http://www.sanfte-zahnklammern.de/spangen/geraetelistea/geraete_a_eng.html

Whoever wrote this, it's amazing.  I had to translate the page here: http://www.sanfte-zahnklammern.de/alterna/alterna.html but I was surprised to see the Quad Helix targeted as problematic.   This person appears to made a notated difference between strong and slower (preferring a slower method) with the distinction of force to break a palate in mixed dentition and not.  I 100% concur with these concepts!  Interestingly, the Crozat is making a comeback for airway with sites such as forwardontics using it.  It was an early brace system as you can see by scrolling down.  It also was one in which the aesthetics of the individual are not so compromised.  The turn-key straight apart approach is less attractive and overly harsh.  This person is really, really educated.  There are other links, too.  Really good stuff.