Here are some x-bows and a handbow that i made some time ago, making and shooting crossbows is one of my greatest passion.

Hey thanks Manu, you know, that is almost exactly what i said right before i started to build that last crossbow with the steel prod and bronze nut, "One day il go for it" i think, that weekend i stumbled upon a leaf spring that i bought for 2 dollars and the rest is history.

Hey thanks Bill, indeed the Handbow is cable backed and the all wooden x-bow has a fiberglass backing, i was originally going for an all primitive wooden crossbow without artificial parts but fiberglass makes wood extremely resistant, and as it is the steel crossbow has a draw of about 150# pounds with a 7.5" inch draw.

Hey thanks Parmenion, seems like you have a perfect size prod for a crossbow, i don't know to much about centimeters but i would leave it as it is, 31" inches is perfect for a first time steel crossbow, 7mm i think is a bit more than 1/4 of an inch, thicker than my x-bow, i also don't know how pre-bent your prod is, but regardless of its size or draw weight, a medieval crossbow is pretty much always strung at 4 or 5 inches and they need to be strung that way because the crossbow is canted upwards a little bit, the power stroke of a medieval crossbows also varies from 5 to 7 inches. If you are using spring steel you should have no problem choosing your own brace height and power stroke(so long as it stays within this parameters), modern spring steel is way better than what they had back then. A more general rule for crossbows is that they can safely be braced at 1/10 their size and then be drawn 1/3 their size, so your bow would have a 3 inch brace with a 7 inch powerstroke, you have a lot of options to adjust your crossbow to your needs, also if it pulls more than 150 pounds you will need to reinforce your nut, just epoxy a tiny steel square to the bottom of the nut. if you have more questions ill be happy to help.

Parmenion, a powerstroke of 15'' with 4'' brace is 19'' draw waaay to hi for a 31'' prod, handbow aren't under as much stress, my recurve bow is 60'' braced at 7'' with a powerstroke of 21'', thats less stress than your crossbow, also remember that a crossbow needs to be more relaxed at all times than a regular bow since it can remain cocked several hours, unlike a bow that is drawn and shot in a couple of seconds,... that being said however and since your prod is already pre-bent and kinda large a 4'' brace with a something like a 11'' or 12'' powerstroke seems reasonable enough (i would't go higher than that tho) so yeah go for it man  , an yeah steel is the perfect nut material, i don't know what material the nut in the picture is made of, i just borrowed that picture from another guy in another forum to show how to make a simple nut, and from what you say about your prod i would gesstimate that i would pull a good 300# or 400# with that setup.

This reminds me of my first only attempt in making a crossbow... it looked like the one in your fourth picture:




here with a friend of mine.

It was nothing more than a rattan bow stuck on a second-choice wood handle, the trigger was a simple wood cylinder moved upward by an iron lever... simple and effective 

But despite the fact that Italian urban militia were renowed in the Middle Age for being skilled crossbowmen, my favorite ranged weapon is still the bow!

So, a while ago someone sent me a message asking for pics of my crossbow trigger, dude if your reading this, im sorry i took so fricking long to reply, i got a wicked stomach infection and stayed in bed for a whole week and after i got better i kinda forgot.., anyways, i tried to send you a PM but i couldn't figure out how to attach the pictures, so im doing this.
In the first picture you can see my trigger before i put it on my crossbow, and in the second a pic i found online of how it alings., in the third and forth ones you can see some examples of a "skane lock" trigger, sometimes called a "hole and peg" trigger and also 3 more pics of skane lock crossbows(including my red one with a pic of my steel prod)., i would highly recommend you start with one of this design since its one of the easiest type of crossbows to make, it can be made entirely out of wood and it doesn't look half bad (my first crossbow was a skane lock), and it works very well with prods(bows) under 150 pounds., and some bonus pics, the last 2 crossbows are some examples of the only "dark age" crossbow ever found (one is made by Todd Todeschini probably the most famous crossbow maker in the world) notice the very simple pivot trigger/lock,.
So yeah, sorry again dude, hope this helps.

Hey Parmenion, i just heated it with one of those 20 dollar "Bernzomatic" blowtorches till it got a bit red and then hit it with a hammer over an anvil, very simple.