Home / News / Blogs / Design specifications for semi trailers(Part 1)

Design specifications for semi trailers(Part 1)

Views: 0     Author: Wondee Autoparts     Publish Time: 2022-11-30      Origin: Wondee Autoparts

1. General principles of semi-trailer design

During the design of semi-trailer, the following 7 principles shall be followed:

1.1 The product meets the requirements of relevant government and industrial standards and regulations. When the enterprise has special regulations, it also meets the requirements of the enterprise.

1.2 The structure design is reasonable and the product safety is emphasized.

1.3 The important parameters such as the distribution of axle load, the layout of center of gravity, the height difference of tractor and semi-trailer shall comply with the relevant regulations of the enterprise.

1.4 The product has good processability and is easy to be manufactured and installed.

1.5 Materials shall be selected reasonably in consideration of economy.

1.6 Pay attention to the appearance, and the product appearance should be beautiful.

1.7 Serialization and universality of parts shall be considered.

2. Complete vehicle design of semi-trailer

2.1 Scheme formulation

2.1.1 The overall dimensions, wheelbase and front and rear suspension of the whole vehicle shall conform to the government announcement as far as possible. The exported vehicle models shall be designed according to the requirements of foreign users. The axle load distribution shall be reasonable, and the vehicle performance shall meet customers’ requirements.

2.1.2 Distribution of axle load and matching of tractor and semi-trailer

Mainly refer to the following table 1 for the driving form of the tractor, the distribution of the axle load, and the matching of the tractor and the semi-trailer, refer to Table 1 below.

Table 1. The axle load distribution of semi-trailer and matching requirements of tractor and semi-trailer

Driving form of tractor

Height difference of tractor and semi trailer(3-axle)

Axle load ratio at king pin

Height difference of tractor and semi trailer(2-axle)

Axle load ratio at king pin
















Front slewing clearance


Rear slewing clearance


Up and down swing clearance

≥0.13*〔Front distance+tractor rear suspension〕

Height of landing gear from the ground

350mm ≤ H ≤ [effective travel of landing gear - 50mm], installation height of landing gear: 890-930mm,

travel: 430-480mm

2.1.3 Design of key points

〔1〕The main load bearing positions of the vehicle shall be designed safely and reasonably.

1] Main load bearing position of semi-trailer: positions of the king pin, the landing gear and the suspension. For the semi-trailers used for hooking and unhooking transportation, pay special attention to the strength of these positions, as shown in Figure 1 below.

Main load bearing positions of a semi-trailer

Figure 1. Main load bearing positions of a semi-trailer

2〕Design principle for main load bearing positions: ensuring the safety of use is the main principle. According to different vehicle tonnage configurations, partial or overall strengthening, dispersing stress, and increasing strength shall be carried out for the positions with stress concentration or weak strength, and complying with the principle of vehicle lightweight principle.

〔2〕 Beating space of tire

There should be enough beating space between the side beam of the frame and the tire. If the space is insufficient, a limit block should be installed on the lower wing plate directly above the center of the leaf spring, as shown in Figure 2 below.

Schematic diagram of Tire beating Space

Figure 2. Schematic diagram of Tire beating Space

Common tire beating space:

Tire : 1100.00R20, beating space: 130mm;

Tire : 12.00R20-20, beating space: 150mm;

Tire : 11R22.5, beating space: 140mm;

Tire: 12R22.5, beating space: 160mm.

〔3〕 The parts and materials selected for key load bearing positions shall match with the vehicle tonnage configuration.

2.1.4 Carriage structure

1〕Carriage structure of the fence semi trailer

The carriage body is composed of the front board, fence boards and stand pillars. The opening mode of the fence is divided into upper and lower opening mode and left and right opening mode. For the number of doors, 3-axle semi-trailer has 11 doors or 13 doors. The stand pillar is divided into interrior box type and exterior box type. As shown in Figure 3 below.

Schematic diagram of the fence semi trailer

 Figure 3. Schematic diagram of the fence semi trailer

2〕Carriage structure of stake semi trailer

The carriage body is composed of front baffle, box boards, flower railing and stand pillars. Opening mode of the bin grid: left and right split type, upper and lower open type, long lock rod split type, short lock rod split type, and slot up lift type. For the number of doors , 3-axle semi trailer has 11 doors or 13 doors. The stand pillar is divided into interior box type and exterior box type. The rear door is divided into box type split door, box plate on the same side and bin grid split door. As shown in Figure 4 below.

Carriage structure of stake semi trailer

Figure 4. Carriage structure of stake semi trailer

3〕Carriage structure of van semi trailer

The carriage is composed of front baffle, fixed box plates, stand pillars, rear door and top box plate. The van type is divided into sealed roof type (i.e. full seal type) and unsealed roof type. Opening mode of left and right doors: 1 pair split type, 2 pairs split type and 3 pairs split type.

〔4〕 Chassis structure

The chassis structure of common semi-trailer can be divided into straight beam type and gooseneck type. The gooseneck type can effectively lower the height of the rear cargo platform and the height of the gravity center of the whole vehicle. See Figure 6-1 and 6-2 below.

Gooseneck semi-trailer

Figure 6-1. Gooseneck semi-trailer

Straight beam semi-trailer

6-2. Straight beam semi-trailer

2.2 Detailed design, proofreading and review

2.2.1 Confirm the relevant parameters of the tractor. The matching of the tractor and semi trailer should be reasonable. For the gooseneck vehicles, pay attention to the matching of the gooseneck length with the tractor, and the height of the rear cargo platform shall be correct.

2.2.2 The length and width of the carriage shall be consistent with that of the chassis, and shall be consistent with the requirements in contract;

2.2.3 The sizes and positions of the stand pillar boxes in diagram and that of the chassis in diagram shall be consistent, and there is a through beam at each stand pillar box.

2.2.4 The opening method of flower railings, stand pillar numbers, box plate form, opening number of doors and relevant technical requirements shall be consistent with the contract.

2.2.5 The length of the front neck, the height of the longitudinal beam and the materials shall be consistent with the contract.

2.2.6 The specification of materials used for edge beams and through beams shall be consistent with requirements in contract.

2.2.7 requirements in contract

Confirm whether all accessories meet the  requirements in contract.

2.2.8 The standard configuration is 2 spare tire racks. Confirm the number of spare tire racks and whether there are mounting positions.

2.3. Drawings shall comply with relevant regulations of the government, the industrial and the company.

2.3.1 Dimensions to be marked in the drawing of a semi-trailer:

overall length, width, height, track, rear cargo platform height from the ground, box height, wheelbase, position of the front and rear suspension, landing gear position, protective rail position, tool box position, spare tire rack position. The parameters such as stand pillar width, box plate length, height and width of flower railing, and step height of gooseneck vehicle are shown in Figure 7 below.

General drawing of semi-trailer

 Figure 7.General drawing of semi-trailer

By  WONDEE Autoparts


Related Products

content is empty!


Xinhe Industrial Park, Xiamen, Fujian, China 361006





Copyrignt @ 2021 XIAMEN WONDEE AUTOPARTS CO., LTD. All Rights Reserved | Friendly Links: www.wondee.com | www.wondeetrailerparts.com