Help and Support Centre - Proper 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 (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. 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. Clutch control also applies to driving a tractor.

 

With the clutch pedal completely pressed, there is no direct link between the engine and the driveshaft, 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.

 

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 when negotiating tight turns, 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.

 

Potential clutch 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 vehicle 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.

 

Riding the Clutch

In a vehicle with a manual transmission, such as a tractor, 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.

 

Fig 1

Fig 2

Overheated, melted and seized thrust bearing (Fig 1) and melted clutch fingers (Fig 2) - a direct result of riding the clutch pedal - the constant pressure on the thrust bearing caused it to overheat, melt and finally seize.

The seized thrust bearing then melted the fingers on the clutch itself as shown.



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 usually 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. While inefficient, most drivers routinely use this technique effectively when driving in reverse (as fully engaging the reverse gear results in velocity too great for the short distance travelled) 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 vehicle to roll either downhill or from inertia. While this is not damaging to the vehicle, 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 inertia.

 

Back to Help Topics

 

Click Here to return to the Homepage..