QUESTION: “What size singletree do I need?”
When used on a set of shafts, either cart or buggy, you would like the ends of the singletree to be just inside the shafts themselves. This allows the traces to be placed through the shaft loops, if present. Most standard cross bars, where the singletrees are fastened, are about 40”. Here is where the 38” size is applicable. (Sword end singletrees are 42” in length, but when the traces are slipped on they will fasten at 38”) If you were on a cutter with a cross bar of 36” width, you would use a 34” singletree.
In the cases where singletrees are mounted in a doubletree set up, you then want to match singletree lengths with horse size. As a general rule, an 800# animal, driven as a pair, will use a 28” singletree and a 1000# animal will use a 30” singletree.
When you get into the heavier working style singletrees, they are fashioned a bit different. These are designed for work harness with heel chains yet can be used individually or in a doubletree set up. These are well ironed on the back side for heavier pulling.(Measurements are wood lengths)