Lets say you wanted to teach kids about why heat cycling new tires is important. It’s up to you to impart important fundamental gear head knowledge to these blossoming young minds. So how do you do it? Maybe we should sit the kids down in the classroom and lecture them about the polymer chain structure in rubber that heats up when distorted. Chances are they’ll end up texting their friends about how much they hate you. You might even make Twitter for being the heinous old person who was forcibly torturing them with the atrocities of totally lame science stuff.

There’s a better way to learn about how rubber works with little risk of being labeled “cray cray”, at least not the bad kind. It involves shooting slingshots, preferably with a huge German dude that runs the world’s largest slingshot forum and builds crazy new slingshots every week for his YouTube channel. Destin of Smarter Everyday did a collaboration video with Jörg Sprave of the Slingshot Channel to explore how to get the most out of a rubber band.

Lets talk about the two questions at the end of the video.

Question 1: Why does rubber lose elastic potential when it cools off?
You would think that when the rubber got hot, it would be softer and not contract with as much force. I was stumped about this until I watched one of the videos left in the comments of the Smarter Everyday video:

What makes this interesting is that the molecules that interconnect the long polymer chains of rubber bands are able to transform thermal energy (heat) into kinetic energy (motion) which is a pretty rare direct conversion. The turbine of a turbocharger can also convert heat into motion. Now you know the answer to the trivia question about what turbos and rubber bands have in common. Jörg took advantage of rubber’s thermal energy conversion to make a crazy new slingshot called The Entropy Slayer 2000. It’s got a heated chamber for the rubber band. He loads a shot, warms the rubber band and then is able to fire the ball at higher speeds.

We should also take this moment to visit the subject of heat cycling new tires while we’re still talking about the heat of rubber deformation. Heat cycling tires is very different from warming up tires or overheating tires from heavy use. It’s something that should be done to break in new tires and it’s more about deformation than friction. You need to deform the rubber in the tires enough to make them generate heat like the rubber band in the slingshot we just talked about. That means smoothly driving in circles in both directions at low to moderate speeds. Putting heat into the tire with abrasion from hard loads will actually do damage instead of helping break in the rubber. It’s also very important that the tires be allowed to cool naturally afterwards. What this does is it breaks some of the weaker molecular bonds in the tire rubber during the deformation process and then reforms them with stronger connections during the cooling process. Doing this with new tires allows them to last longer and perform more consistently.

Question 2: Why does the tapered rubber band contract faster?
I think the best way to think of this is a linear rate spring vs. a progressive rate spring. The spring rate in the straight piece of rubber is the same throughout it’s length. When the shot is released, all of the rubber contracts at the same rate giving a sudden punch of acceleration. The whole rubber band also loses energy at the same time. The same isn’t true of the tapered rubber band. The spring rate is continuously changing through the rubber band (the band is seeing the same force even though the amount of rubber in the cross section is decreasing). That means that different sections of the rubber are transferring energy into the shot at different rates which produces a force multiplication effect. The best intuitive analogy of how this works is to think of the first man-made tool to break the speed of sound: the bullwhip. Just about anybody can flick the handle of a bullwhip to make the tip crack with a sonic boom. Where our rubber band is a continuously changing spring rate, the bullwhip can be thought of as a continuous string of joints. Continuous and tapering energy transfer allows a large body of energy (the rubber band or the handle of the bullwhip) to more efficiently focus on moving a small object (the ball bearing or the tip of the bullwhip). That’s why the tapered band on the slingshot contracts faster and it’s also why a simple flick of a bullwhips handle can result in the tip exceeding the speed of sound.

To see the physics in action, check out this video of martial arts and weapons expert, Anthony De Longis, demonstrating his bullwhip fighting style. He starts with talking about how the whip works and then how it’s able to be used as a pretty fascinating weapon.

 

Sources: Smarter Everyday, ChristopherJSykes, The Slingshot Channel and mattbynumfilms on YouTube