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In the overall truck system, the running gear acts as the truck's "limbs and skeleton". It bears the entire weight of the truck, transmits power, ensures smooth driving, and serves as the core support for the truck to operate efficiently under various complex road conditions. Among them, the five core components—the frame, front axle, rear axle, suspension, as well as wheels and tires—perform their respective duties and cooperate closely, jointly forming the "stable cornerstone" for truck driving. The design and configuration of this system are precisely matched according to the differences in truck models, load capacities and application scenarios, and its structural details and technical characteristics directly determine the truck's load-bearing capacity, driving stability and scope of application. Today, combined with professional design standards for commercial vehicle structures, we will disassemble these five components in detail, understand their functions, classifications and synergy logic, and easily grasp the core principles of the truck running gear.
The core missions of the truck running gear can be summarized into four points: first, load-bearing—it supports the cab, cargo box and loaded goods, and bears the weight of core assemblies such as the engine and gearbox, serving as the "bearing foundation" of the entire vehicle; second, power transmission—it converts the power transmitted by the transmission system into driving force, and feeds back the road reaction force and reaction torque to the frame to realize two-way power transmission; third, shock absorption and buffering—it alleviates the impact caused by road bumps during driving, protects vehicle body components and goods, and improves driving comfort; fourth, guiding coordination—it works in conjunction with the steering and braking systems to ensure accurate driving and stable braking of the truck, and avoid potential safety hazards such as deviation and rollover. The realization of these four core missions relies on the tacit cooperation and precise design of the five components.
If a truck is compared to a human being, the frame is its "spine". It is a frame structure spanning the front and rear axles of the truck, and also the basic carrier of the entire running gear and even the whole vehicle. All key components of the truck—the front axle, rear axle, engine, gearbox, cab and cargo box—are fixed on the frame through special connectors. At the same time, the frame has to bear various loads from inside and outside the vehicle, making it a veritable platform for "bearing everything".
The frame is mainly composed of longitudinal beams, cross beams, engine brackets, clutch brackets and other accessories. According to the manufacturing process, it can be divided into welded frames, riveted frames and riveted-welded frames, among which riveted frames are widely used in micro trucks, medium trucks and heavy-duty trucks. According to application scenarios and design requirements, frames also have a variety of classification methods to adapt to the core needs of different vehicle models:
1. By structural form: There are three types—side beam type, platform type and backbone type. The side beam type frame is the most widely used, featuring easy installation of the cab, carriage and special equipment, and facilitating the modification of variant vehicles, making it the mainstream choice for commercial vehicles; the platform type is suitable for modern passenger cars or commercial vehicles, and the backbone type is adapted to commercial vehicles with independent suspension.
2. By width: Divided into front-wide and rear-narrow frames and equal-width frames. Foton Daimler heavy-duty trucks and Times micro trucks mostly adopt the front-wide and rear-narrow design, while Times light trucks are mainly equipped with equal-width frames.
3. By vehicle model: Can be divided into tractor frames (focusing on the adaptation of saddle installation), dump truck frames (strengthening the fixation of upper loading structures), stake truck frames (matching the length of cargo boxes), bus frames, semi-trailer frames, etc. The frame of different vehicle models will be optimized for specific structures according to core functions.
4. By layer number: Including single-layer frames, double-layer frames and partial multi-layer frames. For example, heavy-duty truck products with 6×2 and 4×2 configurations mostly adopt double-layer frames, with an inner L-shaped plate added at the balance shaft to improve load-bearing capacity.
In addition, the cross beam structure of the frame is also designed differently according to vehicle models and positions: the front/rear cross beams of heavy-duty trucks are mostly channel-shaped, and the cross beam at the balance shaft adopts a back-to-back structure; the front cross beams of micro trucks and light trucks usually use circular tube beams, and the transmission shaft hanging cross beams and shock absorber cross beams also adopt circular tube structures. The core principle of cross beam selection is to ensure strength while avoiding interference with other devices. Cargo trucks are usually equipped with 5 to 6 cross beams, and heavy-duty models will be equipped with an appropriate increase.
The design dimensions and material thickness of the frame are directly linked to the vehicle model grade. The frame parameters of micro trucks, light trucks, medium trucks and heavy-duty trucks vary significantly, and vehicles of the same grade provide multiple schemes according to standard load and heavy load requirements:
1. The height of micro truck frames is mostly between 100mm and 200mm. For example, the longitudinal beam height of Times Xiangling micro truck is 130mm, the material thickness is 1.5mm to 5mm, and high-strength steel such as B510L and B610L is adopted, all of which are single-layer frames;
2. The height of light truck frames is mainly 200mm to 250mm. The frame height of Times light truck New Pilot standard load model is 192mm, the material thickness is 6.0mm, and 610L steel is selected;
3. The frame height of heavy-duty trucks has the largest span, ranging from 200mm to 320mm. The frame height of Auman heavy-duty dump truck can reach 320mm, adopting 1610 high-strength steel, mostly double-layer frames with a material thickness of 5mm to 8mm to meet the demand for large load capacity.
The core function of the frame is not a single "bearing", but to realize the three major functions of connection, load-bearing and protection: first, the connection function—the installation and connection of all systems of the whole vehicle are based on the frame, which is the "link" between assemblies; second, the load-bearing function—it bears the self-weight of the whole vehicle and the weight of goods, serving as the "load foundation" of the truck; third, the protection function—it can absorb impact force in the event of a collision to protect the safety of the cab and its occupants. At the same time, the frame is also the "dividing line" between the chassis and the vehicle body—the part below the frame (including the frame) is the chassis part, and the part above the frame is the vehicle body and upper loading part. Its quality directly determines the truck's load-bearing capacity, driving stability and service life. Once the frame is deformed, it will directly lead to vehicle body skewing, tire uneven wear, and even potential safety hazards such as brake failure and steering deviation.
The front axle and rear axle are the "core executive components" of the truck running gear, connected to the front and rear ends of the frame respectively, and are key devices for transmitting various forces between the frame and the wheels as well as the bending moment and torque generated thereby. Although both belong to "vehicle axles", they have completely different divisions of labor: the front axle is mainly responsible for "steering + load-bearing + braking", and the rear axle is mainly responsible for "driving + load-bearing + deceleration + differential speed + braking". The front and rear axles of some special models can also develop functions such as driving and follow-up. The cooperation between the two enables the truck to drive flexibly and bear loads stably.
The front axle is also known as the driven axle or steering axle. The front axle of most ordinary trucks is a non-driven axle, only responsible for steering, load-bearing and braking, while the front axle of off-road trucks and heavy-duty all-wheel drive trucks is equipped with a driving device, becoming a "steering drive axle". Its core components include brake drum, hub, wheel bearing, steering knuckle arm, kingpin, front axle, etc. For vehicles with non-independent suspension on the front wheels, the front axle also integrates shock absorbers, subframes, tie rods, brakes and other components (excluding tires).
The four core functions of the front axle precisely match the driving needs: first, realizing the transportation function—it bears the sprung mass at the front of the vehicle, including the weight of the cab and part of the engine, and bears vertical load, longitudinal force and lateral force; second, realizing the steering function—it is connected with the steering system through the steering knuckle to transmit the steering force output by the steering gear to the wheels. The ordinary front axle realizes steering through the deflection of the steering knuckle around the kingpin, while the steering drive axle divides the half shaft into inner and outer sections connected by a universal joint, and the kingpin is made hollow to allow the outer half shaft to pass through, realizing the combination of steering and driving; third, realizing the braking function—it completes the service braking with the front wheels through the brake assembly of the front axle; fourth, the optional driving function—the power system drives the front axle through the transmission shaft and transfer case to improve the passability under complex road conditions.
The front axle is divided into single front axle and double front axle. The frontmost axle of a vehicle with double front axles is the first front axle, and the second one is the second front axle. The load-bearing capacity and structural form of the front axle of different vehicle models vary significantly, and all are designed for specific purposes:
1. Micro trucks mostly adopt 1t welded circular tube beam front axles with disc brakes, suitable for vehicles with a total mass of less than 3.5t;
2. Light trucks commonly use 2.4t and 3.2t front axles to meet the demand for standard load transportation;
3. Medium trucks are adapted to 3.6t and 4.2t front axles;
4. Heavy-duty trucks adopt Steyr integral forging front axles of 6.5t, 7.5t and 9t with larger section dimensions and higher brake specifications. For example, the brake size of the 9t front axle reaches φ420×180mm, suitable for heavy-duty transportation scenarios.
The steering principle of the front axle is divided into two types: ordinary mechanical steering and power steering:
· The ordinary front axle drives the left and right steering knuckles to deflect through the steering wheel → steering shaft → steering gear → steering rocker arm → steering drag link → steering knuckle arm to realize wheel steering;
· The front axle with power steering is added with power-assisted devices such as steering oil pump, steering oil tank and steering oil pipe, which reduces the steering resistance through hydraulic power assistance for easier operation, and is equipped with a steering damper to improve steering stability.
The rear axle is the core driving component of the truck running gear and also the "main force" for bearing the rear weight of the vehicle. It is mainly installed at the rear of the vehicle, responsible for bearing the weight of the cargo box and most of the goods, and transmitting the power of the transmission system to the wheels. It also has multiple functions such as deceleration, differential speed, braking and auxiliary steering, being a core component integrating a variety of functions.
The six functions of the rear axle form a closed driving loop: first, the transportation function—it bears the self-weight of the rear part of the vehicle and the weight of goods, and bears various loads and moments; second, the driving function—it converts the power transmitted by the power system through the transmission system into the driving force of the wheels; third, deceleration and torque increase—it reduces the rotational speed of the transmission shaft and increases the torque through the main reducer to adapt to the power demand of truck driving; fourth, the differential speed function—it adjusts the rotational speed of the left and right wheels through the differential to solve the problem of shorter driving distance of the inner wheels and longer driving distance of the outer wheels when turning, avoiding tire wear; fifth, the braking function—the rear wheel brakes are the main force for service braking, and the parking braking is also realized through the rear wheel brakes; sixth, the auxiliary steering function—it adjusts the wheel speed through the differential to improve steering maneuverability, and the rear axle of some vehicles also has an active steering function.
The rear axle has a variety of classification methods, which are mainly divided according to suspension form, deceleration form, quantity and function to precisely match different vehicle models and application scenarios:
1. By suspension form: Integral rear axle and disconnected rear axle. The integral rear axle is matched with non-independent suspension (such as leaf spring suspension), which is the mainstream choice for commercial vehicles; the disconnected rear axle is matched with independent suspension, mostly used for passenger cars.
2. By deceleration form: Central single-stage reduction axle and (central + wheel-side) double-stage reduction axle. The single-stage reduction axle has few parts, light weight and convenient maintenance, suitable for road transportation; the double-stage reduction axle has large torque capacity, strong overload capacity and large ground clearance, suitable for off-road and heavy-duty transportation such as dump trucks and engineering vehicles.
3. By quantity: Single rear axle and double rear axles. Heavy-duty trucks mostly adopt double rear axles to disperse loads and improve load-bearing and driving capacity.
4. By function: Floating axle, follow-up axle, middle (rear) axle and rear axle. The floating axle can float up and down and has load-bearing capacity, with a structure similar to the front axle; the follow-up axle has load-bearing capacity but no driving capacity, divided into passive steering and active steering types; the middle (rear) axle is a through axle with driving capacity; the rear axle only receives power input and realizes driving.
The power transmission and differential speed principle of the rear axle is clear and efficient: the power transmitted from the engine is shifted by the transmission and then transmitted to the main reduction gear of the rear axle by the transmission system, and then transmitted to the differential after deceleration and torque increase; the differential is composed of a cross pin and small planetary gears inside. When driving straight, the cross pin does not move and the left and right wheels rotate synchronously; when turning, the cross pin rotates to adjust the rotational speed of the left and right wheels to realize differential steering. Finally, the power is transmitted to the wheels through the half shaft to drive the truck to run. At the same time, the rear axle transmits the road reaction force and reaction torque to the frame through the leaf spring to realize two-way force feedback.
The common rear axles for commercial vehicles are centered on drive axles. Combined with the differences in vehicle load capacity and application scenarios, a clear selection system has been formed, which is mainly divided into two mainstream types: single-stage reduction rear axles and double-stage reduction rear axles, and derived with special rear axles such as through axles and floating axles, adapting to the full range of commercial vehicles including micro trucks, light trucks, medium trucks and heavy-duty trucks. It is the core component to ensure the power transmission and load-bearing capacity of commercial vehicles, and its selection directly determines the operational efficiency and applicability of the vehicle.
The single-stage reduction rear axle is the most widely used rear axle type for commercial vehicles, suitable for most road transportation scenarios, and is also the first choice for micro trucks, light trucks, medium trucks and heavy-duty truck tractors. Its core advantages are simple structure, mainly composed of main reducer, differential, half shaft and stamped-welded axle housing, with few parts, high transmission efficiency (up to more than 98%), convenient maintenance and light self-weight. It has a minimum transmission ratio of < 3, small ground clearance and driving noise ≤ 89dB, which can effectively reduce the fuel consumption of the whole vehicle, suitable for the demand of high-speed and standard load road transportation. Mainstream domestic commercial vehicles such as Times light trucks, Auman road tractors and Foton Daimler tractors all generally adopt single-stage reduction rear axles. Among them, light trucks commonly use single-stage reduction rear axles with a load-bearing capacity of 2.5t-8t, and heavy-duty road tractors mostly select 10t-16t class single-stage reduction rear axles.
The suspension system is a general term for all force-transmitting connection devices between the vehicle frame and the axle or wheels, equivalent to a "buffer pad" connecting the truck's "spine" and "limbs". Its core components include elastic elements (leaf springs, air bags, rubber springs), shock absorbers (hydraulic shock absorbers) and force-transmitting devices (upper and lower wishbones, anti-roll bars, thrust rods, etc.). The three parts undertake the functions of buffering, shock absorption and force transmission respectively, and none of them can be missing.
The five core functions of the suspension ensure both driving comfort and handling stability: first, transmitting force and moment—it bidirectionally transmits all forces and moments such as supporting force, braking force and driving force between the wheels and the frame; second, buffering and shock absorption—it alleviates the impact force transmitted to the frame or vehicle body by uneven roads, reduces vehicle body vibration, and protects goods and vehicle body components; third, guiding and positioning—it ensures that the wheels move within a fixed angle, stabilizes steering and avoids wheel deviation; fourth, load-bearing assistance—it transmits the weight of the vehicle body and goods to the axle (non-independent suspension) through the frame, and then to the ground; fifth, stability control—it balances comfort and handling stability, and avoids problems such as brake dive and unstable operation through reasonable design of spring stiffness.
The suspension has a variety of classification methods, mainly divided according to connection position, structural form and working principle. Different types of suspensions are adapted to different vehicle models, load capacities and application scenarios, each with its own advantages and disadvantages:
1. By connection position: Front suspension and rear suspension. The front suspension is connected to the front axle, divided into front suspension for single front axle and double front axles, adapted to 4×2, 6×4, 6×2 and other models; the rear suspension is connected to the rear axle, divided into rear suspension for single rear drive axle and double rear drive axles. The rear suspension for double rear drive axles is equipped with a balance shaft assembly and thrust rod. The balance shaft enables the rear axle to bear loads evenly, and the thrust rod transmits the longitudinal force between the frame and the axle, adapted to heavy-duty models such as 6×4 and 8×4.
2. By structural form: Non-independent suspension and independent suspension. The non-independent suspension installs the left and right wheels on an integral rigid axle, with leaf springs as the core and no separate guiding mechanism. Some models are equipped with anti-roll bars to improve lateral stiffness. It has the advantages of simple structure, convenient maintenance and low tire wear, being the mainstream choice for commercial vehicles and adapted to non-load-bearing bodies; the independent suspension connects the left and right wheels to the frame independently to form a disconnected axle, with no mutual influence on wheel jitter, light suspension weight, good driving comfort and ability to lower the vehicle's center of mass, being a symbol of mid-to-high-end models. At present, it is also gradually popularized in domestic micro trucks and light trucks, adapted to load-bearing bodies.
3. By working principle: Leaf spring suspension, rubber suspension, air suspension and hydraulic suspension. This is the core classification method for commercial vehicles, and the four types of suspensions have their own characteristics, adapting to different transportation needs.
The leaf spring suspension is the mainstream choice for micro trucks, light trucks, medium trucks and heavy-duty trucks. According to the leaf spring structure, it can be divided into four types, each adapted to different scenarios:① Ordinary multi-leaf spring: Simple structure and manufacturing process, mostly used for heavy-duty dump trucks and other heavy-duty models. Disadvantages: heavy weight, large friction between leaves, poor comfort and high noise;② Few-leaf variable cross-section spring: It can reduce the weight by 30%~50%, with small friction between leaves, good comfort, long service life and realizing the lightweight of the whole vehicle. It is mostly used for road transportation models such as box trucks and tractors. Disadvantages: complex structure and manufacturing process;③ Gradual stiffness leaf spring: Its stiffness increases gradually with the increase of load, with stable change and improved driving comfort. It is mostly used for medium trucks and models driving on national roads. Disadvantage: complex process;④ Two-stage stiffness compound leaf spring: Simple structure and wide application, mostly used for vehicles with poor road conditions. Disadvantage: the suspension stiffness increases instantaneously when the auxiliary spring acts, which has a negative impact on driving comfort.
The rubber suspension uses rubber as the elastic element, with a working principle similar to the engine mount. It has strong load-bearing capacity, no noise, maintenance-free and low operating cost, being an important lightweight design scheme for heavy-duty trucks. Its cross beams and saddles are mostly castings with complex structure but high reliability. At present, the Hendrickson HUV270 and 271 series are mainstream products, with a maximum static load of 35t for double rear axles, adapted to 6×4, 8×4 heavy-duty truck models such as dump trucks, tractors, cargo trucks and mixers.
Air suspension is a standard configuration for high-end commercial vehicles, composed of air bags, height valves, hydraulic shock absorbers, anti-roll bars, buffer blocks, etc. According to the national standard GB1589, vehicles equipped with air suspension are allowed to increase the total mass by 1t, which is also one of its core advantages.
Its working principle is as follows: the height signal is collected by the vehicle body ground clearance sensor, and the on-board computer controls the air compressor and exhaust valve to realize the inflation and deflation of the air bag, automatically adjust the vehicle body height, and adjust the stiffness of the shock absorber at the same time (three modes: comfort, normal and sport). The air suspension has excellent comfort, can effectively protect precision goods, and has the advantages of light self-weight and small impact force on the road, prolonging the service life of the road. The disadvantage is high cost. At present, it is mainly applied to luxury buses, high-end heavy-duty trucks and special transport vehicles (such as cold chain and precision equipment transportation). The application proportion of air suspension in foreign heavy-duty trucks has reached more than 80%, and it is gradually popularized in China.
Wheels and tires are the only components of the truck running gear in contact with the road surface, equivalent to the "feet" of the truck. They have to bear the entire weight of the truck, transmit power and braking force, adapt to different road conditions at the same time, and ensure driving safety. The two cooperate closely: the wheel acts as the "bridge" connecting the tire and the axle, and the tire acts as the "core medium" for grip, buffering and power transmission. Their performance directly determines the driving safety, stability and economy of the truck.
Truck wheels are mainly composed of hubs, rims and spokes. The hub is connected to the half shaft of the axle to drive the wheel to rotate; the rim is used to install and fix the tire; the spoke connects the hub and the rim to transmit torque. The three jointly form the core of load-bearing and power transmission.
The design of truck wheels is fully adapted to heavy load requirements, mostly adopting steel structures with high strength and strong load-bearing capacity, which can bear huge weight and torque and adapt to various freight scenarios; some high-end trucks adopt aluminum alloy wheels with lighter self-weight, which can reduce the fuel consumption of the whole vehicle, and have better heat dissipation performance, which can effectively reduce the temperature of tires and brakes and improve driving safety.
To further improve the load-bearing capacity and driving stability, the wheels of trucks (especially the rear axle) mostly adopt a dual-tire side-by-side design. This design can increase the contact area between the tires and the road surface, disperse loads, reduce the wear of a single tire, prolong the service life, and improve the grip and braking effect, adapting to the demand for heavy-duty and long-distance transportation.
Most truck tires are radial tires, which have the advantages of high wear resistance, high load-bearing capacity and low rolling resistance, suitable for long-distance freight and heavy-duty transportation. Their core structure includes tread, sidewall, carcass and belt layer. Each part performs its own duties to ensure the performance and reliability of the tire: the tread is the part in contact with the road surface, undertaking the functions of grip and power transmission; the sidewall protects the carcass and has a certain elasticity to alleviate road impact; the carcass and belt layer enhance the tire strength and bear the load to avoid tire blowout during heavy load or high-speed driving.
The pattern design of the tread is the core of the tire adapting to different road conditions, and different patterns correspond to different application scenarios to precisely match the transportation needs:
1. Longitudinal pattern: Good drainage performance, low noise and low rolling resistance, suitable for long-distance transportation on highways and paved roads;
2. Transverse pattern: Strong grip and good anti-skid performance, suitable for engineering transportation and rural transportation on muddy, gravel and unpaved roads;
3. Mixed pattern: It takes into account the advantages of longitudinal and transverse patterns, with balanced grip, drainage performance and wear resistance, adapting to a variety of road conditions and being the mainstream choice for comprehensive freight.
For the truck running gear, the daily maintenance of wheels and tires is particularly important, which is directly related to driving safety: first, regularly check the tire pressure—excessively high or low tire pressure will aggravate tire wear, reduce load-bearing capacity and even cause tire blowout; second, check the tire wear condition, and replace severely worn, cracked or aging tires in time to avoid skidding and tire blowout caused by tire faults; third, check the fastening condition of the wheels to prevent the hub and rim from loosening and causing safety accidents; fourth, do a good job in tire dynamic balance and alignment to avoid tire uneven wear and prolong the service life.
Especially for trucks driving under heavy load and high speed, the tire load is greater, and more intensive maintenance is needed. Once a tire failure occurs, it is very easy to cause serious accidents such as rollover and rear-end collision. Therefore, the maintenance of wheels and tires is one of the core links in the daily maintenance of trucks.
After understanding the individual functions and technical characteristics of the five components, we can clearly find that the truck running gear is an organic whole with mutual support and precise coordination. The five components form a complete driving closed loop, and the lack or failure of any component will directly affect the normal driving of the truck:
When the truck starts, the power of the engine is transmitted to the rear axle through the transmission system. The main reducer of the rear axle reduces the speed and increases the torque, and the differential adjusts the wheel speed according to the driving state. Finally, the power is transmitted to the wheels through the half shaft, and the tires generate friction with the road surface to convert it into the driving force of the truck; at this time, as the core base, the frame bears the self-weight of the whole vehicle and the weight of goods, transmits the pressure evenly to the front and rear axles, and connects all assemblies to ensure the smooth transmission of force; the front axle cooperates with the steering system to realize wheel steering according to the driver's operation and accurately control the driving direction; during driving, the suspension buffers road bumps through elastic elements, the shock absorber suppresses excessive vehicle body vibration, and the force-transmitting device ensures the positioning of the wheels and axles, so that the wheels are always closely attached to the road surface to improve driving stability and comfort; the wheels and tires act as the final executive components, transmitting power and braking force to the road surface, and bearing all loads at the same time to ensure the truck accelerates, decelerates, steers and stops stably.
The five functions of the frame—"bearing and connection", the front axle—"steering and load-bearing", the rear axle—"driving and deceleration", the suspension—"buffering and stability control", and the wheels and tires—"grip and power transmission" are precisely connected to form the core logic of truck driving. According to the truck's model, load capacity and application scenario, the five components will be designed and matched in a targeted manner—micro trucks focus on lightweight and flexibility, heavy-duty trucks focus on load-bearing capacity and reliability, high-end road trucks focus on comfort and economy, and engineering trucks focus on passability and durability. This differentiated adaptive design enables trucks to precisely meet the needs of different transportation scenarios.
The truck running gear is the "driving foundation" of the whole vehicle. The five core components—the frame, front axle, rear axle, suspension, wheels and tires—each have unique structures, functions and technical characteristics, and through precise coordinated cooperation, they realize the all-round driving needs of load-bearing, power transmission, buffering, guiding and driving. Understanding the core principles, classifications and synergy logic of these five components can not only make us more clearly understand the driving mechanism of trucks, but also enable us to accurately judge the adaptability of vehicles according to transportation scenarios in daily use, maintenance and purchase, better protect vehicles, prolong their service life, and improve the efficiency and safety of transportation.
With the continuous development of commercial vehicle technology, the running gear is also moving towards the direction of high strength, lightweight, intellectualization and comfort. The application of high-strength materials, the optimization of lightweight structures, the integration of electronic control systems, and the popularization of high-end configurations such as air suspension have all realized an all-round improvement in the truck's load-bearing capacity, driving comfort and fuel economy, laying a solid foundation for the high-quality development of the commercial vehicle industry.
