Installing wheel rims with inappropriate geometric parameters is one of the most common causes of accelerated wear of the suspension and even accidents on the road. Many car enthusiasts, when choosing a new “casting” or “stamps”, pay attention only to the diameter, width and number of holes, completely forgetting about disc offset or E.T. Ignoring this parameter may result in the wheel starting to rub against the side member or, conversely, protruding too far beyond the arch, collecting dirt and creating instability in control.
In this article we will analyze in detail what overhang is, why it cannot simply be “adjusted” and how to take measurements yourself if the markings on the disk are erased or are in doubt. Understanding the physics of the process will help you avoid costly mistakes when purchasing a set of wheels.
First, you need to understand that overhang is not the physical length of any specific part of the disk, but the calculated distance. Departure (ET) determines the position of the disk mounting plane relative to its longitudinal axis. It is this parameter that dictates how deeply the wheel is “recessed” into the arch or, conversely, how much it sticks out. An incorrect offset changes the break-in arm, which directly affects the handling and life of the wheel bearings.
It is critically important to understand: even a deviation of 5 mm from the factory values can lead to a shift in the center of gravity of the wheel and uneven distribution of loads on the suspension elements. Therefore, before buying discs “by eye” or on the advice of friends, you need to arm yourself with a tape measure and a calculator for accurate calculations.
What is disc offset and why is it important?
Disc offset, often indicated by markings ET (from German Einpress-Tiefe) or OFFSET, is the distance between the vertical plane of symmetry of the wheel and the plane of application of the disk to the hub. This parameter is measured in millimeters. It is impossible to visually determine the offset, since it depends on the internal geometry of the spokes and rim, which is hidden during installation.
There are three main types of departure, and they should absolutely not be confused. Positive departure means that the mounting plane is shifted outward from the center of the disk. This is the most common option for modern passenger cars. Zero departure implies that the mounting plane coincides with the axis of symmetry. Negative departure typical for SUVs and racing cars - here the mounting plane is shifted inward, closer to the passenger compartment, which allows for a wider track.
⚠️ Attention: Installing discs with an offset different from the factory one changes the distribution of forces acting on the suspension. Reducing the offset (the wheel sticks out) increases the load on the wheel bearing and arms, which can lead to premature failure.
Why do manufacturers regulate this parameter so strictly? The fact is that engineers calculate the kinematics of the suspension taking into account a specific run-in arm. Changing the offset changes the trajectory of the wheel when turning and compressing the shock absorber. This can lead to the fact that when the car is fully loaded or on bumps, the wheel begins to touch body elements or suspension parts, which can lead to an emergency stop.
Necessary tools and preparation for measurements
Before starting measurements, it is necessary to prepare a workplace and tools. To carry out precise work, you do not need complex professional equipment; a basic home craftsman’s kit is enough. The main condition is that the disc must be removed from the car, since it is impossible to measure the offset “in weight” or on an assembled wheel with rubber.
You will need the following items:
- 📏 Steel ruler or tape measure - it is better to use a hard metal ruler 50-100 cm long, since a soft tape measure can cause errors when sagging.
- 🔧 Flat, smooth surface - A concrete floor in a garage is ideal, but you can also use a smooth tile floor.
- 🧱 Two identical bars or bricks - they are needed to create reference points to lay the disk flat.
- 🧮 Calculator - for quick calculations, since the formula requires precision down to a millimeter.
It is also important to clean the disc from dirt, especially in the area of the central hole and along the edges of the rim. The presence of dried dirt or old balancing clay can distort the measurement results by several millimeters, which is unacceptable when calculating the geometry.
If there are old markings on the disc but they are hard to read, try gently scrubbing the area with a wire brush or using WD-40 to reveal the embossed characters.
Step-by-step instructions: how to measure disk parameters
The reach measurement process consists of several successive steps. The accuracy at each step determines the final result. Take your time, double-check the ruler readings several times to eliminate the human factor.
First you need to measure the overall width of the rim (W). Place the disc flat on a flat surface. Attach a ruler to the outer edges of the rim (not taking into account the decorative beads, if any, we measure exactly the working part where the rubber sits, but in practice they often measure the full dimension and divide it in half, but for the ET formula we need the full width). Record the resulting value.
Then turn the disc over with the inside facing up. We need to find the distance from the mating plane (the place where the disc is pressed against the hub by the nuts) to the inner edge of the rim. This distance is denoted as A1. Place a ruler perpendicular to the plane of the disc and measure the depth.
Now we have all the data for the calculation. The departure (ET) formula is as follows: ET = A1 - (W / 2). Where A1 - the distance from the mating plane to the inner edge, and W — full width of the disk. The result can be positive, negative or zero.
☑️ Checklist before purchasing discs
For clarity, let's look at an example. Let's say the full width of the disk (W) was 200 mm (about 7 inches). Distance from the mating plane to the inner edge (A1) is equal to 135 mm. Divide the width in half: 200 / 2 = 100 mm. Subtract half the width from A1: 135 - 100 = 35 mm. This means your disk is crashing. ET35.
Conformity table and permissible deviations
Many drivers wonder: is it possible to install a disc with an offset different from the standard one? The answer lies within acceptable limits. Car manufacturers provide a certain margin of safety, but it is not unlimited. Below is a table showing the effect of different ET values on wheel position.
| Departure type | Designation | Wheel position | Impact on the car |
|---|---|---|---|
| Positive | ET > 0 | Recessed into the arch | Standard position, stable handling |
| Zero | ET = 0 | In the center of the arch | Increasing track, increasing load on suspension |
| Negative | ET < 0 | Protrudes out | Risk of hitting the arches, instability on the track |
| Big E.T. | ET >> full-time | Heavily recessed | Risk of friction against the caliper or shock absorber |
There is a plus or minus 5mm rule which states that offset deviation within 5mm of the factory value is usually safe for most civilian vehicles. However, for sports cars or heavy SUVs, the tolerances can be even stricter. If you plan to install discs with an offset that differs by more than 5-7 mm, be sure to consult a specialist.
What happens if you install a disc with a large negative offset?
The wheel will significantly widen the car's track. On the one hand, this will improve cornering stability and add aggressiveness to the appearance. On the other hand, the load on wheel bearings, steering tips and levers will increase sharply. In addition, protruding wheels will actively throw mud at the sills, and on off-road conditions they can damage soft arches or mudguards.>
Impact of offset on handling and safety
Changing the geometric parameters of a wheel rim is not just a matter of aesthetics. Shoulder break-in - a parameter that directly depends on the offset and determines how the car behaves when braking and cornering. At normal reach, the forces generated during braking are compensated by the suspension design. If the reach is reduced (the wheel is pushed out), the force lever increases and the suspension experiences loads it was not designed to handle.
In addition, incorrect overhang affects the operation of the stability control systems (ESP) and anti-lock braking system (ABS). The electronics “knows” the car’s factory parameters and adjusts the braking forces based on them. Changing the geometry can introduce errors into the operation of these systems, making their reactions delayed or incorrect in an emergency situation.
It is also worth mentioning the law. In many countries, including the Russian Federation, installing wheels with parameters that do not correspond to the vehicle model may result in refusal to pass a technical inspection. If the change in offset has led to the wheel protruding beyond the dimensions of the body (beyond the arches), this is a direct violation of traffic rules and faces a fine.
⚠️ Attention: Using spacers to change the offset is a questionable method. Spacers increase the load on the bolts/studs and can cause the wheel to come off at high speed if they are made of poor quality material or are not installed correctly.
Common mistakes when selecting disks
One of the most common mistakes is to focus only on the diameter and number of holes (PCD). Drivers buy discs that physically fit onto the hub, but do not take into account that space inside the arch is limited. As a result, when you turn the steering wheel all the way, the wheel begins to rub against the spar or steering rod elements.
The second mistake is an attempt to “stretch” the arch by installing discs with a negative offset on a lowered suspension. Without installing wide fenders (arch extensions), this will inevitably lead to the fact that when the interior is loaded or on uneven roads, the tire will rub against the body, damaging the paintwork and the rubber itself.
The third mistake is ignoring the width of the rim when calculating the offset. Remember that reach is measured from the center of a specific width. If you install a wider wheel with the same offset (ET), its inner part will still move closer to the suspension elements. Therefore, when increasing the width of the rim, it is often necessary to reduce the offset in order to maintain balance.
FAQ: Frequently asked questions
Is it possible to install a disc with an offset of ET40 instead of the standard ET35?
Yes, such a deviation of 5 mm is considered acceptable for most passenger cars. The wheel will go inside the arch by 5 mm, which usually does not create problems with clearances to the caliper or shock absorber, but may slightly change the steering feel.
What do the J and H markings on the disc mean?
Marking J (e.g. 7J) refers to the shape of the rim flange where the tire bead sits. The number 7 is the width of the rim in inches. Marking H (Hump) indicates the presence of an annular protrusion on the rim flange, which keeps the tubeless tire from dismounting under lateral loads.
Does offset affect fuel consumption?
Indirectly - yes. Increasing the overhang (the wheel moving outward) worsens the aerodynamics of the car and increases rolling resistance. In addition, the increased load on the transmission and engine due to changes in the shoulder forces can lead to a micro-increase in fuel consumption, especially in the urban cycle.
How can I find out the standard offset for my car?
Information about standard wheel parameters (size, offset, PCD) is usually contained in the car’s service book, on a sticker in the driver’s door opening, or on the manufacturer’s official website in the technical specifications section.