| “Some use HOn3 track for On18 – I did. If you run the numbers, that 9 mm (0.354 in) gauge track actually comes out to a 17 in (792 mm). Since there was a real world 18 in gauge track we simply call this On18 .. rather than On17. |
On18 Car Width and heightIn order to ‘Fabricate’ an On18 clearance gauge we need to attempt to find the width of a car/engine. I found a thread on Trainorders.com where it goes:
 That gave me enough information then to calculate the width of the car for On18 .. (Pseudo-Engineering again). If I keep the same proportion to the gauge that the Maine 2′ used .. then … 3.2 x 18 = 57.6″ or 4’9.6″ .. which I rounded up to 58″/4’10″. In O scale that is 1.2″ Fig 1 shows a 2ft gauge locomotive that I re-sized for 18″ gauge. Note that I came within a inch of the calculated width (yes .. dimensions are only ‘sorta kinda’ correct .. but so is this entire guesstamation I am conducting. |
 All well and good. No – we humans don’t resize (other than expanding over time) so let’s keep the vertical dimension for the loco/car height from the 24″ loco of 85.4″ and apply that to our 18″ loco. This is where I am not sure of – we need to know the distance from the top of the rail to the bottom of the car to which we can add our 85.4″ (1.78″ O scale).
I see some use HOn3 trucks and so looking at the NMRA RP-23 Bolsters page I see that the “Center Bolster Bearing Above Top of Rail” is 1/4″. Elsewhere I find that Floor Height for HOn30 is 7.2 mm (0.283″). So .. until told different I am going to go with … 0.283″ (13.6″) + 1.78″ (85.4″) = 2.063″ (99″) from top of rail to top of car, and 1.2″ for the car width.
Fig 2 then is a starting point with width, height to floor and height to roof (Version 2). On the right (Version 1) is the one I made up several years ago. I just noticed while updating this that .. yes .. I had the gauge wrong. *sigh* Thoughts ..
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Dimensions P & E
If we agreed on the current dimensions this could be called something like Gauge A – minimum dimensions .
If we waned to add a slanted/clerestory roof it would really add little weight and would make sense for humans to occupy. I did this with V1 of the gauge. |
On18 Gauge Version II (HOn30 trucks)
Note that this is made for those using HOn30 trucks. I am making another version to use my own On18 truck. Next step is to print one and share the STL file. |
On18 Gauge Version II (On18 trucks)
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Dimension E is 0.283″ – or – 0.317 – or – something else. Dimension P I *derived* from an On18 truck I am working on which measures 18.7 mm across the journals .. so I rounded up to 20 mm and split the diff for a 10 mm / 0.394″ P dimension.
This is designed to use my own On18 trucks with 20″ wheels
I am on the same page. I have been loosely using 20 and 24 in gauge as a guide. I deduced that the car width of roughly 3X gauge (was also backed by Mr. Fairlie) and came up with 5 scale feet for width and a similar 2 foot car height. The cab floor of the 24 in gauge Decauville 3 ton 0-4-0 also has a 24 in height.
Who would have ‘thunk it’ those years in math class but I have a lot of fun with these calculations. I think it may be simply that it’s like a detective in a mystery story .. we are peering into the .. .not unknown .. but certainly murky small narrow gauge past. It can’t hurt to have a fellow molders also part of the investigation!