About Those Brakes

Sun (Auckland), Volume II, Issue 372, 5 June 1928, Page 7

About Those Brakes

Stopping Powers Examined

THE THEORETICAL SIDE

EVER'i day we hear tales of the phenomenal stopping power of four-wheeled brakes. Something like this : “Last night I was bowling along at about 35 miles an hour, when some ass barged out from a side street. I jammed, on the brakes and pulled up in less than a car’s length.”

Now, although four-wheel brakes do give us control of our cars far above anything possible in years gone by, these statements are woefully exaggerated and only serve to lead drivers of moderate experience into trouble, if they heed them. Even in the sacred precincts of the Supreme Court some little while since I heard a learned advocate state in all seriousness that his client had pulled up “dead” from 20 miles an hour when he applied the brakes. These far too general claims anent the stopping power of modern brakes are purely misconceptions. For the actual distance in which any car can pull up from a given speed is governed definitely by physical laws coupled with the conditions existing at that particular moment. Any layman must realise that the maximum braking effect will be obtained when all four wheels do an equal share of work. But in practice this state of affairs seldom holds good. Further, no wheel should lock and skid for reasons that will be shortly explained. Trying It Out Let us now analyse the factors governing braking so that we may see what actually goes on when the pedal is depressed. One of the most important considerations is the co-efficient of friction between tyre and road, and in order to clearly understand this matter, a very simple experiment can be made. Cut a section about four inches long from an old .tyre and firmly secure inside it a metal block, making the total weight up to, say, one pound. Lay the section, tread down, on a smooth table, and attach a cord to the metal, passing it over a pulley on the table end, and to the other extremity attach any required weight. In order to determine the friction coefficient, we must add enough metal to this extremity to just keep the weighted tyre section moving along the table. N-ow while the figure obtained will not greatly interest the average motorist, we can now see how altered conditions affect our braking systems. Say in the first experiment we have added two pounds to the extremity of the cord in order to keep the tyre moving. Now we pull the tyre back to its original position and wet the surface of the table. As soon as we let go the tyre will start off with a rush, proving the friction co-efficient has been altered by the varied conditions. Try again after smearing the surface with lubricating oil. Still less weight will be needed to keep the tyre movOn the other hand, if the experiment be made on a rubber sheet much heavier weight would be needed to overcome the friction between the two surfaces. Skidding While these experiments clearly demonstrate the effect of various surfaces upon the braking efficiency of any tyre, they also show that in no instance wili the weight used in keeping the tyre “skidding” ever start it off from rest. That is why a car with locked wheels will slither for a greater distance than would be the case if the brakes had been properly applied. The stopping power of a skidding wheel is reduced fully one-third. There are three other factors to be taken into account when pulling up. First: Design and condition of tread. Second: Wind resistance. Third: Effect of engine. A new tyre with boldly designed tread is much more effective than one that has worn smooth. Not that the friction co-efficient is any greater, but due solely to the fact that the edges of the moulded tread tend to “bite” into the road surface and give a better mechanical purchase. Wind and Engine Wind resistance is not of great moment at speed below 20 miles an hour, but the effect may be readily observed if the brakes are applied at 50 miles an hour, and the time noted in slowing to 30 m.p.h. When there is a strong wind blowing, try this with and