Generating some power - The engine

The engine in your Bantam is firstly a piston engine. It has a piston inside a cylinder, into which a gas is introduced. Secondly it is an internal combustion engine, because the gas is a flammable fuel/air mixture which we will ignite inside the cylinder. The hot gases expand, pushing the piston to the bottom of the cylinder. The piston is attached to a short rod by a pivot within the piston. The rod is further attached to a shaft which has a pin off centre, to which the rod is attached. Thus the linear movement of the piston is changed into a rotating movement of the shaft.

The engine needs a system to allow the mixture to be introduced to the cylinder; the burnt gases to be exhausted, and the mixture to be ignited.

This can be very a complex system of valves, or we can simply use the piston itself, and introduce the fuel & release the burnt gas through ports in the cylinder.

We call this a piston-ported engine. Using this method, we can achieve the induction, compression, power & exhaust stages of the cycle within two movements of the piston.

We call this a two-stroke engine:

So, the diagram above can help us understand how the two-stroke cycle works. It’s helpful to remember that a ‘stroke’ refers to one movement of the piston from top to bottom or from bottom to top – thus, for a two stroke the complete cycle occurs once for every rotation of the crankshaft.

So to begin:

1. In picture 1 the upward movement of the piston away from the crankcase means that the air pressure drops in the crankcase, sucking air/fuel mixture from the carburetter through the intake port

2. When the piston comes down again, as shown in picture 4, the intake port is blocked. The piston falling increases pressure in the crankcase, and blows the air/fuel mixture through the transfer ports and into the cylinder, above the piston. This important event requires a couple of design changes not necessary in a four stroke engine – one is that the thermodynamic efficiency of the engine is increased if the volume of the crankcase is minimised when the piston is down, and maximised when it is up – the difference is called the ‘crankcase compression ratio’. The second factor is that the fuel mixture needs to be retained in the crankcase, necessitating the use of seals around the crankshaft. Failure of these seals results in the mixture leaking out of the crankcase and air, or gearbox oil, leaking in which means you have an uncertain fuel/air mixture, leading to very poor running.

3. The action of the air/fuel mixture coming into the cylinder has the effect of blowing the burnt mixture through the exhaust port, shown in picture 3. The expanding gases go into the exhaust as a pressure wave, and the reflection of that pressure wave off the structure of the exhaust is used to prevent the incoming charge from passing unburnt into the exhaust as well.

4. As the piston rises again, it closes off the exhaust port and compresses the mixture in the cylinder. At a point where the piston is almost at the top of the stroke, the spark plug ignites the mixture and the mixture starts to burn. As the piston passes over its highest point (‘top dead centre’) the pressure in the cylinder from the burning mixture rises rapidly and forces the piston down again, as shown in picture 2.

5. As the piston moves down, the piston uncovers the exhaust port and the remaining gas pressure pushes part of the burnt remains of the mixture charge into the exhaust

You might have realised that with all this up and down several things are going on at once – we only have two strokes to get all this done! So, when we are sucking mixture into the crankcase in item 1 above, we are also compression mixture on the other side of the piston, in item 4; and at the end of our power stroke described in item 4, we are pushing mixture through the transfer ports in item 2.

Motorcycle Fuel Systems

You might not know it, but we owe this man, Daniel Bernoulli, a considerable debt. Bernoulli was a Swiss mathematician born into a famous family of mathematicians in 1700 (famous to mathematicians and engineers, that is).

What he described, in 1738, was derived from the principle of conservation of energy. This states that, in a steady flow, the sum of all forms of mechanical energy in a fluid along a flowline is the same at all points on that flowline. This requires that the sum of kinetic energy (the energy of motion) and potential energy remain constant. Thus an increase in the speed of the fluid occurs proportionately with an increase in both its dynamic pressure and kinetic energy, and a decrease in its static pressure and potential energy.

What this means, put more simply, is that if an airflow is speeded up (by sucking it through a pipe for example), then more of the energy contained in the airflow is present as motion and since you cannot invent energy from nowhere then it’s pressure energy must decrease.

This means that when an aircraft wing moves through space, the pressure on the longer, curved upper surface is lower than that on the shorter, straight lower surface because the air on the upper surface moves faster just to keep up.

Talking of keeping up, are you getting this?

Back to the pipe with the air being sucked through it. Because the air in our pipe moves faster than the air outside, the pressure outside is higher than the pressure inside. This means that if you introduce a small tube of fluid into the side of the pipe, the fluid will get sucked up the tube due to the lower pressure.

In our world, the pipe and its little tube are called a carburetter!

So there we have a concept that allows us to draw air and fuel into the engine. We now need some refinements: we need to be able to set the ratio of air to fuel for different engine speeds, and it would be helpful for the rider to be able to vary the speed of the engine. The other small problem is that the engine needs different amounts of fuel & air according to its load, its running speed and the air temperature. And the amount of oxygen in the air…and… and…

So, that little tube poking up into the air stream – we can control the size of that by changing the size of the hole through it. That will allow us to control the amount of fuel passing into the air stream for the whole range of the carburettor’s operation. It’s shown in the diagram as item 21, and is properly called the main jet. Of course, we also need a way of opening and closing the carburetter, and for that we have a slide that will block off the airflow, or will allow it to fully open. It’s shown as item 15, and is opened by pulling on the cable K.

Now, remember we said that the air/fuel ratio needed to be controlled across the whole range of operation of the engine. You’ll soon realise that if the fixed main jet provides the correct amount of fuel when the slide is fully open, then it will provide way too much when the slide is not fully open… so we need some more widgets to deal with that, ideally something which is variable. Enter the tapered needle, item 19, to fanfare & drum roll! The tapered needle slides up and down with slide 15, in a tube 18 called the needle jet. The fact that it is tapered offers a variable area between the needle and the needle jet, which gives us a variable fuel flow, and now, we can control the mixture at any slide opening!

More or less. Bantam carburetters of the era we are dealing with are not sophisticated devices, and when you shut the slide, the engine stops (in fact, that is how you stop the engine since there is no ignition switch either!). Most motorcycle carburetters feature yet another jet arranged to bleed some fuel into an air passage around the slide, just a small amount, to let the engine tick over while you put your helmet on and say goodbye to the cat.

But there is more. The most casual observer cannot fail to notice that a motorcycle fuel tank is typically placed above the engine, and, especially if you are inclined to walk the fells & dales of Yorkshire, or you have ever spilt a cup of coffee, that liquids are usually inclined to flow downhill. Thus, we need a device to stop the fuel pouring out of the tank and flooding the carburetter and washing over your feet. We’ll use a float, item 2, coupled to a needle valve, item A to stop the fuel coming in through the fuel pipe, item 7. This is doubly important because the level of the fuel in the needle jet (item 18) affects the ability of our air flow to suck it out – too high, and it will come out too easily and there will be too much fuel in the air flow. Therefore, the float (item 2) and the float needle (item A) are coupled together to set the fuel level at just the right height, just like the float valve in the cistern in your toilet. There are two passages, C and D, to allow the fuel to get to the needle jet.

You will remember also that we said we would need to arrange for the engine to start when it was cold. Engines need more fuel in the air/fuel mixture when they are cold, and to provide this we have spring loaded plunger (item 6) which can be used by the rider to hold the float down and the float needle off its seat – thus allowing a lot of fuel into the carburetter, temporarily.

Motorcycle Electrical Systems

This chap on our right is Michael Faraday, who was born in 1791 in London and, without any formal education other than a persistent and inquiring mind, became one of the foremost scientists of the 19th century. One of the things Faraday explained to the world was the principle of electromagnetic induction. Electromagnetic induction is the production of a potential difference (a voltage) across a conductor (a wire) when it is exposed to a varying magnetic field.

Faraday's principle underlies most of the ability of our machines to run and generate light.

Power Generation

Electrical generators, and let’s not confuse the situation by calling them alternators or dynamos for the moment, operate by the use of energy from the engine to rotate a coil of wire relative to a magnetic field, thus, by the understanding provided by Mr. Faraday, inducing a voltage in the wire. In the simplest installation (including Bantams) a fixed coil is mounted within a rotating drum; the drum is provided with one or more permanent magnets in its periphery.

Because the magnets rotate around the coils, passing them with each rotation of the engine, the current that flows in the coils changes according to the proximity of the magnet (i.e. the strength of the magnetic field) and can flow in either direction according to the polarity of the field. We say the current alternates. The fixed coils make it very easy to connect to the vehicle electrical system. In fact, we’ll connect each end of our coil to each end of a light bulb filament

And that’s it. With some wires and switches, we can connect the coils to the light bulbs and ride at night. Bulbs don’t mind if their current swings both ways.

The other interesting thing that happens is that the current that can be induced to flow in the coils increases with the speed of the passing magnet – thus, if we increase engine speed, we can have more voltage and more current.

This starts to throw up the first problem with our simple system – when we ride too fast in the dark, the bulbs blow…

Voltage Control & Rectification

All Bantams, battery lighting or otherwise, use a form of voltage control common to small bikes of the period. Essentially, the generator coils (and there may be two or three lighting coils depending on the type) are switched in and out of circuit according to the lighting load. If all the coils are in use, and the lights are switched to ‘sidelights’, increasing the speed of the engine will generate considerably more than 6 V and will blow the bulbs; thus, when only sidelights are in use, only one or two lighting coils are used, the system brings the other into service when the lighting switch is moved to the headlamp position. This switching of coils is achieved in the lighting switch in the headlamp.

Battery lighting Bantams have another need, beyond voltage control. The current coming from the generator is alternating current – accepted happily by the bulbs, but no good for battery charging. The battery requires direct current – and thus the alternating current has to be rectified. Bantams had an early Selenium solid state rectifier, a step up from the glass valves they replaced but not as efficient as a modern silicon rectifier, and rather prone to temperature related failures.

So what we need to do is take our generator coils and connect them to the rectifier before we hook them up to the bike. What we need though is to make sure the current can only flow in one direction, so we need a one way ‘valve’. Electrically it looks like this:

The alternating current from the generator is provided at the input to the rectifier, and plotted on an oscilloscope looks like the waveform at the top. Notice that the waves are alternately positive and negative. The rectifier is made of four devices called ‘diodes’ – labelled D1 to D4, which pass current in one direction. When the waveform is positive the input current passes to the output through diodes D1 and D2, but cannot pass through D3 and D4; so effectively D1 and D2 only give us the positive voltage peaks from the waveform. Conversely, when the waveform is negative the input current passes to the output through diodes D3 and D4, but cannot pass through D1 and D2; so effectively D3 and D4 only give us the negative voltage peaks from the waveform but since both pairs of diodes are connected to the same output, all voltage peaks appear as positive, giving us the waveform at the bottom.

Now, this is not true DC current, but since all peaks are positive, it will charge a battery.

And sparks…

Sparks are provided by another neat little Faradaien trick – it’s really the same one again, our friend electromagnetic induction, but there is nothing moving here except a magnetic field.

A clever American chap, by the name of Charles Kettering, invented a system to replace the hot tube ignition systems common in the first road vehicles.

Two coils of wire are arranged around a common core – that is, they are wound onto the same bobbin. One coil has a few turns of thick wire (called the primary winding); the other has tens of thousands of turns of thin wire (called the secondary winding). We arrange for a current to flow in the primary, from a battery (or perhaps a magnet rocking by, in the rotor of a generator), and we end up with a magnetic field in the core of both the primary and the secondary winding. Now, we thoughtfully arrange for an engine driven switch, the ‘contact breaker’ to control the flow of current in the primary winding. When we open the switch, the current in the primary no longer flows and the magnetic field disappears.

Now, remembering how movement of coils of wire in magnetic fields induces current in those wires due to electromagnetic induction, so does movement of magnetic fields in coils of wire induce current in those same wires. So when our field collapses, because we switch off the current in the primary, a current is induced in the secondary. Now, because we have arranged for the secondary to have many, many more turns than the primary, the voltage is very high (but there is not much current, which doesn’t matter.) But where does it go? It jumps the shortest distance it can to earth, as a fat spark.

All we need to do now is arrange for that spark to occur inside the engine (with a special device called a spark plug), and to get the voltage to the plug (with some cable capable of passing a high voltage without losing it), and we are home and dry.

Except for one small thing – remember we induced a current in the secondary when we collapsed our magnetic field? Well guess what. The same thing happens in the primary – it’s only small, but the spark it produces occurs at our contact breaker points and will destroy them if we don’t address the problem. To fix that, Kettering added a small device called a condenser.

The Condenser

An automotive condenser is basically two strips of foil, with an insulating strip between them, rolled up and placed in a tin. The wire is connected to one strip, the tin case to the other and putting a voltage across the two strips causes an electrical charge to be set up, rather like the static charge set up when you rub a balloon on the carpet.

They behave a bit like a battery in that you can charge the two foil strips, much like the two plates in a battery. When the points are closed, the condenser is shorted out and there is no charge across the strips. When the points open, the energy in the primary coil circuit has to go somewhere – it either cascades across the points, eventually burning them out, or it charges up the condenser. The energy is lost to earth when the points close.

Holding it all together - the Motorcycle Frame.

So having spent a lot of time talking about centres of gravity, forces, rake & trail it perhaps comes as no surprise that we might like a nice stiff structure to maintain the shape of this machine.

This is where the frame comes in. Of course it has a lot of other jobs to do but what it is mainly about is:

• Keeping the wheels inline
• Keeping the steering head in the right place

And then it has a lot of other jobs to do, mounting the engine, fuel tank and the saddle. The frame design is further complicated by the need to keep the wheels on the ground over bumps i.e., it needs to have the suspension system built into it.

So, stiffness is the name of the game. Look at this:

Marvel at the thickness of those two pairs of tubes bracing the steering head and connecting it to the swinging arm mounting at the back. Notice the cross bracing between those tubes and the cast & bolted structure in the centre – it’s huge. Then look at that swinging arm – those square tubes make a very stiff structure as well.

Now look at this:

That’s a swinging arm Bantam frame. It is a bit of an unfair comparison, since this is a lightweight commuter and the modern frame is a fast, high capacity sports bike. The point is not to compare modern design with 60’s design, but rather to emphasise the stiffness that is important in a frame.

You can see the headstock, where the forks pass through, with it triangular bracing. Whilst upright this will be relatively stiff, but because the tubes and headstock are all in the same plane it will bend when cranked over – hardly a problem for a Bantam, but this deign would see the wheels out of line on a bigger bike. See the relatively large tubes that go down to the engine front mounting and along to where the seat front is mounted, then curving and dropping down to the swinging arm. This, combined with the bracing effect of the engine, stops the frame bending in the middle – the modern frame is a much stiffer beam, with those big tubes and all that cross bracing.

Notice the lighter structure that supports the seat, and the top of the suspension units. Functional, but the modern frame with its single suspension unit is much lighter, because it doesn’t have this structure, and stiffer, because the rear suspension top mounting is attached to a very stuff area and not to a flexible outrigger.

Then look at the round section (cheap & easy to make) swinging arm and its mounting at the front – a simple tube welded across the main part of the frame. Horses for courses.

For completeness, here is a D1 Bantam frame:

Not dissimilar to the D14 frame, but more rigid around the rear end since the rear wheel is more rigidly mounted – there is no weakly supported pivot for the swinging arm. This particular example is from a plunger framed Bantam.

This one is a rigid frame, from a heavyweight Ariel:

Notice the strong triangular shapes around the steering head and the rear end, and the cross bracing at the rear. This was the state of the art for the rigid frame, built in the early 1950’s as manufacturers started to experiment with suspension systems.

Creature Comforts – the Suspension

For the purposes of accelerating and braking in a straight line it is important to have the tyres in contact with the road. Aerobatics, whilst exciting and dramatic are not really appropriate for the highway. Similarly, for the stability of the machine when cornering, contact with the road is essential.

Now, on a board-flat, dry racetrack we can fit a tyre with no tread. What is important to traction during accelerating and braking is the size of the tyre’s contact patch and the friction between the rubber compound and the road; clearly a tyre with no tread has more rubber in contact with the tarmac. Your tyre pressure is important here:

You can see that the correct inflation pressure (and this is related to the combined weight of the rider and the machine) results in the maximum contact area. But of course, we don’t ride around all day on dry, flat race tracks or even dry, flat roads – we have water to contend with.

Aquaplaning occurs when a combination of speed, tyre wear, tyre inflation or the depth of water on the road causes a loss of traction. Basically, a layer of water creates a cushion between the road and your tyres, drastically reducing the friction between the compound and the road surface.

Tyres are designed with grooves and ‘sipes’ to allow water to disperse away from the contact patch and to allow the rubber compound to maintain contact with the road. At higher speeds, the wedge of water in front of the tyres may pass right under the tyres with the result that the tyres will ride on a cushion of water. This is obviously not desirable since it results in a complete loss of friction between the tyre and the road…

But of course our wet roads are also not flat – we also have bumps to contend with. Newton’s Second Law of Motion tells us that “the acceleration of a body is directly proportional to, and in the same direction as, the net force acting on the body, and inversely proportional to its mass” which means in this context that if you exert a force on your motorcycles front wheel, via a bump in the road, it is going to accelerate in the same direction as the force was applied; and if it is a lightweight motorcycle it will accelerate more for the same bump. Now, having bounced your front wheel clear of the ground, your ability to maintain your tyre’s contact patch and your braking ability is going to be compromised, isn’t it!

What we need then is some form of spring suspension arrangement. Some of this comes in the form of tyres, and some in the front & rear springs.

As you will see from the foregoing, it is most important to keep the front wheel in contact with the road if we want to steer and to avoid falling off. Springs at the rear end are mostly concerned with your comfort and you will see many pictures in this book showing Bantams with no rear suspension. Whilst there are many incarnations of front suspension, the one we see today and on our Bantams is the telescopic fork – essentially a pair of tubes, arranged to slide inside one another and provided with a spring to absorb the energy from the bump which is all you need – except when we realise that the energy stored in the spring by the compression from the bump will be released again as soon as the bump is gone, and if it is of sufficient magnitude will result in the wheel bouncing off the road again. Thus we need a method of releasing that energy slowly, or damping it. In early Bantams this is not really addressed, except perhaps by excessive friction; later machines have hydraulic damping within the fork leg.

The Bicycle – or why you don’t fall off

You will have noticed by now that your average motorcycle is not laterally stable (combinations & trikes aside) in that if it’s not going along or propped up, your bike will fall over. Remember your first push bike? A push bike is a light weight, slow, slender wheeled single track vehicle not dissimilar to a motorcycle with a significant weight (the rider) sitting high above it. A motorcycle is a much heavier machine, faster, with wider heavier, wheels & tyres with the same significant weight (though sitting lower down) and a heavy engine & gearbox slung underneath. So:

• Push bike – light, slow turning wheels with a small angular momentum (mass x rotational velocity) coupled to a light frame with a high centre of gravity
• Motorcycle – heavy, fast moving wheels with a much higher momentum in a heavy frame, heavy engine & tank of fuel giving a low centre of gravity

You might also have notices that the front forks of your bicycle are not vertical, and that the axis about which they pivot is not vertical either. The angle made by the forks, compared to the vertical, is called the rake angle, Θ. You will have seen that where the tyres touch the ground they flatten slightly – and that these contact patches are directly under the axles. Imagine a line drawn parallel to the steering stem, all the way to the ground, through the forks and the front axle – and imagine the point at where that line touches the ground – it’s in front of the contact point for the front tyre isn’t it? The distance between the point at which your virtual line and the tyre contact point is called the ‘trail’, ‘T’.

Rake & trail, as we will see, are very important concepts.

Here is a simple thing – a plan view of a bicycle. Note that the contact points of the tyres are in line with the centre of the frame when the steering is straight ahead.

Now consider travelling at very low speeds – rolling you bike around with the engine off, outside the garage or wobbling about, learning to ride your new bicycle. You remember, on your first ride without stabilisers, how you turned the handlebars to and fro? That is your ears, brain & muscles acting as a system to maintain balance. As you turn the handlebars to the right, the contact point of the front tyre on the road moves to the left of the centre line of the machine (because of the rake angle).

Now, the centre of gravity is still on the machine centre line (assuming you are still in the saddle) so, if you carry on turning right you are going to fall off. So, you turn the handlebars to the left and the contact point of the front tyre moves to the right, as the centre of gravity stays on the machine centre. You stop falling to the right and start falling to the left. So you turn the bars to the right… need I go on?

Your brain of course being a rather smarter machine than your average desktop computer learns that it can control the amount of movement of the bars and starts to compensate the instability with bar movements that are eventually so small and instinctive that you don’t notice yourself making them. You can ride a bike!

I’ve mentioned the centre of gravity a few times now. Centre of Gravity (CoG) is the point at which the weight of an object, or system of objects, acts. The bicycle itself is light – 10-20kg or so and its own CoG is probably just above a line drawn between the two axles, more or less mid-way along:

The CoG is the funky yellow and white circle. The rider however, be he a stout chap riding his bicycle back from the pub or a skinny teenager on his way to school is going to weigh anywhere between 40-100 kg, and he is sitting up high on the seat. Together, the CoG of the bicycle and rider are much higher:

Now, if we compare that to a motorcycle:

We can see that because the motorcycle is relatively heavy compared to the bicycle, the combined CoG is much lower. He’s also changed his beret for a peaked cap.

So, two things happen:

• The motorcycle is much more stable because the CoG is low
• The motorcycle is much more stable because it is much more massive – to move massive loads we need greater forces, because inertia increases with mass

But there are other forces at play, more significant to massive motorcycles than spindly bicycles.

I mentioned wheels a couple of times. A turning wheel behaves in a peculiar and interesting fashion if you try and move it sideways. Look at this diagram:

This is the key to your stability at speed. It is called ‘Gyroscopic effect’ or more properly ‘gyroscopic precession’.

The key to it all is the concept that a force and subsequent deflection made to a given point on a moving wheel rapidly moves around the wheel, to a point on the other side of the steering head where it acts in the opposite direction, thus cancelling itself out. There are a number of scenarios to consider:

• Steering, where you turn the handlebars
• A shift in your position in the saddle, which moves the centre of gravity and causes a lean
• A bump, which might have either effect

So, going back to our stability discussion over stability and mass. Looking at Figure 10‑1, we can see an initiating force, the yellow turning arrow on the left. This is the rider attempting to turn the handlebars – without leaning over. The resultant force, shown in green in the middle diagram, serves to deflect the front of the wheel to the left (as the rider would see it. The amount of movement (the magnitude of the deflection) depends on the magnitude of the force and the mass of the wheel – so if the force increases, the deflection increases and if the mass increases the deflection decreases, and you can see if you put the same force on a bicycle wheel you will get considerably more deflection than you would on a heavier motorcycle wheel.

But the interesting bit is that because it is rotating the deflected ‘part’ of the wheel moves from a point in front of the steering column to a point behind the steering column, assuming the rider applied the yellow initiating force to the handlebars momentarily, or that the yellow initiating force came from a bump in the road acting on the tyre. Of course, behind the steering column that force & deflection becomes the blue resultant force, which is in the opposite direction – and cancels out the green effective force, thus centring the steering.

A similar sequence of events happens when the rider provides a yellow initiating force by leaning, or moving his body & thus the centre of gravity, as shown in Figure 10‑2. The initiating force is effective as the green arrow at the front of the tyre, ahead of the steering column & thus resulting in the yaw, the blue resultant, turning the motorcycle in the same direction as the lean.

The next step is to talk about Holding it all together - the Motorcycle Frame.

Nomenclature

What are all those bits called? Here is a series of pictures to help you get the words right. You are looking at a British bike here – our cousins in foreign lands may be used to different terminology, but when you are buying parts from UK suppliers it helps to know what words we use here in Blighty.

Next we will move on to talking about The Bicycle – or why you don’t fall off.