So which is worse, bored farmers, with
3000 HP pulling tractors and giant sculptures of dinosaurs or bored
engineers with rocket-assisted cars and steam powered motorcycles? Or
both, competing with trebuchets?
D'wife has remarked several times that
Engineers should never be allowed to be bored. The results all too
often wind up making headlines and not always in a good way.
Here's my rather modest contribution: A crossbow, made from leftover parts of other projects involving bicycles and old motorcycle parts. I will warn you up front that this thing is not all that close to being finished. I'm designing this as I go along and most of is is subject to revisions, some of which may be quite significant.
Viewing the top. The notch in the right hand stave is not there on purpose. It was there when I found the piece of aluminum. So far, not a problem.
Bottom side. Here you can see the mechanism. The spring is inside the black PVC tube to keep it from flexing when I cock the bow.
The draw on this is a compound
stringing method that makes the pull relatively lighter while
increasing the pull distance quite a bit. Most crossbows only pull
back 6-12 inches but might be as high as 150 lb. The physics of
springs says that the energy in a spring is 1/2KX^2 where K is the
spring force, and X is the distance it is compressed or extended.
More pull distance means less effort to cock it for the same energy
for the bolt.
This one uses a rear spring from a
motorcycle to supply the force, and the bottom brackets from 2 small
bicycles to operate the bow arms which normally do not bend. The
chain runs from one sprocket, down through the spring, back up, and
around the other sprocket. When the arms are pulled back, the spring
is compressed forward. If you want a more powerful bow, you simply
attach the chain further around the sprockets, increasing the
pre-load on the spring.
It simply cries out for some sort of