Most of the testing that I do is development so that one is given a car that the resident tester has developed as far as possible and it is handed to you to see if you can polish off the last second or two to make it really competitive.
Denis Jenkinson in his book The Racing Driver talks of his "tenths" system, and using that as a grade, I would say the average tester can cope with up to eight tenths, whereas a racing driver goes nine and sometimes ten tenths. Obviously if a new car is running only to eighty percent of its limit of adhesion it is never really in the realms of how it will be used in racing. There is always an exception to every rule and Uhlenhaut of Mercedes-Benz provides this particular one. But normally it is the racing driver tester who must push the car into that final twenty percent.
One does so with a certain amount of care. If a pilot goes out to test a new aeroplane he has gauges to tell him how fast that aeroplane will go and he knows if he goes over mach .92 that the thing will go into a high speed stall and the wings will fall off - for that is something which can be calculated. In a racing car you cannot calculate. No designer can say that this car will lap Silverstone at 117 mph and that at 118 mph something is going to happen, because the waveband of one driver to another is incalculable.
Facts and figures cannot be written down as they can with an aeroplane. Therefore one approaches things fairly easily, and the first thing one does on pulling away is to put the brakes on lightly to see if they pull either way. Then you stand on them hard to see if they pull, or maybe lock on. Then one goes into a corner quite smoothly at a reasonable speed to see what the tendency is, without using too much throttle because you can usually oversteer with a lot of throttle. Then one tries the same thing with too much throttle. All these things you do well inside the car’s limit.
One sets out to provoke the car by doing things to it more viciously and violently than you would normally, so that you can gauge the characteristics of the machine. For instance, if you give the wheel a tremendous wrench to give it oversteer and you lose it at a relatively low speed you realise that you must be very careful up near the limit because it has a quick break away. If on the other hand it is, like most racing cars, reasonably easy to handle until you get up to the ultimate, one tries out the same exercise at increasingly higher speeds.
Once the characteristics of the car are established, which is the first step, one calls upon past experience to make suggestions to improve its drivability. Certain traits will suggest to you certain means of dealing with them, not just the obvious ones like playing with tyre pressures or sizes to overcome too much over or understeer.
Another problem is whether you are setting the car up for yourself or testing it to improve it all round for other drivers. In the latter case one has to get rid of basic faults, try to get the steering ratio and the gear ratios right, effect the best available compromise to which the works driver will add his own final touches. Some like oversteer, some understeer. Roy Salvadori, for instance, likes high tyre pressures, and he would therefore find more modifications to make on the Aston because of this.
The whole thing is trial and error, backed up be the driver’s feeling. There are so many variables and combinations. Damper settings, roll centres, wheel angles, roll bars, tyre pressures and so on; from the handling point of view alone these must all be tried one way and then another and the permutations are endless to produce the ultimate balance or drivability, which one must have. No car is perfect, but when you are on the limit, which in racing driving you are, the important thing to know is what the car is going to do so that you can take advantage of its good points and watch out for its bad ones.
In the actual case of the Grand Prix Aston a number of alterations were made and we found the car handled far better with single rather than double acting rear dampers, and stiffer fronts. We started off doing 1 min 34.8s and by our process of trial and error brought the time down to 1 min 29s.