two types of car were driven at several speeds and with different tyre pressures over a simulated bridge deck expansion joint. four other types of car were driven at several speeds over a bumpy road. measurements were made of peak-to-peak vertical acceleration on the car floor and at the seat/person interface. the results for all the cars used show that above a certain speed, about 30 km/h, a discontinuity will act as an impulse to the car's suspension system, since the time interval between the passage of the front and rear wheel is short. as a result at speeds above 30 km/h two resonant modes are excited - a 'body heave' at about 1.75 hz, and 'wheel hop' at 10-15 hz. below this speed the shape of the discontinuity becomes important. the effect upon the vehicle occupants of 'wheel hop' is largely filtered out by the car seat. (a).

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