Wheel drive selection is not just a matter of aesthetics or a desire to make the car look more aggressive. This is, first of all, a technical necessity, on which traffic safety, suspension life and even fuel consumption directly depend. Many car owners who decided to change the standard “stamps” for alloys or increase the diameter to install lower-profile rubber, face confusion in the numbers. An improperly selected disc can touch the arches, rub against the calipers of the brakes or create a dangerous load shoulder on the hub bearing.
In this article, we will analyze all the nuances of determining the geometric parameters of the wheel. You will learn where to look for factory marking, how to correctly make measurements yourself if the inscriptions are erased, and what tolerances are considered critical. Understanding these processes will help you avoid costly mistakes when buying a new set of wheels.
It is worth noting that modern cars, especially premium brands, often have unique wheel requirements. Therefore, simply taking a disk from a similar machine “by eye” is a risky venture. It is important to understand the value of each number in the code. ET, J and DIA.
Where to look for factory labeling and what it means
The easiest and most reliable way to find out the size of a disk is to find the appropriate labeling on its inner surface. Manufacturers are required to affix this data, and it is a code consisting of alphanumeric symbols. Usually this information is stamped on a spoke or in the central part of the disk facing inside the car. To see it, you often do not even need to remove the wheel - just turn the steering wheel to the stop and look into the arch, although for complete confidence it is better to dismantle the wheel.
The typical marking code is as follows: 8.5J x 18 H2 ET45 PCD 5x112 DIA 66.6. It may seem like a random set of symbols, but each value is strictly regulated. Number. 8.5 It's the width of the rim in inches. J - type of edge (shape of the board), 18 - mounting diameter. Letter H2 indicates the presence of humps (special ring protrusions) that keep the tubeless tire from turning. The most important for selection are the parameters ET (departure) PCD (number of holes and diameter of their location) and DIA (diameter of the central opening).
It is important to understand that labeling can be applied in an inch system (characteristic of the US and Japan) or in a metric system, although the latter is less common. If you see a number of different numbers, for example, 15x6.5This is the width and diameter respectively. Sometimes manufacturers use non-standard designations, so it is always worth checking with the manual to the car.
⚠️ Attention: If the labeling on the disc is completely erased, corroded or painted over, you can not rely on visual comparison. In this case, the only reliable way is to measure independently with a rod and a ruler with the subsequent calculation of the flight according to the formula.
Some manufacturers, such as BBS or OZ RacingThey may be marked with additional certification markings (TUV, JWL). This indicates that the disc has passed crash tests and meets safety standards. However, geometric dimensions always remain in the standard format described above.
Methods of self-measurement of diameter and width
If there is no marking or you want to double-check the data of the old drive, you will need simple tools: a roulette, a ruler or, ideally, a rod with a large sponge. Measuring the diameter is the most obvious procedure, but there are nuances here. Measure not the outer edge of the disc, and the inner diameter of the rim, where the side of the tire sits. This parameter is indicated in the marking (13, 14, 15, 16, 17, 18, 19, 20 inches, etc.).
Measurements of the interior are also required to determine the width of the disk. Apply the ruler to the inner edges of the rim (from side to side). The resulting value in millimeters should be translated into inches, dividing by 25.4. For example, if you get 178 mm, then 178 / 25.4 ≈ 7 inches. Rounding usually occurs to the nearest standard value (6.5, 7.0, 7.5, 8.0).
The most difficult thing is with the definition of departure (ET), since this is the calculated value. The departure is the distance between the vertical plane of the disk symmetry and the plane of application of the disk to the hub. It can be positive, zero or negative. To measure, place the disc flat on a flat surface. Measure the distance from the landing plane (which is adjacent to the hub) to the lower edge of the rim. Then measure the total width of the disk, divide it in half, and subtract the first value. The difference is your departure.
When measuring the diameter of the central hole (DIA), use a bar for maximum accuracy. This hole should fit tightly on the hub of the car. If it is smaller, the disk will not fit. If more, a beat will occur, which will have to be compensated by transition rings, which is not always safe for high-speed driving.
How to accurately measure disk departure (ET) and its impact
Disc departure, designated as ET (from German Einpresstiefe) or OFFSETThis is a critical parameter that is most often ignored by beginners when tuning. It depends on it how much the wheel will protrude from the arch or, conversely, go in. An error in the calculations can lead to the fact that the wheel will touch the suspension elements or body when turning the steering wheel or loading the car.
The formula for calculating departure is simple: ET = a - b/2where a the distance from the landing plane to the inner edge of the disk, and b - total disk width. All measurements are carried out in millimeters. If the resulting value is positive (which happens in 90% of cases for civilian cars), then the landing plane is shifted from the center of the disk outward. If it's negative, go inside. For front-wheel drive cars, a positive departure is characteristic, for rear-wheel drive it can be less or even negative.
The impact of departure changes on the vehicle’s behavior is significant. Reducing the departure (the disc becomes “wider” relative to the hub) expands the track, which theoretically improves stability in cornering. However, it also increases the load on the hub bearings and suspension elements as the lever strength shoulder becomes larger. In addition, wide discs can begin to touch the arches when the machine is fully loaded or on the bumps.
What happens if you put a disk with the wrong flight?
With too little departure (the disc sticks out strongly), the wheel can rub against the underwings or the body when turning. If the flight is too large (the disc is deeply recessed), the disc can touch the brake caliper or suspension elements. It is also possible to change the shoulder of the run-in, which will lead to steering and accelerated wear of the rubber.
There is a myth that the flight can be adjusted with the help of pads. Technically, it is possible, but legally and from a security point of view, it is controversial. The shelves change the geometry of the suspension and can cause weakening of the fastener under dynamic loads.
Determination of drilling (PCD) without special tools
Parameter PCD The Pitch Circle Diameter describes the number of mounting holes and the diameter of the circle on which they are located. This is one of the most important parameters: if the PCD disc does not match the PCD hub of the car at least a couple of millimeters, install the wheel will not work, or (worse) the nuts will not be tightened to the end, and the wheel can fly off on the go.
Standard PCD values for passenger cars: 4x98, 4x100, 5x100, 5x112, 5x120, 6x139.7. The first digit is the number of bolts, the second is the diameter in millimeters. If the number of holes everything is clear visually, then the diameter of the circle is difficult to measure with a conventional ruler, since it is necessary to find the centers of the holes and calculate the diameter of the described circle.
For 4-hole discs, it’s simple: measure the distance between the centers of opposite holes. For 5 holes, the formula is more complicated: measure the distance between the centers of adjacent holes and multiply the resulting number by the coefficient. 1.701. For 6 holes, the distance between the centers of opposite holes will be equal to the desired diameter. For 10 holes (rarely, but sometimes) the distance between the neighboring ones is measured and the result is multiplied by 3.236.
☑️ Checking disc compatibility
Using transition washers to change PCDs (e.g., from 5x112 to 5x120) is possible, but such designs require regular checks of bolt tightening. They are considered a temporary solution and are not recommended for continuous use, especially at high speeds.
Comparative table of popular disk parameters
To make it easier for you to navigate the variety of sizes, we have prepared a table with the most common parameters for different classes of cars. Note that this is averaged data and may differ for your particular model.
| Car class / Model | Diameter (inches) | Width (J) | Departure (ET) | PCD (drill) |
|---|---|---|---|---|
| Small class (Solaris, Polo) | 14-15 | 5.5 - 6.0 | 35-48 | 4x100 |
| C-Class (Focus, Astra, Golf) | 16-17 | 6.5 - 7.0 | 40-52 | 5x108 / 5x112 |
| Business (Camry, Optima, Passat) | 17-18 | 7.0 - 7.5 | 35-45 | 5x114.3 / 5x112 |
| Off-road vehicles (Duster, Creta) | 16-17 | 6.5 - 7.0 | 30-40 | 4x100 / 5x114.3 |
| Premium SUV (X5, Q7, G-Class) | 19-22 | 9.0 - 10.5 | 20-35 | 5x120 / 5x130 |
From the table it is seen that with the growth of the class of the car increases not only the diameter, but also the width of the disk, and also changes the requirements for drilling. SUVs often have smaller departures to provide greater stability and space for larger braking mechanisms.
Common errors in the selection and installation of disks
One of the most common mistakes is to ignore the diameter of the central hole (see below).DIA). Owners often buy disks with a large hole, counting on the fact that “bolts already hold”. That's a dangerous misconception. The centering of the wheel occurs precisely along the hole, and the bolts only press the disk. If there is no centering, vibration occurs that destroys the suspension.
The second mistake is trying to stretch a narrow disk or to squeeze a wide one. Installing a 225 mm wide tire on a 6 inch wide disc (instead of the recommended 7) will cause the tire profile to become rounded ("bubble"). This will worsen handling, increase tread wear in the middle and increase the risk of disassembling the wheel in a turn.
When buying used discs, be sure to check their beat on the balancing machine. Visually, the “curved” disc may look perfect, but when rotating give a strong vibration.
The third mistake is about the fastener. For cast discs, bolts with a longer threaded part or a cone/spherical hat other than the regular one are often used. Using an inappropriate fastener can lead to the fact that the bolt will rest in the bottom of the hole without clamping the disk, or, conversely, will hang out.
⚠️ Attention: After installing the new wheels, be sure to check the tire pressure and make a balancing. After 500-1000 km of run it is recommended to re-stretching bolts of fastening, as the new metal can sit down a little.
It is also worth mentioning the load on the axle. Discs for passenger cars may not withstand the weight of a heavy SUV or minivan, even if the dimensions are the same. Always pay attention to the labeling. MAX LOADThis indicates the maximum load per wheel.
Questions and Answers (FAQ)
Can I put an ET40 drive instead of a regular ET45?
A difference of 5 mm is acceptable for most cars. The ET40 will be outwards 5mm larger than the standard. This may improve stability slightly, but it will also increase the risk of contamination of the sidewalls of the body and grazing the arches when fully loaded. Before installation, it is recommended to try on the wheel and scroll the steering wheel to the point.
What do the letters H, H2, FH mean on the disk marking?
This is a designation of the type of humps - ring ledges on the shelves of the rim. They are needed for reliable fixation of a tubeless tire at lateral loads and sharp turns. H (Hump) is a simple protrusion, H2 is a double protrusion, FH (Flat Hump) is a flat one. For normal operation, the difference between them is insignificant, the main thing is that the tire corresponds to the type of rim (chamberless).
What is the difference between forged and cast discs when choosing a size?
Forged discs are stronger and lighter cast at the same dimensions. This allows them to be wider without a critical increase in weight, which has a positive effect on the dynamics of acceleration and suspension. But forging is much more expensive. When choosing a size for forging, you can consider options 1-2 inches larger than the standard ones, but with a lower rubber profile, while maintaining the overall diameter of the wheel.
Why can’t you use the bolts from stamped disks for casts?
Bolts for “stamps” often have a shorter threaded part or a different head shape. The cast discs are thicker at the site of the fit, and a short bolt may simply not reach the thread in the hub. In addition, the shape of the cone or sphere under the hat should perfectly coincide with the return hole in the disk, otherwise the bolt will not tighten with the desired effort.
The main rule of selection: geometric dimensions (PCD, DIA) should coincide perfectly, and deviations in width and departure are permissible within +/- 5-10% of factory values, but require mandatory fitting.