1 00:00:00,520 --> 00:00:03,400 Speaker 1: Welcome to brain Stuff from house stuff works dot com 2 00:00:03,600 --> 00:00:14,560 Speaker 1: where smart happens. Hi. I'm Marshall Brain with today's question, 3 00:00:14,840 --> 00:00:18,080 Speaker 1: how do thirty pounds of air in your tires hold 4 00:00:18,160 --> 00:00:21,240 Speaker 1: up two tons of car? The next time you get 5 00:00:21,320 --> 00:00:23,800 Speaker 1: in your car, take a close look at the tires. 6 00:00:24,239 --> 00:00:28,080 Speaker 1: You'll notice that they are not really round. They're squashed 7 00:00:28,120 --> 00:00:31,200 Speaker 1: at the bottom. That flat spot on the bottom where 8 00:00:31,200 --> 00:00:34,080 Speaker 1: the tire meets the road is called the contact patch 9 00:00:34,159 --> 00:00:37,040 Speaker 1: for the tire. If you were looking up at the 10 00:00:37,120 --> 00:00:40,320 Speaker 1: car through a glass road, you can measure the size 11 00:00:40,320 --> 00:00:43,159 Speaker 1: of this contact patch. You would multiply the length of 12 00:00:43,159 --> 00:00:45,880 Speaker 1: the contact patch by its width to get the area, 13 00:00:46,240 --> 00:00:48,800 Speaker 1: then add up the area for all four tires to 14 00:00:48,840 --> 00:00:52,240 Speaker 1: get the total area of the contact patch. For your 15 00:00:52,400 --> 00:00:56,279 Speaker 1: two ton car, or your four thousand pound car, or 16 00:00:56,320 --> 00:00:59,720 Speaker 1: your two thousand kilogram car, you will find that the 17 00:00:59,760 --> 00:01:02,720 Speaker 1: air of the contact patch is about equal to the 18 00:01:02,760 --> 00:01:05,880 Speaker 1: weight of the car divided by the tire pressure. In 19 00:01:05,920 --> 00:01:09,720 Speaker 1: this case, four thousand pounds divided by thirty pounds per 20 00:01:09,800 --> 00:01:13,560 Speaker 1: square inch equals add thirty three square inches. That may 21 00:01:13,600 --> 00:01:16,560 Speaker 1: seem like a lot, but your car's tires are probably 22 00:01:16,600 --> 00:01:20,160 Speaker 1: about seven inches wide. That means that the contact patch 23 00:01:20,240 --> 00:01:22,880 Speaker 1: for each tire will be only about four and a 24 00:01:22,920 --> 00:01:26,280 Speaker 1: half inches long. If you go outside and measure the 25 00:01:26,319 --> 00:01:29,440 Speaker 1: size of the contact patch on your car, you'll probably 26 00:01:29,520 --> 00:01:32,160 Speaker 1: find that it's actually even bigger than this. You can 27 00:01:32,160 --> 00:01:34,600 Speaker 1: measure the width of the tread pattern anywhere on the 28 00:01:34,640 --> 00:01:36,639 Speaker 1: tire to get a pretty good idea of the width 29 00:01:36,640 --> 00:01:39,520 Speaker 1: of the contact patch. To get the length, take two 30 00:01:39,560 --> 00:01:42,000 Speaker 1: sheets of paper and slide them under the front and 31 00:01:42,040 --> 00:01:44,880 Speaker 1: back of the tire until they won't go any further. Now, 32 00:01:44,959 --> 00:01:48,160 Speaker 1: measure the distance between the two pieces of paper. The 33 00:01:48,240 --> 00:01:51,720 Speaker 1: reason that the contact patches even bigger than this calculation 34 00:01:51,840 --> 00:01:54,880 Speaker 1: suggests is that at the back and front edge of 35 00:01:54,880 --> 00:01:58,240 Speaker 1: the contact patch, the pressure exerted on the ground is 36 00:01:58,280 --> 00:02:01,360 Speaker 1: not very high. At the point where the tire is 37 00:02:01,400 --> 00:02:04,760 Speaker 1: just barely touching the ground almost no way it is supported. 38 00:02:04,960 --> 00:02:07,360 Speaker 1: As you move towards the center of the contact patch, 39 00:02:07,480 --> 00:02:10,480 Speaker 1: more and more weight is supported. Now, let's say that 40 00:02:10,560 --> 00:02:13,080 Speaker 1: you drop the pressure in your tires to seven p 41 00:02:13,360 --> 00:02:16,160 Speaker 1: s i a quarter of what it was. You would 42 00:02:16,160 --> 00:02:19,760 Speaker 1: find that your contact patch did not get four times bigger. 43 00:02:20,240 --> 00:02:22,800 Speaker 1: This is where the stiffness of the side while the 44 00:02:22,840 --> 00:02:26,200 Speaker 1: tire comes in. When the pressure is this low, the 45 00:02:26,280 --> 00:02:28,600 Speaker 1: structure of the tire starts to bear some of the 46 00:02:28,639 --> 00:02:32,160 Speaker 1: weight of the car. Low profile tires like you see 47 00:02:32,160 --> 00:02:35,880 Speaker 1: in sports cars have short, stiff side walls, so these 48 00:02:35,919 --> 00:02:39,040 Speaker 1: tires will tend to squish less than the big tires 49 00:02:39,040 --> 00:02:41,680 Speaker 1: on an suv or a pickup truck. In fact, there 50 00:02:41,680 --> 00:02:44,280 Speaker 1: are some tires that are so stiff they can run 51 00:02:44,320 --> 00:02:46,840 Speaker 1: with no pressure in them at all. These are called 52 00:02:46,919 --> 00:02:51,200 Speaker 1: run flat tires. Be sure to check out our new 53 00:02:51,280 --> 00:02:54,600 Speaker 1: video podcast, Stuff from the Future. Join how Stuff Work 54 00:02:54,680 --> 00:02:58,239 Speaker 1: staff as we explore the most promising and perplexing possibilities 55 00:02:58,240 --> 00:03:03,080 Speaker 1: of tomorrow. The Houstuffork's iPhone app has arrived. Download it 56 00:03:03,120 --> 00:03:04,440 Speaker 1: today on iTunes.