• DOOR CONSTRUCTION
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THE folding, mechanically operated door, as fitted to the front-entrance bus, is the simplest type from the constructional point of view. The height of the glazed upper portion and the depth of the belt panel are determined by the size of the corresponding parts of the main saloon. The bottom of the door closes on the step, which must not be less than 10 ins. and not more than 1 ft. 5 ins, from the ground.
Each half of the door consists of two uprights and three horizontal members, which are conveniently called the top, middle and bottom rails respectively. Each rail is tenoned into the stiles.
Providing a Foundation for the Belt Mouldings.
The middle rail is about twice the depth of the top one in order to provide a foundation for the upper and lower belt monldings, and on the front half of the door to ensure a firm fixing for the operating gear.
A. substantial bottom rail imparts solidity at the step level, where the door is liable to come into contact with the passengers' feet.
The rear half of the door is moved by the other section, but some form of control is necessary in order that B22 it shall fold and extend in a proper manner. Some builders advocate only a top guide under the cant-rail, so that the step requires no guiding slot. This slot, if an open one, is liable to create a draught and, if enclosed underneath harbours dirt.
On the other hand, a top and bottom guide functioning in unison should ensure smooth working and a minimum amount of strain on the door. Further, an enclosed bottom guide may have curved ends to facilitate cleaning.
The Design of the Door Operating Gear.
The door gear is designed so that the operating handle has still a short distance to travel after the rear half of the door has made contact with the body pillar. The door stile has a rubber tube or channel fastened to its edge, which is compressed during the final movement of the control handle, thus making a weathertight joint.
As the rubber will mark a painted surface, it is a good blart to cover the pillar face with plated metal. ' The modern passenger chassis is seldom more than 2 ft. high from the ground, consequently only one step is required between the ground and floor. This makes ascent and descent safer, because if there be an intermediate step, it must be approximately triangular in plan, in order to provide clearance for the movement of the trailing half of the door.
Determining the Width for the Single-step Tread.
The difference in overall width of the chassis and body determines the maximum width for the single-step tread. Support is provided by the entrance pillar on each side, and the adjacent members of the bottom framework, also the cha§sis itself is available for securing step stays.
From the technical standpoint, the outwardly hinging coach door is one of the most interesting parts of the body. It is curved from side to side as well as from the waist down wards. The door is about 3 ins, thick, because it often accommodates a drop window, on each side of which there must be strong framework.
The Reason for Bevelling the Pillars.
On the lock side the door and body pillars are bevelled. This bevel corresponds with the arc of travel which is determined by the position. of the pins of the hinges and the width of the door. Bevels are necessary in order that the clearances of a comparatively thick door may be reduced to a minimum.
In theory the bevel required at the bottom of the door is different from that at the waist, because the hingepin line is vertical, but the line of contact of increasing the door retreats to an amount from this hinge In practice an average bevel is found which is satisfactory so long the turn-under is moderate. The bevel of the door pillar is also influenced by its position in relation to the longitudinal axis of the body. A door parallel with this axis requires a different bevel from one which forms an angle with it.
A door pillar has to be large enough to accommodate the rebate for the window and the spindle which operates the lock of the bolt. The inner face of the pillar, where the window grl...ork., is formed, is at right angles to the plane of the sheet of glass.
The groove of the other door pillar is also at right angles to the same vertical plane, therefore the window slides in a pair of grooves having parallel faces of contact..
A door sliding lengthwise is suitable for the front entrance of a bus which is of maximum width, say, not less than 7 ft. 4 ins. The 2 ins. or 3 ins, taken up by the thickness of the sliding door and its casing are allowed for by making one row of crosswise seats a little shorter and the central gangway slightly narrower. This is, however, possible only when the other seats and the remainder of the gangway are in excess of legal requirements. A sliding door to a rear entrance will not affect the size of the adjacent seats, unless the back one was intended to accommodate five passengers.
Features of the Floor-level Sliding Door.
A door which slides at the floor level does not demand any special constructional modification of the body, but when it moves at the step level it has to pass a cross-bearer. This cross-bearer is shortened and to it is fastened a pillar with an inside rail at the waist and roof levels, so that a casing is made to enclose the door between this inner framework and the outside of the body.
As the outside pillar is not secured to the cross-bearer a slot is formed, which is curved to agree with the turn-under of the body and door.
Framework Alteration Necessary.
It will thus be seen that if an entrance has a hinged door, and a sliding one, as just described, is to be substituted for it, some alteration of the bottom framework is necessary. This alteration is obviated if the door can be mounted outside the body without exceeding the prescribed limits of overall width.
Too much attention cannot be given to the construction of doors for the bodies of motor vehicles, because they have to withstand harder wear than does, probably, any other part of the coachwork. This is particularly noticeable in the case of stage carriages.