Sunday, 3 August 2025

Experimental Routemaster (XRM): The Overlooked Project of London's Transport History

XRM model bus design
It suggests a livery that is partly red and partly unpainted aluminium.

As a follow-up to my previous articles, "A Brief History of Low-Floor Buses and London Transport’s XRM Project: A Missed Opportunity for Accessibility" and "The Proposed Successors to the AEC Routemaster Before the Three-Door, Two-Staircase Hybrid New Routemaster Bus," this piece will delve into the ambitious yet ultimately unfulfilled project known as the XRM, a London Transport (LT) initiative aimed at designing the next generation of double-deck buses. The Experimental Routemaster (XRM) is indeed a forgotten part of London's Transport history.

The Origins of the XRM Project

In 1974, London Transport began considering a replacement for the Routemaster (RM), codenaming the project XRM. This prospective design was envisioned as the successor to the "second generation" double-deckers, such as the Leyland B15 (later known as the Titan) and the MCW Metrobus, which were expected to enter production in the 1980s. The motivation for an in-house developed vehicle stemmed from LT's unsatisfactory experiences with buses acquired for one-person operated (OPO) conversions in 1973, and a desire to create a bus specifically tailored to its unique requirements. Although LT was then obligated to order standard bus types to qualify for the New Bus Grant, there was a forward-looking perspective that a custom design would be essential once the grant system concluded.

Initially, the XRM was conceived as a side-engined bus, utilising the proven components of the RM family. This layout offered flexibility in door configurations, allowing for adaptation to both one-person operated (OPO) or crewed services. It was also anticipated that a side engine would mitigate the issues encountered with rear-engined buses. The side-engine concept harked back to the AEC Q types of the 1930s; despite initial "teething troubles" with engine mountings and cooling systems in the Qs, considerable knowledge had been gained since then. Design work for the XRM, also internally referred to as QRM, commenced in earnest in 1975.

Four-Axle Power-Saving Bus
At the 1977 British Genius exhibition, NEL showed a future bus model with power-saving features. Removable offside grilles allowed engine access.
Image by National Engineering Laboratory

Addressing Passenger Flow and Comfort

A key driver for the XRM project was the unacceptably high boarding and alighting times experienced with standard bus designs available in the late 1960s and early 1970s. The high saloon floors in these vehicles, necessitated by the transmission shaft from the rear engine, meant passengers faced a second step of approximately 14 inches (35.6 cm) after an initial 10-inch (25.4 cm) step. Even marginal increases in the time each passenger took to board could accumulate into significant delays. Consequently, the XRM design aimed for a low floor, just 17 inches (43.2 cm) from the ground, thereby reducing the height of the second step to 7 inches (17.8 cm).

To achieve this low floor, the use of small wheels was proposed for the XRM. This concept had also been explored through the Moulton coach project and by Leyland in the mid-1960s with a design study for a low-floor, multi-wheeled Leyland Commutabus, which featured four axles and eight small wheels. A mock-up of the Commutabus was built at Leyland, and a partial structure was constructed at Chiswick for boarding and alighting speed tests. Leyland eventually shelved their project due to problems with transmission line components and tyres.

Hydrostatic Drive: A Revolutionary Concept

#London #Transport (LT) News (No. 114 - December 16 1977) clipping: LT was trialling an energy-saving hydrostatic transmission in a Daimler Fleetline bus, developed at East Kilbride and tested in Chiswick. Colin Curtis believed it could reduce fuel use, engine wear, and enable regenerative braking.

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— CLondoner92 (@clondoner92.bsky.social) July 13, 2025 at 1:14 PM
The transmission shaft posed a major obstacle to incorporating a low floor in bus designs. One of the innovative ideas investigated by London Transport was hydrostatic drive. This system involved a pump feeding oil to wheel-mounted motors, with the simplest form only requiring hoses between the diesel engine and the wheel motors in the hubs, thus eliminating the conventional propeller shaft. Such a system also showed potential for regenerative braking, albeit with added complexity, and promised quieter operation compared to conventional transmission and rear-engine layouts. Furthermore, a significant advantage of hydrostatic drive was the flexibility it offered in engine placement, allowing it to be sited almost anywhere on the bus. This was particularly appealing to LT, given their "very unsatisfactory experience" with rear-engined designs.

To test this novel concept for public service vehicles (PSVs), the government-sponsored National Engineering Laboratory (NEL) at East Kilbride was enlisted. In the autumn of 1976, London Transport's DM 1787, a Daimler Fleetline, was sent to East Kilbride for conversion. The project's first stage involved converting the engine to drive a swash-plate fluid pump, which supplied high-pressure oil to four pistons in each wheel-mounted motor. All four pistons in each motor are used to provide the energy needed to set the bus in motion, but once started acceleration is achieved by cutting out two of the pistons to increase the oil flow to the others.

Despite these advancements, initial road trials in late 1977 revealed significant challenges. The "gearchange" mechanism, involving the cutting off of two motors, caused problems with seals blowing out due to oil having nowhere to go at that instant. While an expansion space resolved this, the drive noise was deemed unacceptable, and oil leaks under high pressures were excessive. Crucially, fuel consumption was considerably worse than conventional bus transmission in London conditions, achieving around 4½ mpg (1.57 km/L) instead of 6½ mpg (2.29 km/L). The transmission efficiency of this system was a little lower than that of conventional drive and work is continuing to try to improve this.

The planned second stage of the project aimed to incorporate regenerative braking, a system used on electric vehicles for many years but not successfully applied to oil-propelled motor vehicles. This would save fuel by harnessing energy typically lost during braking. Previous attempts at energy storage in motor vehicles largely focused on flywheels, but these proved impractical due to the size required for effectiveness. The NEL's approach involved converting the wheel motors to pumps during braking, delivering oil to hydraulic accumulators to slow the vehicle and store energy for acceleration. A secondary benefit of this system would have been reduced brake wear, as friction brakes would only be needed for final or rapid deceleration.

However, the conversion of DM1787 never progressed beyond the non-regenerative stage. Without significant fundamental development work, particularly on a control system for optimum efficiency and the incorporation of regenerative braking, substantial improvements in fuel consumption were not achievable. Such development would have been time-consuming and costly, likely delaying the XRM programme. Consequently, in 1980, the decision was made not to utilise hydrostatic transmission for the next generation of buses, and DM1787 was converted back to its standard configuration.

Challenges with Multi-Axle and Small Wheel Designs

Front, nearside, and rear drawings of the XRM.

The multi-axled, small-wheeled vehicle concept, while offering low step and gangway heights, presented other issues. Research was conducted simultaneously with the hydrostatic project into problems associated with small wheels and the steering of multi-axled vehicles. A second-hand Bedford VAL coach was purchased for this purpose in 1975, chosen for its twin steering front axles.

It was discovered that braking efficiency with eight small wheels would be inadequate, and current regulations did not allow retarders to compensate for deficiencies in the basic braking system. While a change in law was anticipated with the increased use of retarders, for the moment, small wheels posed problems for achieving a low, flat floor. Additionally, tyre design issues arose, as no manufacturers were producing suitable small tyres for PSVs at that stage, and including an unproven tyre design in the XRM was deemed unwise.

Investigations into steering geometry also revealed difficulties. The Bedford VAL's mechanical system with power assistance, using several links with ball joints, provided less positive steering than London bus drivers were accustomed to. While a rack and pinion arrangement was considered to eliminate ball joints, this type of steering suffered from poor self-centring characteristics. Unlike most LT buses, where steering resistance increased with the turning angle, providing better self-centring, the general wheel effort on a rack and pinion system was more or less constant.

Assistance from the Cranfield Institute of Technology, including a computer model based on London Transport's E3 route, indicated unacceptably high tyre wear on a four-axled vehicle. With twin rear axles, the steering geometry had to be based on a line midway between the axles, leading to a degree of "scrubbing" on all four tyres. Complications like steered or trailing rear axles were unacceptable. These steering and brake problems ultimately led to the abandonment of the twin-axle front configuration.

The decision to adopt a mechanical drive for the XRM, rather than hydrostatic, further militated against multi-axles, as mechanical transmission would have been too complex and inefficient to drive twin rear axles. The idea of having normal-sized wheels at the rear and small twin-axle wheels at the front to maintain a low second step was also deemed unfeasible due to the customary practice of fitting new tyres on front wheels and then moving them to the rear when partly worn.

By the end of 1977, the obstacles encountered made it clear that the XRM would need to depart from its initial vision. The revised design would feature single front and rear axles, normal-sized wheels, and mechanical transmission. However, the side engine remained central to the design, as it provided the crucial flexibility for door layouts.

Evolution of the Design and Ancillary Systems
With the Routemaster reaching the end of its design life, a review of its performance informed the XRM's development. The principle of front and rear subframes, which facilitated overhaul procedures at London Transport's Aldenham Works, had proven effective. Improvements seen in the FRM, such as the elimination of the front subframe and the direct fitting of the rear axle onto the underbody structure, were to be incorporated into the XRM.

The original concept of the XRM with four axles.

The braking system for the XRM was largely based on the Routemaster's, which had an excellent track record. The possibility of power-assisted handbrakes was considered if weight necessitated it over more common spring brakes. Manual handbrakes, though perhaps old-fashioned, had proven more reliable and provided immediate shoe clearance indication.

While the RM's steering had been satisfactory, the XRM presented an opportunity for rethinking power assistance. Conventional power steering systems were wasteful, circulating fluid continuously. Tests on RM 1368 with an accumulator system showed promise, as it only used energy when required by opening a valve to an accumulator containing oil under pressure. This system also opened the door to hydraulically-driven constant-speed alternators, offering savings in battery weight and cost. If hydraulics were to be used for all auxiliary circuits, air suspension would not be viable.
The four-axle XRM design used mainly longitudinal seats on the lower deck to fit around the wheel boxes.

Regarding suspension, coil springs, fitted to most RMs, had shown good life and were found to be the most cost-effective solution compared to leaf springs and air suspension. While air suspension offered constant step height regardless of load, beneficial for elderly or disabled passengers, it had disadvantages such as delays in pumping up the system and potential damage to airbags. Experiments with constant frequency coil springs on RM7 aimed to improve ride but encountered "coil slap" noise issues.

The Active Ride Control (ARC) system developed by Automotive Products, involving hydraulic struts at each wheel and independent wishbone suspension, was adopted for the XRM. This system not only allowed for reducing step height at bus stops but also provided a considerably smoother ride and automatically adapted to changing loads. The equipment was initially tested on RM1 and later transferred to RM 116 for service trials on route 9 in June 1980. However, issues arose with the hydraulic pump drive and inadequate electrical charging, leading to battery drainage and breakdowns. A gear arrangement was later fitted to the alternator drive to improve charging.

The announcement of MCW's Metrobus in 1977 introduced the use of a hub reduction axle on a bus for the first time. Tests with such an axle on SMS 303 indicated it was as good as conventional axles, and more importantly, it allowed for a smaller differential crown wheel, enabling a lower floor line. Consequently, a hub reduction axle was selected for the XRM.

The side engine concept also presented challenges. Servicing of auxiliaries like the fuel pump and injectors needed to be accessible from the offside, and LT desired the engine to be completely sealed off from the passenger saloon. At the time, no engines on the market fully met the requirements for power, accessibility, and size, although one manufacturer was willing to produce one. The required engine length depended on the vehicle's wheelbase and the need for a satisfactory transmission line to the rear axle, now that hydrostatic drive had been abandoned.

Consideration was given to producing the XRM in two different lengths: a standard 31ft 2ins (9.50 m) version with a 16ft 2ins (4.93 m) wheelbase, and a shorter 28ft 8½ins (8.75 m) version with a 14ft 6ins (4.42 m) wheelbase. The longer XRM would have seated 73 with standing room for 21, while the shorter design would have seated 65 plus 19 standees. The longer version could accommodate a proprietary six-cylinder in-line engine (Leyland L11) if auxiliaries were fitted on the opposite side. However, the shorter design could only fit a V-engine, which required overhead accessibility, making it unsuitable for siting under the stairs. A Mercedes V-engine was investigated and found suitable in other respects, but the shorter XRM would have required a specially modified engine.

The original XRM design was finalised at 32ft 6ins (9.91 m) in length, seating 41 on the upper deck and 26 (or 23 for OPO with AFC equipment) on the lower deck, with room for 22 standees. Most lower saloon seats were longitudinal, with only four sets of transverse twin-seats at the rear. After the abandonment of hydrostatic transmission and small wheels, greater use of transverse seating was envisaged. The design team also aimed to reduce the bus's weight significantly, targeting 8 tons (8.13 tonnes) for the standard XRM and 7½ tons (7.62 tonnes) for the short version, compared to the 9¾ tons (9.91 tonnes) of Fleetlines then being delivered. Lighter buses consumed less fuel and extended the life of mechanical units.
The 9.5 m (31 ft 2 in) XRM’s front and rear doors with twin staircases aimed to improve flow and enable crew use with open-platform, while allowing one-person operation.

The side engine offered theoretical flexibility in door and staircase layouts, allowing for front, centre, or rear doors, any combination, or even all three, along with the possibility of a second staircase at the rear. Drawings were prepared for all layouts, analysing capacities, passenger flows, and boarding/alighting factors. Three configurations were selected for final assessment: front door and staircase only; front and centre doors with a central staircase; and front and rear doors with front and rear staircases.

The two-door XRM featured front and rear staircases, with a lower deck layout for 26 seated and 17 standing passengers. The upper deck had 39 seats, giving a total capacity of 82.

The front and rear door design, with two staircases, aimed to reduce delays caused by standing passengers at a centre exit. It also allowed the possibility of use as an open-platform bus for crew operation, while retaining suitability for one-person operation (OPO) — similar to the three-door, two-staircase New Routemaster, which operated with part-time conductors and open platforms on six routes from 2012 to 2016.

However, this configuration performed poorly in economic assessments, mainly due to the space taken up by the second staircase. It was also unclear whether the Department for Transport would approve a new bus design with an open platform. Additionally, OPO use would have required automatically closing rear doors.

It was also concluded that the operational benefits of two different XRM sizes would be outweighed by manufacturing and maintenance disadvantages and costs, as the engineering differences would be greater than those between RMs and RMLs. While long buses reduced manoeuvrability, other less costly solutions existed than simultaneous production of two bus types.

Abandonment of the Project

By the summer of 1980, many of the revolutionary ideas for the XRM had been dropped, and the design had become much more similar to the Leyland Titan and MCW Metrobus. A crucial decision loomed: whether to invest in prototype production during a period of tight finances. The recession in the bus industry also made manufacturers unlikely to produce another design given their limited production runs. Furthermore, London Transport's standards were often too expensive for provincial operators, limiting the XRM's appeal elsewhere.

From an operational standpoint, it was believed that crew-operated vehicles would still be needed in central London for some time, and evaluations showed Routemasters were structurally sound and capable of many more years of service. Therefore, plans to produce 2,500 XRMs between 1985 and 1990 to replace the Routemaster fleet seemed unnecessary. If a new bus design wasn't needed until the 1990s, London Transport could benefit from the extended timeframe to observe technological advancements and potential alternatives to diesel power.

The XRM later evolved into a shorter, more conventional model with uneven pillar spacing at the centre door, single-stream boarding, and flat windscreens.


Ultimately, at a conference in late September 1980, Dr. David Quarmby announced the likely abandonment of the XRM project, a decision later endorsed by the Greater London Council (GLC). The project was formally terminated in 1981. London Transport decided not to proceed with the XRM at that time, opting instead to continue ordering the now satisfactory MCW and Leyland standard types. The system development undertaken for the XRM project was, however, intended to continue with a view to its potential introduction into a future version of the Titan or Metrobus.

The 8.75 m XRM offered better handling but less capacity. Two sizes proved uneconomical, so the 9.5 m centre-exit design was favoured. The shorter model featured a redesigned windscreen and reconsidered four-piece doors for safety.

The XRM program's only physical legacy was the aging Bedford VAL coach purchased in 1975. The project's abandonment was attributed to engineering difficulties, particularly with the hydrostatic drive and the four-axle layout. The unsuitability of a complex twin-steer layout for a conventional single rear axle and mechanical drive was also a factor. The looming economic recession, which deeply impacted bus operators and manufacturers, further discouraged the development of a completely new double-deck bus.

The Unseen Legacy: QRM and Beyond

Despite the abandonment of the XRM, the core idea was not entirely discarded, and some work continued on a related project, the QRM. This essentially revived the side-engined concept of the Q-type double-decker from the 1930s, incorporating RM and FRM design features, such as the rear suspension. The financial climate, however, was not conducive to converting the withdrawn rear-engined Routemaster, FRM1, into a QRM. The Maxwell gearbox, then in development, offered flexibility in drive line placement, which would have been advantageous for a side-engined bus, but its development was halted due to market recession.

While London Transport ceased its policy of developing its own vehicles, the fundamental ideas persisted. MCW's Metrobus Mark II, with its standardisation and use of glass-fibre, led to considerations for the next stage. The focus on accommodating elderly, infirm, and disabled passengers continued, leading to the development of a split-step entrance concept in conjunction with Ogle Design. There was also a desire to return to independent front suspension for improved riding and stability, which were to form the basis for the Metrobus Mark III.

An underframe was produced in close collaboration with MCW, designed to accommodate a rear, side, or even front engine. However, this idea was reportedly scrapped due to London not ordering Metrobuses that year, facing pressure from Leyland. This concept would have greatly benefited the industry through standardisation in body design, offering operators flexibility in engine placement and body customisation.

The thoroughness of London Transport's investigations into new ideas, even those that did not come to fruition, was a testament to their approach. While some might argue that such work should be left to manufacturers, manufacturers require encouragement and commercial viability for new ideas. The close collaboration with AEC, for example, proved highly beneficial to both London Transport and the wider bus industry.

In conclusion, the XRM project, though ultimately shelved due to a confluence of engineering difficulties, economic pressures, and a re-evaluation of fleet needs, represented a significant effort by London Transport to proactively shape the future of its bus fleet. It aimed to address critical issues of passenger comfort, boarding efficiency, and operational flexibility through innovative design and technology. While the XRM itself was never built, the extensive research and development undertaken contributed to a deeper understanding of bus design challenges and influenced subsequent developments in the industry.

I would like to extend an invite for you to follow me on X (formerly Twitter) for transport-related updates. You can find me by searching for @CLondoner92 or by clicking on the direct link to my X page here. I am also present on BlueSky and Mastodon. I look forward to connecting with you on these platforms. Thank you for your support.

Sources:
London Transport Scrapbook 1977 by James Whiting
London Transport Scrapbook 1980 by James Whiting
Colin Curtis 40 Years of London Transport

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