The question is in the title. I’m aware that this may depend on the driver and how sudden the acceleration/retardation is, but certain luxury buses are for instance way smoother and softer than other. Obviously, this depends on the cost and quality of the engine. But what exactly makes it smooth?
In: Engineering
Many good answers here. All of them are pretty much right. It’s a combination of all of them.
Wheelbase. The rear wheels on a bus are far enough away from the front that when the front hits a bump, the rear passengers don’t really feel it because the angle of the bus changes only slightly. Those sitting in the middle feel the bump but less than the people over the front wheels. When the rear passes over the bump, the front passengers don’t really feel it and those in the middle again feel it less than those in the rear. In a short vehicle, the front moves when it hits the bump, the rear is close enough to feel the change in angle of the car as it goes over so both front and rear passengers feel both bumps almost equally.
Tyre depth. Bus tyres have a deep profile. Loads of tyre to soak up the bump.
Weight of the vehicle versus weight of the wheels. This is a bit hard to grasp. It’s known as unsprung weight. The bus weighs a lot, in comparison it’s wheels weigh very little. The wheels are bouncing up and down as the bus drives along. The suspension has to control the movement of the wheels by damping the movement of the springs. To damp the movement the suspension uses the mass of the bus as an anchor. Because the bus is heavy, the suspension does not move the bus much when it controls the wheels. It’s a bit like a small child trying to control a rabbit in their hands, the rabbit weighs a decent proportion of the weight of the child so the struggling of the rabbit has a great impact on the child. Give the rabbit to an adult and the effect is lessened.
Soft suspension. Busses have long travel, soft suspension which allows the wheels to move a lot over bumps. This is why busses handle the corners, er, like busses. They’re all soft and squidgy because they’re designed to isolate the passengers from bumps but the downside of that is that they leans violently if you drive them like a race car.
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