Now that the Formula One season of racing is under way we are once more hearing explanations of why drivers do not win, as distinct from hearing why the winner did win. Sometimes it is a straight-forward explanation like “I crashed” or “the engine blew up” but quite often it is a semi-technical reason that tends to blind the reporting journalist with science and remarks are quoted verbatim without any effort to analyse them or even understand them. Such observations as “I had too much understeer” or “the car wouldn’t turn into the corners” or “the car won’t come out of the corners”, while there are frequent references to “too much wing” or “lacking speed down the straights”. Tyres get blamed with comments such as “the front tyres went off” or “there was no adhesion at the back”. All these remarks are valid, but it might be interesting to look more deeply into some of them, to find out what they really mean and what the effect would be on the car. To this end we will consider handling characteristics in a form evolved some twenty-five years ago; the basic principles, as far as we understand them today, are unchanged though there could be changes in the near future.
In fairly simple and straightforward terms, what a car does and why can be described in technical language, and can be illustrated by an algebraic equation or it can be set up on a computer programme, as ELF Team Tyrrell intend to do, but of more interest to those of us without degrees in mathematics is that what a car does can be illustrated by a fairly simple graph. We must accept some of the basic principles first of all, and one of these is that when a tyre is deflected from the straight-running path it assumes an angle to the direction of travel, due to characteristics of tyre construction and of rubber. This angle is called the “slip-angle”, and no matter whether a car is being cornered fast or slow, sharply or on a great radius, slip-angles will be generated to varying degrees. Cornering gently at touring speeds will generate small slip-angles, while racing drivers trying hard will generate large slip-angles. For various design reasons involving steering and suspension geometry, which we need not go into, a racing car does not have to generate the same slip-angles at the front as it does at the rear, although it can do so if desired. If the slip-angles of the front tyres do happen to be the same as those of the rear tyres, then the car is in a condition known as “neutral-steer”. If the slip-angles of the front tyres are greater than those of the rear tyres when cornering, then the car is in an understeer condition, but if the front slip-angles are smaller than those of the rear tyres, then the car is in oversteer. This is a very simplified evaluation of the three conditions of cornering under which a motor car may exist. There are numerous factors affecting these conditions, ranging front geometry of suspension movements, through weight distribution and weight transfer, to aerodynamic effects and the modern fully-adjustable racing car can be set to achieve any of the three conditions, understeer, neutral-steer, or oversteer or a combination of any two or all three. Consequently, the driver who can work and communicate well with his designer can have his car adjusted to do whatever he wants. The big problem is that of the driver deciding what he wants and knowing it when he has got it, but that is a human problem that we cannot illustrate on paper.
The aim of the racing driver is to get round corners as quickly as possible, whether they be slow, sharp corners or fast, gentle curves, therefore he seeks to have all his tyres gripping the road as well as possible. As the car corners the tyres resist the side-thrust generated by centrifugal force acting on the weight of the car and the more side-thrust the tyres can resist the greater can be the cornering force. This force we will refer to as Cf and it can be the side-thrust resisted by one tyre or all four tyres (or all six on the Tyrrell P34), depending on what we are discussing. It is also important to be able to evaluate the Cf of the front tyres alone and the back ones alone, but for our purposes we are only interested in the total cornering force, whether from four or six tyres. It would appear that if all the tyres could generate the same Cf then you would automatically get the maximum cornering force possible, which would appear to be desirable. For various reasons beyond our control this is difficult to achieve, and if you did achieve it you would have a neutral-steering car, because the value of Cf is directly related to the slip-angle generated by the tyre. For various practical reasons a constantly neutral-steering car is not the most desirable thing to achieve, mainly because of human limitations in the driver. Because conditions on racing circuits are never constant for very long it is better to have flexible Cf values as one end or the other of a racing car, for has the driver with suitable instincts and reflexes can generate a higher total Cf value with greater safety and stability than an inbuilt high Cf value due to neutral-steer, especially when operating in the maximum area where the tyres are reaching their natural limit of adhesion.
In Fig. 1 is shown the basic “handling characteristic curve layout” and on this graph can be illustrated exactly what sass will do when cornering, or what the driver would like it to do. The horizontal line 0—Cf represents the value we call Cornering Force, increasing from left to right. Cornering Force is related to the speed of the car and the radius of the corner it is taking, or in other words centrifugal force. It is not solely related to speed, nor is it solely related to corner radius, as is often mistakenly assumed, but is a product of the two. The point 0 is where there is no cornering force in action, which means the car is stationary or it is travelling in a straight line. As the car takes a comer the value of Cf begins Is rise and we move along the horizontal line to the right. As we have seen, oversteer is the condition when the rear slip-angles are greater than the front slip-angles and understeer is vice-versa. The vertical has on the left of our graph represents the difference in degrees of the front and rear total slip angles. Subtracting the front slip-angle value from the rear value will leave us with a positive value for oversteer, represented by the vertical line above our horizontal line, and a negative value will give us understeer, represented by the vertical line below our horizontal line 0—Cf. Therefore the area above our base line represents the condition of oversteer and the area below represents the condition of understeer. It should now be obvious that the has 0—Cf is representing the condition of neutral-steer, being the condition between the other two, and the fact that it is represented by a line and not an area, shows that the condition of neutral-steer is a very fine one.
In the upper area the dotted line 0-A represents the characteristics of a car with oversteer at all times, that increases continually as the cornering force increases. The solid line 0,—A illustrates a similar car with the difference that it has oversteer even when no cornering force is applied, which is to say that it oversteers when travelling in a straight line or is practically stationary. This of course, is absurd and purely hypothethical, but it illustrates the point that a car can have a handling characteristic curve that need not start from the point O. Such a car would be inherently unstable and such cars have existed; they were unstable when travelling along a straight road, in other words they could not travel in an exact straight line. A road car with a heavily laden boot and soft rear tyres would generate this condition. Similarly, in the lower area we can illustrate a car with continually increasing understeer, as shown by the dotted line 0—B, while 0,- B illustrates a similar car with a basic amount of understeer when no cornering force is acting upon it, namely when travelling in a straight line. This is a basically stable condition and for practical purposes we can assume that any car which will tread in a straight line without the need to make corrections on the steering has a basic understeer, which means that the front tyres are generating greater slip-angles than the rear. This characteristic is very suitable for normal road cars, especially if they do not achieve high values of Cf, but this is not desirable for a racing car operating at high Cf values to the right-hand end of our base line 0—Cf. With the conventional rear-drive car with steered front wheels, the driver has various things he can do to vary the value of Cf at the front or rear, or both, and this means he can affect the line on the curve, whether it is 0—A or 0—B, but because of the basic layout of the conventional racing car he can have more affect on 0—A than he can on 0 – B, therefore it is desirable to have your car in the upper area, or oversteer condition, at high Cf values. As we have seen that a basic condition of a certain amount of understeer is desirable for straight-line running, it is now obvious that we have got to have our racing car changing front the understeer condition desirable for basic stability to a state of oversteer at the higher cornering forces in order that the driver can maintain control as he nears the limit of adhesion, and it is this change-over point that is critical in the overall balance of the car. On our “handling characteristic curve” this point of change-over is marked M, and it can be almost anywhere along the base line. In Fig. 2 is shown the handling curve for an average saloon car that starts all at 0, with an amount of inbuilt understeer to ensure straight-line running, which increases as the cornering force increases, but that changes to a state of oversteer, due for example to reduced cornering force on the rear tyres caused by roll and weight transfer. Now obviously, if the car is going to change its characteristics from understeer to oversteer fairly suddenly then the handling curve line will cross the base line at a large angle. This angle is called Theta (0) and is an important factor in our analysis, as is the point M at which it occurs.
We all know the type of family saloon illustrated in Fig. 2. It goes nicely in a straight line and starts cornering all right, with a fair amount of steering lock on, but as we increase the cornering force, either by increasing speed or reducing the radius slabs corner, or doing both; then we put on more steering lock as we get further from our base line. There arc numerous design factors that can now affect the car, such as suspension geometry being altered by the leaning (or roll) of the car, or the effect of weight transfer, and if these reduce the effectiveness of the rear tyres and not the front tyres, the car will head all up into the area above our base line, which means the tail has begun to slide outwards and we have to unwind the steering and put on corrective steering lock. If our car makes this change in a smooth manner, allowing the driver to turn the wheel and keep pace with the change and thus keep everything in balance, then it means that the value of tho angle is small. If it all happens at fairly small cornering forces then it means that the point M is not very far along the base line 0—Cf, but it will be appreciated that the nearer the point M is to our base 0, then of necessity the angle will be greater, and the converse applies. If we are going to have any initial understeer at all then M cannot be very close to 0, otherwise the angle will be steep. If we have the point M a long way along our base line, it will give us a shallow angle 0, but it will mean that the car is approaching the limit of the value of Cf, which is a dodgy area to be in, for if you go over the maximum of Cf then it zeroes fairly abruptly and you are left with lots of speed and no adhesion, which spells disaster. If our family saloon does not have any outside features to reduce understeer and change it into oversteer it will follow the dotted line 0,—B in Fig. 2, ending up in terminal understeer as front-wheel-drive cars tend to do. In all normal road use, the production car never gets very far along the base line, as the average owner does not generate a very high value of Cf under normal motoring but the enthusiastic chap having “a bit of a go”, will get to the right-hand side of Fig. 2 and begin to wish he hadn’t, or that the car had been designed with different terminal characteristics.
In Fig. 3 is shown the handling line of a racing car in which it is kept stable by a small amount of understeer for as long as possible to that the point M is a long way along the base line, and when the car does change tit oversteer it does to very gently, with the angle being nice and small. However, by this time the cornering force is at a very high value and tho ultimate limit of adhesion is being approached, so the driver must use his controls, steering and accelerator, to try and keep the oversteer down as close to the base line as possible as the limit is reached, and he must reduce the value of Cf before the limit is reached, or he will lose all control, at point A. He is aiming to keep the car as far as possible along the base line to the right, without actually reaching the end. If our racing car is on a terminal understeer course, as shown by the dotted line in Fig. 3 the situation deteriorates steadily and increases rapidly at the end. This you often see as the front wheels lose adhesion and on full lock the car ploughs off the track in a straight-line. It is arriving at B in Fig. 3. If it suddenly spins and leaves the track backwards then it is at point A in Fig. 3 and the point M was very close to the right-hand end of our base line and the angle was steep.
We can now begin to see what the racing driver is trying to say in our opening paragraph. If he is complaining of too much understeer he means that the line of his handling curve is sinking too far below our mean line 0—Cf and if and when it returns, it will cross the neutral line at too steep an angle. This is what is wrong when a car is described as “turning nasty in the middle of the corner”, the angle is too great and in all probability the point M is a long way to the right, which automatically means that understeer built up to too large an amount before the characteristics changed. Too much initial understeer is the cause of a car not turning into the corner as the driver would like it to, while such a characteristic can also cause high rates of wear on the front tyres, and as the cornering force of the front tyres diminishes with the wear (or the temperature) the car deteriorates into a characteristic that would follow the dotted line 02—B in Fig. 3; if the driver persisted in pressing on into the realms of high Cf he would soon find himself “ploughing” off the track nose first, even though he might have the steering on full lock. Some drivers and designers get their adjustments so wrong that the car can never settle to one side of the neutral-steer or the other and it wobbles from small increments of understeer to small ones of oversteer. It fluctuates back and forth across our base line, generating a number of points M as the cornering force increases, and this is to be avoided, if only for the peace of mind of the driver, though you often see cars doing this.
The shape of the “handling characteristic curve” can be altered by a great number of things and the art of the driver and designer is to choose the right ones for the job. Tyres, naturally, play the largest role and you can alter sizes, rubber compounds and, more importantly, construction, front and rear and side to side. Front and rear aerodynamic devices can be used to load individual tyres more than the static weight does, while static weight distribution plays a vital role and fuel load is very important. Some cars will alter their handling curve quite drastically between full tanks and empty tanks and it is up to the driver to decide whether he wants the car to handle nicely at the start or the end of orate. Anti-roll bars, springs and shock-absorbers all affect weight transfer under cornering forces and these are important choices to make. It is virtually impossible to make the perfect choice of all the variables, but the clever driver is the one who can get more right than his opponent, and equally it is impossible to make the car handle perfectly on all corners on a circuit, so the good driver is the one who can decide which corners arc important and make sure the handling is the way he wants it for them. It is just as important to decide which corners can be discounted and if the car feels bad on them, to appreciate why and to know what can be done to make up for the situation. The driver can deliberately provoke a car into an initial oversteer or understeer, but what happens after that may well he out of his control if he has set the car onto a handling line that is in opposition to its natural characteristics. Also he can exaggerate any part of the natural handling curve of his car, by use of the steering wheel or the accelerator, or even the brakes, or a combination of all three, but he is treading on dangerous ground and may, unwittingly, put the car on to part of its basic curve that is a had characteristic or an unstable one.
There are maany more aspects to this matter of racing car handling, that space does not permit us to deal with here, but I hope this brief grounding will enable the reader to picture graphically, what a car is doing as it goes round a corner, especially when it is approaching the maximum cornering force at the limit of tyre adhesion and road surface.—D.S.J.
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