I. What is a Tricycle Rear Axle? Core Positioning
The tricycle rear axle (commonly known in the industry as the rear axle assembly or rear drive axle) is the core assembly integrating load-bearing, power transmission, steering differential, and braking. Located at the rear of the vehicle, it connects the left and right rear wheels and receives power from the engine/motor. It is the most critical load-bearing and transmission component in cargo and passenger tricycles, often referred to as the "heart" of the tricycle's power system.
It simultaneously performs four core functions:
Bearing Capacity: Bearing the entire vertical load of the cargo box, passengers, vehicle weight, and heavy cargo, as well as lateral forces, braking forces, and impact loads generated during driving;
Reduction and Torque Increase: Reducing the high speed of the engine/motor and amplifying the output torque to meet the needs of heavy-load starts and hill climbing;
Differential Steering: Automatically distributing different speeds to the left and right rear wheels when the vehicle turns, avoiding tire drag, wear, and steering jerking;
Power Distribution and Braking Integration: Distributing single-path power evenly to the left and right wheels, while integrating a rear wheel drum brake/hydraulic brake structure to achieve rear wheel braking.
The overall logic of the rear axle of fuel-powered tricycles and electric tricycles is the same, only the front-end power input structure differs: fuel-powered vehicles input power via a drive shaft/chain, while electric vehicles directly connect to the gearbox input shaft via a motor flange. There are significant differences in the structural strength, speed ratio, and force-adding mechanism of the rear axle structure between cargo dump tricycles and passenger enclosed tricycles.
II. Complete Overall Structure of the Rear Axle for Two- and Three-Wheeled Vehicles (Layered Disassembly, from Outside to Inside)
The rear axle assembly consists of four main modules: the external axle housing load-bearing system, the central differential housing main reduction differential system, the wheel-side braking system of both end half-axles, and the power-adjusting transmission mechanism (exclusive to heavy-duty models). All components are assembled as a single unit, with gear pairs lubricated internally using gear oil.
(I) External Load-Bearing Structure: Rear Axle Housing Assembly (Axle Tube + Differential Housing)
The rear axle housing is an integral load-bearing frame, entirely constructed from thickened seamless steel tubing through stamping and welding. It comprises two main components: the central differential housing (differential housing) and two extended axle tubes on the left and right. Heavy-duty models will have additional external reinforcing ribs and thickened welded flanges.
Central Differential Housing (Differential Housing)
An integral cast steel housing with a sealed internal cavity accommodating the main reducer and differential gear set. It includes a pre-installed filler port, drain plug, and vent valve. Gear oil is added internally to prevent gear dry friction and corrosion. A mounting bracket is provided on the top of the housing for securing the rear suspension and shock absorber springs to the frame. The heavy-duty reinforced rear axle features a larger housing with double the wall thickness and an increased internal gear module, resulting in stronger torsional resistance.
Left and Right Axle Tubes (Half-Shaft Sleeves): Seamless, thickened steel tubes with welded bearing housings at both ends. The half-shafts pass through the tubes. The diameter of the axle tubes on cargo tricycles is generally over 70mm, while that on light-duty passenger vehicles is around 60mm. The axle tubes bear the majority of the vehicle's weight, and the welding process determines the lifespan of the rear axle. A seamless, one-piece welded rear axle is less prone to cracking and oil leaks, while a split-welded rear axle is more susceptible to weld leaks, deformation, and bending under prolonged heavy loads.
External Accessories: Lifting lug brackets (connecting leaf springs/shock absorbers), gear oil filler screw, drain plug, oil seal, dust cover, and brake line mounting bracket.
(II) Core Transmission System: Main Reducer + Differential (Core Inside the Gearbox)
The entire gear set is installed within the sealed cavity of the gearbox and is the core of power conversion. It consists of a primary main reduction gear and a differential assembly. For heavy-duty power-adjustable rear axles, an additional secondary transmission and power-adjustable gear set is built-in.
1. Main Reducer (Bevel Gear)
It consists of a driving bevel gear (angle gear), a driven bevel gear (circular gear), and a pair of bevel gears meshing together:
Driving Bevel Gear: Connects to the front input shaft (drive shaft of a fuel-powered vehicle / output shaft of an electric vehicle motor), a high-speed pinion;
Driven Bevel Gear: A large-diameter gear, fixed to the differential housing, completing the primary reduction.
Gear Ratio Concept: Number of bevel gear teeth ÷ Number of angle gear teeth = Rear Axle Gear Ratio. A larger gear ratio results in stronger reduction, higher torque, and better hill-climbing and load-bearing capacity, but a lower maximum speed; a smaller gear ratio results in higher speed but weaker torque, suitable for light-load passenger transport on flat terrain.
Examples: 1:9.3 gear ratio (general cargo), 1:16 low-speed climbing gear ratio, 1:8 high-speed passenger gear ratio.
2. Differential Assembly (the core of the differential, enabling turning function) Standard bevel gear differential four-piece set: differential housing, planetary cross shaft, 2-4 planetary gears, left and right half-shaft gears.
Differential housing: rigidly fixed to the driven bevel gear, revolving synchronously with the bevel gear;
Planetary cross shaft: fixed inside the differential housing, the planetary gears are mounted on the shaft and can rotate freely;
Planetary gears: mesh with the left and right half-shaft gears on both sides respectively;
Half-shaft gears: with involute splines in the inner bore, tightly meshing with the splines of the left and right half-shafts, outputting power to the wheels on both sides respectively.
3. Power-up Transmission Mechanism (Exclusive to Heavy-Duty Dump Tricycles, Standard Feature on Power-Up King Rear Axles)
Ordinary rear axles only have single-stage reduction, resulting in insufficient power for heavy-load climbing and starting under full load. Power-up rear axles add a high-low speed gear set at the front of the main reduction gear, along with an external shift cable and shift fork mechanism, offering two speeds:
High Speed: Low gear ratio, suitable for unloaded driving on flat roads, high speed, and low fuel consumption;
Low Speed Power-Up: Two-stage superimposed reduction, torque amplified 2-3 times, suitable for fully loaded driving, steep slopes, and muddy roads, significantly improving load capacity and preventing engine stalling and motor overload coil burnout.
High-end 10-speed and 8-speed rear axles integrate reverse gear, power-up, and high/low speed functions, with a single gearbox enabling three modes: high-speed forward, power-up forward, and reverse.
(III) Output Structure at Both Ends: Half-shaft, Wheel-side Bearings, Braking Assembly
Symmetrically arranged left and right, power is transmitted from the differential half-shaft gear to the wheels via the half-shaft, while also integrating a complete drum brake system.
Half-shaft
There are two types: semi-floating half-shaft and fully floating half-shaft, which are key differentiators between light-duty passenger and heavy-duty cargo rear axles:
Semi-floating half-shaft (ordinary passenger and light-duty small cargo tricycles): The half-shaft bears both torque and the weight of the entire vehicle. If the half-shaft breaks, the wheel will detach directly. The half-shaft is thin, has low load-bearing capacity, and is low-cost. The outer end of the half-shaft has a tapered structure, and the brake drum is fixed by a spline. The entire weight of the vehicle rests on the half-shaft.
Fully floating half-shaft (heavy-duty dump trucks and large-tonnage cargo tricycles): The wheel hub is supported by double bearings on the outer side of the axle tube. The entire weight of the vehicle is borne by the axle tube and bearings. The half-shaft only transmits power and is not subject to vertical loads. The half-shaft is connected to the wheel hub only by flange bolts. If the half-shaft breaks, the wheel will not fall off. It is safe, has a strong load-bearing capacity, and is the standard rear axle structure for factory-made cargo vehicles. The inner end of the half-shaft features an involute spline, into which a half-shaft gear is inserted. Common spline tooth counts are 16 or 18 teeth, matching different gearbox models.
Wheel-side bearings and hubs: The heavy-duty rear axle uses double tapered roller bearings, offering strong load-bearing capacity and impact resistance; the wheel hub has welded brake drums and is fitted with steel rims and tires.
Rear wheel braking assembly: Integrates brake drums, brake shoes, return springs, brake camshafts, and brake backing plates, available in mechanical drum brakes and hydraulic brakes; the rear axle tube has a pre-reserved oil passage, with the hydraulic brake's rear axle connected to the oil pipe, achieving synchronous braking of the left and right rear wheels.
III. Complete Working Principle of Power Transmission (Divided into fuel-powered tricycles and electric tricycles)
(I) Power Transmission Flow of the Rear Axle of a Fuel-Powered Cargo Tricycle: Engine → Gearbox → Driveshaft → Rear Axle Input Shaft → Driving bevel gear (high-speed pinion) → Driven bevel gear (first-stage reduction amplification torque) → Differential housing revolution → Planetary gears drive the left and right half-shaft gears → Half-shaft → Wheel hub and tire, completing the drive.
Straight-line driving principle: When the vehicle travels on a straight road, the left and right rear wheels travel the same distance, resulting in equal resistance on both sides. The differential housing drives the cross shaft and planetary gears to revolve. The planetary gears only revolve, not rotate on their own axes. The left and right half-shaft gears rotate at completely synchronized speeds, ensuring both wheels rotate at the same speed, power is evenly distributed, and there is no speed difference.
Turning principle (core function of differential): When the vehicle turns, the outer rear wheel travels a longer arc and a greater distance, while the inner rear wheel travels a shorter distance, creating a difference in resistance. The differential housing continues to revolve, and the planetary gears rotate on their own axes, automatically compensating for the speed difference-the outer half-shaft gear accelerates, and the inner half-shaft gear decelerates. The left and right wheels roll at different speeds, resulting in pure rolling contact between the tires and the ground, preventing drag, tire wear, and stiff steering, ensuring smooth turning.
Core characteristic: Differential without torque difference. Regardless of whether the speeds are the same, the torque transmitted to the left and right wheels is always equal, preventing slippage on one side during heavy-load turns.
Extra-high Gear Working Principle (Heavy-duty Vehicles)** Pulling the extra-high gear cable in the cab activates the shift fork, which pushes the transmission clutch to switch to the low-speed gear pair. Power is reduced twice, through the main reduction gear and the secondary extra-high gear reduction, resulting in a significant increase in output torque. Switching to extra-high gear for heavy-load starts and steep inclines greatly reduces engine load and prevents slippage due to insufficient power. Switching to high gear eliminates the secondary reduction gear, leaving only the primary main reduction gear, suitable for fast driving with an empty vehicle.
Reversing Principle** The rear axle integrates a reverse gear set. The shift fork changes the power meshing gear, reversing the input shaft rotation direction, causing the half-shaft to rotate in the opposite direction, thus achieving reversing.
(II) Electric Tricycle Rear Axle Working Principle** Without an engine or drive shaft, the motor is directly flange-fixed to the gearbox input end, simplifying the process: Drive motor → Motor output shaft → Rear axle active bevel gear → First-stage reduction gear → Differential → Half-shaft → Rear wheel. The principle is exactly the same as that of a fuel-powered rear axle, also featuring synchronous straight-line travel and differential turning functions; the electric dual-speed rear axle reduces high-current output, preventing motor overload and overheating during hill climbing, and extending battery and motor lifespan.
IV. Mainstream Classifications and Applicable Scenarios of Tricycle Rear Axles
1. By Load-Bearing Structure: Semi-Floating Rear Axle / Fully Floating Rear Axle
Semi-Floating Rear Axle: Simple structure, low cost, and light weight; the half-shaft is heavy-duty, with a maximum load capacity ≤ 1 ton; mostly used in passenger tricycles under 200cc and short-distance urban commuter tricycles, unsuitable for long-term heavy-duty cargo hauling. Disadvantage: If the half-shaft breaks, the wheel will detach directly, resulting in a low safety factor.
Fully Floating Heavy-Duty Rear Axle: The axle tube has dual bearings supporting the wheel hub, and the half-shaft only transmits power and does not bear weight; the axle tube is thickened, and the gear module is increased, with a rated load capacity of 1~3 tons, suitable for hydraulic self-unloading cargo boxes, agricultural use, and construction site transportation; impact-resistant and not prone to axle breakage, all freight vehicles in Latin America and Africa are equipped with fully floating rear axles as standard.
2. By Transmission Structure: Standard Single-Speed Rear Axle / Dual-Speed Rear Axle with Afterburner / Multi-Speed Rear Axle
Single-Speed Standard Rear Axle: Only one reduction gear, no high/low speed switching, suitable for light passenger transport and short-distance freight on plains, lowest cost;
Dual-Speed Afterburner Rear Axle: High speed / low-speed afterburner, standard configuration for mainstream factory freight vehicles, balancing speed and heavy-duty climbing;
Eight-Speed / Ten-Speed Heavy-Duty Rear Axle: Integrated multiple forward gears + reverse gear + afterburner, specifically designed for large-displacement 300cc water-cooled heavy-duty dump tricycles, suitable for mining and heavy-duty transportation.
3. By Power Source: Fuel-Powered Rear Axle, Electric Integrated Rear Axle
Fuel-Powered Rear Axle: Reserved driveshaft input interface, larger gearbox space, suitable for high-torque engines;
Electric Rear Axle: Direct motor connection to flange, more compact size, speed ratio matched to the low-speed characteristics of the motor, often paired with a dual-speed afterburner structure.
4. By braking method: mechanical drum brake rear axle, hydraulic oil brake rear axle. Hydraulic brake rear axles have stronger braking force, are standard equipment on heavy-duty cargo vehicles, provide stable braking, and are suitable for long downhill fully loaded conditions; mechanical drum brakes are mostly used in light passenger tricycles.
