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Quick question bugging me:
Obviously the car moves forward when neither applying the brake nor the accelerator.

How does this not damage the transmission when the brake is pressed?

I first thought that perhaps the transmission releases when the brake is applied, but why can I inch forward while applying my brake then?

It seems like it would strip the transmission if it were trying to accelerate while braking.

Thanks, Mac

Edit 1: Thanks for all your responses, I now understand it! I don't think this is a duplicate, because the question on how a torque converter works doesn't include this question, and as a result I wasn't able to find an answer without asking a separate question.

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    @JPhi1618 although similar I think the question does not get answered by the QA for the dupe. The OP is curious how the transmission and brakes interact and why the transmission doesn't get damaged. I would think that how basically a transmission works has the contents of an answer.
    – DucatiKiller
    Commented Mar 2, 2016 at 19:11
  • @DucatiKiller, Honestly, I thought that question I linked to was better than it was... Do you think this one is better? I still think the main answer is "the torque converter allows slippage", no?
    – JPhi1618
    Commented Mar 2, 2016 at 19:54
  • I agree but it never says that so the OP can't make the connection, unless I have that wrong.
    – DucatiKiller
    Commented Mar 2, 2016 at 19:55
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    This doesn't sound like it has anything to do with torque converters. The transmission doesn't strip out because it's designed to withstand way more torque than the brakes apply to the axles. For some reason the answer stating this was downvoted. Also of note, you can damage either a manual or auto transmission. The manual just wears extra fast if you constantly drive with the clutch partially disengaged, while the auto can overheat if you're revving the motor fast enough.
    – MichaelS
    Commented Mar 2, 2016 at 21:30
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    The torque converter is the fluid coupling between the engine and the transmission. Part of it's job is to allow the engine to run while the wheels are stopped. See either of the videos I linked for a more full explanation. But bottom line, when stopped, the transmission is not moving, nor is the input shaft. What allows this is the torque converter.
    – cdunn
    Commented Mar 2, 2016 at 22:02

3 Answers 3

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The somewhat longer answer is that the device that makes what you're asking possible is the torque converter. This device is a pair of curved blades setup as an impeller, and a turbine. The following cutaway pictures illustrate this setup:

Diagram of torque converter internals

Here is a torque converter split in half for a better view:

Torque converter impeller side

Torque converter turbine side

The way this works is explained really well by a You-Tube video here: https://www.youtube.com/watch?v=z5G2zQ_3xTc

Here is the gist of it. The turbine is attached to the engine and rotates at the speed the engine does. The pump is the other set of blades, and it is attached to the output shaft which goes to the transmission. The only connection between the two sets of blades is a viscous fluid (transmission fluid). At idle, the converter is in a phase called stall. No power is transmitted because while the turbine is spinning with the engine, which causes fluid to flow into the pump, the pump doesn't move because it is attached to the wheels which are held by the brakes. When you release the brakes, you move forward because the fluid driven by the turbine into the pump causes the pump to move which gets sent to the transmission.

Bottom line, the reason this works is because there is no solid mechanical connection between the engine and the wheels. One of the links in that chain (the torque converter) only connects the engine to the transmission by two sets of turbine blades that face each other and are only indirectly driven by the force of moving the viscous fluid.

Here is a link to a You-Tube video that demonstrates how the converter works using two fans. It really helps to visualize how the two turbine blade sets work together, and how the system works. https://www.youtube.com/watch?v=pTfipsejqS0

I hope that helps!

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  • TY for clarifying on the comments +1
    – DucatiKiller
    Commented Mar 3, 2016 at 2:23
  • Note that all of this only applies to an automatic transmission, manual transmissions work on a completely different mechanism.
    – Leliel
    Commented Mar 3, 2016 at 3:00
  • I never knew this existed. It sounds very inefficient (but apparently it is not, how comes?)
    – njzk2
    Commented Mar 3, 2016 at 5:49
  • @Leliel Yes, but on a manual transmission you would typically need to disengage the clutch when you brake.
    – Taemyr
    Commented Mar 3, 2016 at 8:54
  • @njzk2: Most torque converters actually have an added clutch that will make a physical connection above certain speeds. That way it's only inefficient while accelerating from a standstill.
    – Joey
    Commented Mar 3, 2016 at 9:02
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This is a great question, and the answer lies here: Why doesn't the engine stall out when you come to a stop with an automatic transmission?

And more information here: How does a torque converter work.

With a manual transmission, you push in the clutch to disengage the transmission. With an automatic, that job is handled by the torque converter.

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    Furthermore, coming to a stop without disengaging the transmission would always stall the engine before it did any damage to the gears in the transmission. This is the same reason a stick shift will stall if the clutch is engaged too quickly, instead of shearing any teeth off the tranny gears.
    – MooseLucifer
    Commented Mar 2, 2016 at 21:30
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While the torque converter on an auto and the clutch on the manual transmission do allow the engine to disengage the drive. I believe that the real answer to this question is that the gears in the gearbox are designed to be strong enough so that the gears won't be damaged either by the maximum torque of the engine driving it, or the forces on it through engine braking.

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  • they will be damaged. Try engaging the first gear on a manual while revving very high, and with the brakes on. (not just engaging it, disengaging the clutch, switching in first gear, then reengaging the clutch, obviously)
    – njzk2
    Commented Mar 3, 2016 at 5:54
  • @njzk2 Which bit of the manual transmission are you saying will break in the situation you describe? I also don't see how that fits with what the question was asking. I am sure you can break anything if you try hard enough.
    – HandyHowie
    Commented Mar 3, 2016 at 8:16

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