Before installing new rims on your car, motorists are faced with the need to choose the right manufacturer and quality material, as well as determine the dimensions and other characteristics. But is it possible to install wheels of a different size on your car without preliminary calculations? In our article, we will focus on such a concept as the departure of rims.
1 The main characteristics of car rims
Alloy wheels have a huge number of advantages. Such models, as a rule, have a more attractive design, which allows you to implement the most daring tuning ideas on a car. In addition, alloy wheels are lighter, which reduces the load on the springs of the chassis of the car and improves traction, since contact with uneven surfaces is more pronounced in this case. However, if they are not matched correctly, the car will be less manageable and smoother. In addition, poorly matched wheels increase the risk of accidents on the road.
Therefore, professional tuners and automotive specialists, when choosing a particular model of disks for a particular car, first of all, take into account their geometric characteristics. They are encrypted in the marking, and in order to recognize them, you need to understand the basic parameters.
2 What parameters are affected by geometric indicators?
The main geometric indicators of the disks are their landing length and diameter. Based on these parameters, experts determine what sizes and tire options to use for a particular wheel model. As a rule, leading manufacturers of automotive rubber allow small deviations in the width of the rim, usually from 0.5 to 1 inch. Thus, a wider installation is only possible in combination with discs with a larger diameter than the previous one.
For example, if the car had 175/70 R13 tires, then the increased figure will be 175/65/R14. This must be measured to increase the outer diameter of the car wheel, otherwise it will rub the wheel arch. In the case presented, the increase was +0.8 millimeters. It should also be understood that car manufacturers adjust the speedometer readings for a certain wheel size, therefore, after installing more “powerful” rims, its readings will have a certain error.
As for the differences in diameter in places where special holes for the fastening bolts are located, the permissible error in their changes should be no more than 0.1 mm. If this error exceeds this indicator, they talk about the need to center the rim with alternate tightening of the fixing bolts (sometimes special centering rings or wheel spacers are included). Otherwise, the disk simply will not enter the wheel hub. If the diameter is smaller within acceptable limits, then the owner has the opportunity to bore on turning equipment.
3 Departure as the main criterion for choosing discs
In scientific terms, reach (measured in ET) is a measure of the distance between the vertical plane of symmetry of the wheel to the plane of application of the disk to the main hub. Thus, this distance exponent can be negative, positive, and zero, respectively. In simple terms, offset is a measure of how much the rim "protrudes" or "flies" out of the wheel arch. Consequently, with an increase in offset, the wheel “backs” more deep into the wheel arch, with a decrease in offset, the wheel protrudes outward.
This means that the disc offset and width determine not only the possibility of installing a certain model in the wheel arch, but also the fact that when these parameters are changed, various suspension characteristics automatically change, such as wheel travel, turning radius, break-in moment arm. These changes directly affect the handling of the car.
The main problem faced by tuners or car owners who are going to install discs with a different offset on it is a change in the behavior of the steering wheel and suspension due to a change in the break-in shoulder (the location of the wheel axis relative to the hub).
The running arm can be internal and external, or positive and negative. This value directly affects the traction force of the steering wheel and the ability of the car to move in a straight line. Thus, the maximum deviation of the offset of new discs from standard ones should not exceed 5 millimeters, otherwise it will be very difficult to drive a car, especially at high speeds.
4 Why is it necessary to install discs with a different offset?
The installation of rims with a reduced overhang contributes to an increase in stability and, as a result, the wheelbase of the car. So, the car is able to confidently take turns and has increased stability on the road. With some changes in the suspension, the overall ride comfort also increases, but springs, levers, brakes, bearings, hubs, etc. are also changed for this, and not just new disks are installed. Sometimes, installing discs with a smaller offset compared to the standard allows you to reduce the weight of the suspension and thereby improve the dynamics and fuel consumption in certain modes.
With an increase in overhang, the track decreases, and if this is done without preliminary calculations, then the wheel can simply rest against the brake caliper or touch part of the wheel arch with its inner side. The increase in reach also affects the operation of the suspension units and the running gear of the car. As a rule, discs smaller in size and diameter are installed on some versions of cars in the winter period of operation, however, the effectiveness of this phenomenon has not been clearly proven, and in the tuning process, reduced versions are practically not used.
If we talk about which wheels to choose, forged or cast, then in this case it all depends on the car model, tuning options and other factors. The former are made by hot stamping, the latter under high pressure and metal machining. Forged ones are stronger and lighter, but this does not mean that they are suitable for all car models, it all depends on the situation. Regardless of the departure, when choosing a disc, it is necessary to take into account the values and interpretation of the marking according to international standards where:
- ET - "Departure" can be designated as Offset or Deport (Internal or protruding);
- B - an indicator of the width of the disk between the flanges of the rims (measured in inches. 1 inch \u003d 2.34 centimeters;
- "J" is a special service symbol that denotes the design features of a particular model;
- "X" - an indicator of "non-separable" rim;
- "D" - the size of the diameter of the landing tire, "d" - the diameter of the hole for the wheel hub.
It should be added that there are cases when discs with increased or reduced offset fit without problems on a standard wheel hub. However, this does not mean that their use will be safe for various suspension elements and car handling. Engineers of the car manufacturer carefully and in detail calculate all the moments of operation of the ball joint, hubs, shock absorbers, steering tips, levers, etc., therefore, the use of discs with sizes that are not recommended by the manufacturer can significantly reduce the overall life of all these parts.
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