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Midland Red: BMMO D10 — Type Overview
Introduction
In September 1960, BMMO had recently begun production of the BMMO D9 double-deck bus, when their experimental department produced a prototype for a double-decker bus with the engine mounted under the floor, the BMMO D10. Until this time all of the company’s double-deck designs had placed the engine in the traditional place at the front alongside the driver, however BMMO were interested in taking advantage of the extra capacity available on vehicles with the engine at the rear but didn’t want the engineering problems, notably overheating, that went with them.
The problem with mounting the engine under the floor of double-deck vehicles is one of height; the vehicle must maintain adequate headroom on both decks without the engine protruding into the passenger area or the vehicle exceeding the maximum overall permitted height of 15′ (4.57m) for public service vehicles at the time. BMMO solved this problem by using a specially modified version of their 10½-litre engine fitted horizontally on the nearside with the cylinder heads facing towards the centre of the bus and the crankcase towards the outside. By doing this the deepest part of the engine, the flywheel housing, was clear of the lower deck gangway and a reasonable headroom of 5′ 11¼″ (1.81m) on the lower deck and 5′ 9¼″ (1.76m) on the upper deck was achieved with an overall vehicle height of 14′ 4½″ (4.38m).
An SCG 4 speed semi-automatic gearbox was fitted with an offset train of gears at the back to bring the output shaft nearer the centre line, driving a type D9 rear axle. Other mechanical systems such as the brakes, radiator, power steering and suspension were all BMMO standard as used on the type D9.
[ Page Top ]Prototype vehicles
4943 (R/No: 943KHA) — First Prototype
BMMO constructed the first D10 prototype entirely in-house at their Central Works at Carlyle Road in Edgbaston, Birmingham. The body design incorporated the same basic features as the BMMO type D9 but with a front staircase and entrance, and therefore full front similar to the BMMO single-deck designs.
Surprisingly BMMO resisted the potential to seat 80 in this prototype, choosing instead to provide 78 seats and a spacious stairway layout that permitted additional luggage accommodation beneath, giving the D10 a 6-seat advantage over the similarly sized D9. Also, unusually for BMMO the emergency exit was on the offside rather than the usual BMMO position in the centre at the rear.
This first prototype was registered 943KHA entered service at Digbeth depot in January 1961 with the fleet number 4943. She underwent lengthy evaluation at a number of depots, being used on busy Black Country services. In April 1964, she moved to Stafford depot were she operated without notoriety on the relatively undemanding services in that area until the end of 1971.
4944 (R/No: 1944HA) — Second Prototype
In 1961, BMMO built a second D10 prototype, registered 1944HA, which was mechanically identical to 4943. The bodywork however was quite different as although it generally followed the appearance of the first prototype, it was fitted with a narrow exit and second staircase at the rear. The idea was to channel passenger flow by using the front door and staircase as an entrance only, with passengers leaving via the rear exit and staircase. This twin stair and twin door layout reducing the seating capacity to 65 and the positioning of the rear staircase requiring the rear emergency door be re-located back to the traditional position in the centre.
1944HA entered service at Sheepcote Street depot, Birmingham in April 1961 with the fleet number 4944, and like 4943 was tested from a number of depots in the area on busy Black Country services. However, the experimental layout was not successful and in November 1962 she returned to Central Works and was rebuilt with a single front door and staircase, similar to 4943. The emergency door was not relocated during the rebuild and as a result, the final seating capacity was one less than 4943 at 77. In April 1964, she joined 4943 at Stafford depot were she worked until begin withdrawn in 1973.
[ Page Top ]Design Limitations
Mounting the engine in the near-side of the vehicle frame may have solved the problem of deck height, but it also created a number of additional problems which seriously compromised the design.
The weight distribution was seriously imbalanced, and therefore brakes and suspension components came under uneven strains and suffered uneven ware. I suspect the weight imbalance and uneven component ware would also have had some unwanted effects of the vehicle handling too.
Also, the chain of gears used to bring the gearbox output in line with the differential was an inefficient solution and would have impacted on power and fuel consumption. And finally, placing the engine within the integral frame created a hole in the structure which caused excessive stresses to the surrounding parts of the frame. This could be seen on the two prototype vehicles which both had cracked stress panels around the engine by the time they were withdrawn.
[ Page Top ]Summary
The concept of the BMMO D10 design, with its under-floor engine layout, was undoubtedly inspired and ahead of its time, and is still considered by some to be the ideal double-deck design layout. Unfortunately, these prototype designs had a few serious flaws that would have required major reworking to rectify and, I would imagine, this was considered uneconomical and almost certainly the reason the design never entered production.
Had the design been successful, it is believed that production of the BMMO D9 would have ceased at the end of the first batch in late 1961, and subsequent double-deck production would have been built to the BMMO D10 design.
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