Although some Clubs own timing apparatus that is used for sprints and hill-climbs that will record to three places of decimals, or one thousandth of a second, the official RAC/ACU timekeepers will only time standing-start record attempts to two places of decimals or one hundredth of a second. The reason for this is that it is impossible to line up a vehicle, or a motorcycle, to any real degree of accuracy on the starting-line. George Hall, who does most of the record-attempt timekeeping these days carried out some experiments on the effect of the position of a vehicle relative to the timing light-beam at the start of an acceleration test. Officially you are allowed 10-centimetres “run-in” to the light beam, for obviously you cannot be lined-up actually on the beam, and everyone takes full advantage of this 10-centimetre rule.
The standard procedure is to use a wooden or metal gauge that you place on the line of the light-beam itself and the foremost part of the car or motorcycle is brought into contact with the gauge. When the gauge is removed and with a chock behind a rear wheel, the vehicle has the allowed 10-centimetres “run-in” to the beam. Hall has calculated from his experiments on initial accelerations that a misalignment of 1 millimetre would give a timing error of .0007 sec., which sounds negligible, but if the alignment error was 10 millimetres, or 1 centimetre, the time error would be .0069 sec., a not insignificant amount on a standing-start quarter-mile and a large factor in timing to thousandths of a second. While no-one is likely to be 10 millimetres out in aligning a vehicle they are equally not going to be accurate to 1 millimetre. Hall’s conclusions were that the human error in alignment, i.e. placing the 10-cm. gauge correctly on the beam-line, positioning the vehicle and holding the chock, could not be to a better accuracy than about 5 mm., so that measuring time to more than 0.01 sec. was not justified. Naturally you can get a record quoted to three places of decimals because all records are the mean of two runs in opposite directions, so that the addition of two times to hundredths of seconds can give you a mean-tittle by division to a thousandth or third place of decimals. At the moment there is no known method of alignment for a standing start that can guarantee complete accuracy, so that times for acceleration tests given to three places of decimals are worthless.
It has been suggested that alignment for a standing start should be done by a secondary beam, as used in dog-racing, but tests have proved that it is not possible to reduce the error to even 1 mm., and that is not sufficient to justify one-thousandth of a second time apparatus, always assuming the clocks had been officially checked to .001 sec. These observations are based on the calculation that even with 1g acceleration a vehicle takes 0.1428 sec. to cover the first 10 centimetres, and would take 0.1435 sec. to cover 10.1 centimetres, which gives us our error of .0007 sec. per millimetre.
For flying-start records it is another matter altogether for there is no question of any fixed distance before the first beam and beam-timing apparatus can read to one thousandth of a second quite easily.—D.S.J