A bumper is a bar attached transversely in front of or behind a car body to prevent contact between an obstruction and the car body or to cushion the shock of collision between vehicles. The impact-bars have various sectional forms, from flat to round and from tubes to channels, and are composed of steel, wood or rubberized fabric. The attaching devices are sometimes yielding, sometimes rigid. The evolution of the bumper is shown in the records of the Patent Office. Early types had yielding attaching-parts and rigid impact-parts. These were followed by types having a rigid bar connected with the frame by only a spiral spring, by those having channel-steel impact-bars and others having round spring-steel extending from the frame-horns. A strip of rectangular spring-steel was then used by a Western blacksmith, and later a similar non-reinforced bumper appeared which was cut in two in the middle, the ends being overlapped and the overlapped parts clamped together. For several years development was limited to these types. Then appeared a loop-end type in which the impact-bar was spread vertically. This led to the introduction of many bumpers having an extended impact-surface. In 1914 or 1915, resilient bumpers that demonstrated their protecting possibilities on cars traveling at the rate of 15 m.p.h. were introduced. These offered an elastic yielding resistance that built-up in intensity in proportion to the magnitude of the shock and to the available clearance.
Designing the most protective bumper consists simply of providing the maximum allowable clearance and arranging the structure to make full use of this clearance under the maximum impact. The limit of protection is determined by the rate of retardation the most expensive parts of the car will withstand and by the practical limitations of cost, weight and clearance.
The Underwriters' Laboratories, applying central impact-tests to a stationary bumper, obtain deflections comparable with those produced in 10-m.p.h. collisions. But the minimum and the maximum deflections should be specified in order that bumpers may be gaged not only for the prevention of contact but for the amount of cushioning that is to be provided.
The tempering of springs is essential to maintain the original shape of the bumpers that take full advantage of the available clearance in their protective action. Soft-steel bumpers may be made which will provide all possible protection for one collision but after several slight collisions soon become objectionable in appearance.
In recent years few other accessories have been so unsatisfactorily attached to automobiles as have bumpers. However, provision for their application has been neglected by car manufacturers. One or two additional holes would greatly simplify the labor of attaching them and would also reduce the cost to the car-owner. Bumpers at the same elevation on colliding cars are twice as effective as bumpers having different elevations. Although specifications cover this height, many bumpers fail to conform to them. An appreciation of the protective power of bumpers and closer cooperation between car and bumper manufacturers would result in a reduction of the growing list of automobile casualties.