Let's talk HO/OO wheels and axles

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Disclaimer:  I am not a design expert or a professional engineer, I just have nearly 40 years modelling railways.  This is a guide only meant to share my experience and hopefully shed some light on a confusing and sometimes frustrating topic.

The most important (and basic) element of any model railway are the wheels and axles on which the locomotives and carriages run.  One would think that it should be a fairly standard and trouble free area, but it can be a labyrinth. 

Firstly there is the height of the track.  Track height is measured in hundredths of an inch.  Most old track was 1/100 of an inch tall and this is known as Code 100.  Unfortunately this is not to HO (1:87) scale.  Track is now available in 83/1000", known as Code 83 and even 70/1000", known as Code 70.  These fine scales are more realistic and more closely approximate the relative size of rails in HO scale modelling.   The upshot of this is that the older Code 100 rolling stock was manufactured with wheels having deep flanges.  These flanges do not run evenly on the shorter Code 83 track, having a tendency to ride on the ties and foul on the frogs of points (turnouts) causing derailments.  Newer rolling stock with the Code 83 compliant flanges should run on Code 100 track, though. 

As manufacturers looked for ways to lessen manufacturing costs and increase profits, they turned away from standard metal wheels and introduced plastic sets.  These were naturally insulating and lighter and the casting was easier.  It did have "cheapness" issue, though.  While metal wheels were machined smooth on the inside surface, many of the plastic wheels still have dags from their molding.  This meant that the supposedly smooth inner surface of the wheels fouls the frogging of points (turnouts) causing a characteristic "hopping" and even derailments when crossing points.  The answer lies in a quick file to smooth the wheels, but the cause of the problem is not always immediately evident.  Additionally, it often led to cracks in the flanges and the "crescent moon" where pieces of the flange broke away.

Secondly there is the gauge.  HO/OO gauge is 16.5mm or 0.65 inch between the inner faces of the two tracks.  Fairly simple that one, until you get to manufacturers and the overall width of the axle. HO scale is 1:87, OO is 1:76.  Consequently OO scale rolling stock is marginally bigger than their HO counterparts.  This means that while the distance between the faces of the wheels will be much the same, the length of the axle differs markedly.  To further muddy the waters, even individual manufacturers have tinkered over the years with the axle lengths of their products!

In my experience this has resulted in three main axle lengths: 24, 25 and 26 mm.  There are ranges within these also, but generally for example an axle 23.8mm fits into the 24mm group, where an axle of 24.4mm fits into the 25mm range.  This might not seem like much, but if you force a 25mm axle into a 24mm spacing, the wheels will not turn and consequently will develop flat spots not to mention the inevitable derailment caused by the added drag.  Slipping a 24mm axle into a 26mm spacing means a carriage that appears "down on its haunches" and often falls off its axles.  OO scale, because of its bigger size generally exclusively use the 26mm axles. 

Unfortunately, and here my experience is with Lima, the same carriage mold manufactured some years apart, can have different axle lengths.  So if you have wheels that find it difficult to turn even after copious and liberal lubrication, the chances are that the axle is too large for the carriage.
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