In reply to geoff.comley:
I thought about doing that too but I think the whole idea of them being on a cord is so you have to use your core muscles to stabilize yourself so you don't swing which will give a better work out, if they were supposed to be fixed then a fingerboard would be better.
plus you can take the rock rings out with you and hang them from a tree for a better warm up before climbing if you so wish to.