The ends of the 70% low-floor car, including the operator’s cabins, are generally at the same height as a high-floor car, allowing trucks of conventional design under the ends of the car. But it is not possible to use conventional trucks beneath the low-floor portions of the car because the floor would be lower than the elevation of solid axles. The usual resolution is to use trucks that do not have conventional solid axles extending from wheel to wheel. Instead, the four wheels are each connected directly to a u-shaped frame that passes beneath the floor.
Each wheel, lacking a mechanical connection to another, therefore rotates independently and is naturally called an independently rotating wheel (IRW). As an alternative to IRW trucks, at least one manufacturer has developed a truck using conventional solid axles connecting very small diameter wheels. This design also ramps the floor of the articulation body section slightly above that of the floor by the doors. However, small diameter wheels will have a smaller contact patch with the top of rail and thereby increase wheel/rail contact stresses, possibly increasing rail wear and corrugation rates.
Because of constrained space, these special truck designs beneath the center sections of 70% low-floor LRVs are generally non-powered. Propulsion is provided only at the conventional trucks under the ends of the car. However, 100% low-floor cars must provide propulsion at trucks under the low-floor, and carbuilders have come up with several ingenious, albeit complex, methods for doing this. Because of this complexity, 70% low-floor cars using conventional power trucks have generally been considered more reliable than 100% low-floor cars.
Nevertheless, the 100% low floor LRV has been almost exclusively adopted for new vehicle purchases by in-street tramway type operations in Europe and also by some of the stadtbahn-type operations. As of 2010, the first 100% low-floor LRV specified in North America was being produced for Toronto Transit Commission. The Toronto cars are also specified to negotiate a 36-foot [11-meter] radius curve. The degree to which the carbuilder succeeds in meeting the Toronto requirements may radically change preferences for light rail vehicle design.
As of 2010, the lowest 100% low-floor LRV was the Vienna Ultra-Low-Floor (ULF) car, with the floor a mere 200 mm [about 8 inches] above the top of the rail. The traction motors of the ULF car are mounted vertically within the articulation sections. As of 2010, this design has not been adopted elsewhere.
The conventional trucks that are under the end body segments of 70% low-floor cars rotate with respect to the carbody. By contrast, the trucks under 100% low-floor LRVs generally do not rotate and are, for all practical considerations, rigidly fixed to the carbody. This configuration has resulted in vehicle designs that are radical departures from high-floor and partial low-floor designs and vehicles that have significantly different steering and curve negotiation characteristics.
