Saturday, November 26, 2022

Car Standards - (Part 1) - Weight, Rolling, CG Allowances & Diaphragms

The following is an excerpt from the LMRC's Car Registration Mechanical Standards.  The data here goes back to at least 1996 when I joined the club.  The first 3-4 years, I was involved in the Car Department learning all the aspects of proper freight car construction to club standards for excellent operational qualities.  In the 2010s, when I designed the F-50-series kit for OwlMtModels, these standards formed the basis for all the mechanical design for the construction of the new cars.  Now, 25+ years later, as I'm continuing to build my SP Jawbone Branch and additional cars for it, I'm still using these standards on the car kits I build.  Cars built to these standards can run on any layout, anywhere and preform well.

Car Shops at Bakersfield, LMRC photo Oct 2016

For now, I want to specifically look at the relationship between car weight, rolling qualities, and center-of-gravity in the cars. 

Layout Standards

I will also say that these car standards are paired with the right-of-way construction standards of the layout.  Full compliance with both of these standards allows for train lengths to climb to nearly 80 cars upgrade and 120-140 cars downgrade.  Of course it takes a good crew to take a train over the railroad without any problems, physics still has a bite if you do anything too fast, or pull something stupid.

Three AC's work upgrade with the Mojave Shorts East at Caliente.  Photo circa 2009, Jason Hill collection

The curves at the club are 48"R minimum with 1/4"-3/8" tangent offsets with 12" long spiral easements.  Grades range up to 2.3% on the steepest parts of the main grades.  Limits of 39 cars without helpers on the ruling grades.  At 40 cars and above helpers need to be cut into the ascending trains.  Around 70-80 cars descending trains also need to have a helper to keep the cars in the train from rolling down against the road engines and causing excessive buffing force, which will eventually push hard enough to derail and buckle the head-end cars sideways.

LMRC Car Standards

Starting at the Weight section...


1. The minimum weight for any car is 3.25 oz. "Inspected" rolling stock (excepting Locomotives) must conform to NMRA Recommended Practice RP20.1, with an absolute minimum weight of 1/4 oz. below or 1/2 oz. the calculated recommended practice (see two weight tables, next page).
2. Track slider (cleaner) cars should be weighted to the maximum weight for their car length and rollablity category, and roll (bench tested) with the track slider disengaged from the rail contact.  Finally, they must be successfully track tested on the railroad with the track slider (cleaner) fully engaged with the railheads.
[3.] Registered equipment must be weighted according to the appropriate weight/body length/rollability/ tables below.  The table tolerance is stated above (Weight 1.).  Further allowances should be made according to the paragraph (Allowances to Weight) below.  Every effort should be made to construct the equipment to its table "target" weight.  Category I Freight cars regularly used in passenger service with predominately Cat. II equipment (e.g. express cars), MAY be required to conform to the Category II weight requirements to ensure reliable operation.
[4.] Model loads added to rolling stock should weight as little as possible and be removable, except as provided in paragraph 'M' below, (Summery of Paragraph M, basically TOFC/COFC and modern containers should not be weighed.  Only bottom containers in double stack well-cars may be weighted).

Allowances to Weight

1. Weight allowances to equipment may be made from the referenced weight tables at the discretion of the Car Department Foreman, on a "proof" basis- as follows:
    a. Cars having a 'Center of Gravity Index' (CGI) Less than 35 degrees (CG35), NO weight reduction.
    b. Cars having a CGI of 35 (CG35) or MORE: allow 0.25 oz. (1/4 oz) reduction to the charted weight requirement for every full 5 degree increment to the CG Index above 35 degrees (CG40, CG45, etc).
2. There is no limit to the maximum weight allowance applied under this formula as long as the car meets all the requirements of Section III. This MAY even be applied to the points where the formula brings the car below the 3.25 minimum weight requirement- subject to testing and approval by the Car Dept Foreman.


1. Rolling stock (excluding locomotives and other track powered equipment) must initiate spontaneous motion on a grade of 2.5% or LESS and have a smooth, steady, uninterrupted roll for at least one full car body length.
2. There is no rollability allowance/exemption for freight cars in regular interchange service.
3. Cabooses, passenger cars, unit trains and MoW cars are not in regular interchange service and MAY be exempted at the discretion of the Car Department Foreman.

Center of Gravity

SP 32451 being built with lead strip in the centersill to keep weight lower to the rails.

1. Cars should be constructed to obtain the lowest center of gravity (NMRA RP 20.1)
2. ANY weight added to the car to make its required weight should be kept as low as possible (RP 20.1).
[3.] The center of gravity of each piece of rolling stock (locomotives excepted) shall be determined on a Protractor or Tilt Table and noted as its Center of Gravity Index, expressed in degrees. i.e. CG35 (or CGI-35). This shall be the farthest deflection from the normal upright (0 degrees or vertical) position to which the car can be tilted from the flat, level, horizontal surface (90 degrees) on which it stands without tipping over.  This will be determined by using a vertical protractor arm moving against the flat vertical side of the car, as the car is tipped sideways, the protractor pivot point center coincident with said horizontal flat surface.  Adapter blocks shall be used to establish a flat plane for the measurement  of equipment not having flat sides.  A high CG index number indicates a low center of gravity. i.e. CG90 (or CGI-90, alt CG=90).  Confusing notations like CGI90 are to be avoided.


Category 1:  Cars of Predominately Non-Metal Construction

Table of car Weights (ounces) by carbody Length (scale feet), versus Roll-ability (Percent of Grade).

% grade >        1/2%        1%        1.1/2%        2%        2-1/2%        Max
Car Length (Scale Feet)
30'                    3.25oz      3.25          3.25        3.5            3.75          4.0
35'                    3.5            3.5            3.5          3.75          4.0            4.25
40'                    3.5            3.5            3.75        4.0            4.25          4.5
45'                    4.0            4.25          4.5          4.75          5.0            5.25
50'                    4.5            4.75          5.0          5.25          5.5            5.75
55'                    4.75          5.0            5.25        5.5            5.75          6.0
60'                    5.0            5.25          5.5          5.75          6.0            6.25
65'                    5.25          5.5            5.75        6.0            6.25          6.5
70'                    5.75          5.75          6.25        6.5            7.0            7.25
75'                    6.0            6.25          6.5          6.75          7.25          7.5
80'                    6.25          6.5            6.75        7.25          7.5            7.75
85'                    6.5            6.75          7.25        7.5            7.75          8.0
90'                    7.0            7.25          7.5          7.75          8.0            8.25
95'                    7.25          7.5            7.75        8.0            8.25          8.5
100'                  7.5            7.75          8.0          8.25          8.5            8.75

Category II: Cars of Predominately Metal Construction (Brass Passenger Cars)

Table of car Weights (ounces) by carbody Length (scale feet), versus Roll-ability (Percent of Grade).

% grade >        1/2%        1%        1.1/2%        2%        2-1/2%        3%        3.5%
Car Length (Scale Feet)
40'                    5.0 oz       5.5            6.0           6.5           7.0            7.5        8.0
45'                    5.5            6.0            6.5           7.0           7.5            8.0        8.5
50'                    6.0            6.5            7.0           7.5           8.0            8.5        9.0
55'                    6.5            7.0            7.5           8.0           8.5            9.0        9.5
60'                    7.0            7.5            8.0           8.5           9.0            9.5        10.0
65'                    7.5            8.0            8.5           9.0           9.5            10.0      10.5
70'                    8.0            8.5            9.0           9.5           10.0          10.5      11.0
75'                    8.5            9.0            9.5           10.0         10.5          11.0      11.5
80'                    9.0            9.5            10.0         10.5         11.0          11.5      12.0
85'                    9.5            10.0          10.5         11.0         11.5           12.0     12.5
90'                    10.0          10.5          11.0         11.5         12.0           12.5     13.0
95'                    10.5          11.0          11.5         12.0         12.5           13.0     13.5
100'                  11.0          11.5          12.0         12.5         13.0           13.5     14.0

Jason's Notes & Comments

I have several thoughts and comments to add about the standards which were noticed and not fully codified as of 2022, but I will list them here in form so they can be implemented if desired.

Notes on Allowances to Weight

I will say that the allowances to the weighting is one of the most important aspects that differentiates LMRC's 1990s standards from the typical NMRA weight plan.  

Proto2000 AC&F Type 21 8k gal tankcar, which can be improved by changing the weighting plan.

Models such as Proto2000 tank car kits have four steel weights held fairly high in the tank's body.  Fully one or two can be removed if the weight is converted into lead strip or bars held in the center frame and below the lower weight in the tank.  This will lower the center of gravity index by about 10-15 degrees, bringing the car barely passable at 30 degrees to 40-45 degrees, which then starts to allow additional weight to be removed from the car: 1/4oz from the extra 5 degrees beyond 35 degrees, and probably 1/2 oz from the Proto2000 trucks rolling at 1 or 1.5% grade.

Athearn RTR 65ft gondola weighing only 3.5oz, but with the allowances below it doesn't need extra weight.

Likewise the Athearn 65ft Mill Gondolas can be allowed into service without adding extra weight to the underside of the car due to the metal frame, resulting in a full 120 degree CG, yes the car can be 1/3 inverted and it will self-right to its wheels!  No need to weight it up to 5-6 ounces!

Passenger Cars with Diaphragms (Narrow/Standard)

I'm going to expand a bit on the diaphragm discussion as used on plastic (Cat. I) cars, typical of modern models on the market.  For operations on 48"R curves with spiral easements, ideally the mating striker face of the diaphragm should be even with the inner pulling face of the coupler knuckle.

Walthers plastic HW Pullman with silver Sharpied striker plate and OwlMtModels 10002 Marker Light installed at Bakersfield depot, 2013 photo by Jason Hill

The function of diaphragms needs to be considered that "weird" interactions between the cars occur which do not with cars without diaphragms.  Basically, when a standard car derails, we look at the end that derails; the wheels, gauge, truck tension, etc.  However, on coupled passenger cars with diaphragms the troubleshooting thought process needs to basically be reversed.  Tension can be transferred through the bodies and throw the far end of a car off the track resulting from a binding of the diaphragms.

This occurs because the model diaphragms proportionally have way more physical force able to be applied to the car body than the prototypes do.  Real diaphragms only have a few hundred pounds of force being applied by their tensioning leaf springs against the striking faces, compared to car bodies that weight in the 30-80 ton range.  On the models we have cars that only weigh 7-12 ounces, but diaphragms that can bind-up and go so far as to provide an almost solid pivot point against the carbody, where the prototype diaphragm would just tear itself apart.  On the models we instead see a derailment.

SP 5199, SC&F kit built with Hi-Tech Diaphragm with a little extra spring distance.

This is why I ideally install the diaphragms to just barely touch.  Ideally the strikers can just contact and any compression forces applied will tend to pivot the other side to press outward roughly keeping the diaphragm from "opening" up visibly on the outside of the curve.

Passenger Cars with Full Width Diaphragms (FWDs)

Full-width diaphragm installed on HW Baggage car - lots of edges to polish with diamond file.

With the introduction of the MTH and BLI plastic Daylight passenger cars with FWDs, some problems were found during operations.  Even though the cars weighs were sufficient for the Cat.I mechanical standards.  However, Lead weights were installed to bring additional weight into the carbodies so the rubber diaphragms will compress better and not push enough to derail the cars.

One of the first steps I found was that the striker plates had micro-burrs on them, around the edges of the metal.  I polished them with a fine diamond file, creating a slight beveled edge to prevent catching against the coupled cars.

In Closing

Blasting through Caliente, No.52 attacks the ruling grade of Tehachapi, circa 1951. - Jason Hill photo

Additional information on wheels and couplers are a bit outside this post's scope.  If there's interest, I can talk about those aspects in a later post.

Jason Hill

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