Combat Archery Crossbow Designs for the SCA
Combat archery crossbows are a different beast than target archery crossbows. Target archery crossbows want to be powerful (for a flat flight trajectory) and smooth (to ensure the crossbowman hits what s/he aims at). Combat archery crossbows have a maximum draw (depending on where you live and/or what war you are attending) that is faily low (ca. 50lbs or 1000 inch/pounds) and are usually fired at targets no more than thirty yards away. As with all generalizations there are all sorts of exceptions, but I'm not going to try to cover every single situation.
A crossbow is essentially an improved handbow. The handbow (your classic stick with a string) requires years of training and conditioning to use properly. You can train someone to use a crossbow fairly well in under a week. The advantages of the crossbow are: it can make use of mechanical advantage (the lever, pulley, or crank) to make possible extemely strong draw weights (i.e. a crossbow might have a draw weight of 600lbs, to draw an equivalent longbow the archer would have to be able to lift 600lbs one-handed [yes, again not entirely true but sufficient to illustrate the point]) for long distance and very flat flight trajectories, the archer can fire while completely relaxed (not while holding a hundred pound stress under tension as you would be with a handbow) which improves accuracy, and the archer can sight down the shaft (handbow arrows actually curve around the bow, look it up, fascinating stuff).
Additional, SCA-centric, advantages of the crossbow are: relatively compact (important when maneuvering in the throng), easier to use when wearing mitten gauntlets (for heavy combat archers [and BTW just so we are clear on this I think ALL combat archers should be required to be heavy]), and they tend to be made of robust materials (not fiberglass/plastic, though this is changing).
The key to all these improvements is the ability to hold the bow in a drawn position using mechanical means. All crossbows use some sort of latch to hold the bowstring back in the drawn position and then release it. There are three primary types of latch: shelf, post, and nut. We'll take a look at all three of these and their comparative strengths and weaknesses.
The shelf latch is the absolute most simple latch in the world. You draw back the bowstring and rest it in a notch cut into the stock (the tiller if you want to use a technical bow term). To fire the crossbow you lift the string over the notch and let it go. People who hunt or shoot with target quality crossbows are wincing right about now. That's because the bowstring 'slaps' the arrow (the bolt, the sharp pointy thing that goes into the bad guy). Slapping the bolt is bad because it creates random motion that throws the arrow off target. In SCA combat archery, who cares? You're firing a projectile with the flight characteristics of a small dress at a target about ten or twenty paces from where you're standing. It matters, but it's not that big a deal. If it were then the Fellwalker Sureshot Crossbows would not be the huge success that they are. That's right. Fellwalker combat archery crossbows use a shelf latch. Take a look at their excellent site and you can learn all you ever need to know about combat archery crossbows. Below is a diagram (reverse engineered by yours truly who is not, by the way, mechanically-inclined) of a shelf-latch crossbow along the Fellwalker lines.
Fig. 1 - A shelf-latch crossbow
See how this works? I've circled the cool part in red to help you out. See? The bowstring rests agains the notch in the stock. When you fire you squeeze the trigger (a simple lever) and raise the post which pushes the string over the notch and lets it propel the arrow (that blue thingy) towards the target. Simple, yes? Look at the one below.
Fig. 2 - Another shelf-latch crossbow
How simple is this? A lever (a piece of metal) lifts
the string. This top trigger design is the most simple SCA combat
crossbow design I have ever seen. I have never fired one so I have
no idea how effective they are. I am unclear about exactly who designed
this, but the webpage where I found it is here: http://jim-diana-hart.home.att.net/cbpics.html
The post latch is almost as simple. It has a piece of something that sticks up above the tiller and holds the bowstring. When you squeeze the trigger the post retracts into the tiller and lets the bowstring go. Generally this is another 'slap' string design, but it might be possible to design a working push string variant. See the illustration below.
Fig. 3 - A post latch crossbow
I saw this design in use at Kingdom Crusades [you know, I always preferred it when they called it Pointless War] (Atlantia/East) in 2001. It was made out of a two by four, had off the shelf (hardware store) components and a fiberglass prod (that's crossbow speak for the bow itself) made from the posts used in electric fences. The bowstring was a piece of cotton twine. The crossbowman loved it and said it was fairly accurate.
The rolling nut is the queen of latches. It was developed with accuracy in mind and has a smooth release. While the design is fairly simple to understand it can be a royal pain [love that term] to build. Part of the reason crossbows were, and are, so expensive. See the design below so you can follow my inexpert explanation.
Fig. 4 - A rolling nut latch crossbow
The rolling nut is that yellow thing. When you pull the trigger you release the nut. The bow string pulls forward and the nut rotates (rolls) forward to allow the bowstring to escape. When you pull the bow string back it passes over the top of the nut and friction pulls the nut back into the starting position. Gravity (or a well placed spring, not pictured here, some assembly required) and the fact that one arm of the lever (our trigger) is longer than the other pulls the trigger back into position and you are ready to go. [In case you are curious, the spring would go a little to the right of the down arrow in the picture.] The nice thing about the rolling nut, aside from that smooth as silk release (by comparison at least), is that you can cut a notch in it and have the end of the bolt (the nock) resting against the bow string. This means the string pushes rather than slaps (and crossbowmen hate slapping, at least where bolts are involved).
Now that we've covered the latch mechanism let's look at the crossbows themselves. Why? Because it's my article and if I want to do things ass backward I will.
Fig. 5 - A basic combat archery crossbow along the lines of the Fellwalker Sureshot (take a look at their website, go on)
Here we see a basic shelf-latch combat archery crossbow. The trigger gets pulled, the bow string is lifted off the notch and away goes the arrow. So what, you are asking, is that curved thing-a-ma-jig over the latch? That is a spring-clip. A spring-clip holds the arrow on the bow and helps to keep it from falling off while you run away from the charging shieldwall. Some people prefer to put the spring clip up front (a side mount) and some people like to have one both fore and aft (sorry, don't mean to be nautical). So what you may ask (being a good re-creationist) did the medievals do? I don't know and, as far as I can tell, neither do the experts. They may have been content to let the nock hang on the string (I don't feel like explaining, ask an archer).
Why, you may ask, is the prod at an angle? Lots of good reasons. It's authentic. It saves wear and tear on the string (ask a crossbowman). It helps keep the bolt in the groove.
The groove? Why, yes, the groove. If you were looking down at the top of this bow you would see a shallow trench from the front of the bow back to the latch. This groove helps keep the bolt on the bow while you are still running away from the charging shieldwall and looking for a little help and helps to guide the arrow.
And yes, I have noticed that there is nothing holding my prod on the bow. The first time it is fired the prod will go merrily chasing after the bolt. Rather than deal with your nitpicking I will address an important issue, the material used to make the prod. Period prods were wood, then laminated wood (horn, sinew and other animal bits come into play here), then steel. Modern prods may also be fiberglass or aluminum. Steel is preferred because it is strong, quick (ask an archer), and durable. Aluminum is reviled. Fiberglass is frowned upon because it shatters easily. Wood is slow and the weather really affects it (ask an archer). Steel is also expensive. In a simple bow the prod is probably the most expensive part of the bow. Alchem Incorporated sells steel prods starting at $30. Add in additional parts, labor, inflation, shipping, phase of the moon, etc.. and it's easy to see how a basic bow can cost you upwards of $200. If you want a good design for a rolling nut crossbow take a look at Alchem Incorporated where they will give you the plans for free (I think they are hoping you'll buy the pieces from them).
There is an SCA combat archery mailing list/newsgroup
By the Sword Inc.
New World Arbalest
A Combat Crossbow By H.L. Johann Wolfgang von Hesse
Simple shelf-latch crossbow design