I have a few kettlebell weights here and I was thinking of using them to prestretch string. If I was to double a string and hang a weight off it, how much weight would be advisable and how long would you stretch it for?
You'd need to know the "creep limit" of the particular string(s) - the tension after which they don't regain their original length after pulling - but 30 lb/14 kg should be fine for almost anything from the Big Boy brands. Time? Tension loss follows a negative exponential, so the longer the better I'd say, but why not experiment? Try 3 hrs, then 6 hrs and compare - there'll come a point of diminishing returns.
Cool so if I tie a 14kg weight to a doubled length of string it should make a difference to tension loss? How long should I leave it?
If I have a few to do, my house will look like a scene from home alone and my devious 4 year old daughter will play the part of Macaulay Culkin
I usually only pre-stretch for 2hrs tops. Whilst extending the time might help in theory, I personally haven't noticed any difference stretching for longer. Also, if you double up on the string, you're effectively halving the load. So you need to double the load for the same effect.
Personally i would be wary of prestretching to the point you loose elasticity in the string, to my mind that would produce a very dead string bed. I could be totally wrong and it might turn out fantastic, but my point it you should try it on your own rackets and see how it goes before you stretch out a bunch of string only to find out that it is all useless. What i do is wrap it around a table leg and pull it by hand (in a starting clamp) just enough to remove the coil memory, then set my machine to 10% prestretch to do the rest.
Am I right in thinking that a prestretch would have to pull the string at a higher tension than the one you intend on stringing the racquet with?
By hand i doubt i pull more than 15-18 pounds, my machine will go a percentage over the desired tension (10-25% iirc, i only use 10). So for a 24lb it will pull briefly to 26.4, then release a little and settle on 24.
Exactly my point - this is the "creep limit" I mentioned; I think the idea is pull until there's only elasticity in the string. Exactly what I do: pull the string until it behaves itself, then let my machine do the over-pulling.
in fact it is reasonable what you say regarding a dead string bed. for the pre-stretch you may just use the same tension as you use for the actual stringing. this will maintain the flex of the string and guarantee that the tension lasts and that the string does not move a lot after the stringing.