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 Post subject: AEC Monocontrol
PostPosted: Wed Apr 17, 2019 11:30 pm 

Joined: Sat Mar 31, 2012 7:34 am
Posts: 60
Location: Mount Maunganui, New Zealand
Whilst looking through the materials on hand when I was compiling the AEC Reliance Export Models thread, I noticed an oddity in respect of the AEC Monocontrol epicyclic gearbox.

The Reliance was announced, along with the Monocoach and Mercury, in Commercial Motor (CM) 1953 October 30.

In the description of the Reliance, it was noted: “As an alternative to the five-speed syncromesh gearbox, a four-speed epicyclic unit with automatic centrifugal clutch is available.”

There was no further elaboration on the nature of the gearbox, and the “Monocontrol” moniker was not used. But it is reasonable to assume that it was the direct air-operated version of the Wilson gearbox, SCG designation RV16, which was by then available. The Leyland version, under the Pneumocyclic name, was announced in CM 1953 November 20. And the SCG gearbox option for the Atkinson Alpha, with Leyland pedestal control, had been announced in CM 1953 November 06.

Returning to AEC, the apparent first appearance of the “Monocontrol” name in CM was in a 1954 September 17 article “New A.C.V. Passenger Range”. Essentially the article covered the Regent V, but included the statement “A new semi-automatic gearbox known as the Monocontrol transmission system and a new power steering gear have also been introduced.”

By then, the direct air-operated version of the SCG gearbox was hardly new, so perhaps the “new” descriptor referred to the Monocontrol name. Is there any evidence of earlier use of that name?

The article provided a description of the Monocontrol gearbox, and mentioned that as compared with the earlier type (presumably the air-operated preselector version), it had been strengthened to enable it to handle the full torque of the AEC 11.3-litre engine. That difference is confirmed in respect of the later export Regal IV chassis variants, for which a choice of 9.6 and 11.3-litres engines was offered, as well as a choice of preselector and Monocontrol epicyclic transmissions was offered, but with only the Monocontrol available when the larger engine was specified.

One possibility that comes to mind is that AEC adopted the Monocontrol name only for its own production version of the SCG RV16. And concomitantly, the epicyclic gearbox initially offered on the Reliance might have been of SCG manufacture, this being an interim measure until it could start its own production.

AEC was not alone in adopting a strengthened design. Leyland did the same, as recorded in a CM 1954 March 05 article about Leyland production of the Pneumocyclic.

What is not clear, though, is whether this strengthening was relative to the basic SCG RV16, or just to the earlier preselector type. It seems less likely that SCG would not have designed the RV16 to accept the torque of the largest bus engines then in service or projected.

The AEC Monocontrol system was well described in AEC sales brochure #559 of 1957 October, which ran to 8 pages. It referred to the fluid flywheel, with no mention of centrifugal clutches. I imagine that AEC had abandoned this not-very-good idea by then.

AEC Monocontrol #559 195710.pdf [1.05 MiB]
Downloaded 319 times

The AEC Automonocontrol system was described in a two-side single-sheet #639 of 1958 September. It had been announced earlier than that, in CM 1957 January 18.

AEC Automonocontrol #639 195809.pdf [256.79 KiB]
Downloaded 301 times

The above-mentioned CM articles are all available at:


 Post subject: Re: AEC Monocontrol
PostPosted: Sun May 12, 2019 3:58 am 

Joined: Sat Mar 31, 2012 7:34 am
Posts: 60
Location: Mount Maunganui, New Zealand
The AEC Monocontrol epicyclic gearbox was used in both close-coupled and island-mounted form.

As best I can determine, the Regal IV (later variants), Regal V, Regal VI, Regent V all used only the island-mounted variant.

The Swift/Merlin used only the close-coupled variant.

But the Reliance seems to have been a mixed bag.

The MU (and by extension, the 2MU) had a close-coupled gearbox, as shown in this except from a 1956 May sales brochure:

AEC Reliance #541 195605 p.04.JPG
AEC Reliance #541 195605 p.04.JPG [ 106.57 KiB | Viewed 3154 times ]
AEC Reliance #541 195605 p.05.JPG
AEC Reliance #541 195605 p.05.JPG [ 98.36 KiB | Viewed 3154 times ]

The HMU (and by extension, the 2HMU) also had a close-coupled gearbox, as shown in this excerpt from a 1956 sales brochure:

AEC Reliance Overseas #545 195607 p.04.jpg
AEC Reliance Overseas #545 195607 p.04.jpg [ 991.67 KiB | Viewed 3154 times ]

There is no reason to suppose that this configuration did not apply to both the 16’0” and 17’6” wheelbase variants.

I do not have any “hard” information about the Reliance 470 4MU/5MU and Reliance 590 2U/4U variants. My best estimate, from interpolation between the earlier MU/2MU and the following 6MU/6U models is that they had the island-mounted gearbox. Specific information would be welcome, though.

In respect of the Reliance 691 6U, both the home (18’7” wheelbase) and export (17’6” and 18’7” wheelbase) variants had island-mounted gearboxes, as shown in these excerpts from respective sales brochures dated 1966 August.

AEC Reliance 691 Home 196608 p.04.jpg
AEC Reliance 691 Home 196608 p.04.jpg [ 478.57 KiB | Viewed 3154 times ]

AEC Reliance 691 Export 196608 p.04.jpg
AEC Reliance 691 Export 196608 p.04.jpg [ 1.43 MiB | Viewed 3154 times ]

Of note is that the 17’6” wheelbase 6U had an island-mounted gearbox, whereas the 17’6” wheelbase HMU/2HMU had had a close-coupled gearbox.

It seems reasonable to assume that the 6U inherited its configuration from the preceding 2U/4U.

In respect of the 6MU, the only datapoint that I have is that from a 1970 July specification sheet for the home model. In the optional Monocontrol case, this was described as having a separately-mounted gearbox. As both the 16’2” and 18’7” wheelbase variants were covered, we may assume that this applied to both.

AEC Reliance 505 Home #0055 197007 p.02.gif
AEC Reliance 505 Home #0055 197007 p.02.gif [ 729.22 KiB | Viewed 3154 times ]

The 18’7’ wheelbase 6MU variant probably inherited its island-mounted gearbox from the 4MU. That the 16’2” wheelbase variant had an island-mounted gearbox is somewhat surprising. Two possibilities come to mind. One is that the 16’4” wheelbase 6MU did not simply inherit the close-coupled gearbox of its 2MU predecessor, but instead was configured the same way as its longer wheelbase brethren. Another is that the 16’4” wheelbase 6MU did have the close-coupled gearbox, but that when the chassis was reconfigured with a 16’2” wheelbase and longer front overhang, the gearbox placement was changed. I suspect that there is some chance of finding out what was the Monocontrol gearbox placement on a pre-1970 short Reliance 505.

The 17’6” wheelbase export 6MU could have followed either the 2HMU (close-coupled) or 17’6” wheelbase 6U (island-mounted). On balance, the latter seems to have been more likely.

That 1970 July Reliance 505 specification shows that the Monocontrol gearbox was available in either 4-speed or 5-speed underdrive form. I am not sure when the 5-speed option was first offered on AEC chassis, although I think it was in the LMC era.

The 5-speed version of the Wilson gearbox was in the SCG lexicon quite early on. When Guy started licence manufacture in 1948(?), it went abundance and built the 4-speed, 5-speed overdrive and 5-speed underdrive forms, each in both spring-operated and internally air-operated preselector form. At the time Daimler was building both the 4-speed and 5-speed underdrive preselector forms, both spring operated, and AEC was building the 4-speed preselector form, mostly with internal air operation but with spring operation at customer request. Leyland started with the 4-speed direct air operated model, and AEC also added this type to its range. Leyland had added a 5-speed underdrive version by 1958. Daimler first moved to the internally air-operated preselector form, 4-speed only as far as I know, and then to the 4-speed direct air operated model as the Daimatic. I am not sure if it added a 5-speed option before the BL era. Guy was last to adopt the direct air-operated form, and when it did, it offered the full range, namely 4-speed, 5-speed underdrive and 5-speed overdrive variants.

Note that at various times Daimler offered its spring-operated preselector gearboxes variously with hydraulically assisted mechanical, external hydraulically operated and external air operated gearchanges,. In all cases the basic spring-operated gearbox itself was unchanged. External hydraulic and air operation required pressure to effect gear disengagement, rather than pressure to effect and hold gear engagement. As I understand it, AEC had tried external vacuum operated gearchange with spring operated gearboxes on the 1930s Q model, but rather unsuccessfully. Assuming the use of air-suspended vacuum equipment, I guess that outcome was not too surprising. Better might have been vacuum-assisted mechanical, perhaps adapting a brake master servo unit for the purpose.


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