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Originally Posted by MikeD I set up my bow last night to discover that the bracing height was down by a centimetre. After checking all the obvious things I decided the string must have stretched (it only had about 5 twists in it before).
How much would a string have to stretch to make a 1cm difference to the brace height (68" bow).
The string is about 3 weeks old and has probably shot about 600 arrows.
I think the stretch happened on Saturday shooting outdoors on a field course. It didn't click at the time but the second 14 targets I started shooting low and looking back I now realise I was shooting 5 meters short, but at the time I just blamed bad form). When I shot sighters last night I had to set my 25 metres mark for the 20 yard target!
I got the bow back with 15 twists on the string.
All a bit disconcerting as I've got my first indoor competition on Sunday!!! |
Disconcerting, yes. But all string materials stretch (maybe not Vectran - not sure about stretch in the liquid crystal type fibres...). The question is whether the stretch was sudden (that is to say: how much stretch/over how much time) and what it means. Polyethylene strings (Dyneema, FastFlight etc), stretch a bit at first, settle in, then creep *very* slowly through their lifetime (which could be years). If they start creeping noticeably then that usually is a warning sign to change the string.
However, don't confuse creep (continual stretching over time) with normal string stretch. Every time you string your bow, the string will stretch. When you unstring it, it will relax again (until next time), so measuring brace height should be done at consistent times (after shooting a few dozen, or before shooting, for example). Try it. Measure your BH before and after shooting each time for a while, and make a note of the differences.
As for amount of stretch affecting bracing height; There's a rule of thumb which suggests that the bracing height difference is twice the difference in the string length. So a 1cm change in BH is the result of a 0.5cm difference in string length, by that rule.
I did some measuring a couple of years back. Dropping the BH on my bow by 1cm increased the arrow speed by 3 or 4 fps - not a great deal. If you see big changes in arrow impact from a BH change, look a little further for a cause...