The two words "model" and "engineering" coming together in model engineering certainly makes for interesting discussion!
The philosophy that we take to build a working miniature entirely governs how long we take, where the detail is required, and the ultimate intended outcome. Thinking backwards from the intended outcome all the way back to the design is crucial, as it will form the rules how to miniaturise the real thing.
For instance, the givens for most of our model locos is that we expect them to run on a club track, but this requirement brings about some key changes to the model that we can't always scale down.
The width of the wheels and flanges needs focus, and thoughtful consideration given to what the loco is expected to do and look like. These wheels must be suited to the track, and a key relationship to understand here (not often discussed) is overhang of the wheel over the rail for 100% contact on curves, out of gauge track, and oil slinging. If the wheel face is shy of or over the rail head, particularly on steam engines, the oil on the wheel from oiling the axleboxes and the side rods will be slung off, greasing up the rail...we've all been to the club runs where second day in, the track is like a skating rink thanks to a few greasing locos!
Another thing to consider regarding wheel width is the flange. Leading flanges especially on 8 coupled driving wheels do suffer significant wear on cornering, and hence why we have a wider AALS flange. This also happens on full size, I can describe a few full size 8 coupled locomotives with lubrication on the leading flanges to try counterract this.
For 5" gauge, combining the width of AALS flange and width of tread to overhang a 10mm flat bar rail on a curve would make a wheel approximately 18mm wide. The full size wheel in 5" gauge would scale from 5-1/2" wide down to 12-13mm. Using a fine scale wheel of 9/16 (14mm) would work and look prototypical with an 8mm flat bar rail, but the fine scale flange has 1/16" less material than the AALS flange.
The wheel width can have a reducing effect to the scale spacing of the frames. This comes into consideration when a smokebox or firebox has to fit between the frames. Not many impacts to a bar frame wide firebox loco.
The second impact of wider wheels is the possible reduction of bearing width on the rods, and tighter clearances of rods and crank pins behind crossheads.
A way to overcome these problems is to consider increasing the scale of the loco. I am one for boosting the scale factor slightly, which I hope to discuss more in future.
So there are some things to consider when building a dream loco, what are the main requirements, and how to get a philosophy to help make life easier building the model. It's always good to know what the implications are of making some key choices.
Happy steaming!
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