OverviewMy On30 layout originated as two 2’x4′ modules from the time they were part of the Carolina On30 Conspiracy. The modules contained only a single siding servicing some coke ovens. To add a bit of .. call it whimsy ..I added Peco HOn30 track to replicate a bit of a line from a mine. Recently I thought to update that a bit with a small loco and some ore cars .. nice for display even if only static. To that end I need bogies .. trucks. There is a phrase that states .. “The definition of insanity is doing the same thing over and over again, but expecting different results.”. Since I have been working on my design for a On18 truck I have kept going back to the NMRA website to look at the dimensions I need for the truck. I suppose that for me, my expecting different results would be that having found the information needed I would use it and not have to return to the NMRA page. Alas .. that is not what has happened .. instead I keep returning to get the information again .. and .. again. To that end .. I have decided to consolidate the needed information to this page so I only have to run through my menu and click once. |
NMRA StandardsThe following is pulled from multiple pages on the NMRA website:
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RP-24 TrucksRP-24 Trucks: Like I said … this is all the link shows. In the Notes it says to see RP-23 for bolster details. |
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WheelsStandard S-4.2: I was looking around the On18 and On9 Modelers Facebook Group. It seems that people will often replace N scale wheels with HOn30 wheels. To give an idea why this can be important let’s consider a standard 33″ freight wheel in various scales:
That means I need to create my own 20″ dia On18 wheels (~Gilpin Tram). My On18 line is simply to simulate bringing ore to an ore bin for the On30 line. A resin wheel would be fine for that. Looking at the NMRA Standards S-4.2 we see Code 110 wheels listed for On2 and On30. We see that the only difference is dimensions K and B (Gauge) with all the other dimensions the same. For On18 I used Code 72 wheels as they will have to use N scale track standards. I added a line for On18 (using dimensions for N scale) . B by the way, is the gauge – flange width (T) x 2 – 0.017″ (clearance). |
RP-25 Wheel ContourI pulled out just the Code 72 data below. The diagram on the left is from the NMRA Wheel Contour page and the one in blue on the right was created using the data in the chart. They aligned with the bottom of the flange. |
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![]() RP-23 Bolsters: Finally we have the bolsters. I modified the drawing on the NMRA RP-23 Bolsters page, removing the bits that are not used.I am going to focus in on HOn3 trucks on the NMRA standards page for bolsters since they don’t have anything on HOn30 .. and it is quite frustrating to find dimensions for a HOn30 truck.
I found an article titled “Victorian Railways Narrow Gauge Underframes in HOn30” where it states that the floor height was 24-3/4″. Fox HON30 bogies shown as having a 7.2 mm height (24.65″ in HO) and N scale standard 7 mm (24″ in HO). There is also the N scale standard coupler height of 5.5 mm (18.9″ HO). What seems to me to get lost is this is the sizing of the wheels. To repeat a bit of what I said earlier, 33″ wheel in N scale is 0.206″ / 5.23 mm. That same N 0.206″ / 5.23 mm wheel is 17.9″ in HOn3 which works ok. In O scale it is only 9.88″. That 5.23 mm wheel will fit nicely under the 7.2 mm height of the HOn30 cars .. clearing the bottom of the car by 1.97 mm .. scaling up to 6.7″. Using the HOn30 trucks for ON18 though and we end up with those really small 9.88″ full-size wheels. Let’s go at it from a different direction. Victorian Narrow Gauge was a 2′-6″ (30″) gauge, using 21″ diameter wheels and a 24-3/4″ floor height. I followed the Gilpin Tram wheels at 20″ diameter so I am happy to with a 24″ floor height. At .5″ in O scale this makes it much easier for maths.
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