Most mules' backs are flatter than a horse's back.  Not in the wither, but down the back bone about 3-5 inches, where the bars rest on the back.  In the pictures, you can see the difference.
The "Mule Bars" that we have designed, allow for less "rock" in the bars, so that they will sit down better on the mule's straight back.  When fitting, please send Back Patterns, and a photo.  If you use a variety of mules, ask for a general Mule Tree.

FITTING MULE:  If it is not possible for me to personally fit your Mule, which I will do in the San Diego area, the next best method is to use a 24" piece of solder wire and shape it down over the withers and through the pockets just behind the scapula bone.  Transfer this shape to a piece of paper by tracing around the wire.  It looks like this: see figure A.  Now beginning at the same point, shape the wire down the mule's backbone for 24".  Like this: see figure B.  Transfer this shape also to a piece of paper.  From these patterns and a photo of your mule, I can assure your mule will be properly fit.
Fig. A
Fig. B