The selection of wheels is not just a matter of aesthetics, but a critical technical task, on which the safety of driving and longevity of the suspension depends. Incorrectly selected parameters can lead to rapid wear of the bearings of the hub, touching the wheel of the body elements or even tearing the wheel off on the move. Owners are often confused about the numbers stamped on the inside of the product, not realizing that each letter and number have a strictly defined meaning.
The marking is applied by the manufacturer in accordance with international standards and contains all the necessary information for identification. Ignoring this data When buying “beautiful” cast discs, it often turns into the inability to install them or hidden problems in operation. We will learn how to read this code correctly to avoid errors.
Understanding the labeling structure will allow you to independently verify that the new drives meet the requirements of your car. This is especially true when buying used kits or ordering goods from online stores, where physical fitting is impossible. Knowledge of the nuances will help save time and money, eliminating the return of the wrong product.
Basic formula for disc marking
The central element of identification is the main string of characteristics, which looks like a set of numbers and letters separated by symbols. For example, a record. 8.5J x 18 H2 ET45 5/112 PCD 66.6 It may seem complicated, but it’s actually structured data. The first number (8.5) indicates the width of the rim in inches.This is a key parameter for the selection of rubber.
Letter J indicates the type of rim edge, which is important for the proper fit of the tire, although for passenger cars this parameter is most often standardized. The number 18 is the diameter of the disc in inches, which must strictly correspond to the landing size of the tire. Symbols H2 They indicate the presence of humps (protrusions) for reliable fixation of a tubeless tire, which is the standard for modern cars.
Next is the parameter. ET45, which indicates the departure of the disk in millimeters. This is the distance from the landing plane to the vertical axis of wheel symmetry. An error in choosing a departure even by 5 mm can lead to the wheel rubbing against the shock absorber rack or, conversely, will protrude too much behind the arch, disrupting aerodynamics and collecting dirt.
Can I change the drive's departure?
Change departure (ET) is possible only in a smaller direction (wheel goes deeper into the arch) and only if it will not touch the elements of the suspension. Increased departure (wheel sticks out) is not recommended without an elevator kit or paddles, as it changes the shoulder of the run-in and creates a huge load on the hub bearing, which can fail for a couple of thousand kilometers.
Fixing parameters: PCD and DIA
One of the most important and error-free parameters is PCD Pitch Circle Diameter is the diameter of the circle on which the fastening holes are located. In the labeling, it looks like a fraction, for example, 5/112. The first digit (5) indicates the number of bolts or studs, and the second (112) is the diameter of the circle in millimeters on which they are located.
Visually distinguishing a disc with a PCD 100 from a disk with a PCD 108 is almost impossible, since the difference in diameter is only 8 millimeters. However, an attempt to install a disc with a mismatched PCD will lead to the fact that the nuts will not be screwed up to the end or the wheel will get skewed. This creates the risk of spontaneous spinning of the wheels. during the movement, which is a direct threat to life.
The central opening, designated as DIA or Hub DiameterIt should correspond exactly to the landing burtices on the hub of the car. If the hole in the disk is larger than the hub, the centering of the wheel occurs only due to the cones of nuts, which is unacceptable for high speeds. In such cases, special centering rings (sparks) are used, which eliminate the gap.
PCD and DIA options are incompatible for different car brands 99% of the time, so their verification should be a priority before buying.
Loading capacity and additional symbols
In addition to geometric dimensions, the critical parameter is the maximum axis load, designated as Load or Max Load. This parameter is often ignored when selecting disks for heavy crossovers or minivans. On the disk you can find a marking, for example, 690 kg. This means that one wheel can withstand a weight of 690 kilograms.
To calculate the required load capacity, you need to divide the total weight of the equipped car with a maximum load by four, adding a margin of safety of 20-25%. The use of discs with insufficient load capacity can lead to their deformation or destruction when falling into a hole or transporting heavy loads.
Also on the inside you can find the designations of the type TÜV or VIA. These are the markings of certification centers (German and Japanese, respectively), confirming that the model of the disk has passed crash tests and meets safety standards. The presence of such stamps indirectly indicates the high quality of the product.
When buying used cast discs, carefully examine the internal shelf. Cracks, chipped paint around bolt holes or welding marks indicate that the disc has been damaged and repaired, making its operation dangerous.
Table of matching sizes and departures
Often, when changing the size of the disks (for example, switching from R16 to R17), other parameters must be adjusted to maintain the overall diameter of the wheel and not to knock down the speedometer readings. Below is a table showing the relationship between the width of the disc and the allowable departure for the average passenger car class C.
| Disc width (inches) | Recommended tyre width | Acceptable departure (ET) | Impact on the rut |
|---|---|---|---|
| 6.5 | 205 / 215 | ET 45-50 | Standard |
| 7.0 | 215 / 225 | ET 40-45 | +5 mm extension |
| 7.5 | 225 / 235 | ET 35-40 | +10 mm extension |
| 8.0 | 235 / 245 | ET 30-35 | Expansion +15 mm |
From the table it is seen that increasing the width of the disc requires reducing the departure (ET) so that the inner part of the wheel does not touch the suspension elements. However, if you reduce the departure too much, the outer part of the disc will go beyond the arch, which is prohibited by traffic rules in many countries and will lead to rapid contamination of the sidewalls of the body.
For each specific model of the car, the manufacturer provides its own tolerance range. Exceeding these limits may result in a denial of warranty service of the chassis.
Instructions for self-measurement of the disk
If the labeling on the disk is erased, is not readable due to dirt or you want to recheck the parameters of the used product, you can make measurements yourself. To do this, you will need a calculator, ruler and calculator. Accuracy of measurements is critical, especially for PCD parameter.
To measure the diameter of the openings (PCD) at an odd number of bolts (3 or 5), the formula is used: the distance between the edges of the adjacent holes is multiplied by a factor (1.155 for 3 bolts, 1.701 for 5 bolts). With an even number (4, 6, 10), the distance between the centers of opposite holes is measured.
Departure (ET) is more difficult to measure without a wheel. Place the disc flat, measure the distance from the mounting plane to the lower edge of the rim, then measure the total width of the disc, divide it in half and subtract the first measurement. The difference is the departure.
☑️ Checking the disk before installation
⚠️ Attention: Never use bolts from an old disc on a new one if the shape of the clamping part (cone or sphere) is different. This will lead to the fact that the wheel will not press against the hub with a plane, but will hold on point, which is guaranteed to weaken the fastener.
The effect of disk size on vehicle behavior
Increasing the diameter of the disk while maintaining the outer diameter of the wheel (tyre profiling) is called scaling. Installation of large discs (R18, R19) with low-profile rubber improves steering stability and accuracy. The car becomes more responsive in corners, the rolling disappears.
But the medal has a downside. Low tire profile worse absorbs road irregularities, passing blows to the body and suspension elements. Risk of disc damage On bad roads, it increases many times. In addition, low-profile rubber is noisier and has a lower mileage life.
In winter, large disks may be inefficient. A narrow tire on a small disk better “cuts” snow porridge and achieves a hard coating, while a wide low-profile tire on a large disk is prone to pop-up (the aquaplaning effect) and rows worse in the snow.
⚠️ Attention: When installing discs with a diameter different from the factory by more than 2 inches, it may be necessary to reprogram the ABS and ESP unit, as the readings of the wheel speed sensors change.
Frequently Asked Questions (FAQ)
Can I put a disk with a large central hole (DIA)?
Yes, you can, but only with the use of special plastic rings. They fill the gap between the hub and the disc, ensuring proper centering. Riding without rings with increased DIA is dangerous due to possible vibrations at high speeds.
What happens if the drive (ET) is smaller than the factory?
The wheel will shift outwards. This can cause the tire to rub against the arch or splasher when turning the steering wheel or fully loading the car. The load on the bearing will also increase.
Will the bolts fit from the old drive to the new one?
Only if the new disk has the same geometry of the holes under the fastener. Often on new cast discs, the holes are made under a cone, and on old stampings - under a sphere (or vice versa). In this case, new bolts are needed.
How do I know the exact size of the disks for my car?
The information is contained in the instructions for the operation of the car, on the sticker in the opening of the driver's door or on the hatch of the gas tank. Also, the data can be found in the catalogs of disk manufacturers by car model.