How does a manual transmission not grind whilst changing gear?

So I understand a bit about a transmission but what I just can't get my head around is that once the clutch is engaged, torque from the engine is no longer being supplied to the transmission, but obviously as if the car were travelling at let's say 80km/h; the wheels would be spinning and therefore the transmission would be too. So firstly why is it that power needs to be disconnected from the engine transmission before selecting a new gear if the transmission will be turning regardless? And if it is turning how does pushing the clutch in still prevent slight grinding amongst the teeth on the output shaft and selected gear?

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  • 1 month ago

    Use  DOT 4  break oil.

  • Anonymous
    1 month ago

    Why, why, why? 

    What does it matter why?

    There should be NO grinding if you drive properly and your clutch disengages totally and your synchronizer brass blocker rings aren't worn out. 

    You said: 

    "once the clutch is engaged, torque from the engine is no longer being supplied to the transmission"

    That's incorrect.

    Once the clutch is DISengaged, torque from the engine is no longer being supplied to the transmission.

    In shorter, simpler terms, when the clutch is disengaged, the transmission is no longer connected to the engine (the flow of power).  The transmission is still bolted to the engine (directly or indirectly). 

    Engage means connect. 

    Disengage means disconnect.

  • Anonymous
    1 month ago

    Clutch is Disengaged. Pushing the pedal Disengages; Not "engages".

    Since no power to gears, they can now be shifted. They synchronize to each other very quickly as rotating mass  of each is quite small. An old " :double clutcher" can often be fooled into noiseless shift, IF  drive shaft sped matched by flywheel speed. Otherwise, most modern trannys are synchro-mesh,  a bronze cone  synchronizes gears when shifter pressed on them.

    My '80 Chevy Still grinds if put into first. . The synchro was Teflon and they wore out in  40K miles.

    EDIT: Not All trannies are constant- engagement. The old double-clutchers sure weren't. Motorcycles have no synchros, their Tiny gears change speed almost instantly,

  • 1 month ago

    Modern Manual gearboxes have a synchromesh to prevent gears from Crashing into each other

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  • 1 month ago

    A synchronizer ring or 'synchro' before the gear helps get the collar and input shaft up to speed during a shift.

  • 1 month ago

    To make it easy to understand I will simplify.   The clutch pushes apart two round metal plates.   Once that is done,  the engine is not directly connected to the wheels or the gears.   Then you can shift the gears as they are still spinning.   Once you get them shifted you let up on the clutch and the two metal plates come back together and are held together by a spring or hydraulic pressure.  The engine power is now transferred from the first plate to the second plate by them being held together.   If you ride the clutch or you have a loss of pressure the plates will slip and you lose power plus eventually one of the plates will wear away and you will need to replace it.   In the old days you would double clutch to keep the gears spinning.   The first time would be so you could shift into a neutral position,  the second time so you could shift into the next gear you want.   Today there is a synchronizer that keeps the gears spinning so you can shift.     The two common mistakes people make are riding the clutch which causes wear and trying to shift before the clutch is entirely pushed in.  

  • Barry
    Lv 6
    1 month ago

    Syncromesh is the answer. There are small friction hubs which help to match the gear wheel speeds as you change gear. This virtually eliminates gear crunching.

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