Tiller help

[stuckinthemud]

Ok, so this is full draw but I think the centre might be too stiff, would welcome any and all criticism.  Also there might be a slight weakness middle of both limbs.  Any thoughts??  Thanks in advance, Andrew

[Pat B]

The bottom limb bends too much at mid limb. Soften the fades a bit and the tips some too. A little of the same with the top limb but not as much. Will this be a hand held bow or a crossbow? For a crossbow I think a stiffer handle would be best.

[stuckinthemud]

Thanks Pat, will take a scrape off as you suggest.  It’s a crossbow so the central 1.5 inch needs to be relatively stiff to avoid the corners of the tiller chewing into the belly.  I will also use a pad of hard leather to help reduce this.  I’m a little bit disappointed with the set (just over an inch in each limb) but I literally just found a reference that said medieval crossbows were normally built from reflexed timber ( this one is based on the Lillohus bow).

[Hamish]

I've never made a crossbow prod, so I don't know if stiff in the middle is ideal or not. Pat's logic for a stiffer handle intuitively makes sense to me, for less shock/recoil.

That being said, it looks like it has acceptable set for a crossbow(if the back is at the bottom of pic), no hinges, and has made the intended draw length, so it's definitely up to the task already, providing there are no signs of chrysalling.
You could perfect the tillering but you will have to lose some draw weight. Its up to you if you want to tweak it. I'd probably leave it as is, but that's just me.

[stuckinthemud]

Quite subtle changes to fades , bigger changes in the tips, how’s it looking now?

[superdav95]

I’ve never made a crossbow either.  I’d have to agree with hamish just on what I am feeling on it.   This seems to make sense.  I have seen other similar project with little more bend in the handle area that being said.  This would spread the stresses and get you more draw length too.  A little bend through the handle may not be a deal breaker after all depending on your build and how the bow is affixed to the stock.  If you are at you intended draw length and weight now I’d be inclined to not mess with it as it looks good now.   Just my 2 cents.  Keep us posted on the rest of the build.  Very cool. 

[stuckinthemud]

Thanks guys, I am going to ponder for a day or three as I have been known to perfect a bow into the bottom of a bin on more than one project.  It is true that if I soften the fades/inner 1/4 I will get more draw/less stress but then, I am at desired draw length.  I probably will take 3 or 4 scrapes off the inners and maybe tidy the tips a little bit, but I might just be better to leave well alone.

[Hamish]

 Looking good. How much weight did you lose with the changes?
Also curious to know the length of the bow, draw weight and draw length?

[GlisGlis]

I made red line first. then cloned, colored in blue and flipped
it may be the picture but top limb looks shorter and maybe flexing a little bit more in the middle section
overall looks good

[stuckinthemud]

It is 37” from nock to nock.  38mm wide and 30mm thick at centre thickness tapers are constant from the fades to the nocks at 1mm per inch.  Nocks should be 15 mm thick and 28mm wide but a couple of knots made the width taper a little curvy. I tillered using a new method (for me) which is actually one of the oldest techniques.  I needed to keep the thickness so I cut hard chamfers down the edges (I think this is called trapping- using a trapezoidal cross section?).  As I am very much still learning I got a chunk of set in the centre early on.  I left the outers too stiff for too long.  Should’ve had more confidence in the tapers.  Draw weight is unknown as I have to extend my bow press to take a longer scales

[Hamish]

Draw length, 12" -14"(?), so I'm guessing you would need at least 150lbs to get the performance of a 50 lbs hand drawn bow?

[stuckinthemud]

Draw length is 12”.  I reckon that due to slow draw and long long long hold time before release compared to a hand bow (maybe a 10 or 15 second cycle instead of 3 to 5 seconds for a hand bow?) the stresses are huge, so you need a draw length to bow length ratio nearer 1:3 than the 1:2.4 for a hand bow. This is inefficient and coupled to sliding the string along the bow table means medieval crossbows push out 1/3 power compared to handbows.  I will be disappointed if it’s much under 200lb .  Am really tempted to give it a gentle heat soak

[stuckinthemud]

Well, that’s disappointing.  At 12” it’s 150lb.  If I pull it to the correct draw for a hand bow that’s 180lb at 14”.

That’s still equal to a 50lb hand bow, so a nice weight for target and hunting and really low stressed/very over built.  If you were me, would you tweak it - flipped tips/recurve/maybe extend the tiller an inch -  or just leave it alone and have a fun, reliable bow?

[superdav95]

Well, that’s disappointing.  At 12” it’s 150lb.  If I pull it to the correct draw for a hand bow that’s 180lb at 14”.

That’s still equal to a 50lb hand bow, so a nice weight for target and hunting and really low stressed/very over built.  If you were me, would you tweak it - flipped tips/recurve/maybe extend the tiller an inch -  or just leave it alone and have a fun, reliable bow?

I might just leave it alone and continue with the rest of the build and enjoy it.  I know its not quite the draw weight you were hoping for but still quite good. 

[pierce_schmeichel]

If you're comfortable with sinew backing and you're not satisfied with the draw weight I would sinew back it. It's the perfect candidate, being a shorty.