Motorcycle Mechanics and Operation
Understanding how your motorcycle works is not only essential for the theoretical exam but also vital for your safety and vehicle maintenance. In this topic, we break down the main systems, from the engine to the tires, focusing on common questions in the DGT tests.
1. The Engine
The engine is the heart of the motorcycle. It transforms chemical energy (fuel) into mechanical energy (movement).
4-Stroke Engine (4T)
The most common type. The combustion cycle is carried out in four distinct phases:
- Intake (Admission): Intake valve open. The piston moves down and sucks in the air-fuel mixture.
- Compression: Valves closed. The piston moves up and compresses the mixture, raising its temperature.
- Explosion (Power/Combustion): A spark is produced by the spark plug. The mixture explodes and forces the piston down (this is the only stroke that generates work).
- Exhaust: Exhaust valve open. The piston moves up and expels the burnt gases.
2-Stroke Engine (2T)
Performs all four phases in just two movements of the piston.
- Upward stroke: Intake and Compression.
- Downward stroke: Explosion and Exhaust.
Exam Fact: Engine displacement (cilindrada) is calculated by multiplying the volume of one cylinder by the total number of cylinders in the motorcycle.
2. Fuel and Lubrication Systems
Carburetor and Injection
The fuel system mixes air and gasoline.
- Carburetor: A classic mechanical system. Includes the choke (estárter), a valve that enriches the mixture for cold starts.
- Electronic Injection: Modern and more efficient. It doses the fuel precisely, reducing consumption and pollution.
Fuel Petcock (Fuel Valve)
On motorcycles without a fuel pump (gravity fed), there is a valve with three positions:
- ON: Open (fuel flows if the engine is suctioning).
- PRI (Prime): Free flow (flows directly to the carburetor to fill the float bowl if empty).
- RES (Reserve): Reserve (used when fuel level is low).
Lubrication System
Oil is vital to prevent engine seizure (gripado) — the melting of metal parts due to heat and friction.
| Engine Type | Lubrication System | Maintenance |
|---|---|---|
| 4-Stroke (4T) | Oil in the crankcase (cárter). Separate circuit from the fuel. | Check level periodically using a dipstick or sight glass (bike must be upright). |
| 2-Stroke (2T) | Mixture. The oil is burned together with the gasoline. | Refill the independent oil tank or perform manual mixing. |
3. Cooling and Electrical System
Cooling
Its function is to keep the engine at an optimal temperature.
- Air-cooled: Cooling fins on the engine and a fan (common in scooters).
- Liquid-cooled: Uses coolant, a pump, and a radiator.
- Maintenance: Check the level in the expansion tank. Never open the radiator when hot! The liquid must contain antifreeze and anti-corrosive agents.
Electrical System
- Battery: If it is "maintenance-free," do not touch it. If it is conventional, monitor the electrolyte level and top up only with distilled water. Keep terminals clean and greased.
- Fuses: Protect against short circuits. Never replace a blown fuse with one of a higher amperage.
- Spark Plugs (Bujías): Check that the porcelain is not cracked and the electrodes are clean.
4. The Transmission
It transfers the power from the engine to the rear wheel.
- Clutch (Embrague): Connects or disconnects the engine from the gearbox. A poorly tensioned clutch can slip (causing wear) or fail to disconnect properly.
- Secondary Transmission: The final system that moves the wheel.
| Type | Characteristics | Maintenance |
|---|---|---|
| Chain | Most common. Efficient but requires attention. | Lubricate and tension frequently. If it becomes loose, it damages the sprockets. |
| Shaft Drive (Cardán) | Rigid shaft. Very durable. | Minimal maintenance (oil level check). |
| Belt | Quiet and elastic. | Monitor for wear and proper tension. |
5. Chassis: Brakes, Suspension, and Wheels
Braking System
- Types: Disc (more effective) and Drum.
- Maintenance:
- Check brake fluid level. If it drops, it may be due to worn pads.
- Check the thickness of the brake pads or shoes.
Important! If the brake feel becomes "spongy," there may be air in the circuit.
Suspension
- Front: Telescopic fork (contains oil inside).
- Rear: Swingarm with shock absorber.
- Function: To keep the wheel in contact with the ground and provide stability. If the shock absorbers are in poor condition, the bike loses stability and braking distances increase.
Wheels and Tires
They are the only point of contact with the road.
- Tread: Must have sufficient depth to evacuate water (legal minimum is 1.6 mm for cars; for motorcycles, follow the TWI wear indicators, though changing them earlier is recommended).
- Pressure: Always measure when cold.
- Low pressure: The bike wobbles (unstable), the tire overheats, and wears on the sides.
- High pressure: Less grip (smaller contact patch) and wears down the center.
6. Eco-Driving
To save fuel and protect the mechanics:
- Use high gears whenever possible.
- Avoid sudden acceleration and heavy braking.
- Aerodynamics matter: loose clothing or poorly placed luggage increases consumption (acts as a brake).
- Maintain a uniform speed. Above 90-100 km/h, fuel consumption increases significantly.