[Will the new silencing regulations restrict motor cycling sport or development ? The following article analyses the salient points of the “Six-to-One” order.]
The few jaded inhabitants of Weybridge who are endeavouring to jeopardise the most exhilarating and useful of sports, have placed the Brooklands track authorities in an awkward predicament.
Embarrassed by the “lightning strike” of competitors on May loth, and finding themselves faced with the alternatives either of going back on the temporary promise given to these clamorous inhabitants, or of having the British Motor Cycle Racing Club cancel the remainder of its 1924 programme, the B.A.R.C. sought to hit upon a happy medium by subjoining to its previous Supplementary Regulation (No. 21) a further statement of inhibition. The entire clause as governing the silencing of machines using the track now reads as follows :—
SUPPLEMENTARY REGULATION No. 21.
” EXHAUSTS.—All motor cycles competing in any race shall be fitted either with a first receiver close to the engine and an exhaust pipe therefrom reaching as far as the back axle, or with an exhaust pipe and efficient silencer.* The first receiver mentioned above must be of a capacity not less than six times the volume swept by the piston of one cylinder of the engine, and if cylindrical, shall not be of greater length than six times its maximum diameter, and if of irregular shape, of equivalent proportions. The pipe leading therefrom to the back axle must extend into this first receiver not less than two inches, and shall not be of a greater area than the exhaust pipe from any one cylinder at its outlet from the engine. It shall not be opposite the inlet pipe to the first receiver in order to prevent a straight through flow. Baffle plates may be added if desired.”
*The clause as it was originally conceived stopped short at this point.
The above regulation in its present form came into force immediately upon its being proclaimed. It will be observed upon careful perusal of this regulation as a whole that it can be invested with myriad interpretations of widely divergent natures. Taken literally, all one is required to fit is “an exhaust pipe and an efficient silencer.” There is no stipulation as to dimensions.
However, the first receiver clause is the one on which the greater number of competitors are now basing their structural embellishments, albeit they are not taking into consideration the fact that it is instructed that “the first receiver” shall be “close to the engine.”
Read in this way, the new regulation necessitates a silencer of 1,500 c.c. for 250 c.c. power-units, of 2,100 c.c. for 350 c.c. cylinders, and of 3,000 c.c. for machines of 500 c.c. In almost every case representatives of these classes on Brooklands have a single-cylinder engine.
However, according to this regulation, a 500 c.c. twin would need only one silencer (of 1,500 c.c.) and is thus no more handicapped than a 250 c.c. single, whereas a 500 c.c. single would require a silencer of double this capacity (3,000 c.c.). A significant feature of this additional proviso is that the bore of the pipe leading from the engine’s exhaust port decides the diameter of the ultimate outlet of the silencer. A few 350 c.c. single-cylinder machines have each a primary exhaust pipe of greater diameter than those of many of the 500 c.c. or I,000 c.c. engines and should therefore be less handicapped in the matter of maximum speed, by fitting the required regulation silencer, than any other class of machines.
It is almost certain that the smallest efficient silencer will be employed in every case, in order to minimise the effects of back pressure, extra weight, and wind resistance, and in this connection it is interesting to note that a 500 c.c. single must not use a silencer smaller than 3,000 c.c., whereas a 500 c.c. twin need not employ one of greater capacity than 1,500 c.c. And this, notwithstanding the fact that the two types of engine discharge the same amount of exhaust gas every two revolutions.
From this it will be seen that the single has ‘a silencer which allows twice the expansion permitted the twin. Unless the bore of its primary exhaust pipes be as large as that of a 500 c.c. single, the 500 c.c. twin will be at a slight disadvantage since the outlet from the silencer of the twin must be smaller if its primary port is smaller.
Probably such machines as the 588 c.c. Norton will feel the effect of this regulation as seriously as any, since they will have to be fitted with enormous silencers of 3,528 c.c. Another martyr, it seems, will be the 600 c.c. Triumph, which will have to carry a silencer of no less than 3,600 C.C.
Recently the well known Douglas exponent, Cyril Pullin, has been experimenting on Douglases with various silencers built to meet B.M.C.R.C. requirements, and he has proved that these regulations detract 3 m.p.h. from the maximum speed of the 746 c.c. machine and 4 m.p.h. from the 494 c.c. engine. Subsequent experiments have modified these results so that with the improved silencer the engine is much quieter and a loss of speed of only 1 per cent. is suffered. Also, the silencer occasions only such a rise in the temperature of the valves as can be adequately dealt with.
In Mr Pulin’s own words “A rough analogy of the silencer is, a diverging rectangular pipe 2″ by 4″, eight feet long, expanding to If by 4″ and which is curled round similar to the shape of a snail and fitted with two deflecting baffles at the extreme end of the pipe, which turn the gasses into two nozzles so formed as to discharge into the ‘ slip-stream ‘ of the silencer when passing through the air. It will thus be realised that the gasses are, firstly, given a rotary motion, and secondly, gradually expanded and cooled during the process.”
We understand that further improvements are now in hand which it is hoped will result in even greater efficiency. Nevertheless, these experiments must of necessity take time, meanwhile, the regulation is bound to have its effect upon record-breaking by British machines.
From the foregoing it may be inferred that the British manufacturers who succeeded in breaking British records before these new new came into force were fortunate, for these I are likely to stand until there has been time for certain research work to mature, whereas world’s records will probably be captured by foreign-built machines during the next few months. It may confidently be anticipated, however, that these foreign triumphs, if they are secured, will endure but for a very short time.
There is very little doubt that, had the B.M.C.R.C. not given the silencing question its attention, the Weybridge residents would have sought to bestlw upon it the attention of the Courts. In this event their chances of success would be a matter of opinion.
There are those who dismiss the whole topic with a Parisian shrug and the laconic remark that “probably the track would have been closed to motor cycles—that’s all.” But these people evidently have not regarded the matter from its broader aspects.
It is said that the majority of the Weybridge and district agitators have taken up their abode there since the track was opened in 1907. In this case locus standi, one may say, would not appear too secure.