Quote:
Originally Posted by Cory
There's a difference between a 38" long bumper segment only supported at the ends and a 38" long bumper segment supported every 12". The latter is never going to break if you're using high quality plywood.
The example I gave is what I believe Craig is referring to.
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I hate to "break" it to you, but that is not quite the case. During early testing, I was driving a robot with bumpers attached in exactly the manner you describe (rugged stiff fasteners, robust stanchions about every 12 inches, bumper proud of the robot frame by about three inches). In the very first drive-from-one-end-of-the-field-to-the-other-as-fast-as-you-can test, the bumper broke and splintered upon impact with another robot at the far end. Within just a few minutes, I did it again while pushing the robot around manually, just to see how fast I could push it against wheel slip.
/edit/I just ran a few numbers out of curiosity. In a "perfect collision" situation (two full weight 151 pound robots hitting head-on at 9 fps, with one of the robots skewed so it impacts the other "corner first") the impact forces get pretty impressive. As the robots collide, they compress the pool noodles down to 20% of their original thickness in about 0.009259 seconds. At a closure velocity of 18 fps, this is a peak change in velocity of 1944 ft/sec/sec, or a 60.75-G impact. Since I said the robot impacted "corner first" I will posit an impact area of 1.5 square inches. Assuming the pool noodles absorb about 18% of the impact energy during compression (not too bad for material of this type), that still means that the localized impact pressure is right around 10,000 pounds per square inch. I haven't looked at the bending moment of 3/4-inch plywood on 12-inch support centers yet. But I am now really not surprised by what happened to the bumpers. /edit/
There is a reason for that rule. Don't count on it changing.
-dave
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