Buying Tomorrow’s Buses Today, Part 3: Doors
Ned Einstein (Transportation Alternatives, New York, NY)
Dennis McNeill (Alternative Solutions International, Thousand Oaks, CA)
Apart from brakes and tires, no pair of bus or coach components is as responsible for as many incidents and injuries as doors and stepwells. This installment of NBT deals with doors. The next installment, Part 4, will address stepwells.
No part of a bus is as needlessly complex as the rear door operating system – including its interlock with the brake system and/or throttle. Current motorcoach rear door configurations involve only “passive lifts” (with the exception of a handful of new motorcoaches with “center stepwells”). However, the “active lifts” common to transit buses loom on the horizon for motorcoaches, particularly in those sectors (commuter/express and sightseeing) where an additional entrance door could accelerate full-coach boarding and alighting. For this reason, the discussion below will encompass both front and rear doors.
Formats and Implications
With slight variations, there are basically six types or formats of bus and coach doors:
- “Plug” doors are comprised of a single panel that swings outward on a vertical, full-door-height hinge. They may or may not be cantilevered. The door’s inside surface typically contains a single, oblique handrail whose upper edge lies near the hinge (so that the handle slants toward the stepwell handrail when the door is open). Part or most of the door surface is often made of glass or Plexiglas. Plug doors may be configured to operate manually and/or pneumatically.
- “Sliding” doors consist of one or two panels which move laterally alongside the outside of the body when pushed or pulled manually. Most sliding doors contain small windows, and handles and locks mounted to both the outside and inside surfaces. Except in rare cases, sliding doors are common to OEM vans, and are left in place during in their “conversion” to larger vans or minibuses. Most of these doors contain a slight cantilever when the door is fully-opened (requiring a slight tug to close them from this position).
- “Jack-knife” doors contain two panels separated by a center-positioned, full-height, vertical hinge. Jack knife doors swing open outward or inward along a second hinge at the body. When they open inward, a single panel may exceed the depth of a normal bottom step. If so, either the bottom step must be deepened, or the next step must be configured to accommodate the “V-shaped” wedge (containing the hinge) intruding into it. Both panels typically contain glass or Plexiglas inserts, often from top to bottom (less a margin of frame area for structural purposes). Jack-knife doors can be configured to operate manually or pneumatically (while pneumatically-configured variations can generally be opened manually with the application of reasonable force, except where “locked out” or locked shut).
- Single-panel “Bi-Fold” doors separate at the center, and fold inward, requiring a deep bottom step (to avoid the penetration of the hinged “V” section). Glass or Plexiglas inserts generally run from the top to the bottom. Bi-fold doors are generally configured to operate pneumatically, but like jack-knife doors, can generally be opened manually with reasonable force (except when “locked out” or locked shut).
- “Dual-panel bi-Fold” doors, which also open inward, contain two pairs of vertically-hinged panels. Upon opening, the pairs of panels separate and fold toward the sides of the stepwell. Because each panel is only one-fourth the width of the total door entry area, the panels do not exceed the depth of a normal step. Therefore, dual-panel bi-folds do not require any modifications to the stepwell.
- “Pantographic” doors are essentially single- or double-panel plug doors operated pneumatically (or hydraulically). Instead of swinging open on vertical hinges, the panel (or panels) is pulled outward and to the side(s).
Rear stepwells with “passive lifts” employ simple plug doors, whereas stepwells with “active lifts” (i.e., the lift platform is transmuted into a stepwell) generally use bi-folds or outward-opening jack-knives. Sliding doors are installed mostly on van and minibus conversions at the OEM level.
Costs generally mirror this same progression, with pantographic doors being the most expensive in terms of capital costs. In contrast, bi-folds and jack-knives are typically the most expensive in terms of operating costs, as they are generally the most problematic – particularly when they include interlocks.
Doors and Interlocks
As a safety feature, many doors (mechanically-operated plug doors excluded) are designed to interface with the power train and/or brake system. This interfacing – referred to as “the interlock” – prevents or curtails the simultaneous movement of the vehicle with the door (or doors) open:
- When the vehicle is stopped with the door(s) open, the interlock prevents the service brakes from disengaging and/or the throttle from supplying fuel to the engine (keeping the bus in place until the door(s) is closed).
- When the vehicle is moving, and the door(s) opened, the interlock engages the service brakes and/or cuts off the supply of fuel to the engine (thereby shuddering the bus to a stop).
Interlocks are not always configured on front doors – since drivers presumably have a direct and proximate view of the front stepwell and passenger/pedestrian activity within and around it. In contrast, drivers must monitor passenger and pedestrian movement within and around the rear door(s) via rear-view mirrors. For this reason, rear doors (including those with “passive” wheelchair lifts) are almost always configured with interlocks.
Because it connects the door operating mechanism to the vehicle’s brake system and/or the power train – and the layers of mechanical, electrical, pneumatic and/or hydraulic components and subsystems between and among them – the interlock increases the complexity of all these components almost exponentially, and provides substantial opportunities for negligent design, engineering, configuration and maintenance. At the maintenance level, trouble-shooting door-operating failures among several subsystems and thousands of parts can be daunting, and can remove a bus from service for long periods of time.
Complexity and Confusion
At the operating level, the complexity created by the interlock may be compounded further by differences in the configuration of door-opening controls from bus-to-bus:
Where drivers operate more than one model of bus, they may not know from day to day how the doors even work. Line inspectors encounter similar problems when checking the interlocks. In passenger service, the ripples from this lack of uniformity can translate into mayhem. To optimize safety, the door control configuration of every bus in the fleet must be configured the same. And this configuration must be explained carefully to new drivers with previous experience driving buses in other fleets – even if the same model of buses.
Doors whose operations are not intuitive to the passengers also require signage. Yet door signage is often inaccurate, and sometimes mystifying. Particularly where multiple operating choices exist (e.g., the door must be “engaged,” and then opened either (a) by physically pushing against it, or (b) by depressing a “yellow tape” for it to open automatically), inadequate signage may compound the confusion that multiple variations already present to alighting passengers. :
Where signage is warranted, it must be both brief and comprehensive, and must describe how the doors actually work: Many or most passengers will not even read it. As a result, boarding and alighting must be logical and intuitive, and must match the passengers’ experiences in stationary buildings as closely as reasonably possible.
The more exotic the door operation, the more precise the instructions must also be for the driver. As an example of the worst (from an anonymous Operator’s Manual), what does this information convey? What risks does compliance with such instructions present?
WARNING: Apply the service or parking brake before placing the door master switch in the ON position. The service brake interlock will be deactivated and the vehicle may roll.
Door configurations also have implications for mirror mounting and usage, as well as for door construction materials. For example, with a plug door or outward-opening jackknife door, the door itself may obstruct the driver’s view of the “danger zone” surrounding the curb-side rear tires via the curb-side, exterior, rear-view mirror. I have actually seen such mirrors mounted to the door (instead of to the body) – giving drivers the choice of viewing the danger zone either while driving or while loading/unloading – rather than during both. Otherwise, when the driver must view the danger zone through the door (via the curb-side exterior mirror), it is important that this vista is directed through glass or Plexiglas – not obstructed by the door’s structural members.
Forces and Foibles
Two caveats will enhance the safety of door designs – particularly in the rear stepwell:
- If the door operates in more than one mode (e.g., passengers can push it open, or trigger it to open automatically by pressing on a strip of tape), design and maintain the door so that, when it swings closed, a similar degree of force can stop it – regardless of the mode by which it was opened.
- Eliminate the notion of door-closing timing intervals for semi-automatically-opening doors (i.e., passengers trigger them open once the doors are “engaged”): Once the passengers open the doors, design the doors to remain open until the driver closes them.
Much of the complexity of rear doors evolved as a response to the driver’s often-compromised visibility of passengers inside or outside the rear stepwell. To compensate, partial control of rear door operations has been given to the passengers. However, the emergence of 360-degree video-cameras (e.g., Tiger Mirrors) provides an opportunity for drivers to view the areas surrounding the rear stepwell even if the passenger compartment is crowded with tall standees. With such innovations, manufacturers can now return control of the rear doors to the driver – reducing much of the ambiguity and complexity.
Maintenance and Mischief
One nagging problem compromising passenger safety is the need to maintain safety features related to the interlock. Paramount among these features is the sensitive edge. In many cases, this device (or certain subcomponents) cannot even be repaired, but instead, must be replaced. Similar problems occur with step treadles – although these devices have fallen out-of-favor (as they should have), and on many buses containing them, have simply been dismantled. As with sensitive edges, many components of step treadles cannot be repaired, and must instead be replaced. Because these features comprise part of the interlock, malfunctions in the brake system or throttle (and in subsystems connected to them) can lead to failures in the sensitive edge or treadle, and trouble-shooting them can be difficult and time-consuming. And the complexity involved may lie beyond the scope of many mechanics. As a result, the regular and reliable operation of these features may require a higher degree, and greater frequency, of coordination and oversight.
From a safety perspective, the best sensitive edge is one that works. And while buses would be better off without step treadles to begin with, those buses which contain them were designed with other features to work in concert with them – including signage, handrails, door-timing mechanisms and other interlock elements. If the step treadles are dismantled, the purchaser must make sure that other door, stepwell and interlock elements are rendered consistent with it. Communicating the disengagement of the step treadle to passengers accustomed to using it is of critical importance, since their stepping onto the bottom step will no longer trigger the doors open (assuming the driver “engaged” them), and they will have to “transition” to the new function (or non-function) to avoid a litany of possible injury scenarios – including walking forward into closed doors. At minimum, if a bus operator dismantles a step treadle, he or she should dismantle all the step treadles on every vehicle in the entire fleet, and conduct a comprehensive campaign (including new signage and driver instructions) to inform the passengers about it.
From a maintenance perspective, it is also imperative to obtain and retain the specific operating and maintenance instructions for every element of the door operation. Keep in mind that the same door manufacturer not only produces the same doors for multiple bus and coach manufacturers, but may supply different configurations for the same doors on the same bus models simply produced for different customers. Operating and maintenance manuals generic to both the bus or the door may not identify or describe the variations on a particular vehicle.
Because most door components are sealed, wear-and-tear may also occur gradually and, often, unnoticeably. Drivers may check the door-opening operation during a pre-trip inspection. However, all drivers do not understand the interlock. Few understand all its intricacies and nuances. Further, a single driver cannot even test certain features of the interlock, like the rear door’s sensitive edge – since the door must be closed from the driver’s compartment while the sensitive edge must be tested from the doorway. Further, opening and closing forces are rarely measured (at either the maintenance or operating levels) to see how easily the doors might be forced open from a passenger’s inadvertent loss of balance or horseplay.
For these reasons, doors must be checked often by mechanics. Along with the proper operation of door features, one should periodically measure the variation from the manufacturer’s established performance criteria. These criteria often include opening and closing forces, opening and closing speeds, and timing. While a mechanic can effect radical changes in door operations by simply shifting the position of an actuator rod, the same changes can and often do materialize over a period of time from both wear-and-tear and abuse.
Management personnel must also be aware of quirks and tricks:
- By depressing the “yellow tape” for an extended period of time (e.g., 30 seconds), the interlocks of some door systems can be dismantled.
- Kneeling a bus with the front door open can jam the interlock even though the front door operation is not directly linked to it.
- Door-opening forces can be altered by rerouting the interlock to bypass the neutral and brake switches (see “The Mysterious Force” in NBT, December, 2001).
Once the interlock is compromised, doors may open with less force than needed, or more force than needed. Either alternative may prove dangerous.
Don’ts and Do’s
While a bit over-simplistic, here are some general principles to follow in selecting doors:
- Simple is usually better than fancy.
- Select features for the least-capable and least-sophisticated passenger.
- Select features you intend to inspect, test, repair and/or replace regularly.
- Maximize the degree of continuity in features and forces.
- Focus maintenance and inspection efforts on the interlock.
At the same time, one must be careful of absolutes: There is no best type of door. And there is no best type of interlock. Otherwise, the best door and best interlock is simply one that works. With respect to both safety and liability: You own it, you fix it.
Kudos and Cautions
Given how advanced buses and coaches already were a decade ago, many recent developments are breathtaking. Some refinements – like the improvements to wheelchair lifts mandated by FMVSS #403 and #404 (many of which were suggested and encouraged by lift manufacturers) – have gone overboard to protect the passengers. Kudos to all these players, regulators and regulatees alike. All members of the public transportation community should be intensely proud of these developments.
At the same time, these developments can haunt us as a liability matter if we fail to take all reasonable and prudent measures to ensure that they are intelligently thought-out, or if we fail to monitor, inspect, adjust, repair and replace them, their components and/or their interrelated parts, when they fail to function as designed and intended. Failing to exercise these responsibilities can transform a tool into a weapon. As we all know, if one fails to keep a gun clean and lubricated, it may backfire.