Andy Goundry raises a glass in celebration of the UK`s most popular PSV chassis in recent times – The Dennis Dart.

The Midi Marvellous:

Ask the man in the street – or even the proverbial man on the Clapham omnibus – to name Britain’s best-selling bus, and it’s odds-on that he would offer up the name of Routemaster.

And yet Routemaster was almost unsold outside London, with export sales being almost non-existent. In contrast, the Dennis Dart sold over its’ twenty-five year history (to date) to almost every bus operator in the land, in large numbers, not to mention significant export volumes to countries as diverse as Hong Kong and Australia. Dart buses infiltrate probably every town centre, village and housing estate in the land, and rarely a day goes by without each of us seeing at least one, and more likely several Darts, Dart SLF’s or Enviro 200’s quietly going about their duties, most likely without recognizing what they were seeing. To put the numbers into perspective, well under 3,000 Routemasters were built, compared to some 17,000 Darts and its successors to date.

So how come a tiny (at the time) bus builder in Surrey came to create what has become Britain’s best-selling bus? In the mid 1980’s, Dennis were a low-volume manufacturer of niche vehicles, mainly fire engines and refuse collection vehicles plus a small number of buses and coaches.

Some years before, Dennis had in fact had some success by spotting an opportunity for an economical, simple and cost-effective coach chassis. Dennis realised that in contrast to the then-current large engines then in use, which took up a lot of space within the coach, it would be possible to use the new Cummins C Series engine which whilst offering suitable power and torque, was sufficiently compact to be packaged vertically under the midships floor of the coach rather than horizontally as was current practice. The engine was also extremely economical, and fuel economy was further improved by the low unladen weight of the chassis.  That coach under the name of the Javelin went on to be a very successful product that remained in production for nearly twenty five years.

By 1987, the UK bus market was at a low ebb, in that the traditional bus grant on which most operators relied to fund the purchase new vehicles had been phased out and the entire market deregulated. Deregulation reduced at a stroke the entry barriers to new operators setting up in business, resulting in large numbers of small panel-van-based buses making an appearance; their increased manoeuvrability allowing bus services to penetrate much deeper into residential estates than had been possible with conventional full-size buses.

Dennis’s much-respected Chief Designer Richard Norman reasoned that these van conversions were built to an altogether less robust standard that conventional buses and were unlikely to stand the test of time. He therefore concluded that a small single deck bus built more in line with traditional standards, yet small enough to access similar narrow winding roads to the “bread vans” as they were disparagingly called, could have reasonable sales potential.

Unsurprisingly there was some resistance internally, on the basis that this was a non-existent market sector, however after Richard developed preliminary proposals, the Dennis team were won over and the project was approved.

Codenamed DM88 (simply Dennis Midibus for launch in 1988), the smaller size of the bus meant that component sizes could be reduced without compromising reliability. The real masterstroke however was that the cumulative weight reduction was such that 19.5” wheels could be used rather than the traditional 22.5” rims, giving much-reduced wheelarch intrusion. This freed up passenger space within the saloon permitting a narrower width bus thus ensuring manoeuvrability was maximised. Dennis marketing team felt that it could be possible to sell a hundred or so of these midibuses a year; predictions which events subsequently demonstrated to fall far short of the mark!

The chassis was to adopt a simple T-drive configuration, using, as with Javelin, a vertical Cummins 6BT 6-cylinder engine together with an Allison AT545 automatic transmission, both well-proven components albeit unusual in bus applications. As Dennis specialism was in chassis design and development, they recognised the need to work closely with a like-minded bodybuilder in order that the unique packaging benefits of the chassis were not compromised by thoughtless bodybuilding. The selected partner was Duple, a sister company of Dennis at that time, who produced  a dedicated, John Worker styled, body, which was duly launched as the Dart midibus at the1988 NEC Commercial Vehicle Show as a 9 metre long, 2.3 metre wide vehicle.

Some success was achieved with sales into London operators, who probably more than anyone had discovered the reliability shortfalls of the van conversions; however, early sales were curtailed by a Group restructuring which involved closure of the Duple factory at Blackpool. Fortunately however, a number of other bodybuilders saw the potential for this new size of bus and took up the challenge. Notable was Plaxton with the Pointer body, and Alexander with their Dash. Alternative lengths of bus were also developed, ranging from 8.5m to 9.5m.

Over the early 1990’s the reputation, and sales, of the Dart grew tremendously, with over 1,000 buses a year being produced at times. The range developed steadily from that original 9m right hand drive only version, to encompass a range of body lengths from 8.5m to 9.5m, with left hand drive variants for a range of markets including North America. Perhaps the most specialised version was a gas-powered variant.

However, the bus market of the early 1980’s was once more undergoing a sea change with the advent of low-floor buses offering improved access to their users. The low-floor buses then available were, however both expensive and unreliable, due to the need to adopt independent front suspension to achieve a lower saloon floor over the front axle. Once again Dennis engineering innovation came to the fore, with the realisation that it might in fact be just about possible to forge a front axle beam with a deeper centre drop than conventional practice, allowing the desired low floor over the front axle to be achieved in a much simpler and cost-effective way than independent suspension.

Dennis’s axle supplier, Kirkstall Forge (later to become Dana) in Leeds accepted the challenge of designing and developing this deep-drop beam with great success, the only compromise being the need to widen the vehicle slightly from 2.3m to 2.4m to accommodate the wider seats mandated by legislation.

Thus was born the Super Low Floor Dart, which took sales to further new heights both in the UK and overseas and in Dennis’s centenary year in 1985, over 1,200 chassis were produced. As well as the increased width, lengths of buses offered grew as far as 10.6m at the urging of the sales force, who were determined to see the Dart concept take sales from full size single deck buses. These lengths pushed the gross weight up as far as 12 tonnes, and in truth were a step too far as they rapidly showed up the limitations of the Allison AT545 transmission, which were only really resolved with the introduction of the more robust Allison B300 on the longer vehicles, and the Allison S2000 unit on the rest of the model range.

Despite suffering severely when Dennis went into administration in 2003 after a period of mismanagement by Mayflower, production was restarted by the new owners Alexander Dennis Limited, or ADL, and continued right up to 2007 when it was superseded at least in name by the ADL Enviro 200, although in truth the chassis of that model continues on virtually unchanged from that of Dart SLF. Enviro 200 has continued the run of success, with almost 4,000 being built to date.

17,000 vehicles over 25 years, and still increasing – not bad for something that was expected to sell a hundred!