Friday, August 10, 2018

Determining the Wheel Diameter

I've been having a bit more of a think about the wheels for Ivor after yesterdays discovery that they are probably too big to both fit inside the splashers and next to each other. It's also been pointed out to me that, normally the centre line of the buffer on standard gauge stock sits 3' 6" above rail height, which would be 14mm in 4mm to the foot scale. While Ivor isn't exactly based on a real loco, it would still be useful to have him sit at roughly the right height if I don't want him to look daft against standard 4mm wagon kits etc. The upshot of this is that I've been doing some maths...

The upshot of all that is summarised in the bottom right hand corner. My original 21mm diameter wheels would put the centre line of the buffer at 14.5875mm above rail head, so about 0.6mm (or almost 2" in real life) too high. To get it to sit at exactly 14mm I need the wheels to be 19.825mm in diameter (excluding the flanges).

I think given this bit of maths (and if anyone fancies checking my working it would be most appreciated) that I would be better off reducing the wheels to 20mm (or 5' in real life rather than the suggested 5'3") which would give a total diameter including the flange of 21.27mm which given the wheelbase of 22.5mm should leave a gap of 1.23mm between the flanges of the two wheels.

Does that seem sensible to everyone?


  1. Go for it. I'll have a quick scribble to check.

    1. I do this as well. I draw in Freecad in imperial then introduce metric components it is a real pain and something I considered an old mans peccadillo. What is your excuse?

    2. Thanks. The combination of metric and imperial can be a right pain, but I can say with 100% certainty it's not my fault! I've no idea who originally picked 4mm to the foot as a sensible scale which mixes the two, but it's not helped by many materials (like the axle rod) still being offered in imperial (1/8" in this case). At times it can get very very confusing. Even worse when looking for 1/8" rod I found some places advertising it as 1/8" or 3.2mm which are not the same, and when you want the rod to fit an 1.8" bearing you definitely don't want a rod with a diameter of 3.2mm!