MisterMMM.com: what does the clutch do?

March 31

I could get very technical about it, because I know what they do in some detail, but limit it to just how it affects the operation of the car. I remember asking one driver who was a car mechanic about it and we had quite an interesting conversation about them!

Why is it called a clutch? That's just a name isn't it? But, of course, something clutches at something else and makes the car go, if it's in gear.

I would like to call it a connector. Its function is to connect the engine to the gearbox. This clip from YouTube, shows a stripped down clutch and gear assembly working. It's quite nice to look at in some ways. When the clutch pedal is pushed down, the gears stop moving, whereas the engine doesn't.

When the clutch is at the biting point, the clutch pedal has been released just enough for the clutch plate to touch the fly-wheel, and then it transmits some torque. The animation on this page for the Diaphragm Clutch about half way down the page, is quite good.

Just remember that the clutch is engaged when the clutch pedal is up, and that at some point on its travel, the plate is just touching the flywheel. It's also useful to remember that in the car, the flywheel is spinning around at between 10 and 100 times a second :-o

The animation here for the Basic Clutch demonstrates what happens when the clutch is at the biting point, and the car is allowed to move off. The plate and flywheel at this point are touching and there is friction between them. The Big wheel is the flywheel of the engine spinning, whereas the small wheel is connected to the gearbox and then wheels, and gradually gets faster as the car gets moving.

Now, the main point of a clutch is in this slipping/biting point in getting the car to move. The car has to be moving at the minimum speed of the engine in that gear before the clutch can be fully released and engaged. If it is brought up too early, then instead of the car moving, the engine is slowed down. If it gets too slow, the engine just stops.

It's a limitation of the internal combustion engine that it can't just get slower and slower. There comes a point when the gases do not move quickly enough around the engine, and the flywheel doesn't have enough momentum to create compression for the next explosion in the cylinders. In other words, the engine stalls. For most cars in first gear, this is about 2mph.

So if you stall the engine moving off from the lights, you have slowed the engine down below its minimum speed. This is done by a combination of too much clutch and not enough gas! Basically, next time, keep the clutch in the biting point for longer, and don't have it too high.

The position of the clutch is the important consideration of how fast you move off. You only have to make sure you have enough gas, but the clutch is in control. If you have the clutch higher, more bite, then you need move gas first.

Oh yeah, using the clutch to change gear makes it easy to do, but it is perfectly possible to change gear without it. When the clutch is not under load, it is easy to slip the gearbox into neutral. Then when the input and output shafts are resynchronised at the new speeds for the required gear, just slip it into that new gear.

It takes an experienced driver who knows their car very well to do that. Just be warned, that when it goes wrong it makes a horrible graunching noise in the gearbox. So much easier to use the clutch though.