Wednesday, February 8, 2017

Racing fire fighter bears


Years ago, I bought a small box of stuff at a yard sale and later discovered six of these wooden toys in with the other contents. Each one measured 7/8" X 1-5/8", X 5/8" wide.  In profile they appear as below:


They were clearly designed to climb down a ladder, but the ladder was missing.  I tried making something up using wooden side rails and rungs made of steel rod just wide enough to clear the slots, but it failed miserably.  The bears would just swing around the first rung, shoot tangentially out into space, and miss all of the rungs below.

So, recently I put on the old thinking cap.  The physics was surprisingly complex. As the toy bears swing around the ladder rung, they create a moment of inertia which swings them out and away from the rung below. Consequently, they have to be prevented from swinging too far outwards. That's what the flat slots are for:  the bears swing around on the rungs, but can only drop straight down.  So, the rungs have to be flat, not round, just wide enough to clear the slots, and also not so wide that the bears couldn't swing around on them. In other words, just slightly smaller than the diameter of the hole through the bears. Next, the distance between the rungs is critical.  It needs to be just wide enough that the toy bear will drop onto the next rung below, with enough clearance above it to begin swinging around it again.  If the distance is too small, the bear will be captured between the rungs and can't move;  to far, and the bear may miss the rung below as it falls towards it. So, I ran grooves in wood and experimented with different rung distances until I got it to work. After that, I had to do some additional fettling with several of the bears, as some needed to have their slots opened up slightly with a file, otherwise they'd hang up on some rungs.  I also had to make sure that there was an even number of rungs, otherwise the bears land upside down (not essential, but it looks better for them to land on their "feet"), and I needed to make sure that there was sufficient room below the bottom rung to remove the bears to start the race over again. Finally, after varnishing the ladder (which naturally raised the grain and created more friction) I sanded it down with very fine steel wood, and then applied floor wax to the rungs.  (Floor wax has become hard to find.  I've still got half a can I found years ago at a yard sale.  It's wonderful stuff for keeping rust off of the top surfaces of table saws and other woodworking bench tools, because it won't stain the wood you subsequently cut.)  Anyway, the bears now race down the ladder wonderfully, sometimes and unpredictably stopping briefly or needing a nudge, so there's often one winner.  It's a fascinating toy! It has enchanted everyone I have shown it to.  Below, a photo sequence from right to left:



If you'd like to try making the game, below a closeup of how the ursine fire fighters appear.  The only change I'd suggest is giving each bear a different number on its helmet.  Mine are all No. 1.


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