The combustion energy of the air-fuel mixture in the cylinders is converted into rotational motion of the crankshaft, but this rotation in itself does not make the car move. In order for engine energy to be effectively converted into forward motion of the machine, it must go through a complex path through many mechanical components. It is this chain of transformations and distribution of traction force that forms the basis for the operation of the entire power plant of the vehicle.
Understanding exactly how energy reaches the asphalt is critical for any driver who wants to understand their vehicle. Torque - this is a physical quantity that determines the traction capabilities of the motor, and it is its transmission that is the main task of the transmission. Without proper distribution of this force, the wheels will simply spin uselessly or slip, without creating the necessary acceleration.
The whole process can be imagined as a relay race, where each node makes its own adjustments to the force and speed of rotation. From the flywheel to the contact patch between the tire and the road, many transformations occur to ensure a smooth ride and acceleration dynamics. Next, we will look in detail at each stage of this journey so that you know exactly what is happening under the hood and underbody of your car.
The role of the flywheel and clutch in transmitting rotation
The first element that receives torque from the crankshaft is flywheel. This is a massive disk mounted on the end of the crankshaft, which not only serves to equalize uneven rotation of the pistons, but also acts as one of the clutch elements. A basket with a pressure disk is fixed on its surface, which presses the driven disk to the plane of the flywheel, ensuring a rigid connection between the engine and the transmission.
Between the engine and the gearbox there is a unit that allows you to briefly break this connection - clutch. When you depress the pedal, you retract the pressure plate, breaking the contact between the flywheel and the driven plate. This is necessary to change gears without damaging the gears and to stop the car with the engine running. At the moment of smooth release of the pedal, friction of the linings occurs, and torque begins to be transmitted to the input shaft of the box.
⚠️ Attention: Sudden release of the clutch pedal at high speeds can lead to instant destruction of the driven disc or breakage of gearbox gears due to the shock load on the transmission.
The quality of torque transmission here directly depends on the condition of the friction linings. If they are worn or oily, slippage occurs and some of the energy is lost as heat before reaching the wheels. Modern dual-mass flywheels further dampen vibrations, making power transmission smoother and protecting the gearbox from jerking.
Transformation of force in the gearbox
After the clutch, the torque reaches the input shaft of the gearbox, where its main transformation occurs. The internal combustion engine has a narrow speed range in which it produces maximum power and torque, so it is necessary step transmission. Gears of different sizes allow you to change the gear ratio, sacrificing shaft speed to increase traction, or vice versa.
In first gear, torque is maximized, which allows you to move a heavy vehicle from a standstill. At this moment, the rotation speed of the output shaft is minimal, but the force applied to the wheels is maximum. As the driver accelerates, the driver shifts into higher gears, where the gear ratio decreases, allowing high speeds to be achieved at lower engine speeds.
In automatic transmissions, the switching process occurs hydraulically or electronically, but the essence remains the same - changing the lever arm in the form of gears or cones of the variator. Torque converter in classic machines it also transmits torque through the liquid, ensuring smoothness, but introducing slight losses in efficiency. It is important to monitor the level and condition of the transmission oil, since it is the oil that removes heat and lubricates the rubbing pairs of gears.
Cardan transmission and drive shafts
From the gearbox, torque must be delivered to the drive wheels, which can be located at a considerable distance. In cars with rear-wheel drive or all-wheel drive, this is used cardan shaft. This is a long pipe with hinges at the ends that compensate for changes in angle and length during suspension operation. The cardan transmits rotation to the main gearbox of the rear axle.
In front-wheel drive cars, the role of the cardan is played by drive shafts (half shafts) with constant velocity joints (CV joints). These assemblies allow the wheels not only to rotate, but also to pivot for steering, and to move up and down with the suspension. External and internal CV joint grenades experience enormous loads and require regular checking of the integrity of the boots.
If a characteristic crunching sound appears when the wheels are turned out, immediately check the outer CV joints - this is the first sign of their wear and the need for replacement.
Balancing the driveshaft is critical to comfort. If the shaft is unbalanced, strong vibrations occur at high speeds and are transmitted to the body. Modern cars often use composite shafts with an intermediate support, which reduces vibration and places the transmission optimally for the interior layout.
Main pair and final strengthening
Before the moment hits the wheels, it passes through final drive. This is the gearbox that finally changes the gear ratio, usually significantly increasing torque and decreasing rotational speed. The main pair consists of a drive gear and a large driven gear (ring gear), which are located in the axle housing or gearbox housing.
Here the rotation vector rotates by 90 degrees in the classic design with a longitudinal engine. The bevel hypoid gear allows the driveshaft to be lowered, reducing the central tunnel in the cabin. The precision of manufacturing the teeth of the main pair directly affects the noise level of the transmission and the absence of hum when driving.
| Drive type | GP location | Transfer Features |
|---|---|---|
| Front (FF) | In the gearbox housing | The torque is transmitted to the CV joints immediately |
| Rear (FR) | In the rear axle gearbox | Requires long driveshaft |
| Full (4WD) | In the front and rear axles | Availability of transfer case |
The gear ratio of the main pair is a compromise between acceleration dynamics and maximum speed. A “shorter” main pair will improve acceleration, but increase fuel consumption and noise on the highway. A “long” pair will save fuel at high speeds, but the car will become sluggish at start.
Differential: torque distribution between wheels
The key element, without which the movement of turns would be impossible, is differential. It distributes torque between the drive wheels, allowing them to rotate at different speeds. When cornering, the outer wheel travels a longer distance than the inner one, and without a differential, one of the wheels would inevitably slip.
In a standard open differential, torque is always split equally (50/50). However, this scheme has a drawback: if one wheel hits the ice, the torque drops to zero on both wheels, as the differential tries to equalize the resistance. This is why on a slippery road a car can get stuck with only one slipping wheel.
How does a differential lock work?
A positive locking system rigidly connects the axle shafts, forcing the wheels to rotate at the same speed regardless of traction. This allows the car to be pulled up if one wheel is hanging in the air, but makes the car uncontrollable on paved corners.
To combat slippage, limited slip differentials (LSD) or electronic locking simulators are used. They redistribute torque to the wheel with better grip, increasing cross-country ability and directional stability. Modern ESP systems often brake a slipping wheel, forcing the differential to transfer torque to the other side.
From axle shafts to wheels: the final stage
In the last stage, the torque is transmitted directly to the wheel hubs through axle shafts. In a dependent suspension it can be a solid beam, in an independent suspension it can be shafts with hinges. This is where the rotational motion is converted into forward motion of the vehicle due to the tire's grip on the road surface.
Wheel size directly affects the resulting torque on the pavement. Increasing the wheel diameter without changing the engine settings is equivalent to lengthening the final drive: dynamics decrease, but the maximum speed can increase. Contact patch The tires are the only point where all the work of the engine and transmission finds its physical embodiment.
☑️ Torque transmission diagnostics
Play in axle shaft connections or wear of spline connections can lead to loss of some torque and the appearance of knocking noises. Regular visual inspection of drive units allows you to avoid sudden breakdowns on the road. Remember that even the most powerful engine is powerless if the torque transmission chain is broken at any stage.
Loss and transmission efficiency factors
No mechanical system operates at 100% efficiency. In the process of transmitting torque from the flywheel to the wheels, inevitable energy losses occur, which turn into heat and noise. Total transmission losses can reach 15-20% of engine power, especially in all-wheel drive systems with multiple intermediate shafts.
The main sources of losses are gear friction, bearing resistance and oil mixing (whiplash losses). Using quality synthetic oils with the correct tolerances minimizes these losses. Temperature also plays an important role: with cold oil, losses are much higher due to its thickness.
⚠️ Attention: Using oils with a viscosity lower than that recommended by the manufacturer can lead to destruction of the oil film and scuffing of gears under high load.
The efficiency of torque transmission also depends on the technical condition of the components. Worn wheel bearings or a warped driveshaft create additional resistance, causing the engine to work harder. Timely replacement of lubricants in transmission units can save up to 5% of engine power, which would otherwise be spent on overcoming friction.
The transmission is not just a rotation transmitter, but a complex mechanism for matching engine characteristics to road conditions, requiring regular maintenance.
Frequently Asked Questions
Why does torque stop being transmitted when the timing belt breaks?
The timing belt or chain connects the crankshaft to the camshaft. If they break, the opening of the valves stops, combustion of the mixture stops, and the engine stalls, ceasing to generate torque.
Does wheel size affect the torque transmitted?
Yes, increasing the wheel diameter increases the leverage of force, which actually reduces traction on asphalt. The car becomes heavier during acceleration, as the engine has to turn a wheel of a larger radius.
How can you tell if the clutch is slipping and not transmitting torque?
When you press the gas sharply, the engine speed increases, but the vehicle speed does not increase or increases very slowly. A burning smell may also appear from the friction linings.
Why do you need a transfer case in all-wheel drive?
It distributes torque between the front and rear axles. In some systems, it also has a reduction range of gears to increase off-road capability.