Why is a loaded bus more comfortable than a empty bus?

Answer 1

"Ride" depends on the ratio of sprung weight to unsprung weight.

A full bus has a lesser percent of its weight unsprung.

Well, not exactly... Handling would be improved with a better ratio of sprung to unsprung weight, since this ratio affects the behavior of the tires when faced with irregular terrain.

The fenomena is that the unsprung weight causes the tires to tend to lose contact when a bump in the the terrain throws them upward; the inertia of the group of masses of the tire+wheel+part of the suspension under the springs tend to keep them moving upward (lessening the contact with the road), until the spring in the suspension system gets compressed enough to stop them and then return them towards the pavement. This inertia also causes the unsprung weight (more properly the unsprung MASS) to lose contact with the road when a void or undulation is encountered.

Therefore, the mass of the wheel+tire+unsprung part of the suspension affects the dynamic Adhesion to the road. Unfavorable ratios cause a loss of contact in irregular road surfaces, specially as speed is raised. Top race cars ussually have a very stiff suspension, but also have a much better ratio of unsprung to sprung mass; that is the advantage of using expensive very light alloys in wheels and suspension arms.

NOW, the ride quality is a matter of many things, but talking in respect to load, it is the RATE of the springs that plays a role. Bus springs are selected to be able to carry a lot of load, because the bus is intended to be able to carry many passengers. When the bus is empty, the springs are relatively unloaded, and they appear to be too stiff for a soft ride.

When the bus is loaded, any disturbances caused by bumps or irregularities of the road have to overcome the force exerted by the suspension springs, but now the sprung mass (which is the mass above the suspension system) is much higher, and the result is that even with stiff springs, the bus body, floor and seats, are less disturbed by the irregularities of the road. This fenomena affects all vehicles with passive suspension (springs).

Buses with pneumatic or active suspension systems can adjust the stiffness of the suspension and keep a better ride within a range of loadings. amclaussen.