Brake System Analysis II: Judging the Three Major Anomalies and Coordinating the Main Mount

The braking system of the tractor is relatively standard when it leaves the factory. In contrast, many trailer manufacturers do not make their braking systems as well. If problems occur during use, it can create a significant burden for later maintenance. This time, we visited the trailer braking system test bench made by Hu Ge to ask him about his views on trailer braking systems and the precautions for installing trailer ABS systems.

This test bench simulates the braking system of a three-axle trailer. From the picture, we can see that three air tanks are arranged on the test bench. According to Hu Ge, national regulations require that the volume of a trailer air tank must reach 80 liters.

For the type of 40-liter air tank installed on Hu Ge's test bench, the standard when a trailer leaves the factory is to be equipped with two. Nowadays, many large trailer manufacturers, such as CIMC Group, equip trailers with three air tanks. Hu Ge told us that this is mainly to ensure an adequate air supply for the brakes during regular braking when driving on long downhill sections. ● Coordination of the tractor and trailer: How to manage it

In terms of brake air circuit layout, the test bench made by Hu Ge provides two methods. One is purely mechanical without ABS, using an emergency brake valve to control a relay valve that connects the three air tanks; the other is equipped with an ABS system, using the emergency brake valve to control the ABS combination valve.

The arrangement of these two air circuits clearly shows that the key component for achieving coordinated braking between the tractor and trailer, as well as attaining linear performance, is the trailer’s emergency brake valve. During normal braking, the main data relied upon is the air pressure in the air reservoir when the vehicle brakes, which is transmitted through the relay valve to the brake chamber slack adjuster.

Moreover, the speed must correspond with the depression of the brake pedal. If, for instance, when the foot brake pedal is pressed one-third of the way, the pressure in the air reservoir has already reached its maximum through the relay valve; or if it has not yet reached the control pressure of the tractor-trailer valve, it indicates that the trailer’s braking response is either too fast or too slow, both of which will affect the coordination of the braking effect between the tractor and trailer.

Based on the current load capacity and speed of tri-axle trailers, Hu suggests that the optimal arrangement is to control the three relay valves through a single emergency brake valve. The emergency brake valve ensures the linear control of the main trailer, while the three relay valves guarantee the response time at the wheel ends. This setup allows coordination of the main trailer through the control of the master brake cylinder, which is what we commonly refer to as the 'one-to-three' method.

In order to prevent brake lock-up, many truck drivers have proactively installed ABS systems themselves. For ABS users, Hu suggests using an installation method that combines an emergency brake valve with an ABS combination valve, and then using the ABS to control the sub-chambers of each brake air chamber. ● Installation Layout and Piping Accessories 

Currently, many trailer manufacturers install the emergency brake valve and relay valve at the front of the trailer. Hu says this is to make installation more convenient. However, in practice, this installation method can result in excessively long piping, which can easily cause various problems during use. Based on personal experience, he recommends installing each valve in the middle of each axle of the trailer. Although installation is a bit more troublesome, it makes later use much more convenient.

For users installing ABS, Hu recommends installing the ABS combination valve between the second and third axles, and the emergency brake valve between the first and second axles, or adjusting the position forward as needed based on actual conditions. 

Since the control lines connecting the emergency brake valve and the trailer ABS combination valve are not part of the load air circuit, their length does not affect braking performance. However, the lines from the ABS combination valve to the individual brake chambers are part of the load air circuit, so their length needs to be more carefully considered.

In addition, different trailers come with varying diameters of piping connecting the air tanks, relay valves, and brake chambers when they leave the factory. Common pipe diameters are 12mm, 16mm, and 18mm, and the difference in pipe diameter can affect the response time during braking.

Brother Hu once conducted an experiment with several pipelines of different diameters and found that their response times differed by tens of milliseconds. Although it is only a matter of tens of milliseconds, the delay during the braking process is already quite noticeable. It is best to choose pipelines with a diameter of 16 millimeters, as they are standard accessories on the market. Pipelines with an 18-millimeter diameter are not easy to purchase domestically, while 12-millimeter pipelines are too thin, which affects the response time of the trailer brakes. 

How to Choose Air Chamber Sub-pumps 

In the trailer brake system, air chamber sub-pumps can be considered wear-prone parts. In a corner of Brother Hu’s repair shop, scrapped air chamber sub-pumps have piled up like a mountain. Based on his repair experience, the push rods and rubber cups inside the air chamber sub-pumps are structures that are easily damaged. The direct result of this is air leakage from the sub-pump during braking, causing the brakes to be ineffective.
 

In addition, the screws for the top cover of low-quality counterfeit air chamber sub-pumps are also parts that are very easily damaged, as their quality is simply not up to standard.

The importance of the air chamber relay valve in the entire braking system goes without saying. Using inferior products not only increases the cost of maintenance in the long term but also affects the coordination of brakes on the trailer, leading to potential safety hazards. Therefore, Hu suggests that truck owners should try to choose products from well-known brands. Among domestic manufacturers, Hu recommends Reili products, which at least have some quality assurance. 

Currently, the market price for Reili brand air chamber relay valves is between 200-300 yuan. Compared to counterfeit products, the price difference can be several tens of yuan. According to Hu's sharing, the first batch of air chamber relay valves used on trailers had a service life of about six months, while Reili's valves can last approximately 1-2 years.

● Editor's Note:

From Hu Ge's test bench, the braking system of the entire trailer is actually not too complicated. The most important standard is whether the trailer's main brakes can achieve a linear effect. At present, the configuration of air reservoirs, the use of emergency brake valves, and the selection of brake chambers are in a state of disorder in trailer braking systems, being quite arbitrary and lacking any standard.

This also stems from users' lack of understanding of this part of the knowledge. The chaos in trailer braking systems can cause severe wear on trailer tires, directly leading to increased operational costs later on. Hu also did a financial calculation: a three-axle trailer has 12 tires, with an average daily usage cost of 100 yuan. If the tires could be used for an additional month, it would undoubtedly save costs.