When the pendulous accelerometer experiences large off-axis acceleration in a direction which produces significant along-the-boom acceleration, the dynamics of the instrument are altered and it becomes nonlinear in its response. The response to an off-axis horizontal acceleration step is derived and found to exhibit small changes in damping, overshoot, damped resonant frequency, and final value of boom position. The size of these effects is negligible for all but the largest accelerations and even then acceptable. The change in overshoot is the largest, about 5 per cent, for an AR-240 strong-motion accelerograph subjected to a 1-g horizontal acceleration step at 45° to the sensitive axis.
An expression is developed for the steady-state response to off-axis sinusoidal acceleration. In addition to the proper response, it contains both a double frequency term and a constant offset. The size of these terms is discussed and an example is given.