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 of the effects of caster and Ackermann geometry on vehicle handling, specifically understeer and oversteer characteristics.
| Identifier | ExFiles\Box 98\3\ scan0077 | |
| Date | 3rd February 1939 | |
| -2- Da{Bernard Day - Chassis Design}/DB.{Donald Bastow - Suspensions}26/N.3.2.39 Cont'd.{John DeLooze - Company Secretary} Next we consider the effect of 6° caster on full lock, and here it is necessary also to consider the poor Ackermann layout of B.V. by which the outer wheel steering angle is less than it should be. The effect of this on full lock is that the slip angle of the outer wheel is 3° less than that of the inner. Two further curves are therefore shown, the first the effect of the 6° caster angle on full lock with good Ackermann and the second with our present Ackermann. It will be seen that the first curve shows the previous understeer changed to oversteer for small sideways accelerations, changing to understeer again for the maximum sideways acceleration, and the second curve as far as can be seen from the rather scattered points, shows very little difference from what we now have except the tendency again to oversteer at small sideways accelerations. The effect of the camber change due to the 6° caster develops more rapidly than the error in the Ackermann angles, so that for a given sideways acceleration, increase of lock angle first tends to give oversteer or reduce understeer, and then increase understeer again at high sideways accelerations but not at low. Broadly speaking, the effect of the increased caster angle is to cancel out the poor Ackermann at high lock angles and at intermediate lock angles to reduce the understeer tendency ( to obtain which a roll rod has been fitted); in other words to give inconsistency in the handling. The poor Ackermann at present does not lead to inconsistency in the same way since it merely increases the understeer tendency already there. I shall be glad if Rm.{William Robotham - Chief Engineer} after reading this, will say whether he still wishes me to proceed with the increased caster project or merely the reduced out-of-centre without this other feature. Da{Bernard Day - Chassis Design}/DB.{Donald Bastow - Suspensions} | ||