If you have already decided in favor of this or that disk type, either it is pressed or cast, etc, it’s high time for you to think over the following matters.
1. Bead width.
Remember the golden rule – it should be 25-30 per cent smaller than the width of your tire’s profile. For instance, you are searching for a disk to fit a 195/70 R15 tire. Its profile is 195 mm /7.68 inches wide. Subtract 25-30 per cent from it and round the given number to the closest value from the line of standard wheel sizes. In this case, you’ll find out that the 195/70R15 tire needs wheel disks measuring 5.5 inches in width.
Warning!
It is not advisable to use either too wide or too narrow wheel disks (as regards the tire profile) as it leads to the violation of designed tire profile, the situation when tire sidewalls are compressed by bead edges or are stretched over it. This worsens the tire’s performance; its steering response, skidding resistance as well as lateral stiffness suffer first and foremost. Maximum allowable deviation from norm equals to 0.5-1.0 inches for disks with up to 14-inch mounting diameter and 1.0-1.5 inches for disks with 15-inch diameter and larger. But of course, you’d better select wheel disks that exactly fir your tires.
2. Disk Diameter.
The complete line up of wheel mounting diameters for both passenger cars and off-roaders includes 10, 12, 13, 14, 15, 16, 17, 18 and 19 inches. The overwhelming majority of modern cars run with 13-, 14-, 16, and 17-inch disks. Lately there has been a tendency towards the increase of a mounting diameter, for instance, cars that are equipped with 13-inch disks by default are now fitted by car owners with 14- or 15-inch disks. They strive to use low-profile tires as their performance is better than that of high-profile ones. The lower the tire is the less rubber is used for it while the outer diameter remains unchanged. It’s not good to increase diameter by using steel disks for it increases the tire weight, which is undesirable. At the same time, light alloy disks allow for diameter increase without making the entire wheel heavier.
Warning!
Sporty car models feature bigger braking systems than that of ordinary cars, therefore, wheel disks should be bigger as well, or the brake will touch the rim.
3. Pitch Circle Diameter and the quantity of these pitches. For example PCD 100/4 means four pitches for a 100 mm diameter.
Warning!
You shouldn’t mount a PCD 98/4 wheel on a PCD 100/4 wheel hub. In this case only one of three nuts (or bolts) would be tightly fixed while the other loosely fixed ones will unscrew sooner or later, which will cause degradation in handling.
4. Diameter of a disk central aperture.
The central aperture of standard wheels exactly fits axle hub. But when you buy replacement wheels you’ll meet wheel disks with bigger apertures. It is done for customer convenience as such disks are equipped with transition rings to match different car models. In this case wheels are centered in accordance to PCD.
5. Wheel offset.
Wheel offset is a distance between a longitudinal plane of symmetry of the rim and a fixing plane of the wheel. It can be zero, positive or negative. Wheel offset is calculated for every car model to ensure optimal handling and minimize load over the hub bearings.
Warning! Don’t ever try fitting your car with wheels featuring a non-standard for it offset value. Smaller offset though enhancing looks and handling overload hub bearings and suspension system. At the same time you cannot make offset bigger as in this case a wheel will be held by a brake.
And the last but not the least - Types and sizes of bolts and nuts for fixing the wheel
If you change from a steel pressed disk to a light alloy one, you’ll have to use longer bolts than standard as light alloy disks are thicker than steel ones.
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