Magic Dobby Loom Mechanical Dobby Program Bars and WeightA mechanical dobby operates from a chain of program bars. Each bar adds a treadling step to your pattern. If you want a longer treadling repeat, you need more bars. Add a weight to the chain to keep tension. For the 40 cm Magic Dobby, use a weight for chains with more than 34 program bars. For the 70 cm loom, use a weight for chains with more than 44 bars.