Would you like to know how the clutch works? What exactly is ‘the biting point'? How does clutch control help you to perform all those driving manoeuvres?
This page will give you a very simple explanation of how the clutch works, but don't be fooled, the basic principles of the clutch really are very simple. Let me show you how the clutch works and you'll understand what ‘clutch control' and ‘biting point' actually mean.
You don't need to know any technical information about the engine or the clutch to learn to drive. Many people have been driving very safely for years and yet they don't have the slightest clue about how the car works
Can you send a message or call someone on your mobile phone? I bet you can, but very few people know how all the electronics inside works. You don't need to, you just need to know how to push the buttons, not how to put together all the parts or how they work together.
So, you don't need to know about the inner workings of the car, but if you take the time to read this section and to study the diagrams, you'll find that a basic understanding of the clutch will help you to understand the finer points of car control enormously.
How The Engine Turns The Wheels
So just what does this clutch thing do ?
OK, well, we all know that the thing that makes the car go along the road is the engine. The insides of the engine spin around and turn all sorts of cogs and things that we don't need to know about.
The engine is connected to the cars drive wheels by a metal broom handle called the drive shaft, it's the long green bar that goes straight up and down in Image 01 on the right.
The drive shaft is connected to the axle via a special device we won't go into here, it's called a differential and it transforms the turning motion of the drive shaft 90 degrees so that the axle can turn the wheels.
The differential in Image 01 is the big red dot connecting the drive shaft to the axle.
The drive wheels are the wheels that the engine sends its power to so the car can move. The drive wheels can be at the front, which is where they are on most cars, or at the back, as they are on BMW's for instance.
If you have a 4 wheel drive car, that's exactly what it means, all the wheels are driven directly by the engine.
For the purposes of this driving tutorial, I'll just create the diagrams as though it's a simple two wheel drive car and that the drive wheels are at the back. That's for no other reason than it makes the diagrams easier to understand (and to create!)
Take another look at Image 01. You'll see that the long green drive shaft is connected to the rear wheels, indicating that this would be a rear wheel drive car. The principles of the clutch are identical, no matter whether the car is rear wheel drive, front wheel drive or four wheel drive. Once you know the basic principles you'll understand all of them.
The drive shaft is made to spin by the engine because its connected to it.
When we press the gas pedal we put more fuel into the engine and that makes it go faster.
Every time you squeeze the gas pedal you'll hear the engine getting higher in pitch as the cogs and other internal bits go round faster and faster. When the engine goes faster the drive shaft goes faster, and because the drive shaft's connected to the wheels the car goes faster.
OK so far?
Now, can you imagine what might happen if the engine was connected to the wheels all the time by the drive shaft?
That's what's happening in Image 01 here, the engine turns and engine power is transmitted to the wheels constantly. No matter what speed the engine was turning at, the power of the engine would always be straining to move the car along.
How would we be able to stop the car?
The engine would always be wanting to push us along, even if we were just trying to stay still on tick over. We'd have to turn the ignition off just to stop the car. Imagine doing that at every set of traffic lights or at every roundabout you were waiting to emerge onto.
And what would you do when you wanted to pull over at the side of the road?
You'd have to perform acrobatics by getting the car into neutral and braking to a stop. But then again, how would you get the car into neutral? Without being able to stop the engines' power being transmitted to drive shaft it would cause enormous damage to the engine just trying to go into another gear. Just trust me on that, it's too complicated to explain fully and you really don't need to understand it.
You've heard the engine change when you move down a gear? It usually revs louder and faster unless you're able to match the road speed to the gear accurately, so trying to do it without being able to switch off the power to the drive shaft would be almost impossible.
So, we need a way of turning off the engine's power to the wheels so that we can stop the car when we want to, without having to actually turn off the engine.
And that's exactly what the clutch does.
The clutch is a very clever little device that you can simply view as a box fitted into to the cars drive shaft.
In reality it's very complicated and has many parts, but just as with your mobile phone you don't need to know all about what's inside, you just need to know how to use it.
Just imagine that the drive shaft is sawn into two pieces and a little box is fitted to connect the two parts, just like in the diagram. The two parts of the drive shaft are now only connected together by the clutch.
In Image 02, the engine is still turning and this makes the drive shaft turn. You can now see that the drive shaft has been sawn in to two pieces and the clutch is placed between them.
In Image 02, the clutch makes no difference to what's happening. All the engines power is effectively transmitted to the drive wheels at the rear, so it's almost like the clutch wasn't there at all.
We may as well not have bothered fitting it because at the moment it's just not doing anything.
We've fitted the clutch box into the car but we need to break our rule of not bothering to look inside it to see how it works, just this once.
As you read on, you'll learn how the clutch controls the transmission of engine power to the wheels.
All will become clear and you'll see that the clutch is an incredibly clever device that's actually simple to understand and very important to know about.
Clutch Plates And Clutch Springs
OK so far, but how does this clutch thing turn off the engines power to the wheels?
Well, the clutch is actually made up of two circular discs called ‘clutch plates'.
In Image 03, I've taken away the box so we can see the two plates. I've called them plate 1 and plate 2.
Plate 1 is permanently attached to engine via the first little bit of the drive chain. Because of this, plate 1 is always spinning and you can think of it as spinning at exactly the same speed as the engine spins.
The clever bit is clutch plate 2. Plate 2 can move, as you'll learn soon.
As I've said, plate 1 is permanently attached to the part of the drive shaft the goes into the engine. That means that this clutch plate is always spinning round along with the drive shaft.
Plate 2 is attached to the other bit of the drive shaft, the one that's eventually attached to the wheels via the differential.
The clutch plates are made of a very hard wearing material and when they are pressed together hard there's a lot of friction between them.
In fact, when they are firmly pressed together there's so much friction between them that they may as well not be there. it's as though both parts of the drive shaft are connected together as before.
Just have another quick look at Image 03.
Here you can see that the engine is turning. The engine makes the first bit of the drive shaft turn and this turns the first clutch plate. The first clutch plate is pressed very firmly against the second clutch plate and this makes the rest of the drive shaft, and the wheels, turn around.
Image 03 shows how the clutch plates are when your foot is off the pedal, when you allow the pedal to come all the way up.
In other words, when you don't press the clutch pedal the plates are pressed very firmly together. This actually happens because there are some really big strong clutch springs that push the plates together.
Think about it, what's the default position for the clutch pedal? I mean, if you don't touch it, where does it naturally find itself?
The clutch pedal likes to be up. All the way up in fact. That's because the clutch springs actively push the plates together and lift the pedal unless you push it down with your foot. On any vehicle, with any drive system, the clutch will always spring back up when your foot is not pressing the pedal down.
So, as above, when your foot is off the pedal the clutch might as well not be there.
What Does The Clutch Pedal Do?
OK, so what happens when we do press the clutch pedal?
What happens when we press the pedal is that the clutch plates are forced apart, so there's a gap between them. When you press the clutch pedal what you're really doing is compressing the clutch springs and moving the plates apart.
Have a look at Image 04, so you get the picture.
Here you can clearly see the clutch plates have been separated.
The engine keeps on turning and it turns the little bit of the drive shaft that's connected to the engine. Plate 1 is attached to this first bit of the drive shaft also carries on turning.
Remember? The first clutch plate is permanently attached to the first bit of the drive shaft and always spins with it.
What about the second clutch plate?. The one attached to the other bit of the drive shaft?
Well, there's a gap between the two plates now so they're not touching. So the second plate can't spin, no matter how fast the engine is turning.
Because the second plate can't spin. neither can the rest of the drive shaft, and if that can't spin the car wheels can't spin.
You can rev the engine as much as you like, with those clutch plates forced apart the car simply will not move anywhere because no matter how much power the engine generates, none of it can reach the wheels. You'd just be making a lot of noise and going nowhere.
We've managed to turn off the engines power to the wheels without turning off the ignition or actually stopping the engine, and all we had to do was to press the clutch down to prevent the power of the engine from driving the wheels. It's actually very clever.
So, the clutch lets us turn the engines power to the car wheels on and off, whenever we want.
When the pedal is up, the full power of the engine is connected to the wheels and the clutch might as well not be there.
When we press the pedal all the way down we separate the two plates so that the second one can't spin. This turns off all the engines power to the wheels and the engine might as well not be there.
But here's the really clever part . . .
By varying how much we press down on the clutch pedal we can vary how much power from the engine is connected to the wheels. It's not an on-off switch. By varying how much we release (lift) the clutch pedal we can control how much of the engines power is being transmitted to the drive wheels of the car.
Using The Clutch To Control Transmission Of Engine Power To The Wheels
Remember how I said that the two clutch plates are made of a hard wearing material and that there is a lot of friction between them?
Well, the clutch works entirely on friction. The second clutch plate is connected, via the second half of the drive shaft, to the wheels. You can imagine that it takes quite a lot of power to turn the wheels and make the whole car move.
Now, let's take the journey of the clutch pedal from all the way down (engine may as well not be there as it's not connected to the drive wheels) to all the way up (clutch may as well not be there as it's now as though there's no break in the drive shaft and the engine is always connected to the wheels)
If we raise the pedal just a little bit and hold it there, the first plate is only just touching the second one and there's not enough friction between them to make the second one move. The first plate actually spins and kind of ‘rubs' against the second one but can't actually make it move.
We lift the pedal a little more to press the plates together a bit more . . . there still may not be enough friction to turn the second plate and move the whole car but the first plate still spins and rubs against the second one.
Lift the pedal and press the plates a bit more . . . a bit more . . . a bit more. Can you see that as we press the plates together the friction builds up until they both start turning?
At some point the plates will be pressed together just tight enough for the second plate to start turning, but they won't be pressed hard enough together for the second plate to turn quite as fast as the first. Lift the clutch a bit more . . . now they're pressed tighter together and the second plate will move a little quicker . . .
By varying how much you press the pedal you are varying how hard the two plates are pressed together and, therefore, how much of the engines power is sent to the wheels. The clutch can be viewed as being similar to a volume control on a CD player.
So, the clutch isn't a simple on off switch. Think of it this way . . . this is a picture of my CD player. It's a bit old now, like me, and I should get a new one, but I put a CD in it and it plays it just fine.
On the front is a volume control. I use the volume control to turn down the sound of the music so we don't wake the neighbours. When I turn down the volume, what I'm really doing is turning down the power that's sent to the speakers.
Let's just think about that for a moment . . . when it's loud the music goes “thump, thump, thump . . .”, but when I turn down the volume the beat of the music still goes “thump, thump, thump . . .” at exactly the same tempo, but It's a lot less powerful. In other words, the CD still spins and plays at exactly the same speed. In fact, I can't change the speed of the CD or the beat of the music by fiddling with the volume control at all.
If you think about it, I can turn the volume all the way up or all the way down, and there are an infinite number of positions in between. But the CD always plays at the same speed and the beat of the music always stays constant.
The clutch is just like that.
I can turn the engines power to the wheels all the way off (clutch down) or all the way on (clutch up). In these positions, the clutch really is just a complicated on-off switch, but it can do so much more than that.
I can vary the power to the wheels as much as I like by varying the position of the pedal, just like a volume control. In exactly the same way, the clutch does not alter the speed of the engine at all, just like the volume control can't change the speed that my CD plays at.
What Is The Bite Point?
move off you always find the ‘biting point' first. Can you now have a guess at what this is ?As a matter of interest, when you get into the car to
Well, you find ‘the bite' by lifting the pedal up very slowly until you just feel the car wanting to move.
What you've done is actually allowed the clutch plates to press against each other just enough so that the friction between them is now high enough to make the second plate want to start turning with the first one. Simple, really.
The biting point will vary depending on a lot of factors.
Different cars have different biting points. The age of the clutch plates is another factor.
Because they spin and rub against each other they do wear out eventually. It's really an incredible feat of engineering that they don't wear out much sooner with all the rubbing against each other they have to do.
Other factors to consider are whether you're on a level road or trying to do a hill start. On a hill start it takes quite a bit more power to get the car moving, so you may have to get the engine spinning faster (setting the gas) and the biting point will usually appear when you lift the clutch pedal a little further than normal. If you're moving off on a level road the car can often pull away very smoothly without much gas being set and the bite point will appear much sooner as you lift the clutch pedal.
It's simply about finding the position of the clutch pedal that's right for your circumstances at the time. With practice it's really easy.
I hope that this introduction to how the clutch works has helped you to understand what the clutch does.
All you need to do is get a picture in your mind of those two plates pressing against each other with varying clutch pressure, depending on how much you've pressed the pedal down. Think of the clutch as a volume control for the engines power being sent to the wheels and it may make it a little easier to understand.