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.