A working diagram of the pivoting bar that balances two horses of unequal pull, hitched to one load.
What's happening. The bar rotates around the trace point so the two horses can walk at slightly different gaits without binding the harness. Each end's distance from the pivot (its moment arm) is fixed, so the horse ahead of its partner briefly carries more of the load — exactly as much more as it is willing to pull. The cart feels the sum, not the difference.
On the ratio. Slide the pivot off the middle and the arms stop being equal. The horse on the long arm carries the smaller share, in direct proportion: arms of two and one give that horse a third of the draft and its partner two thirds. Three horses are hitched exactly this way, one on the long arm and a coupled pair on the short. Readings of pull are taken against each horse's own share, so a team fitted correctly rests with the bar level.
What breaks it. Rigid draft bars, which force both horses to walk in lockstep and lame the slower one by the end of the first mile. The whiffletree, named for the way a puff of wind pivots on nothing in particular, pivots on exactly this.
| Team | Arms A : B | A / B carry | Remark |
|---|---|---|---|
| Pair abreast | 1 : 1 | 50 / 50 | The plate as drawn. |
| Pair, one smaller | 6 : 4 | 40 / 60 | The long arm goes to the lighter animal. |
| Three abreast | 2 : 1 | 33 / 67 | A is a single horse; B stands for a coupled pair. |
| Three, pair on near side | 1 : 2 | 67 / 33 | As above, reversed, for a field with a ditch on the right. |
| Pair, near side heavier | 4 : 6 | 60 / 40 | Bay is the better horse and everyone knows it. |