Steering and handling

While engine performance can be quite clearly defined and measured, the properties of handling, roadholding and ride comfort are not so easy to deal with. The sum total can usually be valued in lap times, or simply in terms of how pleasant the machine is to ride, but often it is not clear what is contributing to what. How is it that one bike is so much easier to handle than another when, superficially, they are so similar?

Much of it is subjective – which does not make it any easier to measure – and what suits one rider does not always suit another. Expressions such as ‘slip’, ‘slide’, ‘oversteer’, etc., often mean different things when they are used by different people. (The definitions of all these terms, as they are used in this book, are given in the Appendix.) On top of all that, the things which control the suspension and the steering are closely related; change one and you stand a good chance of changing two or three others at the same time. This manages to do away with the first rule for test work – only change one thing at a time.

To take a simple example, suppose you decided to change the rear spring. You would expect a softer or a harder spring to give a more supple or a harsher ride and so change the ride comfort. But it could also change the ride height, depending on the length of the spring and its pre-load. This would alter the ground clearance and may restrict cornering speed even though the comfort and the feel of the bike had been improved (with a view to raising the cornering speed). Lowering the back of the bike has the effect of raking out the front and increases the trail slightly; this makes the steering a little heavier and makes the bike more stable. Raising the back has the opposite effect, it reduces the trail and makes the steering lighter and faster to respond. For a given suspension force, the new spring will deflect by a different amount, altering the speed of the damper and therefore the damping force it generates. So just by changing the spring rate you could make noticeable changes to the way the bike handles and steers. You could also alter its straightline stability and its cornering clearance.

Obviously, any changes have to be worked out quite carefully, so that they do what you intend with no unwanted side effects.

With all of these factors – trail, stability, spring rate, etc. – there is an optimum level, which matches the rest of the machine, the rider and the road or track conditions. Where a new design, or a seriously modified machine is being developed it is necessary to work out a series of tests which will produce the best blend of chassis and suspension. On the other hand, if the work is aimed at curing a specific fault, such as weaving or wheel patter, then experiments on a least effort/least cost basis will be as good as any other method. Because one change can have so many effects it is rarely possible to predict the results completely accurately and so test riding is essential. It also makes handling and stability faults very hard to trace; in fact it is often easier to try to make the fault worse so that the responsible part can be identified.

And when it is all finished, one rider may like the resulting ‘feel’ while another may not. A third rider may not care; there is a lot to be said for employing a rider who can go fast on anything, but do not employ him as a development rider.

In some cases the feel of the bike simply makes it more pleasant to ride. In other cases it can make the bike faster because it is able to turn faster, to brake harder or to accelerate earlier, etc. Although it is difficult to measure the individual contributions, the sum total can be seen in shorter lap times, reaching a higher speed at the end of the straight or being able to brake later. On a road bike it is usually enough just to make it feel better, small gains in cornering speeds do not make much difference to journey times. However, it follows that a bike which is capable of greater performance is safer to use at the original level of performance and will not put as much stress or fatigue on its rider.

Figure 1.1 Steering the front wheel makes the machine turn about the instant centre where the axes of both wheels cross. The effect of trail moves the front contact patch, so the centreline of the bike – and the bike’s centre of gravity – is no longer directly above the line along which the bike is supported. The effect of steering castor also makes the front wheel lean (exaggerated in the diagram), so it will generate camber thrust

The way in which a bike steers is the focal point of its handling and its stability. At very low speeds, a bike steers by turning the front wheel into the corner. With the bike essentially vertical, the trail (see Appendix) shifts the front tyre’s contact patch to the left in a right turn. This means that the centre of gravity is now to the right of a line drawn between the two contact patches – the line on which the bike is supported. It would, therefore, try to fall over to the right.

At the same time it is moving in an arc to the right; the centre of this curve would be where lines drawn through each wheel spindle cross one another. It has acceleration (centripetal acceleration) towards this point, even though its speed, as recorded by a speedometer, is constant. The acceleration is v2/r, where v is the speed and r is the radius of the arc which the bike is following. The force which provides this acceleration is mv2/r, where m is the mass of the bike, and it is generated at ground level. The reaction caused by the inertia of the bike equals this force but is in the opposite direction, away from the centre of the turn (centrifugal force) and is considered to act through the bike’s centre of gravity – which is well above ground level.

The centrifugal force sets up a couple which tries to make the bike fall over to the left. The strength of this couple is y mv2/r, where y is the height of the centre of gravity. The couple trying to make the bike topple to the right is mgx, where mg is the total weight and x is the amount the centre of gravity has been displaced from the bike’s line of support.

When the bike is turning in a balanced fashion it does not fall over, so these two must be equal:

or:

Now g is constant, and so is y for a given bike, while x depends on the steering geometry and the steer angle, and r also depends on the steer angle. For a given bike and a given steer angle, rgx/y is constant. There can be only one value for the speed v which will satisfy this and keep the bike in balance (there are two actually, +v and −v, because the mathematics allows for you to be able to ride backwards at the same speed).

If the rider goes slower, then ymv2/r will be too small and the bike will fall to the right, into the turn. If the rider goes faster, ymv2/r will become too big and the bike will fall to the left, away from the turn.

As an aside, the rider can shift the centre of gravity, especially on a very light bike, by standing up (increase y) and by leaning his body to one side (increase or reduce x). So by making x and y variable, a trials rider can give himself a range of speeds for which the bike is balanced in a given turn.

If v is steadily increased and the height of the centre of gravity stays the same, then the term rx will also have to be increased. To increase x, the steering has to be turned further into the turn, but this action reduces r, the radius of the turn, so we will quickly reach a value v for which the steering geometry cannot cope. It happens, on conventional machines, at something in the region of 2 to 4 ft/s (1.5 to 3 mph), but it is worth remembering that this critical speed exists.

Figure 1.2 The cornering force F is generated at ground level, while the reaction of the bike’s inertia R acts through the centre of gravity. The two are equal, so F = R (= mv2/r), and they try to overturn the bike to the left, with a couple Fy. The centre of gravity is also displaced a distance x from the axis through the tyre contact patches and this tries to overturn the bike to the right, with a couple mgx

If the speed is too great – as it must be if the bike is to exceed 4 ft/s – then the effect of a right steer effort is to make the machine fall, or roll, to the left. So now we have a bike travelling at very low speed, but more than 4 ft/s, and the result of applying a right steer angle is that it rolls left. The immediate effect of this is that the centre of gravity is now displaced to the left of the line on which the bike is supported, and both wheels are leaning (or are cambered) to the left.

So far our bike would have steered and generally behaved as predicted if the wheels had simply been thick wooden discs. Now the tyres begin to do something: they generate what is known as camber thrust. Because the tyre can deform to a flat...