Brake disc wear
Content
Brake disc wear is the inevitable result of the friction material of the brake pads acting on its surface. It depends on the health of the brake system, the operating conditions of the car, the driving style of its owner, the mileage in which discs are used, their quality and type, as well as seasonality, since dirt, moisture and chemicals scattered on the roads have a negative effect on the brakes. The wear tolerance of brake discs, often, their manufacturer himself, indicates precisely on the surface of the product.
Brake disc wear signs
It is quite difficult to determine the wear of the discs by indirect signs, that is, by the behavior of the car. However, it is worth checking the thickness of the discs in the following cases:
- Changes in pedal behavior. namely, a major failure. However, this symptom may also indicate other problems with the elements of the brake system - wear of the brake pads, breakage of the brake cylinder, and a decrease in the level of brake fluid. Nevertheless, the condition of the brake discs, including their wear, should also be checked.
- Vibration or jerking when braking. Such symptoms may occur due to misalignment, curvature, or uneven wear of the brake disc. However, the condition of the brake pads must also be checked.
- Vibration on the steering wheel. One of the common causes in this case is deep wear grooves, disc misalignment or deformation. Problems can also be caused by worn or damaged brake pads.
- Whistling sounds when braking. They usually appear when the brake pads are damaged or worn. However, if the latter fail, there is a high probability that the metal base of the pads can damage the disc itself. Therefore, it is advisable to check its general condition and wear.
If one or more of the defects listed above occur, it is necessary to check the correct operation of the brake system, as well as assess the condition of its elements, including paying attention to the wear of the brake discs.
| breakage | Sticky discs | Car skidding when braking | Whistling brakes | Steering wheel vibration during braking | Jerks during braking |
|---|---|---|---|---|---|
| What to produce | |||||
| Replace brake pads | ✔ | ✔ | ✔ | ✘ | ✘ |
| Check the operation of the brake caliper. Check pistons and guides for corrosion and grease | ✔ | ✔ | ✔ | ✔ | ✔ |
| Check the thickness and general condition of the brake disc, the presence of runout during braking | ✔ | ✘ | ✔ | ✔ | ✘ |
| Check the condition of the friction linings on the pads | ✘ | ✔ | ✘ | ✘ | ✘ |
| Check wheel bearings. Check the condition of the steering mechanisms, as well as the suspension | ✘ | ✘ | ✘ | ✔ | ✔ |
| Check tires and rims | ✘ | ✘ | ✘ | ✔ | ✘ |
What is the wear of the brake discs
Any car enthusiast should know what kind of brake disc wear is acceptable, at which they can also be safely operated, and which one is already limiting, and it is worth changing the discs.
The fact is that if the maximum wear of the brake discs is exceeded, there is a possibility of an emergency. So, depending on the design of the brake system, the brake piston can either jam or simply fall out of its seat. And if this happens at high speed - it is very dangerous!
Permissible wear of brake discs
So, what is the allowable wear of the brake discs? Wear rates for brake discs are prescribed by any manufacturer. These parameters depend on the engine power of the car, the size and type of brake discs. The wear limit will be different for different types of discs.
For example, the thickness of a new brake disc for the popular Chevrolet Aveo is 26 mm, and critical wear occurs when the corresponding value drops to 23 mm. Accordingly, the permissible wear of the brake disc is 24 mm (one unit on each side). In turn, disc manufacturers put information about the wear limit on the working surface of the disc.
This is done using one of two methods. The first is a direct inscription on the rim. For example, MIN. TH. 4 mm. Another method is a mark in the form of a notch on the end of the disk, but on its inner side (so that the block does not strike on it). As practice shows, the second method is more convenient, because with an increase in wear up to a critical one, the disk starts to brake in jerks, which will be clearly felt by the driver when braking.
As for drum brake discs, they do not decrease as they wear, but increase in their inner diameter. Therefore, in order to determine what kind of wear they have, you need to check the inner diameter and see if it does not exceed the permissible limits. The maximum permissible working diameter of the brake drum is stamped on its inner side. usually it is 1-1,8 mm.
Many resources on the Internet and in some auto shops indicate that brake disc wear should not exceed 25%. In fact, wear is ALWAYS measured in absolute units, that is, in millimeters! For example, here is a table similar to those given for various cars in their technical documentation.
| Parameter name | Value, mm |
|---|---|
| Nominal brake disc thickness | 24,0 |
| Minimum disc thickness at maximum wear | 21,0 |
| Maximum allowable wear of one of the disc planes | 1,5 |
| Maximum disc runout | 0,04 |
| Minimum admissible thickness of a friction lining of a brake shoe | 2,0 |
How to determine brake disc wear
Checking brake disc wear is not difficult, the main thing is to have a caliper or micrometer on hand, and if there are no such tools, then in extreme cases you can use a ruler or a coin (more on that below). The thickness of the disc is measured at 5 ... 8 points in a circle, and if it changes, then in addition to the wear of the brake area, there is curvature or uneven wear. Therefore, it will be necessary not only to change it at the limit, but also to find out the reason due to which uneven wear of the brake disc occurs.
How is it checked
In order to find out the degree of wear, it is best to dismantle the wheel, since the thickness of the disc cannot be measured otherwise, and if you need to check the wear of the rear brake drums, you will have to remove the entire brake mechanism. When carrying out a further check, it must be taken into account that the disks wear out on both sides - external and internal. And not always evenly, so you need to know the degree of wear of the disk on both sides of the disk, but more on that below.
Before checking, you must know the information about the thickness of the new brake disc for a particular car. It can be found in the technical documentation or on the disc itself.
Limit wear of brake discs
The value of the maximum allowable wear will depend on the initial size of the disk and the power of the vehicle's internal combustion engine. Typically, the total wear of the entire disc for passenger cars is about 3 ... 4 mm. And for specific planes (internal and external) about 1,5 ... 2 mm. With such wear, they already need to be changed. For brake discs consisting of a single plane (usually installed on the rear brakes), the procedure will be similar.
Checking the wear of brake discs involves checking the thickness of both planes of the disc, the size of the shoulder, and then comparing these data with the nominal value that a new disc should have, or recommended parameters. also assess the general nature of the abrasion of the working area of the disk, namely, uniformity, the presence of grooves and cracks (the size of the cracks should not be more than 0,01 mm).
During a scheduled inspection, you need to look at the size of the grooves of the working and their structure. Small regular grooves are normal wear. It is recommended to replace discs paired with pads if there are deep irregular grooves. In case of conical wear of the brake disc, it is necessary to change it and check the brake caliper. If cracks or other corrosion and discoloration are visible on the disc, it is usually associated with thermal phenomena that occur due to frequent and excessive changes in the temperature of the disc. They cause braking noise and reduce braking efficiency. Therefore, it is also desirable to replace the disk and it is desirable to install better ones with improved heat dissipation.
Note that when the disc wears, a certain edge forms around the circumference (the pads do not rub on it). Therefore, when measuring, it is necessary to measure the working surface. it is easier to do this with a micrometer, since its “encircling” working elements allow you not to touch it. In the case of using a caliper, it is necessary to place any objects under its gauges, the thickness of which coincides with the wear of the pads (for example, pieces of tin, metal coins, etc.).
If the value of the thickness of the disk as a whole or any of its planes is below the permissible value, the disk must be replaced with a new one. A worn brake disc must not be used!
If you do not have a micrometer at hand, and it is inconvenient to check with a caliper due to the presence of a side, then you can use a metal coin. For example, according to the official Central Bank of Russia, the thickness of a coin with a face value of 50 kopecks and 1 ruble is 1,50 mm. For other countries, relevant information can be found on the official websites of the central banks of the respective countries.
To check the thickness of the brake disc with a coin, you need to attach it to the working surface of the disc. In most cases, the critical wear of one disk surface is within 1,5 ... 2 mm. Using a caliper, you can find out the wear thickness of both one half of the disk and the total thickness of the entire disk. If the edge is not worn out, you can measure directly from it.
What affects brake disc wear?
The degree of wear of the brake discs depends on many factors. Among them:
- Driving style of a car enthusiast. Naturally, with frequent sudden braking, excessive wear of the disc and wear of the brake pads occurs.
- Vehicle operating conditions. In mountainous or hilly terrain, the brake discs wear out faster. This is due to natural causes, since the brake system of such cars is used more often.
- Type of transmission. On vehicles with a manual transmission, the discs, like the pads, do not wear out as quickly. Conversely, in cars equipped with an automatic transmission or a variator, disc wear occurs faster. This is explained by the fact that in order to stop a car with automatic transmission, the driver is forced to use only the brake system. And a car with "mechanics" can often be slowed down due to the internal combustion engine.
- Type of brake discs. Currently, the following types of brake discs are used on passenger cars: ventilated, perforated, notched, and solid discs. Each of these types has its own advantages and disadvantages. However, as practice shows, solid discs fail the fastest, while ventilated and perforated discs last longer.
- Wear resistance class. It directly depends on the price and the type of disk indicated above. Many manufacturers simply indicate the minimum mileage for the car for which the brake disc is designed instead of the wear resistance class.
- Brake pad hardness. The softer the brake pad, the more gentle it works with the disc. That is, the disk resource increases. In this case, the braking of the car will be smoother. Conversely, if the pad is hard, then it wears out the disc faster. Braking will be sharper. Ideally, it is desirable that the hardness class of the disc and the hardness class of the pads match. This will extend the life of not only the brake disc, but also the brake pads.
- Vehicle weight. Typically, larger vehicles (eg crossovers, SUVs) are equipped with larger diameter discs and their brake system is more reinforced. However, in this case, it is indicated that a loaded vehicle (that is, carrying additional cargo or towing a heavy trailer) brake discs wear out faster. This is due to the fact that to stop a loaded car, you need more force that occurs in the brake system.
- The quality of the disc material. Often, cheap brake discs are made of low-quality metal, which wears out faster, and may also have defects over time (curvature, sagging, cracks). And accordingly, the better the metal from which this or that disk is made, the longer it will last before replacement.
- Serviceability of the brake system. Failures such as problems with the working cylinders, caliper guides (including the lack of lubrication in them), the quality of the brake fluid can affect the rapid wear of the brake discs.
- The presence of an anti-lock system. The ABS system works on the principle of optimizing the force at which the pad presses on the brake disc. Therefore, it extends the life of both pads and discs.
Please note that usually the wear of the front brake discs always exceeds the wear of the rear ones, since they are subjected to significantly more force. Therefore, the resource of the front and rear brake discs is different, but at the same time there are different requirements for wear tolerance!
On average, for a standard passenger car used in urban areas, a disk check must be performed approximately every 50 ... 60 thousand kilometers. The next inspection and measurement of wear is done depending on the percentage of wear. Many modern discs for passenger cars easily work for 100 ... 120 thousand kilometers under average operating conditions.
Reasons for uneven wear of brake discs
Sometimes when replacing brake discs, you can see that the old ones have uneven wear. Before installing new discs, you need to figure out the reasons why the brake disc wears unevenly, and, accordingly, eliminate them. The uniformity of disc wear greatly affects the braking performance! So, uneven wear of the brake disc can be caused by the following factors:
- Material defect. In rare cases, especially for cheap brake discs, they can be made of poor quality material or without following the appropriate manufacturing technology.
- Incorrect installation of brake discs. Most often, this is a banal distortion. This will result in conical disc wear as well as uneven brake pad wear. At the initial stage, the disc can be pierced, but it is still better to replace such a disc with a new one.
- Incorrect installation of brake pads. If any of the pads was installed crookedly, then, accordingly, the wear will be uneven. Moreover, both the disc and the brake pad itself will wear out unevenly. This reason is typical for already worn brake discs, since the pads wear out much faster than the disc.
- Dirt getting into the caliper. If the brake caliper protective boots are damaged, small debris and water will get on moving parts. Accordingly, if there are difficulties in movement (uneven stroke, souring) in the working cylinder and guides, then the uniformity of the pad force over the area of \uXNUMXb\uXNUMXbthe disk is disturbed.
- Curve guide. It may be uneven due to incorrect installation of brake pads or mechanical damage. For example, as a result of repair of the brake system or an accident.
- Corrosion. In some cases, for example, after a long period of inactivity of the car in atmospheric conditions with high humidity, the disc may become corroded. Because of it, the disk may wear out unevenly during further operation.
Please note that it is possible, but not recommended, to grind a brake disc that has uneven wear. It depends on its condition, the degree of wear, as well as the profitability of the procedure. The fact that the disk has a curvature will be prompted by a knock that occurs during braking. Therefore, before grinding grooves from the surface of the disc, it is imperative to measure its runout and wear. The admissible value of the disc curvature is 0,05 mm, and the runout appears already at a curvature of 0,025 mm. The machines allow you to grind a disc with a tolerance of 0,005 mm (5 microns)!
Hack and predictor Aviator
The wear of the brake discs must be checked approximately every 50 ... 60 thousand kilometers, or if problems arise in the operation of the vehicle's braking system. To check the wear value, you need to dismantle the disc and use a caliper or micrometer. For most modern passenger cars, the allowable disc wear is 1,5 ... 2 mm on each plane, or about 3 ... 4 mm across the entire thickness of the disc. In this case, it is always necessary to evaluate the wear of the inner and outer planes of the discs. The inner side of the disc always has slightly more wear (by 0,5 mm).
