# English subtitles for clip: File:Around the Corner (1937) 24fps selection.webm

 ```1 00:00:01,267 --> 00:00:02,807 Formations, like these, 2 00:00:03,224 --> 00:00:06,599 require long hours of intensive drilling and careful judgment. 3 00:00:07,314 --> 00:00:09,095 When the troop goes around the corner, 4 00:00:09,367 --> 00:00:12,743 the riders on the outside of the turn have to adjust their speed 5 00:00:12,744 --> 00:00:15,061 to keep even with the riders on the inside. 6 00:00:16,637 --> 00:00:20,139 The man on the outside has to ride a lot further and a lot faster 7 00:00:20,140 --> 00:00:22,085 in order to keep up with the parade. 8 00:00:24,052 --> 00:00:28,196 The outside wheels must spin faster than the wheels on the inside 9 00:00:28,758 --> 00:00:32,666 because they have a greater distance to travel in the same length of time. 10 00:00:33,511 --> 00:00:38,052 When a wagon turns a corner, the wheels can travel at different speeds 11 00:00:38,053 --> 00:00:41,808 because each one can turn freely on the axles. 12 00:00:41,859 --> 00:00:45,821 And in the early automobiles, the rear wheels turned separately 13 00:00:45,822 --> 00:00:48,820 and only one wheel was connected to the engine. 14 00:00:49,459 --> 00:00:52,631 But when only one wheel was driven by the engine, 15 00:00:52,631 --> 00:00:54,662 it had to do all the work, 16 00:00:54,703 --> 00:00:59,206 and it couldn't get a good enough grip on the road to do its job properly. 17 00:00:59,845 --> 00:01:02,308 So the one-wheel drive was soon out of date. 18 00:01:03,151 --> 00:01:09,101 But if two wheels are locked on an axle, so that they are not free to turn separately, 19 00:01:09,298 --> 00:01:11,869 one or the other has to slide. 20 00:01:12,491 --> 00:01:15,851 So engineers had to find a way to connect both rear wheels 21 00:01:15,852 --> 00:01:19,580 to the engine, without sliding and slipping on turns. 22 00:01:20,870 --> 00:01:25,113 The device which makes this possible is a part of the rear axle. 23 00:01:25,585 --> 00:01:27,892 It is called the differential, 24 00:01:27,893 --> 00:01:32,209 because it can drive the rear wheels at different speeds. 25 00:01:32,822 --> 00:01:34,804 The differential looks complicated. 26 00:01:35,140 --> 00:01:39,215 But once we understand its principle, it is amazingly simple. 27 00:01:39,855 --> 00:01:42,763 These two wheels are mounted on separate axles 28 00:01:42,764 --> 00:01:44,593 and supported by a frame, 29 00:01:45,391 --> 00:01:49,798 so that they can revolve freely at different speeds. 30 00:01:50,350 --> 00:01:55,415 Let's fasten a spoke on the inner end of each axle. 31 00:01:56,754 --> 00:02:02,461 So that by turning the spokes, we can turn each wheel separately. 32 00:02:06,923 --> 00:02:11,614 With a bar or cross piece, we can turn both wheels 33 00:02:11,614 --> 00:02:15,973 in the same direction at the same rate of speed. 34 00:02:16,869 --> 00:02:19,418 Let's get something to hold this bar in place, 35 00:02:19,419 --> 00:02:22,509 so that it will press against the spokes. 36 00:02:23,313 --> 00:02:26,468 Notice that this support is not locked to the axle, 37 00:02:26,469 --> 00:02:27,842 it turns freely. 38 00:02:29,145 --> 00:02:33,129 Now, we can spin the wheels by rotating the support. 39 00:02:34,066 --> 00:02:39,111 This is fine, as long as both wheels are able to turn at the same speed. 40 00:02:39,587 --> 00:02:43,264 But let's see what happens when we go around the corner. 41 00:02:44,090 --> 00:02:48,927 With this arrangement, we cannot drive one wheel faster than the other. 42 00:02:53,620 --> 00:02:58,802 And if we stop one wheel, the other wheel won't budge. 43 00:03:01,590 --> 00:03:07,533 Let put this bar on a pivot, so that it can swing in either direction. 44 00:03:13,458 --> 00:03:18,996 Now, the bar can still turn both wheels at the same speed. 45 00:03:19,583 --> 00:03:24,697 And, because it pivots, it lets one wheel turn 46 00:03:24,698 --> 00:03:27,523 even when the other is stopped. 47 00:03:28,619 --> 00:03:33,075 But, if turned too far, the bar will swing around 48 00:03:33,076 --> 00:03:37,004 until it won't drive the spokes that turn either wheel. 49 00:03:37,601 --> 00:03:42,766 We need another crossbar and more spokes to carry on the job. 50 00:03:43,708 --> 00:03:45,434 When we stop one wheel, 51 00:03:46,239 --> 00:03:51,121 the crossbars will continue to push the spokes of the free wheel around. 52 00:03:52,683 --> 00:03:55,245 As long as both wheels are free to turn, 53 00:03:56,621 --> 00:04:02,744 the bars do not swing on their pivots and the wheels move at the same speed. 54 00:04:04,500 --> 00:04:08,034 Now we have the working principles of a differential. 55 00:04:08,767 --> 00:04:14,012 To adapt the model for use in an automobile, we will have to make a few changes. 56 00:04:14,944 --> 00:04:20,981 In order to reduce the jerky action, caused by wide spaces between the spokes, 57 00:04:21,626 --> 00:04:24,276 we will put in more spokes. 58 00:04:28,022 --> 00:04:33,844 Further filling in the spaces between the spokes gives steadier, more continuous action. 59 00:04:34,225 --> 00:04:39,002 And changing the shape gives firm, constant contact. 60 00:04:40,109 --> 00:04:43,012 Now we can make the gears thicker and stronger, 61 00:04:43,144 --> 00:04:46,348 and we have differential gears. 62 00:04:46,923 --> 00:04:51,881 The edges are cut so that they will fit together more smoothly and silently. 63 00:04:52,261 --> 00:04:56,659 And another gear is added to share the work of driving the axles. 64 00:04:56,695 --> 00:04:58,234 The principle is the same. 65 00:04:59,466 --> 00:05:02,949 In order to turn the support and drive the wheels, 66 00:05:03,172 --> 00:05:09,229 we can fasten a large gear here, connected by a smaller gear 67 00:05:10,195 --> 00:05:11,663 to a source of power. 68 00:05:18,760 --> 00:05:21,565 Notice that the power is connected to the differential 69 00:05:21,566 --> 00:05:23,263 at the center line. 70 00:05:25,519 --> 00:05:30,535 We can make our model more compact by moving the gears closer together. 71 00:05:31,868 --> 00:05:34,316 When we put our differential in an automobile, 72 00:05:34,317 --> 00:05:36,899 we have to leave room for the drive shaft, 73 00:05:36,900 --> 00:05:38,978 which carries the power from the engine. 74 00:05:40,502 --> 00:05:43,875 We may build the floor of the car above the drive shaft. 75 00:05:45,035 --> 00:05:49,151 But, if we do, we won't have much room inside, 76 00:05:49,152 --> 00:05:51,839 unless we make the top of the car high too. 77 00:05:52,920 --> 00:05:55,105 Of course, we could lower the floor and ceiling, 78 00:05:55,106 --> 00:05:58,129 but the drive shaft would be higher than the floor. 79 00:05:58,703 --> 00:06:00,440 This would have disadvantages. 80 00:06:01,536 --> 00:06:04,315 A shaft in the middle of the floor of an automobile 81 00:06:04,316 --> 00:06:09,535 would be inconvenient for passengers, and would be awkward for carrying luggage. 82 00:06:10,028 --> 00:06:15,118 Today, engineers have found a way to make the car roomier and closer to the road 83 00:06:15,706 --> 00:06:18,396 without a clumsy shaft above the floor. 84 00:06:18,885 --> 00:06:24,542 The drive shaft from the engine to the differential is lowered out of the way. 85 00:06:25,226 --> 00:06:29,386 And the drive shaft is connected to the rear axle at the bottom. 86 00:06:29,830 --> 00:06:36,199 The new low center drive makes the rear axle quieter, stronger, and more durable 87 00:06:36,501 --> 00:06:41,334 because it gives better, smoother contact between the gears. 88 00:06:42,343 --> 00:06:45,789 The automobile of today with the low center drive 89 00:06:45,790 --> 00:06:48,989 is stronger and more rugged. 90 00:06:49,592 --> 00:06:54,356 Every part of the rear axle has been build to withstand strains, 91 00:06:54,357 --> 00:06:57,591 far greater than it will ever meet on the straight way 92 00:06:57,592 --> 00:06:59,455 or around the corner. 93 00:07:16,509 --> 00:07:18,578 Inside Wheel 94 00:07:19,077 --> 00:07:20,579 Outside Wheel 95 00:07:21,128 --> 00:07:23,408 Inside Wheel 96 00:07:24,267 --> 00:07:25,969 Outside Wheel 97 00:07:26,504 --> 00:07:28,572 Inside Wheel 98 00:07:28,967 --> 00:07:30,365 Outside Wheel```