What happens when you change gear without pressing clutch?

As many of us drive vehicles and motorbikes, We all have questions in mind related to working of clutch and gears. The common question everyone have is what happens if you change the gears without pressing the clutch. So let us understand this and before that we need to understand function of clutch.

Clutch works on the principle of frictional transmission , It is made up of pressure plate and clutch plate. On one side of clutch has engine while other side has gearbox so basically clutch transmits power from engine to gearbox.

When clutch is at idle stage (Not pressed by driver), The clutch is in engaged condition which means power of engine is continuously coming to gearbox and when clutch is pressed by driver, clutch comes to disengage condition and disconnects the engine power to gearbox.

Gearbox changes the speed and torque at its output which is coming from engine by conversion of gear ratios. Gear ratio is changed by sliding the gears over shafts and meshing different size of gears.

Gearbox has gears mounted on input output and intermediate shafts, input shaft is connected to output of clutch , Intermediate shaft is connected to input shaft (has same speed as that of input) and output shaft connects to axle.

Gear box
Gearbox layout

Now when vehicle is running, speed of engine, gearbox and transmission system up to wheel is synchronised and if driver tries to change the gears without pressing the clutch, for a small period of time, link between input and output shaft will break and at that instance output shaft will remain at speed of wheels while speed of input shaft will increase rapidly because of the sudden reduction of load at engine.

Due to this speed difference at input and output shaft meshing of gears becomes tedious. In this case, gears will rub against each other and make loud sound resulting to rough engagement or no engagement at all. Failure of gears also can occur in severe cases.

Why clutch is pressed during gear change?

When clutch is disengaged, input shaft (also the intermediate shaft) becomes idle because it is not connected to engine anymore while output shaft remains at wheel running speed. In this case, with change of gear by driver, gears of input shaft matches the speed of output gears very smoothly because intermediate shaft is idle and can change its speed very easily and match the speed of output shaft.

Synchroniser type gear boxer are very smooth at gear change because synchroniser incorporated in these gears helps them match their speed even better.

So at the end Clutch disengagement by pressing the clutch is very important for gear change considering safety and life of gears.

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