The problem is reported in an article in Transit Australia: "New Articulated Buses and the Adelaide O-Bahn" (John Beckhaus, December 2007). The Obahn busway was opened in 1986. The buses are steered mechanically along its 12 kilometers of concrete tracks. This has been very successful but new buses are now needed.
Standard length buses can be fitted with the guide wheels needed to use the O-Bahn track, but new articulated buses have proved to be a problem. In the time since the original track was built, buses have been redesigned to have low height floors to allow easier passenger access without steps. This required the joint between the bus and trailer on articulated buses to be redesigned. The O-Bhan allows buses to travel much faster than they do on a regular road and this, combined with the smooth concrete Obahn track causes problems. The regular small joints in the track create a slight up and down motion in the trailer, making the passengers motion sick.
Bus maker Scania, along with Simon Smiler, DTISIA, and Peter Clarke produced a report on the problem: "Dynamic Behaviour of Modern Articulated Buses on the Adelaide OBhan". The oscillations causing the problem are between 5 and 7 Hertz (per second). Apparently this happens to be the frequency which causes the most discomfort to the passengers.
Adelaide is the only place where this problem has been found as other busways are not built from concrete track and do not have buses traveling at such high speed. The Cambridgeshire Guided Busway being built in Cambridge England, will be longer than the Adelaide system, but have a lower speed.
Mr. Beckhaus' article discusses possible solutions to the problem by using different design of buses or by modifying the buses by moving the engine to change the weight distribution. But as he points out these changes would be prohibitively expensive for the small number of buses needed for Adelaide.
The limitations of the suspension on low floor articulated buses are a well known problem:
Also, modern, low-floor pusher articulated buses usually suffer from suspension problems, because their wheels lack ample travel to absorb street unevenness, leading to passenger discomfort and relatively rapid disintegration of the superstructure.However, I suggest that without fundamentally changing the suspension of articulated buses, it should be possible to remove the small regular oscillations induced by the O-Bhan track, with some form of damper. This is done with structures such as suspension bridges where oscillations are found after construction. Small devices are attached to the cables of the suspension bridge to detune it, so the oscillations do not build up. The same should be possible with a bus and computer control can be used to do the tuning continually, to adjust to road conditions. The electromechanical devices needed for this are already mass produced for passenger cars and the expertise to fit them exists in Australia.
From: Articulated bus,Wikipedia, 21:24, 23 December 2007.
To minimize the amount of mechanical work needed, electronically controlled dampers could be used. These can be controlled by a small computer, detecting the oscillations using an accelerometer and applying a signal to the dampers to cancel it out. That may sound very complex and expensive, but accelerometers are now used in the inexpensive hand controllers of the Wii video game and electronically controlled dampers are fitted in some models of GM Holden cars made in Australia (also knows as "magneto rheological dampers").
It may be possible to use the same control computer as used in passenger cars to control the dampers, or if a custom computer is needed, one costing under $1000 should be sufficient. As the dampers are only needed to supplement the exiting bus suspension, they need not be large and those for passenger cars should be sufficient.
Adding computer controlled active damping to the trailers of articulated buses could be useful in improving the ride generally and overcome problems with this type of bus. The suspension could be made softer where the road allows and then firmed in milliseconds when required.
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