In reply to Mike Nolan:
Well my view, for what it's worth... Tests have shown that ice screws, with high profile threads, placed slightly angled in the direction of pull in good ice have shown to be stronger placements i.e. screws were stronger when placed at a positive angle of 0 to 20 degrees in good ice i.e. the screw was placed perpendicular to or with the direction of loading.
Tests also found that if screws were placed at a negative angle i.e. placed against the load, then the screws failed at significantly lower loads. This was due to the fact that a tube when loaded across it’s axis is not a strong structure. When an ice screw is placed at a negative angle in the ice then the load can cause the ice around the head of the screw to crush, causing the ice screw to bend and then either pull out or fail.
These tests were done by Chris Harmston for Black Diamond and a US Guide called Craig Leubben. Related article with references which include the tests quoted above is
http://www.ukclimbing.com/articles/page.php?id=4315
What I teach? Well first off you have to decide what is good ice. Takes a bit of time to get this call right (lot's of placing screws etc) so I teach novice ice climbers to place the screws perpendicular to the ice. Easy to judge and falls into the strong enough category. It also seems to be what BD are advocating in their ice screw instructions - I guess because it neagtes the whole issue of people making a judgement about 'what is good ice'.
Once climbers have a working grasp of ice types then I introduce placing screws angled in the direction of pull. FWIW on the lead it's easier just to place screws straight into the ice - less chance of fumbling the placement and dropping £40+ worth of kit!
So key thing is placing screws angled in the direction of pull only seems (according to tests) to hold true for a/ good ice b/ screws with high profile threads.
