I heard that ice screws should be angled slightly down to maximize the strength of the threads. If this is true, why don’t the threads run the full length of the screw, rather than just the first couple of inches?
Like the existence of life after death or true love, the jury is still out on that one. Limited testing by Chris Harmston of Black Diamond and Craig Luebben about 10 years ago showed that ice screws with the hanger end angled down (negative angle) up to 20 degrees were stronger than screws with the hanger parallel to the ice or angled slightly up (positive angle). A negative angle does not, however, increase the strength of the threads; rather, it decreases the chance that the ice will blow out from under the screw.
A negative-angle placement relies on the gripping power of the threads, which vary among screw designs: more and deeper threads equal more grip (presuming solid ice). A negative placement is stronger because the shaft of the screw cannot lever against and crush the ice beneath it like a bartender making a mojito. When the ice supporting a screw blows out, the shaft bends and the screw fails. There are, of course, other failure modes—in soft, rotten or hollow ice the screw will pop out like a bad cork.
But before you start thinking negatively, know that the caveats to a negative-angle placement are as varied as the ice itself. For instance, how much do you angle the screw down? What is the ice like (rotten surface, air pockets, mushy)? In bomber ice it might be better to angle the screw down or perpendicular to the predicted angle of loading (more guesswork? … Noo!), although just about any screw placement in solid ice is going to be bomber.
The long and short of it is you need Old E (experience, that is … not the stuff quaffed by Valley bums) to be able to safely judge the quality of any ice-screw placement. Since this is as realistic as conducting your own auto- crash tests, use common sense to guide your ice-pro placements (looks like you’re a goner).
Other tidbits to glean from Harmston and Luebben’s testing are: Use the proper length of screw—tie offs break. And ice hooks don’t hold, although I’ll still use them as quickie pro to cover me while placing a real screw.
Running threads along the entire length of the screw, like time machines, could be a brilliant idea. Why haven’t the manufacturers thought of this? They have, actually. Most screws are or should be designed for optimal placement ease and strength. Running threads the entire length of the screw would mean you’d have more resistance when placing it. Since getting a screw in as quickly as possible is usually your priority, once you’re on the sharp end a screw with full-length threads is suddenly going to seem like a shitty idea. Moreover, in solid ice, a screw with full threads isn’t going to be stronger than a screw with partial threads because neither is likely to pull out. In poor ice, full threads might increase security simply because they will grip into more ice. Then again, in bad ice you’re more likely to be saved by an angel than by a screw. Besides, who falls on ice?
I suggest that instead of staying up at night pondering the merits of screw designs, your time will be better spent on your knees begging the Almighty for divine intervention next time you’ve got a screw loose out on the business end and it all goes to shit. Next!