From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Technical analysis and comparison of variable rate and conventional leaf spring designs for the 1939 Pontiac.
| Identifier | ExFiles\Box 154\2\ scan0143 | |
| Date | 8th September 1938 guessed | |
| - 2 - Some advocates of the Leaf Spring Institute variable rate spring have induced the Pontiac Motor Car Company to adopt the variable rate rear spring for the 1939 models. This spring is made up with a main plate and second plate of .237 steel, the third fourth and fifth plates of .214 steel. These plates are fitted together to form a spring with an opening of approximately 9". The sixth plate of the spring is a single plate approximately .291 thick with a camber of 5/8". The seventh plate is .323 thick, cambered flat, the eighth plate is .291 thick, cambered flat. As load is applied the main spring rolls into contact with the sixth plate so that at 2" opening the rate of the spring is 113 lbs. per inch. When straight the spring has a rate of 135 lbs. per inch and when in 2" reverse camber the rate is 155 lbs. per inch. You will note the idea is to supply a spring in which the rate of deflection is directly proportional to the load applied. Before mentioning any other objections, I want to call your attention to the problem the spring maker has in getting these three sections of the springto so co-ordinate that the three rates of deflection are obtained under three different loads and heights, which are also specified. In practice, the first and second stage of the spring are always in contact, since the shock absorber arm does not permit a rebound sufficiently great to separate the first five leaves from the sixth leaf. With this construction the first five leaves of the spring are subject to concentrated and irregular stresses due to the fact that the initial deflection causes appreciably more bending at the center than towards the end of the spring. It is to be noted that the static stress in the main leaf is more excessive than in the conventional spring, which will in all probability occasion settling. This assumption is substantiated by our experience with bus springs of similar design. The use of heavier leaves in the shorter plates, which are subject to minor deflections, necessitates using approximately 10% more steel than would be necessary in a conventional spring designed for carrying the same load. In our opinion, a sufficient range of rates can be obtained by the use of a well-built conventional leaf spring used in conjunction with a tension shackle with the proper angle. In regards to variation in spring rates to be obtained by shackle angles, please see Maurice Olley, who has been instrumental in collecting most valuable data on this subject. To briefly sum up, I would say that the advocates of the Leaf Spring Institute's variable rate spring are correct in stating that their spring will ride as well with one passenger as with five, but I will say that a conventional spring with tension shackles set at the proper angle will ride better both with one passenger and with five passengers than the variable rate spring and that such a spring will cost less and last longer. I am not so prejudiced as to entirely condemn the variable rate spring as under certain circumstances it may be desirable to add one heavy plate to the bottom of the spring which will act as an elongated spring set and prevent undue bottoming. | ||