Re: Wood underframes and truss rods - a few questions

The wood frame is like a bridge that will sag if you put too much load on it. If you wanted to carry that amount of load without sagging the frame, one method would be to make the frame timbers taller.

But there is only so much room available for the frame height. So another way to resist sagging the frame is with truss rods. So instead of making a frame with say 3 foot high timbers, you place the truss rods 3 feet below the tops of say 8 inch high timbers.

Since the loading creates the greatest sagging effect at the middle of the car, the truss rods can dip down to 3 feet below the top of the frame near the middle, and then rise up to tie into the end sills. That way the rods can clear the trucks. It is nothing more than a classic truss bridge design to carry the weight of the car and its load in the span between the trucks.

The turn buckles are to adjust to pull the rods tighter in tension in order to compensate for timber shrinkage over time. Without this tightening, the car begins to sag in the middle. However, not all truss rods had turnbuckles. Some had a non-adjustable clevis connection between the two rods where the turnbuckle would normally be. These rods without trunbuckles were re-tensioned by tightening the nuts at the rods where they come through the end sills.