Clutch control
Encyclopedia
Clutch control refers to the act of controlling the speed of a vehicle with a manual transmission
by partially engaging the clutch
plate, using the clutch pedal instead of (or in conjunction with) the accelerator pedal. The purpose of a clutch is in part to allow such control; in particular, a clutch provides transfer of torque between shafts spinning at different speeds. Because an internal combustion engine
produces no torque at zero RPM, a clutch is needed to start a car moving, allowing a nonzero torque to be applied to the wheels at zero RPM. In the extreme, clutch control is used in performance driving, such as starting from a dead stop with the engine producing maximum torque at high RPM.
, so no power can pass from the engine to the driveshaft and wheels. With the pedal entirely released, there is full contact between the engine and the driveshaft, via the clutch plate, which means that the engine can apply power directly to the driveshaft. However, it is possible to have the clutch plate partially engaged, allowing the clutch to slip. As a result, only a fraction of the power from the engine reaches the driveshaft.
translate to large changes in acceleration and engine braking. However, with a combination of clutch control and careful use of engine speed, a much smoother ride can be achieved: by allowing the clutch to slip, variations in engine revs are not immediately translated into changes in drive shaft rotation speed, but rather the friction on the clutch plate allows the drive shaft to gradually equalize with the speed of the engine.
Once the car reaches a suitable speed, the clutch can be fully engaged and speed can then be controlled either by varying the engine speed or by partially disengaging the clutch again if necessary.
This particular use of clutch control is frequently taught to learner drivers as a way to control acceleration when pulling away from a complete stop or when driving at very slow speeds while minimizing the chance of stalling the engine.
When pulling away on an uphill slope the chance of stalling the engine is greater, and as a result it can be beneficial to engage the clutch more slowly than normal while revving higher than normal.
It is not necessary to maintain high engine speed to properly operate a motor vehicle. Any vehicle with a standard transmission can operate with an engine speed slightly above idle when following the proper gear sequence. The clutch can be operated normally with no sacrifice in performance; however, in certain off-road and racing conditions, to attain higher speed and better performance, the practice of clutch control plays an integral part in doing so.
See also: Engine braking
surface when such an action is performed. Slipping the clutch is known to be hard on the clutch surface due to the sliding friction
created.
Drivers can frequently be observed slipping the clutch when they are trying to stay stationary on a hill without using neutral
and the brake
. They apply the clutch to climb a bit, then release to roll back, then apply again, etc. so that the car stays in about the same place. With enough practice, alternating is no longer needed. Applying the correct amount of clutch pressure and throttle causes just enough force from the engine to counter gravity and keep the vehicle stationary (See Balancing the clutch
). The alternative to this technique of staying stationary on a hill would be to put the vehicle in neutral and apply the brake.
Slipping the clutch is a popular term in drag racing
culture and is done when launching a car, usually in a drag race. Some contend that slipping the clutch is the best way to launch a front-wheel drive
(FWD) car as it prevents Torque steering
that many FWD cars experience when too much power is put to the front wheels.
, riding the clutch refers to the practice of needlessly keeping the clutch partially disengaged. This results in the clutch being unable to fully engage with the flywheel and so causes premature wear on the disc and flywheel.
A common example of riding the clutch is to keep slight continual pressure on the clutch pedal whilst driving, as when a driver habitually rests his/her foot on the clutch pedal instead of on the floorboard or dead pedal
. Although this slight pressure is not enough to allow the clutch disc itself to slip, it is enough to keep the release bearing against the release springs. This causes the bearing to remain spinning, which leads to premature bearing failure.
When shifting properly, the driver "shifts" to another gear and then releases pressure on the clutch pedal to re-engage the engine to the driveshaft
. If the pedal is released quickly, a definite lurch can be felt as the engine and driveshaft re-engage and their speeds equalize. However, if the clutch is released slowly the clutch disc will "slip" against the flywheel; this friction permits the engine a smoother transition to its new rotation speed. Such routine slippage causes wear on the clutch analogous to the wear-and-tear on a brake pad when stopping. Some amount of wear is unavoidable, but with better clutching/shifting technique it can be minimized.
Riding the clutch occurs when the driver does not fully release the clutch pedal. This results in the clutch disc slipping against the flywheel and some engine power not being transferred to the drive train and wheels. Most drivers routinely use this inefficiency effectively when driving in reverse (as fully engaging the reverse gear results in velocity too great for the short distance traveled) or in stop-and-go traffic (as it is easier to control the throttle
and acceleration
at very slow speeds).
Riding the clutch should not be confused with "freewheeling" or "coasting", where the clutch is pressed down fully allowing the car to roll either downhill or from inertia. While this is not damaging to the car, it can be considered a dangerous way to drive since one forgoes the ability to quickly accelerate if needed. It is, however, a common practice to roll into a parking space or over speed bumps via momentum.
Manual transmission
A manual transmission, also known as a manual gearbox or standard transmission is a type of transmission used in motor vehicle applications...
by partially engaging the clutch
Clutch
A clutch is a mechanical device which provides for the transmission of power from one component to another...
plate, using the clutch pedal instead of (or in conjunction with) the accelerator pedal. The purpose of a clutch is in part to allow such control; in particular, a clutch provides transfer of torque between shafts spinning at different speeds. Because an internal combustion engine
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
produces no torque at zero RPM, a clutch is needed to start a car moving, allowing a nonzero torque to be applied to the wheels at zero RPM. In the extreme, clutch control is used in performance driving, such as starting from a dead stop with the engine producing maximum torque at high RPM.
Overview
With the clutch pedal completely pressed or a motorcycles lever pulled entirely towards you, there is no direct link between the engine and the driveshaftDriveshaft
A drive shaft, driveshaft, driving shaft, propeller shaft, or Cardan shaft is a mechanical component for transmitting torque and rotation, usually used to connect other components of a drive train that cannot be connected directly because of distance or the need to allow for relative movement...
, so no power can pass from the engine to the driveshaft and wheels. With the pedal entirely released, there is full contact between the engine and the driveshaft, via the clutch plate, which means that the engine can apply power directly to the driveshaft. However, it is possible to have the clutch plate partially engaged, allowing the clutch to slip. As a result, only a fraction of the power from the engine reaches the driveshaft.
Low gear and low speed
When a car is in first gear, small variations in engine speedRevolutions per minute
Revolutions per minute is a measure of the frequency of a rotation. It annotates the number of full rotations completed in one minute around a fixed axis...
translate to large changes in acceleration and engine braking. However, with a combination of clutch control and careful use of engine speed, a much smoother ride can be achieved: by allowing the clutch to slip, variations in engine revs are not immediately translated into changes in drive shaft rotation speed, but rather the friction on the clutch plate allows the drive shaft to gradually equalize with the speed of the engine.
Once the car reaches a suitable speed, the clutch can be fully engaged and speed can then be controlled either by varying the engine speed or by partially disengaging the clutch again if necessary.
This particular use of clutch control is frequently taught to learner drivers as a way to control acceleration when pulling away from a complete stop or when driving at very slow speeds while minimizing the chance of stalling the engine.
Uphill start
- For mechanical aids to hill-starting, see Hill-holderHill-holderHill-Holder is a name for the mechanism invented by Studebaker that holds the brake until the clutch is at the friction point, making it easier to start up hills from a stop in manual transmission automobiles. It was first introduced in 1936 as an option for the Studebaker President...
and Hill Start AssistHill Start AssistHill Start Assist is a variant of Hill-holder used by some semi-automatic, clutchless transmissions. The system prevents the car from rolling away when trying to pull away on an up or down gradient, simulating a "handbrake hill start" manual drivers will be familiar with...
When pulling away on an uphill slope the chance of stalling the engine is greater, and as a result it can be beneficial to engage the clutch more slowly than normal while revving higher than normal.
Adverse road conditions
In adverse road conditions, notably snow or ice, it is recommended to pull away in as high a gear as possible to minimize torque on the wheels and thereby maintain traction with the road. Pulling away requires progressively slower engagement of the clutch as the gear increases, and in a high gear it is necessary to engage the clutch slowly to avoid the increased risk of stalling the engine, or, in the case of adverse weather conditions, spinning the wheels.Changing from first to second gear under maximum power
As the clutch slips, engine speed is lost but torque is not lost except through the effect of the new engine speed on the engine torque itself. In most car engines the torque output is higher as the engine speed increases up to 4500RPM or more http://www.auto-ware.com/combust_bytes/p_goal.htm. So slipping the clutch actually gives more torque to the wheels even though the fraction of power wasted in the clutch increases much faster.It is not necessary to maintain high engine speed to properly operate a motor vehicle. Any vehicle with a standard transmission can operate with an engine speed slightly above idle when following the proper gear sequence. The clutch can be operated normally with no sacrifice in performance; however, in certain off-road and racing conditions, to attain higher speed and better performance, the practice of clutch control plays an integral part in doing so.
Balancing the clutch
Normally, when a vehicle is stationary on an uphill slope it is necessary to use the handbrake in conjunction with clutch control to prevent the vehicle from rolling backwards when pulling away. However, in situations where the vehicle must be stopped briefly, for example in slow moving traffic, the clutch can be used to balance the uphill force from the engine with the downhill force of gravity. The benefit of this is that there is no need for the hand- or foot-brake, and the driver can pull away more quickly. Using this technique will, however, wear out the clutch faster.Deceleration
Typically with motorcycles and in motor sport, the clutch is often used to facilitate the use of resistance from the engine spinning at high speeds to decelerate the vehicle more quickly, often accompanied with normal braking. This is achieved by placing the vehicle in a gear that would ordinarily be too low for the current speed and momentum of the vehicle and by partly engaging the clutch. When this happens momentum energy from the inertia of the vehicle is taken away to spin the engine as close as possible to its maximum capability. As the vehicle is decelerating the clutch can be further released to transfer more energy to keep the engine spinning as quickly as possible. Once the clutch is entirely released this cycle proceeds downwards through the gears to further assist deceleration. If the clutch is controlled improperly while this is being attempted, damage or extra wear to the engine and gears is possible, as well as the risk of wheels locking up and a subsequent loss of proper vehicle control.See also: Engine braking
Engine braking
Engine braking is where the retarding forces within an engine are used to slow a vehicle down, as opposed to using an external braking mechanism, for example friction brakes or magnetic brakes....
Problems
Even normal use of clutch control increases the wear (and decreases the lifespan) of the clutch. Excessive use of clutch control or riding the clutch will cause further damage.Prolonged use
While the use of clutch control at low speed can be used to obtain greater control of acceleration and engine braking, once a car has picked up sufficient speed the clutch should be fully engaged (pedal released).Excessive engine revolutions
Excessively revving the engine while using clutch control, or keeping the clutch partially engaged while accelerating with the gas pedal, can cause unnecessary damage to the clutch.Slipping the clutch
Slipping the clutch (sometimes referred to as feathering the clutch) is a term used by automotive enthusiasts to describe when the driver alternately applies and releases the clutch to achieve some movement of the car. It's called slipping because the clutch plate will slip against the flywheelFlywheel
A flywheel is a rotating mechanical device that is used to store rotational energy. Flywheels have a significant moment of inertia, and thus resist changes in rotational speed. The amount of energy stored in a flywheel is proportional to the square of its rotational speed...
surface when such an action is performed. Slipping the clutch is known to be hard on the clutch surface due to the sliding friction
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...
created.
Drivers can frequently be observed slipping the clutch when they are trying to stay stationary on a hill without using neutral
Transmission (mechanics)
A machine consists of a power source and a power transmission system, which provides controlled application of the power. Merriam-Webster defines transmission as: an assembly of parts including the speed-changing gears and the propeller shaft by which the power is transmitted from an engine to a...
and the brake
Brake
A brake is a mechanical device which inhibits motion. Its opposite component is a clutch. The rest of this article is dedicated to various types of vehicular brakes....
. They apply the clutch to climb a bit, then release to roll back, then apply again, etc. so that the car stays in about the same place. With enough practice, alternating is no longer needed. Applying the correct amount of clutch pressure and throttle causes just enough force from the engine to counter gravity and keep the vehicle stationary (See Balancing the clutch
Clutch control
Clutch control refers to the act of controlling the speed of a vehicle with a manual transmission by partially engaging the clutch plate, using the clutch pedal instead of the accelerator pedal. The purpose of a clutch is in part to allow such control; in particular, a clutch provides transfer of...
). The alternative to this technique of staying stationary on a hill would be to put the vehicle in neutral and apply the brake.
Slipping the clutch is a popular term in drag racing
Drag racing
Drag racing is a competition in which specially prepared automobiles or motorcycles compete two at a time to be the first to cross a set finish line, from a standing start, in a straight line, over a measured distance, most commonly a ¼-mile straight track....
culture and is done when launching a car, usually in a drag race. Some contend that slipping the clutch is the best way to launch a front-wheel drive
Front-wheel drive
Front-wheel drive is a form of engine/transmission layout used in motor vehicles, where the engine drives the front wheels only. Most modern front-wheel drive vehicles feature a transverse engine, rather than the conventional longitudinal engine arrangement generally found in rear-wheel drive and...
(FWD) car as it prevents Torque steering
Torque steering
Torque steering is the influence of engine torque on the steering for some front-wheel drive vehicles. For example, during heavy acceleration the steering may pull to one side, which may be disturbing to the driver. The effect is manifested either as a tugging sensation in the steering wheel, or a...
that many FWD cars experience when too much power is put to the front wheels.
Riding the clutch
In a vehicle with a manual transmissionManual transmission
A manual transmission, also known as a manual gearbox or standard transmission is a type of transmission used in motor vehicle applications...
, riding the clutch refers to the practice of needlessly keeping the clutch partially disengaged. This results in the clutch being unable to fully engage with the flywheel and so causes premature wear on the disc and flywheel.
A common example of riding the clutch is to keep slight continual pressure on the clutch pedal whilst driving, as when a driver habitually rests his/her foot on the clutch pedal instead of on the floorboard or dead pedal
Dead Pedal
In an automobile, the dead pedal is located to the left of the other pedals. Unlike the throttle, brake or clutch pedal, the dead pedal does not move and is not connected to anything...
. Although this slight pressure is not enough to allow the clutch disc itself to slip, it is enough to keep the release bearing against the release springs. This causes the bearing to remain spinning, which leads to premature bearing failure.
When shifting properly, the driver "shifts" to another gear and then releases pressure on the clutch pedal to re-engage the engine to the driveshaft
Driveshaft
A drive shaft, driveshaft, driving shaft, propeller shaft, or Cardan shaft is a mechanical component for transmitting torque and rotation, usually used to connect other components of a drive train that cannot be connected directly because of distance or the need to allow for relative movement...
. If the pedal is released quickly, a definite lurch can be felt as the engine and driveshaft re-engage and their speeds equalize. However, if the clutch is released slowly the clutch disc will "slip" against the flywheel; this friction permits the engine a smoother transition to its new rotation speed. Such routine slippage causes wear on the clutch analogous to the wear-and-tear on a brake pad when stopping. Some amount of wear is unavoidable, but with better clutching/shifting technique it can be minimized.
Riding the clutch occurs when the driver does not fully release the clutch pedal. This results in the clutch disc slipping against the flywheel and some engine power not being transferred to the drive train and wheels. Most drivers routinely use this inefficiency effectively when driving in reverse (as fully engaging the reverse gear results in velocity too great for the short distance traveled) or in stop-and-go traffic (as it is easier to control the throttle
Throttle
A throttle is the mechanism by which the flow of a fluid is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases , but usually decreased. The term throttle has come to refer, informally and incorrectly, to any mechanism by which...
and acceleration
Acceleration
In physics, acceleration is the rate of change of velocity with time. In one dimension, acceleration is the rate at which something speeds up or slows down. However, since velocity is a vector, acceleration describes the rate of change of both the magnitude and the direction of velocity. ...
at very slow speeds).
Riding the clutch should not be confused with "freewheeling" or "coasting", where the clutch is pressed down fully allowing the car to roll either downhill or from inertia. While this is not damaging to the car, it can be considered a dangerous way to drive since one forgoes the ability to quickly accelerate if needed. It is, however, a common practice to roll into a parking space or over speed bumps via momentum.