I use something like this to do diagonal measurements to check cabinets for squareness during construction. You could make your own with 2 straight sticks and a couple of binder clips. All it does is tell you that a measurement differs from one taken perpendicular to the first (or, if you know what the measurement should be, it will allow you to do a go/no-go sort of check for a number of rims. Are the Wheelsmith ones worth the price? I'd say no way given what they do.