Bow tension - how do you know when you have it right?

[Bilbo]

Simple question: what's the answer?

[hubrad]

I know for sure that too slack is when the hair can be trapped between strings and stick - possible damage to hair.
Too tight? Maybe too bouncy or some such. If the bow snaps, [u]definitely[/u] too tight!
I have mine so about half a dozen twizzles takes me from slack for storage to tight for playing.
Honestly? Not a clue!

[SubsonicSimpleton]

I'm an arco noob so take this with a pinch of salt;

My understanding is that when you play at fff (loudest possible dynamic) the bow hair should be deflecting towards the stick, so that in the middle of the bow the hair and string are just a whisker from touching the stick - any looser and you can't play at full volume without the string clattering on the stick(possibly causing damage), any tighter and you are just shortening the service life of the hair.

[JoeEvans]

It might not be all that important - I think you would instinctively increase or decrease the weight I put on the strings with the bow to produce the right degree of pressure at the contact point; if the bow was a bit slacker you'd need to weight it a bit more to get the same volume and tone, which I think you'd probably do without noticing that you were doing it, if that makes sense.
When I first started playing arco (only quite recently so I am in no way an expert, barely a beginner) I did experiment quite a bit with bow tension and I couldn't detect much difference in feel or tone as I tweaked the tension up and down, within a reasonable frame.

[Gareth Hughes]

IMO it's a personal preference, like how tight do you like your shoelaces. As a rough guideline though, I'd have the hair tight enough that the tip of your little finger could just about go through the gap between the hair and the stick at the middle of the bow. Like others have said - you'll know if it's too loose because you won't be able to apply much weight from your arm before the stick clatters off the string.