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).
Analysis of spring stress, safety margins, and potential for breakage in different vehicle models.
| Identifier | ExFiles\Box 67a\1\ scan0049 | |
| Date | 31th December 1925 | |
| - 2 - That this is so may not be obvious, but it would certainly happen, and the range of stress would be increased to somewhere in the neighbourhood of 15%. This means we would increase our stress and on top of this increase the stress range both of which are changes which could not be lightly entered upon, as the liability of springs to break would become a very grave possibility. 6. The margin of safety on our front springs is on the balance or zero. The margin on the rear cannot be very much as when we fitted thick leaf type springs breakages were common. The reduction of stress by use of thin leaf springs was only 17% on original stress. It is therefore obvious that we will be running a serious risk of breakage if we increase, as suggested. There is another factor which puts up the stress on the thick leaf type of spring a little disproportionately, namely, the reduced damping due to the decreased number of plates, and unless we were to entirely redesign our springs we would be slipping back to the old position, as the differences in springs are for variations of 250 to 300 lbs. in the existing range of springs, mainly arrived at by reducing the number of plates which would result in increased movement for any given disturbance, and therefore further increase of stress. 7. There is another aspect of the case which does not appear to be understood, or it would have been raised in the criticism, namely, that whilst it is the 40/50 which is complained of, the 20 HP. is relatively the most strongly sprung of the two cars. The figures are as follows:- On a 40/50, with a gross spring load on rear of 2800 lbs, only 2000 lbs. springs are fitted. (Equivalent to 1000 semi-elliptic). Whereas on a 20 HP of 2300 lbs. gross loads on rear springs, we fit springs of 1000 lbs. poundage. In percentage, this means that on the 40/50, we deduct 28.5% from load actually carried and only spring for the balance, while in the case of the 20 HP car, we only deduct 13%. Contd. | ||