Common mistakes in deflex/reflex bows?

[Aussie Yeoman]

Hi all,

I'd like to get into deflex reflex bows, having made longbows and flatbows (straight and perry-reflexed) pretty much exclusively for almost three decades, with one or two recurves in the mix. I've been teaching bowmaking classes for about 15 years.

So I'm not new to bow making by any means, but only to this particular style. I've tried once or twice, but it didn't end well. I think the problem was in having the belly slat too thick, and/or trying to go for too much reflex, in the glue up.

I'd like to hear from the experienced, what common mistakes beginners can make when constructing and tillering this kind of bow. Maybe even some of the uncommon but important to know mistakes too.

Cheers!

[Badger]

     R?D is still my favorite, but not as extreme as shown in the diagram. I have built quite a few like those pictured, and I have had a very high failure rate. My biggest mistake is getting a hinge in the middle of the limb where it just suddenly appears. I believe my biggest mistake on those is trying to go for too much reflex. If the tips are level with the back, they are not too bad to tiller out, but they are a nightmare with a few inches of reflex. My favorite R?D design is more like an R&D with a semi-recurve. I use a very similar form to the one Gary Anderson used. Hard to beat.

[Aussie Yeoman]

Thanks Badger, always good to hear from you.

Had you meant to attach a picture or link to one?

[Badger]

I was referring to the photos of bows in the other thread about bow shapes. I find that if the limb is straight at brace, I am in pretty good shape, but if it is still showing the dip from the deflex, it is under extremely high tension on the string and will twist itself into a knot.

[Aussie Yeoman]

Ah! I see. You mean #6 here? http://www.primitivearcher.com/smf/index.php/topic,73421.0.html

Or perhaps Post #13 in the same thread?

The D/R bows I've seen that are done well do seem to look pretty much dead straight when braced. I did some analysis of a couple and at full draw the bent limb looks to have the same degree and shape of bend as a straight bow with a ~7 inch brace. Doing that bit of graphics was pretty informative, because an elliptical bow at brace has very little bend at the inner limb. Which is something I probably didn't do well the time I tried.

[Aussie Yeoman]

Here's one of the two designs I'm currently playing with. The tips are essentially dead-even with the back of the handle at rest, the limbs pretty straight at brace.

To get this profile, it needs to have an amount of reflex equal to about 80 mm (3 1/4") of reflex on a straight bow. Which I assume would need maybe five inches of deflection during glue up, to account for spring-back.

[sleek]

Biggest problem is over stressing the limb. A limb will always have a specific amount it can bend no mater its profile. Make sure the bend radius never is exceeded.

[willie]

Biggest problem is over stressing the limb. A limb will always have a specific amount it can bend no mater its profile. Make sure the bend radius never is exceeded.

A few forumites have mentioned already how difficult it is to not create a hinge when tillering a R/D design. How would you go about actually making sure the bend radius is never exceeded?

[sleek]

You make sure the reflex is even thought the bending portion of the limb. Whe you bend the limb, you make sure the bend stays even as it straightens out, eventually becoming a straight line as the limb flexes flat. After its a straight line, you make sure it bends evenly into a perfect curve as you pull it through towards full draw. You set yourself up for success by evenly reflexing the bow. Start even, finish even.

[Aussie Yeoman]

So to drop down on that further, how do you ensure that? Tiller only with a scraper? Some other stratagem?

[Tuomo]

I think the biggest mistake is having too much reflex (tips bent too far behind the back). The second mistake is not using the VirtualBow program to model the bow and find the correct taper rate for a specific front profile.

In my opinion, and what I prefer to do, is keep the reflex at about 0–50 mm—no more in normal bow. If I want to make a flight bow or a bow just for speed, and I can trust my materials, then more.

I always make deflex–reflex bows as laminates, because then the correct taper rate for evenly distributed stress is easy to achieve. Usually, a taper of 0.006 or 0.008 works well. I have a good taper sled, so the tiller is almost always spot on. If the tiller needs correction, a scraper works well and is almost the only tool I use.

[willie]

Start even, finish even.

knowing what bend to eyeball for is not as intuitive with a R/D as it is with a straight bow.
Removing wood to the correct  tapers will produce the desired bend.