Tips for Defining Trams

All axles on a tram must, by definition, be guided. Each front or rear axle group is reduced to an effective contact point (ECP) and these are used to calculate the movement of the tram. However, it is not always easy to determine whether an axle, or its wheels, are fixed or free to turn, and if you define them incorrectly, the tram may be undriveable. To avoid undriveable vehicles, two rules must be observed when defining a tram:

• Each tram must have at least one unit with 2 guide points (i.e. a front axle group and a rear axle group) but this need not necessarily be at the end of the tram.
• There must be only one guide point adjacent to each articulation point. For example, if you define a rear guide point (axle group) on unit 2 then there must not be a front guide point on unit 3; and conversely, if there is no rear guide point on unit 2 then there must be a front guide point on unit 3.

Note that it is implicit in these rules that the first unit must have a front axle and the last unit must have a rear axle.

You should consider the following questions when defining a new tram:

Which unit is the 2-ECP unit?

Most trams have a clear candidate for the required double ECP unit. However, if it is unclear you should bear in mind that a two axle bogie cannot be regarded as two separate contact points if the bogie articulates with respect to the unit chassis. If the bogie does not articulate with respect to the unit chassis then you can consider the centres of the two axles of the bogie as separate effective contact points.

Where are the pivot points ?

The pivot points are where the tram chassis articulates. The tram chassis (and therefore the tram body) is assumed to be rigid between the pivot points.