How to determine the offset of a car rim

What is disk offset ET and what does it affect?

Overhang is an important geometric parameter of a disk. The disc simply cannot be put on the hub if it does not fit in size. The discrepancies, as a rule, turn out to be small - the installation of the wheel can still be carried out. But are such experiments permissible? To what extent can the disc offset not correspond to the recommended one? In what direction is the deviation permissible, if it is permissible at all? We will talk about this in the article.

ET disc ejection: what does it mean?

Overhang is the distance from the middle of the disk to the plane of its alignment with the hub. Denoted by the abbreviation ET. The smaller it is, the more the rim will protrude from the outside of the car. The more significant the ET, the more the disk will be recessed. The crash is not affected in any way by the disk parameters. To calculate the load on the suspension mechanism, you only need to know the distance from the middle of the wheel to the hub.

ET must meet the car manufacturer's recommendations. Deviations are unacceptable - even minor ones will cause additional loads on the suspension units. This can cause a reduction in the life of the suspension, and in some cases even leads to its destruction.

You can hear the opposite from sellers. There are many variations of offsets, and therefore store employees don’t really want to select wheels specifically for your car - especially if everything is in order with the other parameters.

Here are some tips for drivers on choosing wheels:

The appearance of the product should be in the background - technical characteristics are more important.
You shouldn’t trust sellers too much - you can’t always get reliable information from them.
Pay attention to the labeling.

What does disc ejection affect?

The offset of the ET affects the wheelbase of the vehicle. If the parameter is changed, the wheel will begin to go outside the body - or, conversely, go inside. All manufacturers clearly regulate it and do not advise allowing even the most minor deviations in any direction. Problems can appear even with a deviation of 5 mm.

Cars vary in handling and stability characteristics. Therefore, each car has its own ET value. Otherwise, the following would happen: with a negative value, the wheel touches the body, and with a positive value, some suspension elements. Only at the values specified by the manufacturer will the level of pressure on the suspension be acceptable.

Here's what happens when there are deviations:

the steering axis moves;
bearings wear out prematurely;
controllability deteriorates;
tires wear out faster;
The service life of the suspension is reduced.

What is the flight like?

The parameter can be positive, zero or negative. With positive offset, the center axis of the wheel is located behind the connection to the hub. At zero, the axis coincides with the mating plane. A negative value indicates that the axis is in front of the contact surface.

Nowadays most cars have positive offset. Other options, of course, also occur, but rather as an exception. Negative and zero ET can be found on racing cars, both on track and in full off-road conditions. Their pendants are very different from standard ones.

How is ET disc offset measured?

The parameter is measured only in millimeters. You will need a ruler and a wooden (or metal) strip, the length of which coincides with the radius of the wheel.

First of all, you need to remove the wheel from the car and put the car on the handbrake. If the wheels have alloy wheels, the procedure will be greatly simplified, since all the nuts on them are open. Otherwise you will have to remove the cap.
Now you can remove the disc from the wheel. This must be done with a sharp movement.
The wheel should be placed on the ground with the side opposite the hub. We place a wooden strip on top of the disc rim.
Then, using a ruler, measure the distance from the surface in contact with the hub to the bottom of the rack - this will be distance A.
Next, we turn the other side of the wheel towards the ground, and also place the rack on the rim.
We measure the distance from the bottom of the rack to the plane beyond which the hub is distance B.

Labeling and formula

Calculations should be made using the formula:

The values obtained during the measurement must be substituted into it.

The ET value is prescribed individually for each machine. All the necessary information on this matter is in the car’s operating instructions. Wheels will not fit the vehicle if the measured value differs from the data in this document. “Non-native” components are not worth buying, even if the seller actively convinces you otherwise.

The markings on the discs must be carefully studied - this is the only way to make sure that they are safe to use. Product markings are standard. In any case, the designation contains the letter I or S. The letter I means that the wheel is “identical” and is installed on production cars. S indicates that the wheel is special, that is, its certification is not tied to a specific brand of car. In some cases, there is no letter designation - instead, the name of the plant where the car was made and its catalog number are applied to the rim.

As an example, consider the rim marker 7.5 j x16 H2 5/112 ET 35 d 66.6:

The first numbers are the width of the disk. For example, 7.5 means the width is 7.5 inches. To convert to centimeters, you need to multiply by 2.54.
The letter J means that the wheel has some design features. This information is of no interest to consumers.
X indicates the indivisibility of the disk.
The number 16 is the wheel caliber corresponding to the tire caliber.
H2 reports that there are 2 humps on the rim.
The number 5 is the number of holes for fasteners, 112 is the diameter at which they are located.
ET 35 speaks of a plus offset, the size of which is 35 mm.
d 66.6 - central hole gauge. Ideally it should be identical to the hub caliber. If this is not the case, an additional ring must be used to center the fit. It is also called transitional.

How to determine the offset of a wheel rim?

The value obtained from the formula can be either positive or negative (or zero). The parameter determines the distance between the axles of the rear and front wheels, forming a gap between the wheels mounted on the same axle. Rubber, rim and tire parameters do not affect ET at all.

The load to which the car's suspension is subjected can be calculated from the applied load arm - the distance from the middle of the rim to the hub. For each specific car model there can be only one ET - the value of this parameter should not depend on the size of the rim and the tires installed on it. The offset value is written on the wheel. The marker could be like this: ET35. The number 35 means the distance in millimeters. In this case, the distance has a positive value. The distance will be negative if the ET-35 marker is applied, or zero if ET0 is applied.

Conclusion

When buying a wheel rim, don't limit yourself to a visual inspection. Look at the markings. Remember that driving safety depends on the right choice. Use only those items recommended by the manufacturer. And write it down somewhere in the most visible place: departure deviations are unacceptable!

How to measure disc offset?

Disk offset is an important geometric parameter of the product, which determines the size of the mating plane, which is taken into account when joining the product to the hub. There are several types of departure:

Positive - if the mating plane does not intersect the imaginary middle of the disk, if it intersects the middle - negative , coincides with the center of the disk - zero.

What depends on departure?

The width of the wheelbase, as well as the symmetry of the wheels, depends on it. Any offset errors have a detrimental effect on the suspension elements and contribute to rapid wear of the working mechanisms of the vehicle. Each car brand has its own Et value, which is calculated using a special formula that eliminates possible errors.

ET=a-b/2 , where a is the distance between the inner part and the contact plane, B is an indicator of the width of the disk. ET – these letters indicate the offset on the marking. The numbers following this designation determine the offset in millimeters.

For example, ET45 means positive offset, 0 means zero, and 15 means negative. Near-zero and negative values are typical for wheels of SUVs and sports cars. In other words, for vehicles with a wide gauge.

How to measure the disc offset correctly?

When purchasing new wheels, it is very important to take into account the compliance of their offset parameters with the parameters specified by the car manufacturer. ET parameters of new disks by looking at the inside of the product. Most often, the departure is denoted by the letters ET, but some French manufacturers may denote DEPORT , and English-speaking ones - OFFSET.

Many drivers want to reduce ET parameters. What is this connected with? Firstly, due to the widening of the track, the appearance of the car becomes more “cool”, since the wheels extend beyond the boundaries of the arches, and cornering stability also increases.

How to measure disc offset manually?

To measure this parameter, you need to have a flat rod and a measuring tape on hand. To measure, the disc is turned over and the rack is applied to the rim. Using a tape measure, you need to measure the distance between the lower edge of the rail and the mating plane - the rear offset, which we will call for the formula - A.

After this, the disk is turned over with its back side. A wooden strip is applied to the rim, after which the distance from the mating plane to the edge of the strip is measured - the frontal offset, which we will call B.

Next, the ET formula is used, where A is summed with B, the resulting number is divided by 2, and the indicator B is subtracted from it. For example, the rear indent A is equal to 118 millimeters, the front indent B is equal to 118 millimeters, we use the formula ET = (118 + 100) /2 – 100 and we end up with the number 9. This is exactly what the flight will be.

They are used to change ET , which makes it possible to expand the wheelbase and give the vehicle a more interesting look. In addition to this change, ET improves driving performance. Spacers are often used in cases where a tuning body kit or sports suspension is installed.

In the second option, installing spacers prevents the wheels from touching the fixed parts of the suspension. In addition, such products help to install wider tires and wheels on a vehicle, because it often happens that the car wheels you like in a store do not have the necessary ET, but otherwise coincide with all the characteristics of standard products.

There are two types of metal spacers. In one embodiment, they simply have holes made for the hub studs. Typically, such “pancakes” have a thickness of ten to twelve millimeters and are called universal. They are used in cases where the disc slightly touches the caliper. Through spacers with a thickness of fifteen to twenty millimeters are used both in cases where the disc rests on the caliper and to expand the wheelbase.

In the second option The product has holes for hub fasteners, as well as threaded holes for disk fasteners. Such a “pancake” is fastened autonomously, and the disk is also independently attached to the pancake. In this fastening option, you can change not only ET, but also the number of fasteners. This product can have different thicknesses, which is very convenient.

About disc offset - permissible deviations

Reading time: 6 minutes

Quite often, car owners install new rims, and many do this not because of breakdown or wear of previous products, but in order to improve the appearance of their “iron horse”. So, when purchasing a new wheel, car enthusiasts always look not at its drilling, that is, the diameter of the mounting hole on the hub, the bolt pattern, or the number and length of the studs on which this wheel is installed, but few people pay attention to the offset of the product (ET), and this is very important indicator for normal operation of a wheel on a specific car model.

What is ET on rims? This question is asked by many car enthusiasts, especially those who purchased their cars relatively recently and, until today, have never encountered the problem of replacing wheels on them.

Geometric characteristics of the wheel rim

Disc ejection: what is it?

Disc offset, or ET indicator, is a dimensional parameter that is indicated on the rim of the product, regardless of its radius or material of manufacture (stamped, cast or forged), and indicates the distance from the mating plane of the wheel to the attachment point to the hub. This dimension is usually set by the car manufacturer.

ET offset on wheels: what is it and how does it affect the suspension and other parts in the car? Depending on the wheel offset, the load on the hub and the bending moment applied relative to it at the base of the suspension are distributed differently. Thus, each automobile concern dictates the strength limit for its parts, which determines the range of wheel offsets.

Some cars, especially when it comes to SUVs and sports cars, are equipped with additional plastic mud flaps, which determine the wheel offset, which in such cases can be zero or even negative, which gives the “iron horse” a very impressive look.

ET departure using 3 indicators as an example

ET on disks - what does it mean and how is it calculated

The designation in the form of two letters of the Latin alphabet ET is not accidental, since this value is international and is determined by the following formula and is expressed in mm, regardless of the country of the disk manufacturer:

Where X is the distance from the outer mating plane of the disk to its inner edge on the side of attachment to the hub or the size that is determined by measuring from the side edge of the wheel along the sides to its grille.

Y is the overall width of the product along the rim.

How to determine the permissible ET deviation for a disk

As a rule, each automaker dictates its own permissible deviations for disc offset, and they depend only on the design of the frame, suspension, calipers, wheel arches and other elements of the vehicle. This means that for each car caliper there is a certain indicator of compatibility of various sizes, expressed in the range from minimum to maximum ET in millimeters. So, below are the permissible deviation indicators for the 35 most popular car models in Russia:

Measurable indicators for calculating departure

From this table it can be seen that negative offset is a familiar parameter only for full-size SUVs, and the smaller it is, the more the wheels stick out on them, however, this gives them additional stability on very difficult sections of bad roads; plastic linings around the perimeter of the wheel arches are often as standard. In addition, these brands of cars have a reinforced suspension, a bolt pattern of at least 5x115, which is better than on passenger cars and absorbs bending moment.

What problems can arise due to incorrect selection of disks?

The danger of incorrect selection of this dimension is especially relevant when operating expensive modern cars. Thus, the position of the vehicle on the road is carefully monitored by the on-board computer and various sensors. If a tire goes flat, the driver receives a signal about a loss of pressure; when the brake pedal is pressed sharply, the wheels do not lock, as the ABS is activated.

The same can be said about the directional stability stabilizer, which controls the position of the car on the road and the straightness of its movement, and also prevents skidding on the road, alternately blocking one or another wheel. In this computer, as a rule, engineers enter certain indicators of the wheel rim dimensions - ET, and as the final result - the values of bending moments.

Measuring the jack of the disc

How to correctly measure the offset of an ET disc

What is ET on disks and how to measure it correctly if circumstances are such that there is simply no other way to determine this indicator? Quite often, worn or damaged wheel rims make it impossible to correctly read the markings on their surface, and in this case, vehicle owners have to resort to their measurements.

To select the right wheel rim to replace a product that has expired, you need to determine the ET value on the old wheel by following these steps:

If the disk is installed on a car, it must be removed using a wheel wrench or a special spanner tool for removing the wheel locks, if any were used when mounting the wheel on the hub. Before dismantling, it is necessary to raise the car with a jack so that the wheel can rotate freely in a hanging position.
It is necessary to measure the back indentation on the disk, and to do this you must first carefully place the disk on a flat surface with the outer side down.
The side of the disk that is attached to the hub is on top, and you need to put a wooden measuring rod on it, the length corresponding to the diameter of the wheel. Accordingly, the entire tool must be located on the steel sides of the wheel, and not on the rubber, otherwise the offset will be determined incorrectly, which will lead to errors when purchasing a wheel.
Using a tape measure or ruler, measure the distance from the mating plane of the disk to the edge of the wooden product. The result is recorded in millimeters.
The procedure must be repeated, turning the disk upside down, and as a result, the car owner will have 2 indicators recorded - front and rear overhang, which add up to the overall ET indicator through simple calculations.

With the described measurement, the formula ET = (A + B)/2 – B is available to the car enthusiast, where A is the first measurement - the amount of indentation from the rear side, B is the same indicator, but from the front.

Measuring the jack of the disc

Zero offset wheels

Thus, to measure the offset, regardless of whether it is possible to read the markings on the disk or not, the car enthusiast can use the simplest techniques and get a fairly accurate result.

A specific example: the first measurement showed the value A = 143 mm, B = 43 mm. Total value ET = (A + B) / 2 – B = (143 + 43) / 2 – 43 = 186 / 2 – 43 = 93 – 43 = 50 mm. Accordingly, based on this indicator, the vehicle owner must choose the wheels he is interested in in the store.

Of course, in such tables the ET indicator will be present without fail, and, as a rule, engineers do not recommend going beyond the proposed size ranges and absolutely disclaim any warranty obligations in the event of a breakdown of the suspension or other parts.

What does wheel rim protrusion mean?

There are several parameters by which car owners select “shoes” for their vehicles. For a passenger car, these can be small-diameter alloy wheels; for SUVs, these can be massive wheels with deep tread. When choosing, the width of the rim, the number of bolts for fastening to the hub, and the material from which the product is made also play a role. But there is one more parameter that many car owners often overlook. It was called departure. But how can you determine the offset of a car’s rim in order to select the most suitable part for a specific vehicle model? This will be discussed in this article.

Disc Specifications

Many people believe that if a wheel has the required number of mounting holes, then by default it is suitable for installation on a car. This is not entirely true. Of course, installing such a product on the hub will not be difficult; the disc will be securely fixed and will allow you to almost fully enjoy a comfortable ride. Moreover, in most stores specializing in the sale of tires and wheels, they will assure you that the offset size declared by the manufacturer does not affect anything, and therefore does not play a fundamental role when choosing a wheel product.

Definition and method of calculating disc offset for a car

Overhang refers to the geometric parameters of the disk. It characterizes the distance between the plane of application of the disk to the hub and the vertical center line of the wheel and is measured in millimeters. It would seem that calculating this indicator is very problematic, but to do this you will need to determine and measure only two quantities:

The distance between the plane of the disk (inner) and the plane by which the disk is applied to the hub (a);
Disc width in millimeters (b).

The formula for calculating the departure in general is as follows:

Geometric parameters of the disk

It is worth understanding that this indicator provides information about the width distance between the centers of symmetry of the wheels - the wheelbase of the car. It is this parameter that is indicated by the manufacturer in the technical documentation for the vehicle, and its non-compliance with the declared one can negatively affect the operational capabilities of the car.

If in the process of calculating the disc offset a negative value is obtained, this in no way means that the calculation was made incorrectly. This parameter can take the following values:

Positive (the most common option);
Negative;
Zero.

So, for the convenience of buyers, wheel manufacturers have developed special markings. A code of the type ETxx is applied to the inner surface of the disk, which allows one to judge the size and type of overhang. The fact that this indicator is negative will be indicated by an inscription like ET-20, but if the value of the parameter is positive, then the code will look like ET15. The ET0 marking means that the selected disc has zero offset.

Disc offset pattern

Will it be a problem to install a disk whose offset does not correspond to the recommended one?

Yes. There are many reasons why the use of incorrectly selected products is not acceptable. First of all, this concerns the correct functioning of the suspension. By installing beautiful, but incorrectly selected wheels for your car, you risk running into problems.

It is known that all functional components of a vehicle have their own service life and are designed for a certain effort. By using a disc whose offset does not quite suit the design of the car, you thereby upset this balance, which leads to premature wear of suspension parts. It’s good if it’s just a reduction in the service life of some component - in this case it can simply be replaced. But if a breakdown occurs while driving, then sad consequences cannot be avoided.

Quite often, disc sellers neglect accuracy in relation to the offset value. Thus, they try to offer you a non-specific model, which may very likely not meet your aesthetic requirements, but show the maximum of products in order to increase the likelihood of purchase.

Are different parts needed for different configurations of the same car model?

A huge number of people work on the creation of one car - from designers to interior assemblers. Of course, special attention is paid to the chassis. During its development, hundreds of parameters are taken into account, each of which will determine the requirements for the design of individual elements.

Today, there are many car brands and even more models on the automobile market. The situation has become quite common when a manufacturer launches models on the market that differ only in engine size. It would seem that the components for them should also be the same, but this is not the case. Many details may differ, including:

Such decisions are very natural. The slightest difference in the mass or overall dimensions of the engine affects the distribution of forces that affect the chassis and suspension, in particular. Based on this, to ensure the necessary maneuverability and comfortable ride, it is necessary to use other components. The way out of such situations previously was to create a good safety margin for the driving components of the car. Now this approach is becoming a thing of the past, giving way to precise calculations, which results in a significant reduction in the cost of developing and manufacturing vehicles.

What are the consequences of using discs with an incorrectly selected offset?

As mentioned earlier, the offset value has a direct impact on the size of the vehicle's wheelbase. This does not concern the indicator stated in the technical passport of the car, but the actual one that the car owner has to deal with. If the offset value increases, the wheel will “sink” into the body, sitting on the hub as much as possible. If it decreases, the wheeled product will protrude beyond the body.

Car owners should understand that using rims with an incorrect offset negatively affects the suspension and inevitably leads to the following consequences:

Steering axis displacement;
Changing the provided and intended parameters of steering wheel rotation;
Premature wear of bearings;
Deterioration in controllability;
Premature tire wear;
Reduced suspension life.

Based on the above, we can say that not all discs that are suitable for installation on the hub of your car are suitable for the car as a whole. Therefore, carefully select the wheels for your car to make every train ride on it not only comfortable, but also safe.

Disc ejection (ET) - what is it and what does it affect?

Offset is the most important geometric parameter of a wheel rim. And this is by no means an exaggeration. We will try to explain the reason for this, as they say, with our fingers. So, if a car wheel does not fit in diameter, number of holes for mounting bolts, or the interval between these holes, then it simply cannot be put on the hub. But usually such discrepancies with the standard (declared by the car manufacturer) offset are not very large, which makes installation possible without difficulties. Will the wheel fulfill its role one hundred percent in this case? And if not, then what will such an experiment lead to? On the Internet on thematic websites, vehicle owners often discuss the topic of how much the offset of the installed disc can differ from the recommended one, and if this discrepancy is acceptable, then in what direction? Often the points of view expressed have diametrically opposite directions.

As for the distributors of car wheels, be it a specialty store or a car market, in nine out of ten cases they will say that a small deviation of the offset from the standard parameters is acceptable. And they will certainly add that if the assembled wheel is easily mounted on the hub, without catching or touching either the body or the suspension during rotation, then it can be used without any doubts or risks. Moreover, people selling wheel spacers will assure that reducing the offset, regardless of the recommended parameters, is not a problem at all and does not pose any danger. All this is easily explained by their desire to quickly sell their goods, and often by banal ignorance. But how are things in reality? Let's start with the basics.

How to determine disc offset?

Disc offset is the distance from the central axis of the disc to the plane of attachment to the hub. Determining it is elementary, because there is a simple formula that looks like this:

ET=XY/2 (calculated in millimeters)

ET – required value (offset).
Y – width of the rim itself (total).
X is the distance between the plane of application of the disk to the hub and its internal plane.

Obviously, the resulting number can be either with “+” (the most likely option), or with “-“, or even go to zero. An important point is the fact that offset directly determines the width of the wheelbase, since it forms the interval between the centers of the wheels located on the same axle. Analysis of the formula also shows that it is not affected by either the disc diameter, width, or tire dimensions.

Car suspension loads are calculated based solely on the force application arm, which is the distance from the hub to the center of the wheel.
This means that there can be only one wheel offset required for a particular car model. Regardless of the tire size and the size of the wheels themselves. The offset value is indicated on the surface of each disc. This is the ETxx marker, where xx is the distance in millimeters. It, as already mentioned, can be zero (ET0), positive (ET35) or negative (ET-35)

Are disc offset deviations allowed?

Regardless of how convincing the sellers' arguments are, you must clearly understand the fact that the offset of the disc you purchase must be 100% consistent with the vehicle manufacturer's instructions .

In no case are the slightest deviations in any direction allowed. It is very simple to explain such a categorical statement. Even with a tiny discrepancy in values, the operating conditions of absolutely all suspension elements without exception change automatically. In this case, forces arise for which these nodes are not designed. In addition, the vectors of application of these forces change, which is also not provided for by the chassis design. As a result, the service life of the mechanisms is significantly reduced, and when critical loads occur, the suspension units may completely collapse, which is very dangerous to life. Statements by CD sellers about a variety of options and nuances are just an attempt to sell you any product, in the absence of one that is ideally suited to your needs. Words about possible permissible deviations significantly expand the offered range of discs, and therefore increase the opportunity to earn money. No more.

Different configurations of one car model

Some car enthusiasts have noticed that different parts for different versions of the same car model are often used. This is due to the fact that when designing and calculating the parameters of components of each modification, a huge number of variables are taken into account, which can differ markedly for cars of the same line. An example of this is various power plants having different dimensions and weight. According to these calculations, taking into account in each case the acting forces and vectors of their application, the final suspension design is formed. This allows us to guarantee the client reliability, ride comfort, high-quality handling and other characteristics, with minimal production costs.

In the old days, most vehicle manufacturers manufactured parts in such a way as to provide a large margin of safety in the main structures of the car, including the suspension. Today, the trend in the market is such that there is a demand for reducing the cost of transport, which is achieved through more accurate calculations. This resulted in a decrease in the safety margin of most parts.

Forces acting on suspension elements

Absolutely any element of the suspension is subject to several multidirectional forces. And it is quite natural that this list increases with the complexity of the design, which is very different from modern machines. Therefore, we offer for consideration the simplest example, where the hub is attached to the body by means of a lever and a strut with a shock absorber (McPherson system).

The force exerted on the wheels is directed upward from the plane along which the car is moving, and the mass of the car is distributed between all wheels. In this case, the points of application of these forces are the centers of the tire contact patch area. And if we assume that the suspension and camber angles are in ideal condition, and the wheels are well balanced, then these centers will be located on the axis of symmetry of each wheel. It is in this place that the axis of the shock absorber strut should fall.

Everything else is simple. The acting force corresponds to the proportion of the car's mass attributable to the wheel. It is directed away from the ground and creates moments in the levers, wheel bearing, and struts with shock absorbers. In the first two cases it will be tension, and in the last case it will be compression. All these points are carefully calculated at the design and construction stage. Naturally, a safety margin is provided for each part, but it was already mentioned above that it is constantly decreasing due to the widespread desire to reduce production costs.

When the calculated offset changes, the forces change their magnitude and direction, because a decrease in offset expands the wheelbase, and an increase - narrows it. This entails a displacement of the steering axis and a change in the parameters of steering wheel rotation, moments of forces and vectors of their application. This aspect also negatively affects the wear resistance of tires, maneuverability and controllability of the vehicle. Taken together, all of these factors lead to the fact that the suspension is operated in a mode that was not intended by the automaker. The level of driving safety decreases, and the service life of most structural elements decreases sharply.

In conclusion, let's say the following. If a new wheel with an offset that does not coincide with the standard one easily fits on the hub of your car, this is not a reason to use it without fear. It cannot be said that the operation of vehicles in such equipment will be safe. Wheel spacers can be a solution, but only if the offset is greater than standard, and you were able to find suitable spacers, which is often very problematic.