Backed oak crossbow

PLEASE HELP TO FUND ARCHERY INTERCHANGE

4d4m

Active member
Hi all, I'm taking my first tentative steps in bow making. So far I've just made two crossbows (after watching a video - below), a little one not much more than a toy, and another more normal size one, mediaeval style but very rough and ready, and only about 15lbs draw. All just made from scrap timber I found in my garage during lockdown (no chance to source anything else at the time). The best material for the bow (well, the only stuff I found that worked) was some surplus Ikea venetian blind slats! No idea what they're made from but they smell quite fragrant when I sand the white paint off them.

I glued two together for the mini bow stave and 3 for the larger one. This sort of worked but I had a lot of failures: all snapping on the back, until I found the video (below) showing making a backed bow from DIY store wood, using layers of fibreglass scrim tape as a backing. Sounded crazy to me but it worked. The problem is the slat wood is a bit soft; the belly is collapsing a bit and it's taking a set. So now more shops are open I've been to the local B&Queue (!) and got some of the only hardwood they seemed to stock: planed oak.

I got two 90cm long pieces: a 25mm x 11 and a 36 x 11 for crossbow laths, and a 2.4m x 46 x 21mm to have a go at a longbow (not an ELB though, a flat bow style probably). I'll do the crossbow ones first to get more experience at tillering on the cheaper pieces before having a go at the longer one.

My question, probably the first of many, is how to know what dimensions to target? Any rules of thumb?

My mediaeval style crossbow has a power stroke about 8" and a "draw length" if that's the right phrase, of about 10.5". The limbs are about 12.5" long, with a non bending section of about 2" in the middle where it's secured in the stock.

I think the new bow should be a little shorter with (hopefully) stiffer limbs for more speed, but what is a sensible target? Aiming for at least 30lbs from the 25x11mm oak.

Original inspiration for my crossbow build:

Scrim tape and DIY store bow:
 

4d4m

Active member
I read that one rule of thumb for a flatbow type profile is for each limb to be roughly the draw length, then add the length of the non-flexing handle and fades.
I've cut the 25x11x900 piece (for the crossbow) to 25" to give a bit of headroom. A 2" non-bending section where it mounts in the stock leaving two bending limbs of 11.5". If I take the draw length for this crossbow to be the back of the bow it's 10.5" which would indicate a 23" long bow. I can always cut it down slightly later.

The next question is the profile. I'm thinking of leaving the stave full width for about half its overall length, maybe just under, before tapering down to about 1/2" at the tips. I think that I started the taper too soon on the previous ones, so they bent in the middle too much.
 

Del the Cat

Well-known member
If you want to leave your timber at constant thickness, it should taper from full width to zero at the tip. Obviusly zero width is difficult to attach a string to you need to compromise. Note the taper can be just on the lower edge, to keep the string nearer to the track/barrel/channel of the crossbow.This video demonstrates the taper:-
Del
 

4d4m

Active member
Thanks Del. Interesting. I hadn’t thought about the thickness yet but makes sense as this is a thin piece of wood.
What do you reckon for tip width? I’d feel a bit wary of going down below 10mm. No science or experience to that, just gut feeling.

For this crossbow I’ll be happy with 30lb but I’m sure that’s achievable. It’s a rising peg mechanism and the concern is too much draw might squash the wood at the notch and may make it unsafe. The stock is only pine 3x2. Or spruce, or something but probably pine.

For the first one I did taper from one side only but it seemed to twist, so the last one I tapered evenly.
 

4d4m

Active member
Nice crossbow there Del! What is the bow backed with?
Just read some of the other blog posts, rawhide binding. Thanks for the link
 
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4d4m

Active member
On the tiller. It's starting to bend but it looks a little stiffer on the right to me and still too stiff near the tips?
It'll have to be thinned down a bit more as I can barely get it into that second notch. My luggage scale reckons it's north of 65lb at that deflection, but it's hard to be accurate. I want to sort out a fixed tiller board with pulleys but this will have to do for now.

IMG_1728 - Copy.JPG
 

Del the Cat

Well-known member
Looks like all the bend is in the middle to me.
Show us the front view of the prod too, 'cos it looks like it has virtually no thickness taper.
Del
 

4d4m

Active member
Hi Del, yes I've only just started tapering the thickness. I realised thanks to your pyramid video it needs to taper a lot more in either one or both dimensions to bend better.

Originally the taper started about 5" from the centre and the limbs were parallel until then. You can still see in the right hand of the picture (labelled L) where the change hasn't been rounded out. I've concentrated more on profiling the right limb (labelled R) in the pic below. L/R labelling is arbitrary at the moment, the labels are so I don't get mixed up. As the limb tip of R has ended up a little lower than that of L it might even end up the other way up when on the bow. Orientation in the pic on the tiller is L-left, R-right :)

IMG_1730 - Copy.JPG
 

4d4m

Active member
I just tried your curve fitting trick and realised an optical illusion that the right limb is bending less. They seem to be bending the same in total but as you say mostly in the middle. If anything the left one looks straighter now!
crossbow curve 1.png
 
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Del the Cat

Well-known member
Hi, yes you can see why it's bending too much in the middle, if we extend the outer limbs into a straight taper (pyramid fashion) IMG_1730a.jpg
it shows how much wood is missing from the centre, which of course makes it weak.
Adding thickness taper will correct this tho'
PS. you'd, be better keeping the top edge straight across and only tapering from the bottom... this helps get the string line closer to the top of the track where the bolt runs, this helps prevent too much friction between string and track.
Del
 

4d4m

Active member
Does tapering from one side only not make it more prone to twist though?
The first one I made from laminated Venetian blind slats seemed to twist and I wondered if that was the reason.
 

Del the Cat

Well-known member
Does tapering from one side only not make it more prone to twist though?
The first one I made from laminated Venetian blind slats seemed to twist and I wondered if that was the reason.
yeah but no but ... like almost all of this stuff... it's a compromise. because the track is above the bow there is a twisting component anyway, the 2 ways .... 3 ways ... errr 4 ways to minimise this are:-
1. Have two individual limbs mounted high to give centre shot.
2. Angle the prod such that at full draw the tips are level with the track, but this won't be the same at brace. (see pic)
3. Make the prod with the top edge flat or even curved slightly up.
4. Shoot 2 fletch bolts which avoids having a downward pointing flight, which needs extra clearance between track and prod.
Often a compromise is used, a little of 2,3 and 4
Del Prod Angle.jpg
 

4d4m

Active member
A bit more work on it. This is yesterday after some thickness tapering on the limb tips (mainly last 5")

IMG_1732 - Copy.JPG

And drawn a little more to the next notch

IMG_1735 - Copy.JPG

Today, little more thinning and a shorter string. Looking better... starting to think about bracing it.
IMG_1737 - Copy.JPG

But, oh no what's this!? Grain split on the corner. Thought it was bending too much there.

IMG_1741 - Copy.JPEG

IMG_1738 - Copy.JPG

I guess it's not terminal. Only about 2mm in from the corner and would lose it if I rounded but don't want to lose any there just yet. I've poked some glue inside with a blade when it was on the tiller and clamped it. Just need to keep a closer eye on things all over. Didn't notice it at first as it's on the belly.
 

4d4m

Active member
Some ups and downs. The split above has been repaired fine, so well I can't see where it was now.
On the other hand I think I pulled it too far on the tiller when it was bending more in the middle, started to get a hinge on one side (same distance from the centre but the opposite side as the split above). Looking closely there was a micro crack starting at the back just under the backing at one side (2.5mm long). The backing was holding but I did what you suggested Del and added a slat on the belly.

I sanded the slat down it tapers to nothing at the ends and did some more thickness tapering. Now got it braced using a string I made (my first Flemish twist, woohoo!)

I think it's looking ok. Here it is with the string, at 6, 7 and 8"

IMG_1751 - Copy.JPG IMG_1752 - Copy.JPG IMG_1753 - Copy.JPG

It did look like there is a straight bit on the right limb starting about 2" from the tiller, but looking closer I think it's because there's a slight twist at the tip and you can see more of the belly in shadow. The back seems to have a nice constant arc to me.

However I really don't think I should be drawing to 10.5" for the existing tiller/stock I've got as was the plan. My gut just screams no. So I'm going to make another stock for this one and draw to 8" as above. I measured the weight roughly by making pencil marks on the tiller when drawing the string with luggage scales. 35lb at 5.5", 45lb at 6.5", 55lb at 7.3". So I reckon about 60lb at 8" as above in the third pic.

Oh yes, nearly forgot. I've swapped the left and right designations of the limbs as when braced the string was closer to one side than the other.
 
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Del the Cat

Well-known member
Good progress, maybe knock up some sort of an adjustable stock for test purposes?
It's all a good learning process, and character forming ;)
Del
PS there's a guy (I think he's in Wales?) on the primitive archer forum doing a wooden prod (link below)...
Tiller check please
 

4d4m

Active member
It's finished, kind of.

57lb at 8.5" draw, with about 6.5" powerstroke. Shoots nicely, the trigger is better than I thought but would still benefit from a smoother surface for the string notch. The wood for the stock is terrible stuff. Just an offcut of a 2x4 but it's like iron hard growth rings about 6-7mm apart with basically cheese in between. Easy to saw and chisel but it just won't smooth properly, even 280 grit just roughs it up like bobbles on a cotton t-shirt. And you can see how the serving has dug into the deck and made grooves!

I'm not totally happy with the bridle but I've never done it before and it does the job.

Anyway I'm very happy with having a functional crossbow from scrap timber plus a few quid's worth of oak stripwood. Doing about 100 fps from what I'd guess are roughly 230 grain bolts. 100gn brass piles, 10" of 11/32 broken arrow shafts and a couple of cheap plastic vanes.

Kind of finished but not quite, it really needs a bolt clip. I know they aren't really historical before the mid to late 16th C but it would make it a better shooter.

Thanks for the advice Del I've learned a lot making this, and started on the next one already! It's going to be bigger and I want to do a much nicer hardwood stock, with a rotating nut this time. (25mm delrin bar is cheap as chips for short lengths and looks a bit like bone or horn if you squint!)

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