Fast Flight Stretching????

[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!!!

[Small Monkey]

Originally Posted by MikeD

How much would a string have to stretch to make a 1cm difference to the brace height (68" bow).

Based on a bracing height of 9", and the following assumptions:

(1) Bracing height measured to centre of bow, at a right angle to the string
(2) Distance from centre of bow to top/bottom of limb constant
(3) You meant one inch not one cm change (so sue me, I can't do metric conversions..)

and using Pythagorus' Theorem for right angled triangles, I'd say about half an inch. Approximately.

[MikeD]

68" is the combination of medium limbs and 25" riser. I don't know the actual length of the string before and after the stretch. 62.5" rings a bell. But it then had a few twists, which would shorten it a bit.

However I think Pythagorus shows it would've streched about 2.47mm ish as the change in brace height was from 22cm to 21cm.

That sounds reasonable. Could it be the string settling inside the serving?

I knew geometry would come in useful some time. Only took 30 years after school.

[Small Monkey]

Originally Posted by MikeD

That sounds reasonable. Could it be the string settling inside the serving?

I think that's what causes stretching. I don't know how long it takes to shoot in a new string normally, but I suspect it varies.

Good luck at the competition!

Originally Posted by MikeD

I knew geometry would come in useful some time. Only took 30 years after school.

Useful? It's not meant to be useful - it's for fun!

[Little Miss Purple]

I would hazard a guess that the weather conditions would effect a string as well??

[Andy1960]

I've never had any problems with fastflight changing dramatically with the weather and I've shot in some awful stuff (county champs some years ago where we had thunder & lightning, the judge didn't think it bad enough to delay the shoot ). Mind you - it may be that I'm just so bad that I didn't notice!

[MikeD]

It was damp on Saturday, but not too bad. The change was dramatic though. I shot 14 targets very well (for me anyway). I stopped to get something to eat and about 30 minutes later started again. I immediately started shooting low (in the 1 ring on GNAS field targets). It didn't occur to me to check the bow. I thought I was just tired and dropping my bow arm or something

But if the serving was quite tight I suppose the string could just 'ping' at somepoint and end up a bit longer. Perhaps the damp drizzly conditions contributed to that.

Anyway I'm getting a second string to shoot in at practice sessions, so I have a backup.

I did work out that my 30 yr old Geometry was wrong and I'd forgotten to multiply by 2 at the end. So it is about 5mm that pinged...

[Kevinf2349]

I am not sure if you already do this or not but.....

Whenever I was going to go shooting I would ALWAYS string my bow up at least 24 hours before I went to shoot.

Try it....you will be surprised how much the bow 'settles' between stringing and shooting.

In fact my bow mostly remained strung all season.

The only other thing I used to do was to keep checking the bracing height throughout the shoot.....if only to reassure yourself that things aren't changing on you.

[Rik]

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...

[MikeD]

Hi Rik,

Definately some food for thought there. I usually string the bow and draw it a few times without shooting, which seems to settle it down. Then I check the brace height and if necessary restring the bow after putting in a couple of twists to bring the height to the 'usual value'. I then check every so often while shooting to see if it changes. Sometimes the brace height will drop 2 or 3mm and I ignore that. On that paricular case the brace height dropped by a lot more than normal. I recently saw the same effect again and put more twists on the string to fix the brace height.

I have now changed to new limbs and a 66" bow lenght instead of 68". I'm using the same make of commercial fast flight string. It'll be interesting to see if it behaves in the same way. In both cases there was a change in weather conditions (the second one indoors happened after the hall warmed up dramatically after starting out cold). May be the string expanded due to the heat...