Adding Truss Rods

In this section we'll add the truss rods to the car along with adjusting the car's weight.


Your model should look like this - basic body core assembled.

Step 1 - Truss Rods Overview

Until the introduction of the steel under frame in the 1920's, the truss rod design was used extensively on wooden rolling stock to increase the load capacity of the cars.

Refer to the following diagram to see how the truss rod is incorportaed into the prototype design.
Truss rods are used to create a truss structure, just like a truss bridge enabling longer cars to carry more weight while weighing less. The loaded truss will have the top member made of wood in compression (where wood is strongest) and the bottom member made of iron in tension (where a long slender piece of iron is strongest). You could build a car solely out of wood but the size, number and weight of the beams to do that would be prohibitive. By using a truss you can get a much lighter structure with a larger capacity.

The truss rod originates at the End Sill, travels over the Truck Bolster then heads diagonaly down to the Queen Post then across to the second bank of Queen Posts, up over the other Truck Bolster the terminates on the other End Sill.
There is a large washer and nut on the end of the truss rod and a turn buckle installed between the queen posts. With this truss design any weight in the middle of the car would press down on the truss rods through the queen posts. This inturn will cause the truss rods to pull the end sills towards the centre of the car, that is the longitudinal stringer will be under compression. Typically there were 4 truss rods running the length of the car though some cars had more depending on their length and load capacity requirements.
Should the truss rods stretch over the years they can be tightened by turning the turn buckles or tghtening the nuts on the end sills.


Example of the truss rod routing on a railway car.

Step 2 - Locating the Truss Rod Holes

Our challenge is to simulate these truss rods as best we can on the models without compromising the operability of the car.
In past, modellers used thin brass wire bent to represent the truss rods. Though these did an admirable job, they required some fiddling to get them to lay right. Today's prefered choice is to use nylon fishing line.


Determining where the truss rod should terminate near the truck bolster.

Before we begin installing the truss rods we must determine the routing of our "fishing line" truss rods.
Though we will include the Queen Posts as the prototype, we avoid routing the rods over the Truck Bolsters as the truss rods would interfere with the rotational swings the trucks will experience on the tight curves found on our model layouts. Thus we need to terminate the rods shortly before the trucks. This is achieved by drilling holes through the car floor near the trucks outer limits for the fishing line to pass through. See above photo for an example of this on a completed model.


Determining where to drill the floor holes.

Find your needle beam / queen post assembly and attach it to the frame where you had previously marked their location. Ensure they are mounted perpendicular to the floor of the car.

Now place the truck bolster on the frame (in the previously marked location) and place your truck on top of the bolster.
Next, using small piece of material (e.g. tooth pick), prop one end on top of a queen post and the other end near the bolster. Try to move this end as close to the truck as possible yet not interfering with the truck wheels. When you find the target location, mark this on the floor and duplicate this dimension for the other end of the car.

Step 3 - Drilling the Truss Rod Holes

Now that we have determined where the truss rod will terminate on the floor of the model, we can drill the holes.
I used 0.015" fishing line for my truss rods so I drilled my holes using a #77 drill (0.018") so the holes were slightly over size ensuring the line would feed through easily.
People typically drill the holes at 90 degrees to the floor, I prefer to drill them at an angle inline with the direction the truss rod is taking as the fishing line will lay better.


Using a pin vise, carefully drill the truss rod holes.

Drill the truss line holes in line with the spacing of the queen posts. The outer truss rods were typically located just inside the outer floor stringers while the inner truss rod were located just outside the inner floor stingers.
Refer to your kit plans.

Step 4 - Install The Fishing Line

I find it easier to thread the fishing line if you first cut the end diagonally with a sharp knife thus creating a point on the line.
Insert one end of your fishing line into an outer truss rod hole from the underside of the car.
Tie a knot into the end of the line, then pull the line tight until the knot is on the floor of the car. Apply a drop of ACC glue and set aside until it has set solid.


The fishing line has been inserted into the first hole (lower right in this case).

Next, this is a very, very important step, thread one of your turn buckles onto the other end of the fishing line and slide it down until it is between the queen posts.

Thread the fishing line into the same hole at the other end of the car. Rest the fishing line over the needle beams but not over the queen posts - we'll do that later. Pull the line snug but not tight.

Thread the fishing line back down the adjacent hole, slip on another turn buckle and thread up through the corresponding hole at the other end of the car.

Repeat this process for the remaining truss rods.

After the last truss rod is threaded, pull the slack out of the line (not tight) and clamp the end.

Apply a drop of ACC on each part of the fishing line as it passes inside the car. Set aside to thoroughly dry.

Cut off the left over fishing line.


All of the truss rods have now been threaded, the end of the fishing line is clamped in place while the glue sets.
Note the turn buckles lay between the needle beams.
This particular car has 6 truss rods.


Here's the completed model - all truss rods have been installed and the turn buckles are threaded on the fishing line (but not glued in place yet).
Note the truss rods cross over the needle beams but are not looped over the quueen posts.

Adjusting The Car Weight

In this section we'll adjust the car's weight so it will operate well on our model railroad.

Fortunatley for us, the NMRA (http://www.nmra.org/) has done extensive research in determining the optimum weight for our models thus all we need to do is refer to the Standards and Recommended Practices (http://www.nmra.org/index-nmra-standards-and-recommended-practices) to identify the correct weight.

RP-20.1 Car Weight (http://www.nmra.org/sites/default/files/standards/sandrp/pdf/rp-20.1.pdf) outlines the benefits of proper weight along with the formula used to calculate the required weight which happens to be a function of scale, gauge and length.

I've taken the mystery out of the math and created a Weight Table for the more popular scales. Just click on the link to down load a PDF copy The first step is to weigh the model, this includes what we have built so far and all the remaining pieces.
To do this I place a styrofoam meat tray (cleaned of course) on top of the weigh scale (as it weighs next to nothing) then placed all the reaming parts to the car in the tray along with the shell that we have built so far.
Don't forget to include the trucks & couplers.

The horse car in the picture is 6.5" long (48 HO scale feet) so by looking up the car length on the HO Standard Gauge Weight Table we see that it should weigh around 4.25 ozs.
As it turns out, the car with all the parts comes in around 3.25 ozs thus we're short by 1.0 ozs. This is what we need to add.

I use the bar style weight bars that you find in hobby shops specifically designed for thhis purpose. They come in 1/4 or 1/2 oz sections where you break off what you require.
For this model I used two 1/2 oz weights and fastened one to the floor at each end of the car. It is best to keep the weight as low as possible in order to keep the centre of gravity low.

The weights should also be alingn the centre line of the car so that it doesn't list to one side.

You don't have to use commercial weights. Anything that has the appropriate weight can be used, e.g. large nuts, pieces of lead, fishing weights, etc.


Here is the car shell & kit parts being weighed on a postal scale.

I initially used the postal scale to weigh my model. This is a spring actuated device which has some pretty wide tolerances!
I ultimately used a small digital scale (from Lee Valley) which provided more accurate reading.


Here is the weight fastened at one end of the car.
Be sure to position your weight away from any windows or open doors.

This complets this section of the kit aassembly.


This Web page is maintained by Grant Knowles and was last updated on Oct, 2015.