Perception of Ride

Passenger opinion regarding what constitutes good ride quality obviously is extremely subjective.
What one person considers the optimum ride may be completely unacceptable to
another. The person who prefers sports cars will be appalled by the handling of a large luxury
vehicle, whereas the owner of the luxury vehicle will be quite dissatisfied with the ride of a
sports car.
Other factors come into play when people evaluate the ride quality of a vehicle. Certainly, the
acoustic quality is a factor, and although not a direct result of the suspension, people object to
noises, rattles, and squeaks in their vehicles. The "feel" of the seats is another important
consideration and has an impact on the level of force or vibration transmitted to the occupant's
body. The climate control system, while not at all influenced by the suspension design, influences
perception of ride, too. If a person is uncomfortable because of the interior temperature,
his or her subjective evaluation of ride quality will be affected. Thus, one of the challenges
facing the suspension engineer is to take highly subjective evaluations and convert them into
numerical standards.
346
Some debate exists as to what quality of motion people find objectionable. Displacement is
not an issue. If it were, climbing steps does not in itself produce discomfort, although the
effort required may. Likewise, velocity is not uncomfortable, as evidenced by pilots who
operate aircraft at speeds greater than Mach 1, with no ill effects. Constant acceleration is felt
as a constant force. While pulling a constant 8-9g in an aircraft is less than comfortable, the
levels of acceleration produced by most passenger cars do not induce great discomfort.
Automotive Engineering Fundamentals
However, the rate of change of acceleration, or the jerk, can produce discomfort. A parachutist
feels discomfort at the moment of opening his or her parachute due to the shock, although
it is usually mitigated by a profound sense of relief as the parachute inflates. But the jerk is
not the only element that produces discomfort. The frequency of acceleration and its direction
influence comfort. A car that pitches drastically when encountering a bump is seen as
less comfortable than one that bounces in a more flat attitude, even if both motions continue
for similar amounts of time.
A substantial body of literature is devoted to quantifying ride quality and human perception
of ride. Studies and data have been collected by bodies such as the Society of Automotive
Engineers (SAE) and the International Standards Organization (ISO), as well as by individual
researchers. Gillespie (1994) provides a succinct overview of the literature. Although the
sources are numerous, Gillespie concludes that there are no accepted standards for judging
ride quality due to variables such as seat position, single versus multiple frequency inputs,
multi-direction input, duration of exposure, and audible or ocular inputs.
The bottom line is that all of the research and comfort curves are a starting point for the
suspension engineer. There is no substitute for the subjective evaluation provided by a road
test. We could conclude that the suspension engineer should eliminate all vibration from the
car, but this tends to be an infinite problem. As surely as one vibration is removed, the
occupants become aware of another, more subtle vibration. As a result, suspension engineers
appear to have solid job security for the foreseeable future.