design ?

[AkDan]

So I've taken a few nites from building that bbo and have been thinking about a question I've never had answered or figgered out on my own.

It regards limb length and tiller.

I've noticed some guys on all wood bows or laminated make limb lengths equal, and some make the lower limb shorter.

With the lower limb shorter do you tiller for even tiller?  or with both limbs the same length do you tiller for even tiller?

Also your shelf in regards to this?  Even limb length vs on limb shorter...or will it be in the same place every time (give or take a little fudge factor)?

I need to sharpen my scraper tomorrow nite so I can get to tillering again, but this question has sat in my bran for quite awhile now.  So as simple the answer is I'm sure...what's your thoughts?

[Art B]

The subject of longer/short limbs, arrow pass layout and hand placement, is in my opinion, one of the most confusing and least understood aspect of bow making. Hope I don't add to that!

Dan, the only way you can get by with an even tiller is if you shoot three finger under where the drawing forces on the bow/string conicide close to their dimensional center. This places equal strain on both limbs. Even here I believe a slight positive tiller is warrented. Even limb strain should be maintained (using the proper grip per arrow pass layout) even as the drawing forces separate for split fingers. This is where your grip and amount of hand/heel pressure comes into play to help maintain even limb strain. And also, the amount of positive tiller needed to ensure good tiller over the life of the bow, is largely dependent on how well we manitain that even limb strain.

Your arrow pass/shelf placement (and limb length) is dependent upon your preferred grip. This bears repeating, "you design a bow for the way you grip it and you grip a bow for the way it was designed". The "fudge factor" should be in hand placement and not shelf placement.

Here's some examples I use. For a straight wrist/light grip a measurement of 1" above the bow's dimensional center is good. A medium grip is around 1 1/2" above center and a 2" above center is about right for a full hand/ low wrist grip.

Assuming a 4" handle (and equal fades) here that 2" above center rest will make equal length limbs. The 1 1/2" above center rest will leave me with a 1" shorter lower limb. That 1" above center with make 2" shorter lower limb (picture a sliding handle). ART

[AkDan]

So if I'm understanding this.

You'd tiller a shorter limbed bow for positive tiller just like you would an even limbed bow (shooting split fingered).   Even though one limb is shorter then the other.  

If that's the case what would be the point in uneven limb lengths?  Could how much heel you put into a bow dictate even or uneven the lower limb length is also?   Or would you take this up in tiller alone?  Just thinking out loud here being more limb would have less strain per its length then a shorter one..so if I guy really heeled a bow you'd want more of an even limb length to help accomadate this?

The grip part makes perfect sense.

Sorry for all the questions.  Seems I've read and watched more then I can get my hands on and this is one part of the bow build I've definatly never understood.

[Art B]

That's correct Dan, with the split finger draw you will need varying degrees of positive tiller depending on how short the lower limb is. For same length limbs and arrow pass at 2" above center you will need a greater positive tiller because the heel pressure of the hand is creating extra strain on the lower limb.

"If that's the case what would be the point in uneven limb lengths? Could how much heel you put into a bow dictate even or uneven the lower limb length is also?"

Vision that imaginary sliding handle I mentioned earlier (dimensional center never moves) and lets use that 2" above center/same length limb bow as an example. Here you're using a low wrist/full hand contact. Now imagine that sliding handle moving down the bow and the lower limb getting shorter. As the handle slides down, the heel of the bow hand has to left up to maintain equal limb balance (and lets don't forget about limb mass too). And as the heel of the hand lefts up, you come to a medium grip. Handle slides down some more and you're at the straight wrist/low contact position. Again, here the arrow pass is closest to center (dimensional center of bow and string) and less stress being placed on the lower limb. So less positive tiller is needed here for a low contact/straight wrist grip for long term tiller health.

So the reason for shorter limbs is to accommodate a preferred lighter grip while maintaining equal limb strain/balance (and equal limb mass).

Another thing to get right is "where" to strenthen the lower limb for a positive tillered bow (again, different degrees of strengthening here according to ones grip). And that is just right outside the lower handle fade or inner third of the lower limb. ART

[AkDan]

It makes sense reading it...now lets see if once I start gluing up my own blanks it'll make sense in the building it department   .

Thanks Art!