Cheers for your posts all, esp the links Basileus!
Ive recently got my greedy little hands on two shorter length of spring steel that I would like to make into a prod (no doubt i will wreck one of them in the process).
I would ultimatly like to make something in the heavier draw weights 200-300lbs as will be using a crank to help n loading (im lazy and love electric come-a-long's).
The leaf spring is from a light automotive trailer. Dimensions as follows. its rather deflexed....
Length (end to end): 560mm
Length (end to end along the back) : 600mm
current deflexed brace height: 90mm (straight line tip to tip with a length of string)
Or...... if someone knows the conversion formula to scale up and to scales down STEEL leaf spring material of the same thickness and center widths... please feel free to help
Ok, so you'll definitely be needing some cocking device for this... As a comparison I got one crossbow made from 6,2mm leaf: it's 900mm long (along the back), 45mm wide at the center and around 7mm wide at the tips. It slightly (~60mm) deflexed and pulls around
150 pounds at 40cm draw (bow's belly to nut). Given that your leaf is only 600mm long expect higher draw weight and much shorter draw.
I'd cut the leaf into following dimensions:
- 37mm wide at the center
- 7mm wide at the tips
- 600mm long (along the back)
- 8mm wide nocks/ears at the tips (or more, if string loops are thick)
Try to place the tips as high as possible, so that you don't have to cant the bow (in it's stock) any more than you absolutely have to.
Even though 7mm at the tips sounds too little, it's not. It should be 0mm if we wanted to stress the bow equally at every part, but that's obviously not possible. If you leave the tips too wide, the center of bow will be overstressed for no good reason. Also, the dry-fire speed of the bow will be low. With heavy bolts you might not notice a difference, though.
One word of warning about using very thin tips... make sure the width taper is very even all the way up to the tips. I managed to bend the tip of one of my steel bows with an accidental misfire coupled with too strong linen string. Of course the bow bent at the exact point which was left slightly
too narrow (~0,5mm) compared to surrounding areas.
I also warmly recommend making a wippe cocking lever - it's fast to use and (relatively) simple to make:
Unfortunately I have not had time to finish the above article although my wippe now works perfectly.