Design of chassis underframe of the hottest low en

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Design of low entry city bus chassis underframe

Abstract: This paper introduces the structure and characteristics of low entry city bus chassis underframe with specific models, and expounds its design points

key words: city bus; The structure and characteristic of low entry city bus chassis frame is explained in this paper, and its design keywords is discussed on the same time

Key words:City-bus; Low entry frame

city buses are the main carrier of urban public transport in China. In recent years, with the increasing support of the state and governments at all levels for the development of urban public transport, as well as the huge business opportunities brought by Beijing and Shanghai's successful bid for the Olympic Games and the WorldExpo, the renewal speed of urban passenger cars in China has accelerated and the grade has gradually improved. On October 1st, 2003, the Ministry of Construction announced and implemented the "grading technical requirements and configuration of urban passenger cars", which included the urban low floor passenger cars in the ministerial standards for the first time. Our company also successfully developed low entry and low floor urban passenger cars and introduced them to the market from 2003 to 2004. On this basis, this paper emphatically analyzes the structure and characteristics of the low entry chassis underframe, and expounds its design points

low floor city buses have been widely used in developed countries such as Europe and the United States, but they are just in the initial stage in China. The low-level front axle, front wheel independent suspension and low-level rear drive axle assembly developed by German ZF company for the city bus chassis have been widely used in Europe in the early 1990s, so that the floor height of the bus can be reduced to a minimum of 320~360 mm. The front door and the middle door are flush with the floor in the car, and the door steps are cancelled. The front aisle and the middle floor form a flat channel. Wheelchair and stroller can easily enter and exit the car. Space is also reserved in the middle for parking wheelchair, and there is a hook to position the wheelchair, so as to avoid the wheelchair moving back and forth or shaking during the travel of the bus

the difference between low entry vehicles and low floor vehicles mainly lies in the rear axle area of the chassis underframe: low entry vehicles are only first-class pedals in the floor area of the front door and middle door (in front of the rear axle), which are connected to the front floor channel through 1-2 steps in the rear axle area; The low floor truck keeps the first step from the front axle to the floor channel behind the rear axle, or ensures the continuity of the floor channel by means of slope transition in the rear axle area. Generally, a third passenger door can be added behind the rear axle. The foreign advanced low floor vehicle adopts the wheel drive technology, which can fully realize that the floor of the whole vehicle is a first-class step

in terms of the domestic public transport market, low entry vehicles are more suitable for the current situation of the public transport market than low floor vehicles. First of all, compared with the low floor car, the first level step flat display of the low entry car enters the test state of 0.00, showing that the flat floor area is almost the same, and the rear axle rear area can also be fully utilized through reasonable layout; Secondly, the complexity of the chassis structure of low entry vehicles is reduced. For example, it is not necessary to choose the low floor special portal rear drive axle instead of the general type of rear drive axle; Moreover, due to the increasing international influence of China's plastic machine industry behind the rear pillar of the rear door, the plate of the low entry car has been raised, providing more space for the placement and layout of various parts on the chassis, and overcoming some difficulties in the general layout; Finally, it reduces the design difficulty of the chassis, facilitates the localization of the main assemblies of the chassis, greatly reduces the price of the supporting chassis, and is more suitable for the affordability of car purchase funds in China's current urban public transport industry

Taking Zhengzhou Yutong zk6120gcr series city bus chassis as an example, this paper briefly expounds the structural design of low entry city bus chassis chassis and its key points for attention

The chassis configuration of zk6120gcr series city bus is as follows:

engine: Cummins isbe220 31

transmission: Allison t270r

front axle: ZF front axle RL 85A

rear axle: Xiangfan 13T rear axle

steering gear: ZF 8098 955467

angle steering gear: zf7860 955900

front suspension: 2 air springs + two-way barrel shock absorbers, the upper thrust rod is approximately parallel, and the lower thrust rod is approximately parallel.Rod V-shaped arrangement, with Φ 50 lateral stabilizer bar

rear suspension: four air springs + two-way barrel shock absorbers, the upper thrust bar is V-shaped, and the lower thrust bar is approximately parallel, with Φ 50 lateral stabilizer bar

braking system: service braking; Double pipe air braking system, front disc and rear drum, parking braking and emergency braking

tires: Michelin 275/70 R22.5

centralized lubrication: Ningbo Sanlang KFU

the underframe structure and section size that affect the structural strength and stiffness of the whole vehicle and the necessary passing performance of the whole vehicle are mutually restrictive and contradictory. Our purpose is to maximize the minimum ground clearance of the underframe on the basis of ensuring the structural strength and stiffness of the whole vehicle. Carbon nanotubes have good heat transfer performance

according to the specific use conditions of urban public transport in China and the conditions of infrastructure such as roads, the strength and stiffness of the overall structure should be reasonably improved on the basis of the carrying capacity of foreign vehicles of the same type. Generally, the rated passenger capacity of foreign 12m city buses is mostly 50 ~ 60 people (including standing), and we determine the level of 110 ~ 120 people. Taking this analysis as the starting point, the mathematical model of structural strength and stiffness of low entry city bus is established, and through the mature mathematical analysis methods such as finite element method, calculation, comparison and analysis are carried out to determine the size of the frame, section size and wall thickness of the overall structure, which affect the strength and stiffness

the whole vehicle structure is a complex statically indeterminate structure, and the rigidity of the whole vehicle should be balanced to make the internal load distribution of the whole vehicle structure tend to be reasonable. Any change in local strength and stiffness will cause the redistribution of load borne by the structure. Sometimes local strengthening or weakening of one part will cause changes in the strength and stiffness of other parts, which must be paid enough attention. Not all parts can be made as strong as possible. On the basis of theoretical analysis and calculation, on the premise of meeting the minimum clearance from the underframe to the ground, the bottom grille should try to enlarge the section size of its structure, appropriately increase the wall thickness, and select high-quality high-strength steel as the profile material. So as to maximize its bearing capacity

1. The front section area

because the floor is 360mm above the ground, taking into account the approach angle and the ground clearance below the longitudinal beam, the front longitudinal beam of the front axle adopts 120x60x4.0 rectangular steel pipe, and the front end is chamfered (some manufacturers also use channel steel, which can increase the ground clearance of the lower plane of the longitudinal beam by increasing the wall thickness of the longitudinal beam and reducing the section height)

the floor frame of the driving area is mounted on the left front longitudinal beam of the front axle. The steering gear is arranged horizontally, and the steering gear bracket and other parts are welded on the floor frame in the driving area. As shown in Figure 1:

in the area directly above the front axle, considering the need to leave the jumping space of the front axle and the layout space of the chassis pipeline harness, the floor channel here is directly connected with the front and rear crossbeams by stamping trough steel plates. As shown in Figure 2:

a trough shaped longitudinal beam, a vertical sealing plate, an arc-shaped wheel housing frame, a reinforced support, etc. are erected above the floor channel, and the supports of various parts in the suspension assembly (airbag support, thrust rod support, shock absorber support, etc.) are installed, as shown in Figure 3

note: This is where the cross-section of the frame longitudinal beam changes greatly. Appropriate reinforcement (such as adding diagonal braces, connecting plates, etc.) should be considered in the design to make the cross-section transition smooth and avoid large stress concentration

2. The longitudinal beam in the middle area

adopts 160x60x6.0 rectangular steel pipe, and the connection mode between the longitudinal beam and the transverse beam adopts the mode of longitudinal beam segmentation and transverse beam penetration. The through beam forms a closed ring structure with the left and right side wall columns in the body frame and the top frame beam to improve the torsional strength of the whole vehicle. See Figure 4

note: the manufacturer also adopts the structure of longitudinal beam through and cross beam segmented, or when the longitudinal beam and cross beam are trough beams, they can be through, mainly depending on the torsion and bending resistance of the frame and the complexity of the manufacturing process

in the area where the longitudinal beam intersects with the transverse beam, the left and right middle longitudinal beams and transverse beams are welded into one by strengthening the diagonal brace to increase the overall strength of the underframe. At the same time, all transverse beams correspond to the body side pillar and the roof beam. See Figure 5:

note 1: when processing the welding of diagonal braces and longitudinal and transverse beams, the diagonal brace notch treatment can be as shown in Figure 6 to avoid welding stress concentration

note 2: the form of reinforcing plate can be as shown in Figure 7, and plug welding holes can be opened to weld with longitudinal and transverse beams to reduce welding stress

3. Rear section area

the transition area between the middle and rear sections of the underframe is the key area in the treatment of the underframe structure. Usually, the lap girder structure (see Figure 8) is used to raise the floor channel of the rear axle through two steps to avoid the rear axle. At the same time, in order to make the transition of the frame section smooth and not cause stress concentration due to sudden changes, some treatment is made in the shape of the front and rear longitudinal beams of the lap girder, In addition, when designing the floor frame of the car body, the frame of the secondary step is strengthened as shown in Figure 9. While realizing the transition of the floor channel from the middle flat plate to the rear floor, the section transition is smooth, reducing the possibility of stress concentration, so as to ensure the overall strength of the whole car frame to the greatest extent

in the general layout of the chassis of low floor city buses, it is quite difficult to arrange the fuel tank, battery, air tank and other parts. This is mainly due to the reduction of the floor, which significantly reduces the space between the chassis structure under the floor and the ground, and it is difficult to arrange the above-mentioned parts with large volume and weight. However, for low entry city buses, because the rear column and rear floor of the rear door have been raised, it provides more space for the placement and layout of various parts on the chassis and overcomes some difficulties in the general layout

in the frame layout of the rear section of the underframe and the engine area, due to the use of the conventional rear engine and rear axle, the rear longitudinal beam has a very high degree of versatility design, which can be interchangeable with the rear longitudinal beam of the conventional bus chassis. There is no specific description here, but only the structure of the later section is shown in Figure 10

see Figure 11 below for the structural diagram of the last chassis underframe


it is an inevitable trend for the development of technology and the improvement of use requirements to lower the floor of public transport vehicles and then unload the oil collector. As a special form of low floor city bus, low entry car is more suitable for the affordability of car purchase funds in China's current urban public transport industry, and can also meet its various functional requirements. It is the most advantageous and feasible technical scheme at this stage and for a considerable period of time in the future


[1] Ma Jingjie, et al., development and research of urban bus Bus technology and research, 2001.2

[2] automotive engineering manual [m] Beijing: People's Communications Press, 2001.6

Zhengzhou Yutong Bus Co., Ltd. Xu Zhiqiang, Zhang Hongtao

Jin Xin, Wang Yaoqian (end) of Henan Communications vocational and Technical College

Copyright © 2011 JIN SHI