Wednesday, 8 February 2017

Reflexed Tips and the Magic Brick

I had someone ask why I used a hot air gun and oil to reflex the tips of the Wonky Warbow rather than steam.
Here are some pics showing how I did it and a list of pros and cons.
1. Dry heat is quicker.
2. You can see what is happening while you are applying the heat.
3. The heat can be taken up to about 200C giving some tempering as well as plasticising the wood. Being hotter it penetrates quicker.
4. You don't have to worry about getting from the steam onto a jig quickly.
5. Gentle even pressure can be applied and the wood allowed to relax into the bend at it's own pace by using the "magic brick".
6. Getting the wood hotter seems to set in the bend and have less bounce back than with steam.
1, The wood can get over heated and scorched.
2, Heat can leak round to the back and effect the delicate sapwood.
3. You can't leave it and have to hold the hot air gun. Whereas you can leave it steaming for half an hour or longer with no problem.

I've used both, I just went for the hot air gun this time (with sunflower oil brushed onto it) so I could get both ends done in the evening and then leave it overnight.

Note the side cheeks used to keep the heat on the belly, the back and sides were also protected with a few layers of masking tape.
With the brick pulling down on the bow, I could see it starting to pull slowly down after less than about 5 minutes of heat!

Reasons for reflexing! (A response to the explain more and the comment)
In this case it is to compensate for the deflex in the bow and to bring the tips back in line with the grip. The more technical aspects of the deflex/reflex design found in modern bows is basically down to improving the force/draw curve of the bow and making maximum use of the materials.
The maths and physics is beyond me, but I have good ways to visualize it and can maybe explain it.
There are several factors coming into play.
Let's consider a simple longbow, if we reflex the tips 2" this will make it require more force to pull the tips back to brace it, this will add early draw weight and give a faster bow... but we may overstrain the wood if we try and pull it back as far as we did before. So what if we deflex the middle of the bow by 2" we are back to where we started but has it made any difference? Well it turns out it has! The angle that the string pulls back on the tips has changed throughout the draw and this makes a difference.... why?
Well imagine a beam sticking out of a wall with a length of string dangling from it. If you want to bend the beam you'd pull straight down at right angles, if instead you angle the string back towards the wall (or away from it) you can't exert the same bending force even though you are pulling just as hard. Take it to the limit and pull the string inline with the beam, you are now exerting no bending force at all! You can pull as hard as you like but you are effectively just trying to stretch the beam not bend it.
That's what happens with a short bow at a long draw, the tips are starting to pull back parallel to the arrow and it becomes harder to pull than would be expected, this is often called "stacking".
Taken to extremes, reflex or recurve can uncoil as the bow is draw effectively lengthening the limbs as the bow is drawn and giving more leverage. This effect is utilized by the cams in a compound and by the levers Asiatic style composite bows which tend to be short, the characteristics of horn and sinew also accounts for their extreme shape.


  1. Could you say sth about reasons for reflexing?
    V interseting so far

  2. I'll add some more to the post :)