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Does it matter if engine power goes through the gearbox before the clutch, or through the clutch before the gearbox?

I'm not sure if there has to be a specific order of clutch & gearbox, or if it's just a matter of what's easier to manufacture.

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If you were to put the clutch after the gearbox, you would likely need a far bigger clutch due to the much larger torque that it would need to transmit.

Pulling away even on level ground would be like trying to do a hill start in a 'normal' car.

Other than that I can't see any reason why a gearbox couldn't be designed to have the clutch on it's output. As long as the clutch allows the gears to sync when changing gear, it should work.

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  • Not sure if the clutch would have to be "bigger" -- it still transmits (roughly) the same power, just merely more torque for less (clutch) rpm (which might also reduce clutch wear too due to less rotations). But, I suppose there could be a difference in "feel" between fast-spinning low-torque clutch, compared to a slow-spinning high-torque clutch transmitting the same amount of overall power.
    – ManRow
    Sep 5, 2018 at 7:22
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    @ManRow Torque is the main factor, speed it not really an issue - engineering.com/calculators/torque_transmitted.htm Notice how diameter and number of plates are important when calculating maximum torque. There is no reference to RPM.
    – HandyHowie
    Sep 5, 2018 at 7:32
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    @ManRow to transmit a higher torque by (static) friction between the clutch plates, either you need a bigger radius or a higher normal force, i.e. bigger clutch springs - which would mean more powerful hydraulics etc to operate the clutch. Either way, the complete clutch system would finish up bigger, heavier, and/or more expensive.
    – alephzero
    Sep 5, 2018 at 7:52
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    +1, this is exactly it. Clutches are rated for torque, it's as simple as that. Power is completely irrelevant.
    – Shamtam
    Sep 5, 2018 at 15:08
  • @alephzero @ HandyHowie +1 +1 Indeed, now I see it is generally torque, not so much rpm, that is the limiting factor in the performance of a given clutch configuration! A clutch can only handle up to a certain torque difference between input/output, so it is best to put it before the gearbox. + alephzero Thank you for the explanation!! : )
    – ManRow
    Sep 5, 2018 at 19:55
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The clutch disengages what follows which is the transmission. If the transmission were connected to the engine, one would not be able to disengage the transmission which is necessary in order to change gears.

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    Well not really since then you would just disengage the transmission from the drivetrain using the clutch that way, just not the engine. What difference does it make if the transmission is disengaged from the engine but engaged to the drivetrain, or vica versa -- disengaged from the drivetrain, but not the engine? I.e., is there any big significant difference between engine->gearbox->clutch->drivetrain, or engine->clutch->gearbox->drivetrain besides the ordering?
    – ManRow
    Sep 5, 2018 at 2:11
  • @ManRow as long as there is active power input to the gearbox from the running engine, how would the gearbox be able to shift gears? In order for the gears to move around they need to be unloaded. Alternatively you'd need a really beefed op synchromesh system to deal with all the forces and speed differences but that would wear out over time.
    – MadMarky
    Sep 5, 2018 at 7:47
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    @MadMarky The same way a regular gearbox does when there "is active power input" from the drivetrain, also known as back torque. A gearbox always has active power input whenever the vehicle is in motion -- whether it's gonna be back torque/power from the drivetrain (standard configuration) or from the engine (the alternate configuration that is the subject of my question).
    – ManRow
    Sep 5, 2018 at 8:32
  • Just a nit to pick: you can happily shift without using the clutch. Just takes some practice. (And isn't always a great idea, but it works wonders on the track.)
    – 3Dave
    Sep 5, 2018 at 18:22
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If the car has a clutch, engine power will always go through the clutch and then the gearbox.

The clutch interfaces with the flywheel (which is spun by the engine), which in turn transmits the power to the input shaft of the gearbox.

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