In the bustle of the city, we often see a huge transport carrying hundreds of passengers, which is popularly affectionately called an “accordion”. However, if you address this question to design engineers or bus depot employees, they may not understand you without clarification. The common name is firmly entrenched in the lexicon, but technical documentation uses completely different terms.
The correct name for this type of public transport is articulated bus. This name comes from the word “articulation”, which means a movable connection between two rigid parts of the body. It is this unit that allows the long car to maneuver in dense traffic, folding and unbending like joints.
In this article we will analyze in detail the structure of this mechanism, consider the types of drives and find out why articulated circuit It is considered one of the most difficult to maintain urban transport. You will learn the differences between models with different engine locations and how the safety system in the moving joint is designed.
Technical nomenclature and classification
The official classification of vehicles clearly divides buses by body type. A standard single body is called a monoblock, while a vehicle consisting of two or more rigid sections connected by an articulating device belongs to the class articulated buses. In international practice, the term “articulated bus” is also often used.
There is also a variety that has three body sections, called two-section articulated bus or "double accordion". Such giants are used on routes with extremely high passenger traffic, for example, in BRT bus rapid transit systems. Structurally, they are a tribunal, where the middle section does not have its own wheelbase, relying on the turntables of neighboring parts.
It is important to understand that the term “accordion” describes only the visual effect of movement, but does not reflect the essence of the engineering solution. Articulation is a complex mechanism that requires regular lubrication, checking for leaks and monitoring the condition of the bearings. Errors in classification may lead to misunderstandings when ordering spare parts or technical documentation.
Movable joint design
The heart of the entire system is the swivel unit, which connects the front and rear parts of the body. It consists of two main platforms: one mounted on the front section frame, and the second mounted on the rear section frame. Between them is turntable with a powerful bearing providing horizontal rotation.
To protect passengers from the external environment and create a comfortable microclimate, it is used fur (accordion made of rubber or polymer materials). This element not only closes the passageway, but also takes on tensile and compressive loads. Modern models are equipped with bellows with a multilayer structure, including reinforcement to increase strength.
⚠️ Attention: The tightness of the joint bellows is critical. If it is damaged, exhaust gases may enter the cabin, especially if the engine is located at the rear, which poses a direct threat to people's health.
The system also includes vibration damping elements. Since the rear part of the bus can move along a complex trajectory when turning, additional dampers and rods that prevent sudden jerks and ensure smooth running. This is especially important when driving on uneven road surfaces.
What is accordion fur made of?
Modern bellows are made of multilayer polyurethane or special rubber reinforced with cord. Insulation is often laid inside, and the surface is treated with compounds that are resistant to ultraviolet and reagents.
Layout diagrams and engine location
Articulated buses can have a different powertrain arrangement, which directly affects their driving performance. The most common layout is a rear-engine layout, where the engine is located at the tail of the last section. In this case, the driving wheels are the third axle, and sometimes the second one if all-wheel drive is implemented.
There is also a front-engine design, where the engine is located in the “nose” of the first section. Such models are less common due to difficulties with weight distribution and organization of passenger space. In this case, the torque is transmitted through a long driveshaft running under the floor of the entire cabin to the rear axle.
Deserves special attention hybrid and electric models. Electric ships often use a scheme with motor-wheels on the second and third axles, which makes it possible to abandon the mechanical connection between sections in terms of transmission. This simplifies the design of the rotary assembly, since there is no need to transmit powerful rotational motion through the joint.
When choosing a bus for a route with frequent climbs, pay attention to the presence of a retarder (hydraulic retarder) or energy recovery, since the weight of an articulated bus is high.
Control systems and rear axle steering
One of the main problems of long vehicles is the rear section cutting off when turning. To prevent the rear of the bus from “driving” onto the sidewalk or into the oncoming lane, modern models use a system rear axle steering. It can be implemented mechanically or electronically.
In mechanical systems, the rotation of the wheels of the rear bogie depends on the angle of rotation of the steering wheel of the front section through a system of rods and levers. In more advanced electronic systems sensors read the angle of rotation of the front axle and the speed of movement, transmitting a command to the hydraulic or electric drive of the rear wheels. This greatly improves maneuverability in narrow city areas.
Failure of the steering system can cause the bus to become uncontrollable at small turning radii. Therefore, diagnostics of this unit is included in the mandatory list of maintenance work. Drivers undergo special training to operate such equipment, since the feeling of size here is different from driving a regular bus.
Types of transmission in articulated buses
Transmitting torque in an articulated bus is a non-trivial task, especially considering the mobility of body parts relative to each other. Let's look at the main types of transmissions used in this technology:
- 🚛 Mechanical transmission: The classic version, where torque is transmitted through a driveshaft passing through a hinge joint. Requires the use of special universal joints with compensating capabilities.
- ⚙️ Hydromechanical transmission: The most common option in modern city buses. The automatic transmission (AT) ensures smooth shifting, which is important for the comfort of standing passengers.
- ⚡ Electromechanical (hybrid): The internal combustion engine operates at its optimum level, producing electricity that powers the motors on the wheels. Allows flexible distribution of traction between axles.
The driveshaft running through the articulation area is a critical element. It must have high elasticity and strength to withstand constant kinks when the bus turns. Any vibrations in this area can lead to rapid destruction of the support bearings.
The choice of transmission type affects fuel consumption and smoothness: hydromechanics are more comfortable for the city, mechanics are cheaper to repair, a hybrid is more economical in traffic jams.
Maintenance and diagnostics of components
Operating articulated buses requires more frequent and thorough maintenance compared to single models. Particular attention is paid to the section junction area. Condition is checked regularly turntable bearing, the presence of lubrication in friction units and the integrity of rubber-metal elements.
The sealing system (fur) is inspected for cracks, tears and abrasions. Even a small hole can cause a draft in winter or water entering the interior. Fur repair often requires dismantling the entire assembly and carrying out work under special conditions, since precise adjustment of parts is necessary.
Diagnostics also includes checking the operation of the section angle sensors and the rear axle steering system. Failure of these systems can lead to increased tire wear and accidents on the road. Mechanics use specialized stands to check the geometry of the wheel pairs.
☑️Daily inspection of the articulated bus
Feature Comparison: Single vs Articulated
To better understand the specifics of the “accordion”, it is useful to compare its parameters with a regular large-capacity bus. The differences relate not only to length, but also to dynamic characteristics, turning radius and infrastructure requirements.
| Parameter | Single bus | Articulated bus |
|---|---|---|
| Capacity (persons) | 80–100 | 150–200 |
| Length (m) | 10–12 | 18–25 |
| Turning radius (m) | 11–12 | 12–14 (with steering) |
| Number of axes | 2 | 3 (sometimes 4) |
| Fuel consumption (l/100km) | 30–40 | 50–70 |
As can be seen from the table, articulated bus has a significantly larger capacity, which makes it indispensable on busy highways. However, its operation requires wider lanes and special stopping pockets. Maneuverability in cramped conditions is inferior to single models, despite the steering system.
In conclusion, it is worth noting that the correct choice of terminology helps to more accurately formulate queries when searching for information or spare parts. Articulated bus is the pinnacle of engineering in the field of ground passenger transport, combining the capacity of a tram and the flexibility of a car chassis.
Why don't they make an accordion longer than 25 meters?
Increasing the length of more than three sections makes the bus unstable at high speeds and creates a “whiplash” effect when maneuvering, which is dangerous for other road users.
Frequently asked questions (FAQ)
Is it possible to drive an articulated bus with a regular category D?
Yes, to drive a standard two-section articulated bus (up to 25 meters long) in most countries, including the Russian Federation, the category D. However, three-section ("double accordion") or extra long models may require an additional mark or category to operate D1 depending on local regulations and gross vehicle weight.
Why is it always cold or hot in the joint area in an accordion?
The joint area is the most vulnerable point for thermal insulation. Rubber bellows has lower heat capacity and insulating properties than metal body sides. In addition, it is in this zone that microcracks and depressurization most often occur, which leads to drafts. In modern models, this problem is solved by installing additional heaters and air conditioners directly in the sections.
What is the name of the mechanism that allows the rear end to turn?
This mechanism is called the system rear axle steering (or servo axis). It can be hydraulic, pneumatic or electric. Its task is to change the angle of rotation of the wheels of the rear bogie in antiphase or in phase with the front axle in order to minimize the turning radius and prevent the bus from “steering” onto the curb.
What is the difference between an accordion bus and a tram?
The main difference is in the type of propulsion and the route. Articulated bus uses pneumatic tires and drives on public roads, subject to motor vehicle traffic regulations. The tram moves along a rail track, has electric traction and priority at intersections, but is limited by its track. Structurally, the articulation unit may be similar, but the chassis is fundamentally different.