How Your Car Turns Corners
May 1946 Popular Science

May 1946 Popular Science [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Popular Science, published 1872-2021. All copyrights hereby acknowledged.

Have you ever watched an animated video of how the differential joint in a car or truck works? Its simplicity required a spark of genius to come up with. The wheels on the inside of a turn need to turn at a lesser rate than those on the outside. Recall that for a fixed rotational rate, ω_turn (rad/s), about a pivot point (the center of the turn) and a distance, ω_turn, from the pivot point, the radial speed, s_turn, is ω_turn*r_turn. Since ω_turn is the same for all points along the radial, s_turn is a linear function of r_turn. Assuming the tires are of equal diameter, d_tire (or r_tire), the rotational rate, ω_tire, of each wheel is s_turn/ r_tire. Without a differential gear system, then, the wheels on the inside and outside of a turn would skid due each needing to rotate at a different rate, but being forced to turn at the same rate. If a picture is worth 1,000 words, then this video is worth many times more than that. Here is a great animation of how a differential gear works. Be sure to also watch the limited slip differential video as well. What about electric vehicles with independent drive motors on the wheels? The computer systems takes care of sensing the rotational speed and torque requirements for each wheel. Analog versus digital. Like Joe Walsh of the Eagles, I'm an analog guy.

How Your Car Turns Corners with Differential Gears - Mechanics of Living

If your car had to travel only in a straight line, it could be almost as simple as an oldtime wagon. A basket full of gears, shafts, pinions. levers, and other parts, front and back, could be discarded. A steering gear and a few rods could come off the front. But the rear has some complicated machinery for going around corners. On a turn, one rear wheel needs more speed. It has farther to travel - like the man on the outside file of a marching column of men. The other wheel needs less speed; it has to slow down on a turn-like the man on the inside of a marching column.

So a unit called the differential stands between the two halves of the rear axle and takes charge of the engine speed. When your car is traveling on a straight road, the differential divides the engine speed into two equal parts - one to each rear wheel. When your car turns a corner, the differential divides the engine speed into two unequal parts. The differential takes some speed away from the wheel on the inside of the turn, and gives it to the wheel on the outside. Follow the drawings on the next page and see how the differential works - and how your car turns corners.

This is how your differential would look if the axle housing were cut away to show the complicated gears.

1. Main parts of a differential work like the pinion A mounted between two racks B and C. All three parts are free to move. As hand pushes down, pinion won't revolve, but pulls racks down with it.
2. When the hand holds back slightly on rack B and the pinion is pushed down, the free rack C will be depressed by the pinion. But the rack B that's held will start the pinion rotating, moving rack C faster.
3. Here the two racks have been turned into round gears, B and C, more like the actual differential. Pinion is still mounted between the gears, and acts the same way. As the hand pushes on pinion A, it won't revolve but pulls both gears down and around.
4. Slow down one round gear, the same way the hand held back on rack B in 2. Now when the pinion is pushed down, it starts to rotate. The pinion is pulling the free gear C around, but the rotation gives that gear a boost, and slows down to gear B.
5. Compare this combination of gears with drawing to the left. Instead of a hand, engine power is pushing pinion A down. When your car is going straight ahead, the differential pinions are pulling both gears B and C around, driving the rear wheels.
6. When making a right-hand turn, the tire on the right wheel pivots and slows down. This slows the round gear B and affects the pinion. It starts to rotate. The rotation gives the gear C a boost and retards gear B. That's how the rear turns corners.

Posted November 20, 2023