3 under tiller

[Dale in Pa]

I've been shooting a stickbow for over 50 yrs. and building them for over 20. Just last year I switched to 3 under, (I guess you can teach an old dog new tricks).

When building, I've always gone by the conventional wisdom of tillering the bow even for 3 under shooters. Can anyone explain why.

Seems to me when you shoot 3 under, your fingers are slightly lower on the string, putting more pressure on the lower limb, seemingly needing a stronger lower limb, not weaker.

I started building bows mainly from the knowledge I gained from the book, Traditional Bowyers of America, by Dan Bertalan back in the early 90s. In the book there was actually some disagreement among bowyers. Some actually stated they tillered a 3 under bow weaker on the top limb than a bow for a split finger shooter.

Been trying to wrap my head around this for a while.Would appreciate any insight.

[macbow]

Dale look through Roy in PA posts.
There is a good discription on tillering for split versus 3 under.

The traditional way we set up bows by measuring at the fades is not used.
Setting up the pulley style tillering tree is used.
I have been using this method since reading their post and it is the way to go.

Maybe Roy can post the link.

[Dale in Pa]

Don't know if it makes any difference, but I'm referring to glass bows.

[macbow]

Mostly wood. But if you can adjust the tillering on your glass bows then the concept would work.

[Bowjunkie]

Dale, here's how I see it... wood or glass, same same. When shooting 3 under, the fulcrum point of the string hand is generally closer to bow center. In fact, depending on design, it may be exactly at bow center. If the bow hand and string hand fulcrums are at bow center, the bow will end up tillered even in order for the limbs to flex evenly... assuming they started with the same profile and let's say, both with 1" of reflex for instance. It's when we depart from this symmetry that things get harder to follow.

My split finger bows usually end up a little weaker in the bottom limb than the same bow tillered for 3 under... and I know that goes against what some recommend, but I balance them relative to my holds and that's how they end up. Folks who tiller them to a predetermined measurement and presume they'll act a certain way have yet to convince me of it, or explain why mine are often different or 'opposite' from theirs yet feel perfectly balanced in the hand, why tiller doesn't shift, and why I don't have to move the nock points to 'tune them'.

There's a little more involved than simply where the string is held, and notions like 'moving the hand down puts more stress on the lower limb' aren't telling enough of the story.

Moving the string hand influences where the dynamic fulcrum is, i.e. the bow's balance point during the draw, up to and including full draw. This balance point dictates the virtual limb lengths. For instance, if we do nothing more than move the string hand up higher on the string, the dynamic fulcrum moves too... and when it moves up, it makes the virtual bottom limb longer. Since a longer lever has a mechanical advantage, it now needs weakened in order to match the limb movement of its counterpart.

[Dale in Pa]

That makes sense Jeff, and is kinda the same thing  I was thinking. I don't have a tillering tree since I just build glass bows (so far) so I can't observe string travel as it's drawn.

You stated "my split finger bows usually end up a little weaker in the bottom limb than the same bow tillered for 3 under"

I guess I'm wondering than why it's become the norm for 3 under bows to be tillered even instead of even stronger bottom limb than for split finger.

[stickmonkey]

You can always make a hook designed to represent the width of three fingers. Doing so  will give you a very good visual early into the draw seeing how finger placement affects the limbs and what they will look like vs a single tied loop to drw from.