The arrangement of wheels and axles under a tram has a great effect on the quality of ride, the ability to go around curves and the extra road space needed to allow for bodywork overhang on curves.
The distance between axles is known as the 'wheelbase'. On a four-wheeled tram, the wheelbase is often an unhappy compromise.
If the wheelbase is too great (Fig 1)
On a sharp curve, the front axle will be attempting
to pull the car around the corner but, because both axles are
rigidly maintained parallel (there is no steering) the axles will
be turned across the track and the wheels will attempt to climb
out of the rail groove. In practice a situation is reached where
the front wheels are trying to mount the rails towards the outside
of the curve and the rear wheels are attempting to climb the inside.
More on guidance systems
A special track cross-section used to be made for curves (the British Standard number being suffixed 'C') with a wider than normal groove, which prevented the wheels binding in the grooves when they ran at an angle but did nothing to counteract the tendency to climb out..
If the axles are too close together (fig 2)
The long body of the car will act as a lever,
magnifying any small irregularities in the track into wild swinging
movements. On corners, the bodywork will perform a wide sweep.
Attempts were made to avoid excessive body movement on older four-wheeled designs by limiting the compliance of the springing. Whilst this reduced the swinging and plunging movements to a less nauseating level, it was not particularly effective against vertical vibration and many humorous stories relate to anatomical effects caused by the shaking to which these early vehicles subjected their passengers.[Ref]
Picture of 4 wheeled tram truck and springs, Black Country Museum, Dudley
The compromise was avoided by the use of 'bogies' or pivoting trucks, one under each end of the vehicle (Fig 3) .
The wheelbase of a bogie could be made relatively
short, to reduce binding on curves, but the placing of two pivoting
bogies, one at each end of the vehicle, stopped the undesirable
bodywork movement and allowed the use of softer springs.
This resulted in a much more comfortable ride and less stress on the track and bodywork.
With only two motors, this arrangement made
the vehicle prone to slipping on hills because only one axle on
each bogie was driven, so only half the weight was on motor-driven
To overcome this problem, without increasing the cost by adding more motors, the 'maximum traction' bogie was developed (Fig 4).
The pivoting centre, which supported the weight of the bodywork, was moved towards the motor-driven wheels so that these took a greater share of the weight. At the same time the non-driven wheels could be reduced in size as they suffered less wear and merely acted as 'steering' for the powered wheels.
Most London tramcars were on 'maximum traction' bogies which were an economical compromise, sufficient for the flatter terrain. Hillier parts of London were served by special 'HR' (hilly route) cars, fitted with equal wheel bogies (as discussed above) but having four motors instead of just two. This gave full traction on all the wheels to cope with the gradients and the extra motor power also provided good acceleration on the flat.
In Bath there are many steep hills which preclude the use of any trams which don't have all wheels driven. Many articulated trams with modern low-floor design are unsuitable for Bath because they do not have room for motors underneath the floor, so it would not be possible to drive all of the wheels. An examle of this is the prototype T.P.L. Pullman vehicle, intended for Bristol, it has 80% traction because the centre wheelset, which carries 20% of the weight, is not driven (Fig 5). However, this vehicle is available in other formats which are more suitable.
and specification of prototype
Design of prototype
Track heating on hills
Most articulated trams in an affordable price range have less than 100% traction so another solution was sought for Bath.
In order to eliminate non-driven wheels, the articulation may be dispensed-with and a single body substituted. The passenger carrying capacity lost by this can be restored either by a motorised trailer, which has many disadvantages, or by adding an upper deck.
Double decked trams have been suggested for
Bath as they also have a number of other advantages
.More about double-deckers?
Double-decker tram pictures:-
Artist's impression of modern vehicle
Older design at Grand Parade
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