mini elm bow - first "sucessful" tiller

[Richard B]

Here is a miniature bow I made from part of the elm stave I posted previously. I made it partially to test the suitability of the elm wood and get an idea of what poundage I might be able to expect from a full sized bow from the stave. Also as a go at tillering before I risk breaking my full size stave. (my first go, with a bit of pine broke, I think I was expecting too much from the wood)

The dimensions are scaled from the full sized bow design that I am planning from the stave. At full size these are:
length 69" NTN
Width 2" at fades continuing at 2" for 12" down the limb before tapering to 1/2" at the tips.

I downloaded a rather neat series of spreadsheets (by David Dewey) which contain loads of wood data (based on the bend test described in TBB 1) and also a design spreadsheet. This spreadsheet suggested that I could achieve a 50 -55lb draw weight for my design at 28" with the outermost fibres at 77% of their rupture strain (based on his data for "Texas Elm").

The scrap of elm that I cut from my stave only allowed for a 16" bow. This represents a scale factor of 0.225. At a scale draw length with a dimensionally scaled bow a draw weight of 55lb/0.225 squared should result in the same strain in the outermost fibres as a 55lb draw at full size. Hence if I can achieve a draw weight of 2.78lb with the mini bow, I should be able to do the same full size.

[Richard B]

The next series of shots show the bow after tillering with a plot from the spreadsheet showing the target tillered shape (to ensure that the bow is evenly strained to 77% rupture along its length). The first picture shows that I have ended up with a little more string follow than predicted, perhaps I overstrained the bow during tillering (or the wood has a lower elastic limit than the "Texas Elm"). The next two shots show the bow strung and at full draw.

 The final draw weight was around 2.4 lb equivalent to around 47lb at full size. I think I could have done better. I had a bit of a problem moving from the long tillering string to the short one. With the long string I carefully tillered the bow to exactly the shape it should be when strung with a short string. This meant that I had to remove material from mid limb. When I put the short string on the bow was too stiff at the ends and too flexible at mid limb, so I had to remove more material at the tips in order to hit the desired shape, resulting in missing my target draw weight!

Have other people noticed this?

Should one be working towards a different tillered shape with the long string than one does with the short one?

[Badger]

  That looks good, scale models are cool to mess with.

[okie64]

Cool project! Yes the limbs will sometimes have a different shape with the long string. Only use the long string till you get it to where you can brace the bow and make sure it is still heavy. After you build enough bows you can go from floor tillering to a short string and skip the long string tillering.

[PatM]

 Most Elm can take far more than that data suggests. I have a shorter Elm with static tips that almost doubles that weight at a longer draw.

[Richard B]

Cool project! Yes the limbs will sometimes have a different shape with the long string. Only use the long string till you get it to where you can brace the bow and make sure it is still heavy. After you build enough bows you can go from floor tillering to a short string and skip the long string tillering.

That could work. I tried to keep below the final draw weight target throughout the tillering. Which meant that when I transferred to the short string (after tillering beyond the brace curvature at below the draw weight target) I didnt have enough material left in the middle of the limbs to achieve the target draw weight with the target tillered curve.
Only concern is will ill tillering on a long string at a weight greater than the final draw weight overload the wood?