There are only three possible scenarios where the OBS wheels will run true and without interference with various issues resulting from the rubber wheels compressing on the surface of your bowl: Anywhere along a parallel line to the spindle.....and with convex and concave surfaces, only a single point where it will intersect perpendicularly with a parallel line to the spindle, at only one single point. Ideally, these are the theoretical points where the OBS wheels should be located for best results, and should be maintained as close as possible. At the moment, I only have one real world example of a convex, or concave surface where it illustrates the point where the OBS wheels should be located for best results. The photo illustrates where the wheels should be positioned on a concave surface, similar to the middle example in the above drawing. Note that the further the distance from the spindle to the outermost point of the bowl rim, the more tendency to vibrate.....so that should be taken into consideration when positioning the wheels. This is the point where the wheels will run as true as can be had, and will have the most influence on dampening any vibrations. (In the photo, if the wheels were positioned just to the right and on the very narrow parallel surface of the rim.....that would also be a good place to position the wheels.) This all brings to the forefront another incongruity, which is not consistent with what is logical.......specifically because the width of the scissor arms themselves results with the two wheels not running true to one another. In order to solve this problem I have made up a special jig for positioning the two wheels so that they are running true to one another: In this photo below, it shows that longer bolts have been substituted for the shorter bolts that came with the OBS as supplied by the manufacturer. This substitution allows for multiple washers positioned strategically.....with the specific purpose of bringing the two wheels in perfect alignment with each other. There is an ascribed line on the jig that allows you to see exactly where the wheels make contact, and align with each other. Use this visual indication to add and subtract washers to accomplish the goal of both wheels running in alignment with each other, as close as possible. ko = burning the midnight oil.....!