About disc overhang - permissible deviations. What is disc offset and how its dimensions affect the characteristics of the machine Et40 what is it. How to choose the optimal disc offset for your car
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 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.
First of all, the wheel must be completely hidden under the wheel arch, and it is the ET indicator that regulates its position - the larger it is, the more the wheel is recessed under the wing; the smaller, the more noticeably the disk protrudes beyond the dimensions of the body.
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 Before purchasing a wheel rim, the driver needs to read the owner's manual for his car or study detailed information on numerous Internet resources in order to make the right choice and not regret it later.
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:
ET = X – Y/2,
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.
As a marking, wheel manufacturers, as a rule, prescribe this value as ET20, ET35, ET42, etc., and any professional can always read it and define these values.
Disc with negative offset 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:
| Item no. | Car model and modification | Overhang ranges, ET, mm |
| 1 | Audi A4 | 35 |
| 2 | Audi A6 | 35 |
| 3 | Audi Q7 | 53 |
| 4 | BMW 3 | 15-25 |
| 5 | BMW 5 | 18-20 |
| 6 | BMW X5 | 40-45 |
| 7 | Citroen Evasion | 28-30 |
| 8 | Citroen Xantia | 15-22 |
| 9 | Daewoo Nexia | 38-42 |
| 10 | Daewoo Matiz | 38 |
| 11 | Dodge Caliber | 35-40 |
| 12 | Fiat Bravo | 31-32 |
| 13 | Ford Focus | 35-38 |
| 14 | Ford Mondeo | 35-42 |
| 15 | Ford Explorer | 0-3 |
| 16 | Honda Civic | 35-38 |
| 17 | Honda Jazz | 35-38 |
| 18 | Honda CRV | 40-45 |
| 19 | Hyundai Accent | 35-38 |
| 20 | Hyundai Sonata | 35-38 |
| 21 | Kia Ceed | 38-42 |
| 22 | Kia Sportage | 0-3 |
| 23 | MercedesBenz A-Class | 45-50 |
| 24 | MercedesBenz E-Class | 48-54 |
| 25 | MercedesBenz ML-Class | 46-60 |
| 26 | Mitsubishi Lancer | 35-42 |
| 27 | Mitsubishi Pajero | from -25 to -15 |
| 28 | Nissan Almera | 35-42 |
| 29 | Nissan Maxima | 35-42 |
| 30 | Nissan Patrol | from -25 to -15 |
| 31 | Toyota Corolla | 35-38 |
| 32 | Toyota Camry | 35-38 |
| 33 | Toyota Land Cruiser 200 | from -15 to 3 |
| 34 | Volkswagen Golf | 35-40 |
| 35 | Volkswagen Tiguan | 20-32 |
As for Russian VAZs, these cars are universal with the exception of the iconic Niva. Thus, the ET dimension for these car models is mainly 35-38 mm, which also corresponds to many indicators of the world's leading automakers.
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 ET indicator is important, since the calculated bending moment on the suspension in inexpensive cars can be exceeded, which will lead to system failure and deformation. This means the need for major and expensive repairs, which not every driver is ready for.
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 If a car owner does not follow the manufacturer’s instructions and incorrectly determines the ET size for the wheels on his car, then the sensors may become confused, which is why the system may give a command to lock the wheels at a completely inopportune moment, and, as a result, the road user will get into an accident , having lost control.
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, from which the overall ET indicator is made up through simple calculations.
You can also measure the overall width of the rim to get the value using the formula ET = X – Y/2.
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.
All ET dimension indicators are specified in the operating manual for a particular vehicle, and often in two versions at once. Thus, the owner of the “iron horse” can see what parameters the wheels are installed on the car at the factory, and what exactly is offered by the manufacturer as analogues.
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 ranges of dimensions and absolutely disclaim any warranty obligations in the event of a breakdown of the suspension or other parts.
Vasilich, a famous fisherman and hunter, bought himself a Niva. The season of ducks and crucian carp is over, but we need to travel around the city. Vasilich didn’t like driving around the city in the toothy VLI-5. He looked and looked at other “nivovods” and installed “Volga” wheels for himself. “And the car looks more solid and drives more stable, and there’s more choice of tires,” he said. But Vasilich was not happy for long - after 1000 kilometers the front wheel bearings were requested to be replaced, a week later he changed the rear ones, and a month later something began to rumble in the suspension. Vasilich was upset, but he didn’t understand what was going on. But the neighbor turned out to be meticulous and found out the reason for the crop grower’s grief. The issue turned out to be an abnormal departure of the Volga wheels.
What is wheel offset?
Wheel offset (ET) is the distance from the imaginary center of the rim to the mating plane, i.e. to the plane that contacts the hub. The offset is calculated using the simple formula ET=a-b/2, where b is the total width .
What kind of departure is there?
The offset can be positive, zero and negative. A positive offset indicates that the center of the wheel is BEHIND the mating plane, a negative offset indicates that it is AHEAD, and ZERO indicates that they coincide. Reach parameters are strictly regulated by the manufacturer and are strictly linked to the suspension kinematics. When it changes, the loads change sharply. For example, it is believed that when the offset is reduced by 50 mm, the load on the suspension increases by 1.5 times. Simply put, the wheel begins to act as a lever, which is especially pronounced in turns when dynamic loads increase. This is due to the fact that the “turning line – wheel center” relationship established by the manufacturer is violated, resulting in a negative or positive rolling shoulder. Its effect is that an additional moment arises, which must be compensated by the steering wheel. The result is heavy steering and unpredictability in corners. And of course, wear and tear on the suspension. This is exactly what Vasilich faced.
Within what limits can you safely change the offset?
The general rule that applies to all cases is to follow the manufacturer's recommendations. This is all the more important because changing the wheel offset may result in the car being removed from warranty. The “popular” rule is simpler - plus or minus 5 mm. But even this can be excessive for modern cars! Very often, even a few millimeters that are invisible at first glance are enough for the wheel to start touching the suspension elements (when the offset changes in a positive direction) or the body (when the offset changes in a negative direction). Therefore, it is better to trust the recommendations of the factory. Note that very often the manufacturer recommends different offset values for different disk sizes. For example, Maksimych would be suitable for orienteering in a car (with the standard ET of the Niva wheels being 58 mm) with rim sizes of 5jx15 with an offset of 45 mm or 6jx15 with an offset of 35 mm. Note that the “Volga” wheels have ET = 0.
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 trading will assure that reducing the size of the departure, 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=X-Y/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?
No matter how convincing the salesperson's arguments, you must clearly understand the fact that the offset of the purchased disc must coincide 100% 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 (MacPherson 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.
For a true car enthusiast, wheels are not only a necessary attribute of the chassis of any vehicle, but also the main element in imparting exclusivity. However, there is one nuance here that is directly related to one parameter - disk offset. Experienced drivers know very well what they are talking about, which cannot be said about many beginners. But the safety of car operation depends on this characteristic. This article will discuss what ET disk flare is.
Disc offset detection
This parameter directly affects the driving performance of any car, regardless of the manufacturer. There are drivers who confuse this characteristic with the width of the rim. Overhang refers to the distance between two planes:
- plane of symmetry of the disk;
- mating area (the plane that comes into contact with the wheel hub).
The width of the vehicle's wheelbase depends on the magnitude of this difference. Departure is indicated by the following markings: Deport, OffsET and ET, with the last two letters being used most often. Actually, this is an abbreviation for two German words Einpress Tief, which translates as “indentation depth”. Experts recommend not to exceed the standard value of the parameter, otherwise it may negatively affect driving. We will talk in more detail about what ET disk offset is and why it is important to observe it a little later, but for now we will get acquainted with the types of offset.
Departure types
There are a huge number of cars all over the world, and therefore wheel manufacturers have something to offer potential buyers. There is a wide range of wheels on the automotive market, which differ not only in appearance and size, but also in offset. And it can be of three types:
- Negative - the mating plane is shifted forward relative to it, that is, it protrudes outward.
- Zero - both the mating plane and the axis of symmetry are on the same line.
- Positive - here the mating plane is already shifted forward relative to the central symmetrical axis.
Disc markings can be different: ET -10 - negative offset, ET 0 - zero offset, ET 47 - positive disc offset. For example, it may look like this in full: 7.5J x 15H25 x 100ET40d54.1. In this example, the departure designation is ET40. Determining the type of departure visually is not difficult. If it is negative, the disc hub will be pressed inward. With a positive offset, as you might guess, the hub, on the contrary, will protrude outward. With a zero parameter it is already clear - the hub is located exactly in the middle of the disk.
Parameter calculation
- A is the distance from the mating plane to the axis of symmetry.
- B is the overall width of the disk.
If for some reason there is no marking on the disk, then you can measure the offset value yourself. To do this you should prepare:
- You need a rail with a length equal to the diameter of the disk.
- You can’t do without a measuring tool (tape measure or ruler).
- You will need a jack to remove the wheel. You cannot unscrew the nuts without a wheel wrench.
- Tackles will also come in handy.
How to measure the offset of the ET disk? First you need to measure the value of the rear indentation. The wheel is placed face down and a rack is placed on the inside of the rim, with its ends positioned on the rim and not on the tire. The distance from the mating plane to the bottom edge of the rail is measured. Now you can proceed to measuring the frontal indentation, for which the wheels are turned with the front part up. The rail is placed in a similar way: the edges are on the rim, not on the spokes. The measurement is the same - from the mating plane to the edge of the rail.
When the measurements are completed, you can proceed to the calculation using the formula (in millimeters): ET = (A + B) / 2 - B, where
- A - rear indentation;
- B - frontal indentation.
For example, A = 153 mm, B = 53 mm, then ET = (153 + 53) / 2 - 53 = 50 mm.
To understand what ET 46 disc offset is, you need to understand how many people work on one car. As you know, each model is created through the efforts of several specialists: engineers, designers and others. And not only appearance is important here, because the driver who will operate the vehicle must be confident in his safety. In this regard, the car suspension is far from the last place. Each machine is individual, almost like a person - no two people are absolutely alike. For this reason, the manufacturer sets an individual offset value for each model, deviations from which are unacceptable.
Violation of the ET disc ejection tolerance inevitably leads to consequences of varying severity. In the best case, what can happen is that the service life of suspension components and parts will be reduced, and the tires will wear unevenly and quickly. At worst, the entire suspension will begin to rapidly collapse.
Responsible node
The suspension system of any vehicle is one of the most critical components in the car. Designers do their calculations for a reason regarding the loads that the suspension absorbs. As noted above, they are strictly individual for each model, and for this reason, any adjustment will only be destructive - the wheelbase decreases or increases. Neither one nor the other is good for the car.
To understand what ET disk failure means, it is worth giving the following arguments. The entire mass of the vehicle is distributed evenly between all wheels. In this case, a force directed from the surface acts on each wheel, in the place where it is connected to the road surface, which is called the contact patch. But it doesn’t end with the wheel, the load then acts on the wheel bearing, then is transferred to the shock absorber strut, and then to the lever and tie rod ends. When the wheel axle is shifted even by the smallest distance in any direction, the load on the bearing will increase. The steering axis also changes its position, which does not have the best effect on the car's handling.
What else is the risk?
Almost all modern cars are equipped with Changing the disc offset value in any direction leads to the fact that the car's electronic control unit operates with inaccurate data received from the sensors. As a result, the entire system begins to malfunction, which leads to incorrect calculations of fuel consumption; the automatic transmission begins to malfunction during gear shifts. This is what ET disk failure means!
In addition, the torque is distributed incorrectly, the ignition timing changes its value. Other problems may also be present. In a word, all this is not good for the car.
Drivers' needs
What is the need for changing the standard disk offset parameter? This is largely due to the banal need of car owners. This act is dictated by a disdainful attitude towards safety, but there is a great desire to improve one’s status through visual improvement of the car. Installation of fashionable wheels, which, although they do not correspond to the standard offset, look very cool and cause delight from others.
In addition, many people intend to expand the wheelbase of the car in this way, which gives it a more sporty look. The situation is made worse by some sellers. They may know what ET disk offset is, but to please themselves they prefer not to strictly follow the manufacturers' recommendations. As a result, they persuade car enthusiasts to make a purchase, but whether it fully matches the car or not is no longer important.
The main thing is that the disc will be purchased. Usually, when installing disks with an inappropriate offset, no problems arise. However, this is not yet a reason to think that the operation of the car is completely safe. And some kind of trouble can still happen.
Way out
Is it really now impossible to buy the wheels you like for your car, whose offset does not meet the standards? Fortunately, everything is not so bad and there is a way out of this situation. It involves using wheel spacers. Thanks to this engineering solution, it is possible to reduce the overhang by moving the wheels outward. Also, using such parts, you can install those disks for which the location of the fastening elements relative to the hub does not coincide.
With such details, you don't have to worry about what ET disc offset is. In appearance, metal spacers resemble pancakes. They are installed between the disk and the hub, due to which the offset value changes. Such details make the choice much easier for drivers. But situations are different. For example, all parameters correspond to the norm (width, diameter, centering hole, and the number of mounting holes coincides with the hub ones), but the offset is different. In this case, spacers come to the rescue; you just need to select the desired thickness of the pancakes.
Finally
Some domestic drivers, despite cautionary warnings, still change the departure values, hoping for the Russian “maybe”. This approach is very risky, because we are talking about personal safety, and losing control of the car puts other road users at risk. And this is extremely undesirable.
An interesting parallel to draw when it comes to walking out is wearing high heels. On the one hand, it looks amazing, but on the other hand, there is a certain risk. Caution and accurate calculation are required here - the formula for calculating the offset of the ET disk has already been given above. Then you can find a compromise solution that will only benefit you, without jeopardizing your own life and everyone else.