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[ Compound bobber ]
[ Compound bobber ]
What is a "bobber"?
Shooting at static targets can get pretty boring
after a while - you run, you shoot, blah, blah, blah. Bobbers are the simplest
type of moving target - essentially a pendulum with target(s) at one end
and a weight at the other.
The typical bobber I'd seen before was a pretty crude device, normally the pendulum pivot was just a plastic block with a hole through it. As might be expected, these tended to slow down quite rapidly - if you left the bobber to last in a stage you would expect it to have slowed down to the point where it was easy to hit. A month ago (September 2002) I can across a different class of machine - constructed from welded metal and with a proper set of bearings. It was a work of art, the Rolls Royce of bobbers, able to run for over a minute before slowing down.
Unfortunatly I have neither the skill, dedication or materials to duplicate this masterpiece. What follows is my take on the problem: not so much a Rolls Royce, but a [insert cheaper car manufacturer here]. It may not be as pretty, but it does the same job (with almost the same performance).
The typical bobber I'd seen before was a pretty crude device, normally the pendulum pivot was just a plastic block with a hole through it. As might be expected, these tended to slow down quite rapidly - if you left the bobber to last in a stage you would expect it to have slowed down to the point where it was easy to hit. A month ago (September 2002) I can across a different class of machine - constructed from welded metal and with a proper set of bearings. It was a work of art, the Rolls Royce of bobbers, able to run for over a minute before slowing down.
Unfortunatly I have neither the skill, dedication or materials to duplicate this masterpiece. What follows is my take on the problem: not so much a Rolls Royce, but a [insert cheaper car manufacturer here]. It may not be as pretty, but it does the same job (with almost the same performance).
Parts list
- ~1m 50x50mm rough timber (for main post)
- 2-3m 20mm x 50mm rough timber (for everything else)
- 1 inline skate wheel and bearing
- a length of M6 studding and 2 long M6 bolts
- wingnuts and normal nuts (also used as spacers, you may need more, depending on wheel design)
- big washers
Build instructions
1: The standMultiple choice here - there are any number of ways to do this. I'm going to describe a transportable model.
A picture paints a thousand words, so...
When folded (see below) the unit is quite a compact package. Notes: all connections are bolted on to allow for easy packing. If you can find bolts long enough to go through the frame and struts use them - I could not and thus used sections of studding (which obviously increases the numbers of wingnuts).
2: The Pivot
Bolt your rollerblade bearing near the top of the post. Depending on the design of your wheel you will have to use one or more spacers (bolts, layers of wood) to let the pendulum swing free.
3: The Pendulum
Cut a length of wood about a quarter longer than the height of the stand. Position a rollerblade wheel about a third down from the top, fix by any means (I used cable ties, wire may be better). This needs to be as solid as possible. Attach the pendulum mass (eg a small drinks bottle full of sand or water) to the pendulum (eg a bolt for it to sit on and more cable ties - if you put in multiple bolt holes the weight can be moved up and down the pendulum, changing it's frequency).
4: Finish
A short piece of wood should be bolted to the top of the pendulum to hold targets - this prevents wild shots damaging your masterpiece. All done!
Deployment
You will find that the unit rocks slightly during use - this must be stopped! Each loose joint saps energy from the pendulum during every oscilation, dramatically reducing it's swing time. Be sure to tighten up the bolts and weigh down the base.If you need to slow down the freqency, put the weight further from the pivot (you did make the stand tall enough, didn't you?).
Compound bobber
This is Ewok's idea, so don't blaim me!Bobbers are predictable. Even when fast they are predictable. The solution to this is to use a compound rather than a simple pendulum. A compound pendulum is one where you have a joint and a second weight some way up from the first, giving it a much more complex swing. Construction details to follow if we can make it work!