Accurately identifying the markings on the inside of the spoke is the only way to know the actual size of the wheel disc of the car if the technical passport is lost or the data in it is irrelevant. Often drivers see the numbers "R15" or "R16" and mistakenly believe that this is enough to buy a new tire or a replacement wheel, ignoring the critical departure parameters (ET) and the diameter of the central hole (DIA). Wrongly selected disk-out can lead to friction of the tire against the body elements or shock absorber rack, which will cause accelerated wear of the suspension and even an emergency situation on the road.
Car manufacturers indicate the permissible parameters of the wheels in the service book, on the sticker in the doorway or gas tank hatch, but these data are often general and do not take into account the nuances of installing non-standard disks. In the process of operating the car, the owner may face a situation where a visually suitable wheel is physically impossible to install on the hub due to a mismatch of holes under the bolts or insufficient internal diameter. Understanding how to decipher PCD (Pitch Circle Diameter) and other codes, avoids the purchase of inappropriate accessories and unnecessary financial costs.
There are several proven methods of dimensional identification: visual inspection of factory markings, use of a bar for manual measurements or reference to electronic catalogs by VIN code. Each of these methods has its own characteristics and degree of accuracy, depending on the safety of factory labels and the availability of equipment. In this article, we will discuss in detail the algorithms of actions for each method so that you can independently and accurately determine all the necessary geometric characteristics of the wheel drive of your vehicle.
Decoding of factory markings on disk
The fastest and most reliable way to find out the size is to find the digits on the inside of the spoke or on the rim of the disk. Marking is usually cast or engraved and contains a complete set of technical parameters in a certain order. A standard string looks like a set of numbers and letters, for example: 8.5J x 18 H2 ET35 5x112 DIA 66.6. Let’s look at each element of this chain, as understanding the syntax of writing is essential for reading data correctly.
The first number indicates the width of the rim in inches, and the letter "J" indicates the shape of the edge, which is important for landing a tire. The next number after x is the mounting diameter, the same R15 or R18 that everyone is looking for first. Next up is the code. fly-out (ET or Offset) which shows the distance from the landing plane to the vertical axis of wheel symmetry. An error in determining this parameter even by 5 mm can lead to the fact that the wheel will protrude too much behind the arch or, conversely, touch the brake caliper.
- 🔍 Rim width (e.g., 7.0 or 8.5) shall correspond to the width of the tyre profile specified by the manufacturer.
- 📏 Diameter. It is specified in inches and must be strictly consistent with the outer diameter of the tyre.
- ⚖️ ET (Einpress Tief) - departure, a critical parameter for loading on the hub bearing.
- 🔩 PCD - the number of holes and the diameter of their location, without the coincidence of which the disk simply will not be worn.
⚠️ Note: If the marking is erased or the disc is repainted, it is absolutely impossible to rely only on the visual assessment "by eye". The difference of 10 mm in departure is not noticeable visually, but it is dangerous technically.
Often, disc manufacturers use additional designations such as “H2” or “H,” which means the presence of humps – ring protrusions on the rim for reliable fixation of a tubeless tire. Maximum load (MAX LOAD) may also occur, expressed in pounds (lbs) or kilograms. For passenger cars, this parameter is rarely critical if the disc is original, but when selecting reinforced wheels for SUVs or commercial vehicles, it is worth paying close attention to.
Analysis of drilling parameters and central hole
Drill parameters denoted by the abbreviation PCDThey are second in importance after the diameter. The "5x112" record means that the disc is attached to 5 bolts located on a circle with a diameter of 112 mm. Incorrect determination of this size will lead to the impossibility of installing the wheel: even if the holes coincide in number, the displacement in diameter will not allow the bolts to twist to the end or will lead to the wheel beating when moving.
The central hole, or hub hole (DIA, CBD), should perfectly match the diameter of the car hub. If the hole in the disc is larger than the protrusion on the hub, the wheel is centered only by the cones in the bolts, which creates vibrations and the risk of cutting off the fastener. In such cases, transition rings are used, but factory-fitting diameters are the preferred option for safety.
Table of popular drills
Autoprom |Driller (PCD)|Central hole (DIA)
Volkswagen, Audi, Skoda|5x112|57.1 mm
BMW |5x120|72.6 mm
Mercedes-Benz |5x112|66.6 mm
Toyota, Lexus|5x114.3|60.1 mm
Ford, Volvo|5x108|63.4 mm
To measure a drill without marking, a bar and a simple geometric formula will be required. The distance between the edges of opposite holes (for a disk with 4 holes) or between neighboring ones (for calculation using the coefficient) is measured. However, given the complexity of manual calculation and the high probability of error, it is easier to use ready-made matching tables for your car brand, which are easy to find on the Internet on the model of the car.
Manual measurement methods of the disc
If the marking is missing completely, the only way to know the size is to make physical measurements using a measuring tool. You will need a long sponge bar or millimeter-division roulette, as well as a ruler. Accuracy is critical because the standards allow for minimal deviations that can be decisive in installation.
To determine the width of the disk, do not measure the total width from edge to edge. It is necessary to measure the distance between the inner edges of the rim (the place where the side of the tire adjoins the disk) and translate the result from millimeters to inches (divided by 25.4). Rounding is done to the nearest standard value (e.g. 6.0, 6.5, 7.0 inches).
Departure measurement (ET) is a more complex procedure. First, the disc is placed flat on a flat surface. The bar gauge measures the distance from the landing plane (the place of contact with the hub) to the inner edge of the rim. The total width of the disk is then measured. Formula of calculation: ET = A - (B / 2)where A is the distance from the plane to the edge, B is the total width. The result may be positive or negative, indicating the direction of the wheel axis displacement.
- 📐 Use a metal ruler to measure the diameter of the bolts if there is no precise tool.
- 🔢 Transfer all dimensions to millimeters to avoid confusion with the inch system.
- 🛑 Check the landing depth of the brake calipers, attaching the disc "on weight" before puffing.
⚠️ Note: With manual measurement of departure error of several millimeters is inevitable. If you plan to install forged disks with minimal gaps, it is better to trust professionals or catalog data.
Using a VIN code to select disks
The most accurate and safe method is the selection of the VIN code of the car. This 17-digit identifier contains complete information about the equipment of the machine, including the type of wheels installed at the factory. Online services and dealer directories use this code to issue a list of original items and their parameters, which eliminates the human factor in measurements.
To use this method, it is enough to enter a VIN in the search bar of a specialized store or spare parts catalog. The system will give the exact dimensions: diameter, width, drill and departure, recommended by the engineering department of the manufacturer. This is especially true for cars with non-standard options, where, depending on the engine or type of transmission, different braking systems and, accordingly, discs of different diameters could be installed.
However, it is worth remembering that the factory parameters are designed for standard wheels. If you are planning to install tuning disks with changed geometry, VIN data can only be a starting point. In such cases, changes in suspension design and possible limitations associated with the expansion of the vehicle's track must be considered.
The effect of disk size on controllability and flow
Changing the size of the wheel drive directly affects the dynamic characteristics of the car. Increasing the diameter of the disc while maintaining the overall external diameter of the wheel (by reducing the profile of the rubber) improves the course stability and accuracy of steering. The car becomes sharper in corners, the risk of aquaplaning is reduced, but the load on the suspension increases due to a decrease in the shock absorbing capacity of the tire.
The width of the disk also plays an important role. A wider disc allows you to install a wide tire, which increases the contact spot and improves traction. However, this leads to an increase in rolling resistance and, as a result, to an increase in fuel consumption. In addition, wide wheels on wet roads are prone to pop-up (aquaplaning) more often than narrow ones, since it is more difficult for them to divert water from under the tread.
| Parameter | Increased importance | Influence on the car |
|---|---|---|
| Disk diameter | Growth (R15 -> R18) | Improved handling, increased flow, stiffness |
| Disk width | Growth (6J -> 8J) | Better grip, increased flow, risk of arching |
| Departure (ET) | Decrease (ET40 -> ET30) | Expansion of the track, increasing load on the bearing of the hub |
| Disc weight | Growth (stamp -> forging) | Deterioration of acceleration dynamics, growth of fuel consumption |
It is important to maintain a balance between aesthetics and functionality. Too large wheels with a low tire profile on bad roads can lead to a rapid failure of not only the rubber itself, but also the suspension elements. Air suspension Adaptive shock absorbers may partially compensate for the stiffness, but do not override the physical laws of inertia of unsprung masses.
Frequent errors in selection and installation
One of the most common mistakes is to ignore the diameter of the central hole. Drivers buy wheels with a large DIA, counting on transition rings, but often forget that the rings must be of high quality and metallic. Plastic spacers may not withstand loads at high speed or when they fall into a hole, which will lead to a shift of the wheel relative to the hub.
Another mistake is to try to squander holes under bolts or use bolts of a shorter length. Shortened bolts do not provide the necessary area of engagement of the thread, which is fraught with spontaneous unwinding of the wheel on the move. It is also dangerous to use bolts from other cars with a different cone angle (for example, a cone of 60 degrees instead of 45), as this leads to disc skewing and the destruction of holes.
☑️ Checking before buying a disc
When installing pads to change the departure, it is necessary to use elongated bolts or studs. Standard fasteners in conjunction with the prostension have no right to life, since the thread simply will not enter the hub to the desired depth. Always check the length of the threaded part after installing any parcel elements.
⚠️ Attention: The use of paddles thicker than 20 mm requires the mandatory replacement of regular bolts with elongated ones or the installation of studs. Ignoring this rule can lead to the wheel being pulled off.
Seasonal nuances and wheel storage
The correct selection of discs is often dictated by the seasonality of operation. For the winter period, it is recommended to use discs of smaller diameter and with a higher rubber profile. This provides better comfort and less chance of disc damage if it is hit by the holes hidden under the snow. A narrow winter tire more effectively "cuts" snow porridge to a hard coating, improving traction.
Summer kits, by contrast, tend to be large diameters and low profiles for maximum performance. However, when storing such wheels in the assembly (with rubber), it is important to follow the rules: they are better stored in suspension or on special racks to avoid deformation of the sidewall of the tire under the weight of a heavy cast or forged disc.
The ideal disc is not only a beautiful design, but also an exact coincidence of all geometric parameters with the requirements of the car manufacturer. Safety is more important than appearance.
Do not forget to regularly check the condition of the fasteners. Nuts and bolts lose their tightening moment over time, especially after the first 50-100 km of running on new wheels. Control stretching is a mandatory procedure that will save you from losing a wheel on the track and ensure a long service life of the hub.
FAQ: Frequently Asked Questions
Can I put discs with a departure different from the factory?
Deflection (ET) of ± 5 mm for stamped discs and up to ± 7 mm for cast discs is allowed, unless this results in the infringement of the bodywork or suspension. Large deviations change the load on the hub bearing and can accelerate its wear.
What if the central opening of the disc is larger than the hub?
Transitional (center) rings should be used. They are made of aluminum or high-strength plastic and exactly match the diameter of the hub of your car and the inner diameter of the disk, ensuring the correct alignment.
How to find out the size of the disk, if there is no marking at all?
In this case, only accurate measurement of all parameters (width, diameter, drilling, departure) and comparison with tables of standard sizes will help. You can also try on the disc on the car, checking the free rotation and the absence of friction.
Does the size of the disc affect the speedometer readings?
The size of the disk does not affect the overall outer diameter of the wheel (disk + tire) is important. If you choose the rubber when changing the disk so that the total diameter remains the same, the speedometer readings will not change. If the diameter has changed, there will be an error.