the control of the vehicle.
The Geometry of Departure: How It Works Physically
To understand why the wheel is bulging, you need to visualize the structure of the disk. Imagine the disc in the cut. It has two main planes: the internal (which looks at brake calipers and shock absorbers) and the external (facing). In the middle is the symmetry plane. Flight out ET It is the delta between the center of the disk and the point where it is screwed to the hub.
If the flight is very important (for example, ET50), this means that the landing plane is shifted far from the centre towards the inner part of the disk. As a result, the wheel sits deep into the arch. If the flight is reduced (for example, before the ET35), the landing plane is shifted closer to the center of the disk or even goes beyond it (with a negative departure). Since the point of attachment to the machine (hub) remains stationary, the displacement of the landing plane of the disk causes the entire rim to shift along with the tire.
Remember the simple rule: less ET means more wheel outwards. The ET35 is more expensive than the ET45.
Thus, reducing the departure figure is mathematically equivalent to increasing the shoulder on which the wheel is held relative to the hub. This creates the effect of a lever pushing the wheel pair beyond the original track of the car. It's important to distinguish disk-width (J) and his departure (ET), although they are interrelated in the final dimension.
Mathematics of displacement: calculation of the difference of positions
To understand exactly how much the wheel will become “wider” after replacing the disks, you need to conduct simple calculations. The difference in departure is directly converted into millimeters of the wheel out. If you change the drives with departure ET45 fly-by ET35, the difference is 10 mm.
This means that the outer edge of the disk (and, accordingly, the edge of the tire) will shift outwards exactly by these 10 millimeters. At the same time, the inner edge of the disc will also shift outward by the same 10 mm, which can create a gap between the disc and the suspension elements, but remove the gap between the tire and the wing arch. The table below shows examples of displacement at different departure values relative to the standard parameter.
| Staff departure (ET) | New departure (ET) | Difference | Removal of the wheel outwards | Risk of arching |
|---|---|---|---|---|
| ET45 | ET40 | 5 mm | +5 mm | Low. |
| ET45 | ET35 | 10 mm | +10 mm | Medium. |
| ET45 | ET30 | 15 mm | +15 mm | High-pitched |
| ET45 | ET20 | 25 mm | +25 mm | critical |
It is worth noting that when calculating the final departure, it is necessary to take into account not only the parameter. ETBut also the width of the disk itself. If you are installing a larger disk (for example, 8J instead 7J) with the same departure, the wheel will also expand in both directions. However, it is the change in departure that gives the vector displacement of the entire structure in one direction - outward.
Impact on suspension and steering
The shift of the wheel’s center of gravity relative to the hub has a direct effect on the dynamics of the car. Increased shoulder (reduced departure) changes quinopause. Levers, which were originally designed for a certain load vector, begin to experience increased tension. This can lead to accelerated wear of the Silentblocks and ball supports.
Special attention should be paid to the steering. With a decrease in departure, the so-called “shoulder run-in” increases. This is the distance between the point of intersection of the wheel axis with the road and the center of the tyre contact spot. The larger the shoulder, the stronger the bumps from road irregularities are transmitted to the steering wheel. The car can be more emotional.ruttingThe steering wheel will start to “scour” in the hands.
⚠️ Attention: Excessive reduction of the departure can lead to the fact that when turning the steering wheel to the stop, the outer part of the tire will start to touch the splasher or splasher, which is fraught with rupture of rubber.
The nature of the work is also changing. shock absorbers. Due to the increased moment of inertia and change in the load lever, shock absorbers may not work in optimal mode, which reduces the efficiency of damping the oscillations and comfort when driving.
Problems with arches and bodywork
The most obvious problem faced by car owners when installing discs with a small departure is the contact of the wheel with the body elements. Even if there is a gap in a static position (the car is flat), it can disappear in the dynamics. When loading the car with passengers or cargo, the suspension is compressed, and the wheel, already displaced outward, comes even closer to the edge of the arch.
The situation is aggravated by the passage of turns. At this point, the body is heeling, and the outer wheel is pressed to the arch as tightly as possible. If the flight is selected incorrectly, the tread comes into contact with the plastic of the arch or, in the worst case, with the metal of the wing. Not only does it make an unpleasant grin, but it also destroys paintwork and the structure of the tire.
☑️ Checking disc compatibility
Winter operation should also be considered. Snow porridge and ice sticking to the inside of the arch can reduce the available gap by a few millimeters, turning a safe flight at first glance into a problem.
Impact on the behaviour of the car on the road
Changing the drive departure is not only a matter of aesthetics, but also a serious interference with the running gear settings. Increasing the track (the distance between left and right wheels) should theoretically increase the stability of the car in corners. The car becomes less prone to rolls, which may appeal to fans of active driving.
However, this medal has a downside. The increased run-in shoulder, which was mentioned earlier, makes steering more "nervous." The slightest gust of side wind or slope of the road requires more frequent thrusters. Driver fatigue It's increasing on the track. In addition, when braking on a slippery road, a car with a non-standard departure may behave less predictable.
Impact on fuel consumption
The increase in the drive's departure also affects aerodynamics. Wheels bulging outside the arches create additional air vortex, which may slightly increase fuel consumption at high speeds.
Changing the geometry of the wheel installation may require more frequent adjustment wheel-mounting anglesAs standard values may no longer ensure proper tire wear.
How to choose the right drive departure
Disk selection is a process that requires accuracy. You can’t rely on visual assessment or advice from friends. The first step should always be to study the technical documentation for your car. It specifies the permissible departure ranges. Deviations within ± 5 mm of the standard value are generally allowed without significant consequences.
If you want to put discs with a significantly smaller relegation for an aggressive style, you need to try it on. Ideally, install one wheel and check the gaps with the steering wheel completely turned and with a compressed suspension (you can use a jack to simulate the load). Also, it should be considered tyre-width: A narrower tire on a low-relegation disk may not touch the arch, while a wide one will rub.
⚠️ Attention: Using spacers for discs is a way to artificially reduce the flight. Remember that the wires increase the load on the hub bolts and require the use of longer bolts.
When choosing, also pay attention to the shape of the inner shelf of the disk. Even if the numbers of the departure fit, design features (ribs, shape of spokes) can lead to contact with the brake caliper or suspension elements.
Frequently Asked Questions (FAQ)
Can I put discs with departures less than the regular 10 mm?
In most cases, a 10mm deviation (e.g., ET45 instead of ET35) is considered a permissible limit for many cars. However, it depends on the specific model, tire width and arch condition. It is recommended to try it before buying a set.
How does the flight affect the load on the hub bearing?
Reducing the departure (removal of the wheel outwards) increases the lever acting on the hub. This leads to increased load on the hub bearing, which can shorten its service life, especially on bad roads.
What happens if you put a disk with a very small flight?
The wheel will strongly protrude beyond the dimensions of the body, soiling the sidewalls of the car with dirt. The risk of grazing arches increases during loading, handling at high speed deteriorates and wear of suspension parts increases.
Does the disk width affect the actual flight?
Yes, the width of the wheel (parameter J) affects the final position of the wheel. An 8J-wide ET40 drive will be different than a 7J-based ET40 drive. When calculating, you should always take into account the “width + departure” bundle.
Do I need to make a break-down after the change of the disks departure?
Preferably. Although the departure change does not break the suspension geometry directly, it changes the nature of its operation and the position of the wheel relative to the road. Checking the angles of the wheel installation will help to avoid uneven wear of the rubber.