My 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 Standards

The following is pulled from multiple pages on the NMRA website:

RP-24 Trucks

RP-24 Trucks: Like I said … this is all the link shows. In the Notes it says to see RP-23 for bolster details.

RP-24.1 Journals : This gives dimensions in multiple scales, O through N which is why they are labeled V, W, X and J. I replaces those letters with N scale dimensions. The most important for me are the Journal Face Spacing of 0.469″, the Journal Bore Depth and the Journal Bore Span. J which is Journal Bore Diameter not so much as it refers to using blunt axles (Type II) and I will be designing to use pointed (Type I) axles.
RP-24.2 Wheelsets : This labels a wheelset for various dimensions : Wheel Back to Back, Check Gauge, Wheel Width, Axle Overhang, Tread Depth and Tread Width. The dimensions for these labels are found on the tables in the S-4.2 Wheels – Standards Scales page., I will be using Code 72 wheels (N scale wheel contour profile) – which are 0.072″ in width (see ‘Wheels’ section below).
RP-24.3 Axles: I used the dimensions for axle and shoulder diameters for O scale but used the length dimensions for N scale. Note 1 states that “Shouldered axles may be used to ensure proper wheel assembly“. Makes sense to me (this is the ‘Back to Back’ dimension from RP-24.2 Wheelsets). It further states that the “Enlarged center section may be contoured similar to the prototype“.


Standard 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:

  • N scale 0.206″ which then is 9.89″ in O scale. This will obviously appear very small. To put it in perspective a common Railroad Speeder has 16″ wheels.
  • HOn3 26″ wheels would be 0.3″ which then is 14.4″ in O scale .. a bit smaller than a Railroad Speeder wheel.

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 Contour

I 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.

Wheel Section: I found a 1898 drawing of a Union Pacific 33″ freight wheel. I got the profile cross-section of the wheel .. simplifying but ignoring the fluting on the inside of the wheel. I then brought over the NMRA Contour sectional drawing and applied that, modifing dimensions where I had to such as the axle bore (1.6 mm) and added a slight shaping to the outside face but keeping the minimum section thickness to 1 mm or greater (for strength in a 3D print) – ended up with Section.
On18 20″ Wheel: I used the “Follow Me” tool in Sketchup to spin section (B) into a 20″ On18 wheel..
On18 20″ Wheel Printed: I ran a test print of the Version I of the wheel and it came out pretty good. In Version II I flattened the back-side and enlarged the axle hole to 1.6 mm

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.

  • B : Center Bolster Bearing above Top of Rail = 1/4″ for Freight Cars and 9/32″ for Passenger Cars.
  • D : Center Plate Dia = not shown. The next larger shown is TT where it is 5/32″
  • E : Truck Bolster Hole = .094″ (#42 Drill)
  • P : Kingpin Size (screw) = .086″ (#2)

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.

I am pretty happy with the design. next step is to print one.

  • B : Center Bolster Bearing above Top of Rail = 10 mm / 0.394″.
  • D : Center Plate Dia = 0.156″
  • E : Truck Bolster Hole = .094″ (#42 Drill)
  • P : Kingpin Size (screw) = .086″ (#2)
  • Wheel Dia: 20″ – 10.6 mm / 0.417″
  • Bottom of Car : 14″ = 12.7 mm / 0.5″
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