How to choose wheels for your car - FAQ

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10 months 1 week ago - 10 months 1 week ago #47 by Steeve
Wheel rim geometry. How to choose a rim


1. ET - disc departure (or removal) (measured in mm).

This is the distance between the longitudinal plane of symmetry of the disc and the plane of the hub. The ejection can be zero (the symmetry plane of the disc coincides with the plane of the mouse hub), positive (the symmetry plane of the disc is closer to the center of the machine than the plane of the hub - as shown) and negative (further). For each model of the car, the departure has been calculated by the manufacturer so as to ensure optimal stability and controllability of the machine, as well as the lowest load on the hub bearings.

Germans denote ET departure (for example, ET30 (mm) if its value is positive, or ET-30 if it is negative), French - DEPORT, manufacturers from other countries usually use English OFFSET.

The desire to put the appeared in recent years, wider low-profile tires (sometimes on disks of larger size R) leads to the fact that it is necessary to pick up a disk with a reduced ET (ie, the bus has become wider - you need to move it out of the body - otherwise it will start to hurt him). Reduced ET makes the wheel track wider; - it increases the stability of the car and gives it a stylish racing look. Excessive reduction of ET is undesirable, as it can lead to increased load on the hub bearings. As a rule, it is impossible to increase the outreach, i.e. to narrow the track - the disc will be reproached in the brake caliper.

2. Disc mounting diameter (measured in inches, 1 inch = 25.4 mm).

Corresponds to the inner (seating) diameter of the tyre. The vast majority of modern vehicles run on 13", 14", 15", and 16" discs. In recent years, there has been a trend to convert cars to larger mounting diameters - cars with 13-inch rims as standard, 14-inch, 15-inch and 16-inch rims, etc. This is due to the desire (despite the higher cost) to use low profile tyres of interchangeable size, as their driving characteristics are, for a number of reasons, better than high profile tyres.

And the lower the tire profile, the less rubber in the wheel and therefore the more metal in the wheel - because the outer diameter of the wheel has remained unchanged. If you use steel rims, the mounting diameter will not increase much - this will increase the weight of the wheel, which is undesirable. And the use of light alloy rims allows you to increase the mounting diameter of the disk, without weighing the wheel as a whole. On sports versions of cars brake calipers are larger than on non-sports (powerful engine - reliable brakes), - therefore, and the disks must be larger mounting diameter, otherwise the brake caliper will be reproached in the rim from inside the disc.

3. the D-diameter of the center hole of the disk (hub holes).

OEM rims often have the centre hole precisely aligned with the axle hub, i.e. the rim is mounted on the hub with the minimum clearance and tightened with mounting bolts or nuts. But, buying a new disk, do not be afraid that the central hole may be larger than the one you put - the accuracy and reliability of fixing the disk to the hub is ensured by drilling holes in the hub with high-precision machines on the conveyor of the manufacturer. Disc manufacturers often make the central hole of a deliberately larger diameter, this allows the disc to be used on different models of cars (different cars have different diameter hubs - there are no standards for this parameter, and the range of PСD standardized). In this case, the installation of an adapter ring between the hub and disc allows normal operation of the vehicle. Very often alignment rings are sold with a disc.

4. PCD - diameter of centers of fixing holes (measured in mm, indicated by PCD - Pitch Circle Diameter) and the number of these holes.

For example, PCD4x100 means - 4 holes located on a circle with diameter of 100 mm (radius 50 mm from the center of disk axis). These parameters must exactly match the hub geometry. Eye and ruler measurements are not permitted here.

For example, a PCD 4x98 hub can be fitted with a PCD 4x100 disc. 98 mm from 100 mm per eye can not be distinguished (if you are not a turner with 20 years of experience). As a result, only one of all nuts (or bolts) will be completely tightened; the rest of the holes 'will take away' and fasteners will remain undrawn or tightened with a skew - landing of the wheel on the hub will not be complete. The wheel will 'hit' on the way, and the nuts that are not fully tightened will be unscrewed by themselves, which is no longer safe. Manufacturers can produce discs of different diameters for the same machine (see the Toyota Camry example above), but the PCD for any disc diameter must match the hub geometry.

5. Rim width (measured in inches).

Must be 25-30% less than the width of the tyre profile. Suppose you are looking for a rim for a 195/70 R15 tyre. Its profile width is 195 mm. In inches it would be 7.68 (195 divided by 25.4). Subtract 25% or 30% from this value and round up the resulting number to the nearest standard value.Get a 5.5 inch rim - this is the width you need for a 195/70R15 tyre.

The use of both too wide and too narrow rims (relative to the width of the tire) is extremely undesirable: the design geometry of the contact patch between the tire and the road is broken (the sides are either compressed by the rim's edge or stretched on it), which causes the tire to deteriorate its driving characteristics - the tire's turning response, driving resistance, lateral stiffness, aquaplaning resistance, and the tire wears out incorrectly - and the mileage of the tire is sharply reduced.

The allowable rim width deviation is 0.5 - 1.0 inches for rims with mounting diameters up to 14 inches; and 1.0 - 1.5 inches for rims with 15 inches or more. But it's better, of course, to take a rim exactly under the bus. This is especially true for low-profile bus series.

Thus, the disc marking for example 6/15 4/114,3 ET46 D67.1 is deciphered as 15-inch diameter, 6-inch thyroid, having 4 fixing holes, located on a circle 114,3 , disc carrying 46 mm, 67,1 mm diameter hub hole.

To sum up - the algorithm of buying a wheel looks like this: you find out which tire is suitable for your car in terms of size, find out the PСD of your car (car manufacturers almost never change this parameter on machines of the same class), select a disk suitable for the car by PСD, and to the tire diameter and width of the rim, while taking into account ET (depending on the bar profile of the tire) and D (disk with a small D simply does not fit on the hub).
Last edit: 10 months 1 week ago by Steeve.

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