CARAVAN BRAKING.

Auckland Star, Volume LXV, Issue 239, 9 October 1934, Page 16

CARAVAN BRAKING.

SURPRISING THEORY.

EXPERIMENTS IN ENGLAND,

LAWS OF GRAVITY,

Matters concerning the laws oi' momentum, of acceleration and deceleration are apt to become somewhat technical. Ono starts an argument by postulating a weightless body of infinite mass, running in a vacuum on friction - loss bearings, or with some other such supposition. Or when one considers braking, one may find oneself confronted with a, theoretically perfect surface, only bestrewn with Greek symbols and other fauna and flora. Yet Mr. L. H. Dawtrey, A.M.1.A.E., has succeeded in making out a case, only resorting to some of such artifices, for the surprising theory that a car with a caravan in tow will stop more quickly than a car running free by itself. What is_morc, he backs up his theory with practical tests, undertaken with a standard model car. Acceleration and breaking forces are closely allied. Acceleration and deceleration both represent an increase or decrease of speed with respect to time. Allowing Mr. Dawtrey sonic theoretical matters to start off bis argument, if an object at sea-level is acted upon by a force equal to its own weight, it accelerates, if free, at the rate of 32.2 ft per second; in other words, a rate equal to the force of gravity, or 22 m.p.h. every second. If the body were not stationary, and this force were brought to act upon it in the opposite direction to its motion, then it would decelerate at the same rate. Tho magnitude of acceleration or deceleration depends upon the ratio of the force to the mass of-the object. These arguments are based upon the forco of gravity only, and upon some of the other suppositions contained in the first paragraph of this article, but in the case of a car there are other things to consider. There is the force supplied by the power of the engine for acceleration, which will be assisted by gravity on a downhill gradient, and there is the retarding force of the brakes, assisted by gravity on an uphill gradient.

One hesitates to mention sueh mundane matters as gradients, for a. theorist usually only considers level and frictionles<s surfaces. There is also the coeflicicnt of friction of the tyres with the road surface, and it is over this point that so many theories remain merely theories, without fulfilment in practice. It is difficult to define or measure the coefficient of friction, and for purposes of argument it is often written down as 1 or unity— that is to ea-y, a retarding force of 1001b will be required to lock the wheel of some such moving object as a bicycle weighing 1001b. This represents 100 per cent braking efficiency, and using this value of the coefficient of friction, one should be able to stop an object travelling at 30 m.p.h. in 30ft. The only thing is that the coefficient of friction docs not in practice ahvavs equal 1, and this is the reason why" in tests of cars "100 per cent braking.efliciency" is sometimes exceeded. Which is absurd. Now to start again, this time with Mr. Dawtrey's practice, which is extraordinarily interesting. The normal weight on the front and rear wheels of the car he used is lOJcwt and 14-Jcwt, but, for the degree of braking applied, the transfer of scwt from rear to front wheels alters the loading to lSJcwt and DJcwt respectively. "Weight must always bo transferred forward when an object is retarded (who has not been nearly thrown forward through the windscreen in his time), and this is the reason, why the rear wheels tend to skid more readily than the front wheels. Caravan brakes arc actuated by the coupling of the caravan to the car, in euch a manner that a thrust on the tow-bar lever applies them. In this way when the car itself is braked the caravan tends to overrun, and so applies its own brakes. Car and Caravan. Mr. Dawtrey assumes that his car- is coupled to. a caravan weighing llcwt, and that the caravan brakes are arranged to give a ratio of 8 to I—that1 —that is to say, a thrust of lewt on the towbar produces a braking force of Sewt at the caravan wheels. Since all the weight is. now coupled up together, rr soon as the brakes of the are are applied more weight is transferred to the car's rear wheels, which will skid less easily, and it will be the caravan's wheels which will be the first to slide; 2.Sscwt, says Mr. Dawtrey, is transferred to tlic car's rear wheels, pressing down the tail of the ear, &nd allowing the driver to press harder on the brake pedal without causing his wheels to skid. In the case chosen, Sir, Dawtrey finds that the car brakes alone produce 19cwt of brakinj force, whereas their force is 21.85cwt for the car-coupled caravan, plus 9.l7cwt braking force from the caravan brakes, giving a total of 31.02cwt of braking force against the 3Ccwt total weight of ear and caravan. Thus for respective deceleration figures of car and "ir with caravan we have the following table:— Car Car and -" ' Only. Caravan. Deceleration m.p.h. per second 16.72 IS.Oo Stopping distance in;feet from 30 m.p.h ' 30Jft 30ft