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Everman uses T type belts. A T type is a square tooth in a square slot rather than the curved tooth of the GT type. The problem Everman ran into with GT belts is that they wear simply because the curve of the tooth is not actually the same curvature as the curved slot. But, the teeth of the T type do not actually fill the belt's slot. While that appears to be a no brainer failure for T types, it's actually an asset. The fact is, no belts truly engage themselves. What Everman does is engage only one lateral side of the tooth/slot for positioning. He does that by making the pinion pulley/motor the adjustable part of the design rather than the idlers. As the pinion is moved upwards to tighten the loop, one lateral side of the top belt tooth is pulled against the corresponding lateral side of the bottom belt slot. The mating of those two sides is what holds the top belt in lateral position with the lower belt. Since this happens throughout the ten tooth length of the belt between the two idlers on each side of the pinion, this effectively engages and positions the two belts. Using that mating Everman can move hundreds of pounds at 3 meter/second with .001" accuracy for hundreds of hours without belt wear (another plus of using the 3M double sided tape is that the bottom belt CAN be replaced by scraping the tape off of the rail). That said, routers do not weigh hundreds of pounds and routing is not done at 3 meter/sec. For that application, GT belts appear, from the above comments, to give adequate long life and accuracy.

GRAYUK: Suggestion, instead of milling down the width of the idler wheel to fit inside a v slot to engage the two belts, consider laying a metal or plastic insert on the floor of the slot to raise the floor so the wheel can engage the two belts. That way the bottom belt can be afixed to that insert - surely an easier task than afixing it to the slot bottom itself. That way the wheel can act as both an idler and as a v-slot guide wheel because the beveled edges of the wheel will still engage the bevel of the rail's slot. OK, I admit that I've not found such an off-the-shelf insert the right height and width - but it's out there somewhere.... That said, this cobble-together does work - Using thin doublesided tape, afix a GT2 belt to the slot bottom with the teeth up - then, using tape, afix two GT2 belt smooth sides to each other to form one piece with teeth top and bottom - engage that bonded set to the teeth of the belt earlier afixed in the slot - then use a 4th belt with teeth facing down as the pinion driven top belt. In otherwords, a GT2 belt doubled back on itself is the right height to act as an insert on a v-slot extrusion and still engage the bevel of an openbuilds solid wheel with the v-slot bevel for guidance.