Thanks to everyone who’s chimed in, not least cav551 and Geordielad.
cav551:
The first ‘semi auto’ gearboxes, epicyclic boxes called pre selectors, were introduced for buses came in the 1930s and far from being mainly for the benefit of the operator, the main beneficiary was actually the driver. They did noticeably improve journey times but used more fuel doing so and were a more expensive fitment and time consuming to overhaul. […] The early 'boxes were purely mechanically operated, employing a very powerful spring to hold the brake band applied. Failure by the driver to operate the engagement pedal properly would result in the pedal returning with considerable force, inflicting painful injuries to the unfortunate driver’s ankle or shin.
I’m no expert; I’ve watched many a video of preserved 30s/40s buses with the pre-select type box and wondered to myself how the driver could manage operate such a device (in town, stops every 1/4 mile) for an 8-hour shift without wanting to bail out.
Geordielad:
As cav551 has pointed out, Auto boxes are a more older design than many thought. Original ‘Crash Boxes’ pre Constant Mesh, were not only difficult to master but they were also noisy and it was the Bus Companies, especially in London, that pressed makers to find quieter alternatives, the Constant Mesh box was a big help but even then drivers took a little time and use to enable smoother changes (still mistakenly referred to as crash boxes by those that infact drove Constant Mesh box vehicles) but were quieter. AEC went a bit further by developing a box that used a chain drive on the gears similar to a watch strap which had small teeth inside the chain to engage with the gears, however it still needed dog clutches to engage and disengage gears but these boxes proved very reliable. Of course the Automatic gearbox solved many of the problems of the older sliding or constant mesh boxes, very quiet, less work involved on the drivers part, smooth gear changing, less wear on the drive line and probably more. Why drivers ■■■ ■■■ the Auto box is simply maybe a non understanding or a macho thing where your not believed to be a real driver unless your stirring a metal stick while pumping both feet like a drummer in a rock band and making just as much noise! Ok maybe that’s over playing it as many drivers were quite adept in using a constant mesh box and many were a delight to use but the auto box today is a great advancement and an inevitable replacement for older manual boxes due to better materials and technology. Franky.
Thanks for correcting my somewhat glib “leap from crash boxes to semi-auto” comment, I sort of knew I was not telling the entire truth when I wrote it. Perhaps one thing that you wrote that answers essexpete’s contention “I still suspect the motive for development would have been for the operators with any improvement for the driver Incidental!” is
less wear on the drive line and probably more
If the driver has an easier time of it and the Corporation doesn’t have to incur costs replacing worn drive lines (not to mention the costs of one of its buses off the road for the meanwhile), who cares who prompted what?
Your comment re: manual gearboxes and “a macho thing”: yes, there’s a lot of that, here (Oz) especially. But there is a real satisfaction for many of us using a (complex) manual box and getting it right, every shift. It’s a self-critical thing for many of us, every time time there’s a slight “snick” (let alone a grind) we think “bugger”.
cav551:
…The reasons for the lack of popularity in lorry chassis are probably to do with cost, weight and the trend as time went by for more ratios which in turn meant more weight… Certainly another factor would have been that bus chassis at around 14 tons gross coped with a bottom gear ratio of around 4.5 to 1 and a 5.2 to 1 axle ratio, which was really too high for lorries grossing 24 tons and upwards withiut a much lower axle ratio.
I don’t think I was referring to auto/ semi-auto boxes in heavier applications (I may not have been all that clear in my original query) but I grant you that auto/ semi-auto boxes would not have worked at 24T+GVW. My question was really more about the application for lorries on distribution/ urban work.
Carryfast:
I’ve driven sliding mesh, fuller and zf constant mesh and Allison torque converter semi auto but not the fluid flywheel preselector fitted in AEC buses.
The fact that manual fuller constant mesh worked perfectly well in US buses says that would have been the best all round choice given an air assisted clutch.
In my own case the Allison was only fitted in some fire trucks because of doubts about driver quality or more like ease of packaging in long rear engine vehicle types.
All well and good: my query is why auto/ semi-auto transmissions in medium-weight rigid lorries on distribution/ urban didn’t take off (until decades later). BTW Sydney, Melbourne, Brisbane, Adelaide and Perth all preferenced semi-auto buses for their fleets going back to the 70s.
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