Paul Bennett's models

The meaning of "highbridge", "lowbridge" and so on

If you are not a bus-O, you may not know the difference between "highbridge" (normal-height), and "lowbridge" (low-height) double deckers. (The trade names "Lodekka" (Bristol), and "Loline" (Dennis) are also applied to lowbridge buses.) This picture shows three low-height buses between two normal-height buses, and you can see how low the centre buses are. The red beam-edge across the top shows that there are variations between lowbridge buses, too.

"Lo" buses are specially designed to pass beneath bridges that were built long before anyone had thought of building tall vehicles.There are different ways to make a bus lower than normal, and they can be combined: (a) You can lower the headroom of both decks generally (a bit). (b) On the upper deck, replace the twin-seats normally fitted on either side of the central gangway, by a 4-seater bench seat on the left side, and a sunken gangway on the right side. With the gangway lowered like this, you can lower the roof further. (One drawback is low headroom for the lower-deck passengers sitting under that gangway). (c) The chassis frame can be "underslung" at the rear (by running the frame under the rear axle instead of having it pass over the axle in the usual way. (d) By using a hypoid (low-set), horizontal, or offset drive to the rear axle, the driveshaft can be cleared out of the way more, allowing a lower floor. (e) Using a small crownwheel, and reducing the resulting high gear ratio by epicyclic gears in the hubs, also lowers the driveshaft.

If you'd ever woken up in the 4-seater upper-deck seat of a low-line bus on a wet November night, and realised that you had to get off quickly, you'd know just how awful they were — much worse than having a window seat on an Airbus when you need a pee in the middle of the night.