8

While I was in my automatic car, I was on a steep upwards slope and I took my foot off of the brake and put it onto the gas pedal.

My car first slid backwards, but as the engine applied power to the wheels, the car slows down its descent and starts climbing up the slope.

My loose understanding of a transmission is that the crankshaft is geared down to the driveshaft. However, it seems like it would be impossible for something like this to happen since the wheels have a negative rotation.

How is this possible?

14

The torque converter picks up the slack

Torque Converter image

TC Cutaway

In essence, this device connects the engine crankshaft to the transmission's input shaft through a fluid-type coupling. Once the turbine spins fast enough relative to the pump, the pump will start turning as well.

Since the turbine and pump are not mechanically coupled the gear ratio between the engine and transmission is not fixed, which explains why your car slides back on an incline initially until sufficient engine speed is reached, at which point the inertia of the fluid inside the torque converter slowly turns the pump and transmission-side shaft to allow the vehicle to change speed and move up the incline.

  • 2
    Good answer, but the OP may have a dual-clutch transmission without a torque converter. – jedd.ahyoung Mar 2 '17 at 20:31
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    This is what puts the slush in Slush Box! – Ukko Mar 2 '17 at 20:52
4

This is possible, because, at low speeds, the fluid flywheel is acting as a clutch and, since it is a fluid linking the engine to the transmission, it will allow the car to roll backwards while the engine is rotating. Something a manual transmission cannot do unless it is in reverse or you disengage the clutch.

  • Oh, you can use a clutch to arrest a backward roll and turn it into forward motion. It helps if you enjoy replacing clutches, though. – Harper Mar 3 '17 at 3:13
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    Especially if you cannot co-ordinate releasing the handbrake and releasing the clutch!!! Or take 2 minutes to release the clutch... – Solar Mike Mar 3 '17 at 6:50
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    "You " is general and not you specifically... – Solar Mike Mar 3 '17 at 7:05
0

It's even possible for the driveshaft speed to be independent of both of the axle outputs as well. Some differentials have limited slip, which use clutches submersed in oil. They will slip as well when under heavy loads. That's why that Corvette pulled away from my Porsche....

  • 1
    That doesn't sound right. The 'limited slip' function reduces the maximum speed difference between the left and right wheel by introducing a soft coupling between them. It does not allow the L/R wheels to slip relative to the driveshaft. – Hobbes Mar 3 '17 at 10:11
  • I agree that with a conventional diff this is true. some 4wd cars use a viscous coupling in the prop shaft, to allow but limit a speed differential between the front and rear wheels. In this case the speed of the propshaft after the viscous coupling may not be directly related to the crank speed. – Kickstart Mar 3 '17 at 13:46
  • This answer isn't related to the question. Although interesting, conversational answers like this are not what these stackexchange sites are for. – Steven Rogers Mar 3 '17 at 20:37

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