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Something strange that I noticed on my manual transmission car (Renault Clio 3).

Any gear can be smoothly disengaged without use of the clutch pedal, but only when the car is decelerating (ie. when the engine acts as a brake, instead of applying thrust).

Can someone explain it from a mechanical point of view? I roughly know how transmission works, but I cannot explain why it would only work when decelerating. Did someone notice the same thing of different cars? Also, is it harmful to the gearbox?

  • I can pull my car out of gear whether accelerating or decelerating... But neutral throttle is best. – Solar Mike Dec 16 '19 at 15:15
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    There are many similar questions previously asked. You can also put it into gear without the clutch, with practice, if you match the engine speed correctly. Either way, when there is no force between the gear teeth. i.e. the gears wheels which are engaging are rotating at the same rate. When done perfectly there is no harm to the transmission. When done imperfectly there is a great danger of wearing the gear teeth, or far worse. – Weather Vane Dec 16 '19 at 21:36
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Having had a clutch failure in my early driving days, I can attest that one can change gears or disengage without use of the clutch pedal, but not without the use of the accelerator pedal.

The answer is in the difference of force being applied. When you are accelerating, you have a substantial amount of force being transmitted from the engine. In order to safely disengage, you have to step on the clutch.

When you are decelerating, the engine is slowing the vehicle, not as well as by braking and only the inertia of the vehicle (along with road grade) is applying force to the transmission.

If you were on a severe downgrade, traveling at a substantial speed and released the accelerator pedal, more force would be applied to the transmission. If your timing was not spot-on, there may be too much force to remove engagement.

Upshifting without a clutch is possible. Accelerate to the point at which a gear change is desired. Release the accelerator pedal, disengage the gear and move the stick to the next gear position. As you are doing this, the engine is slowing and will eventually reach the rpm required to match the transmission rpm, at which point the synchromesh will assist engagement and you have completed the shift.

For downshifting, the process is a bit more complex. Disengage while slowing in a similar manner, but while transiting to the next lower gear, it is necessary to blip the throttle to raise the engine rpm to better match the transmission rpm. The synchromesh will again assist, but timing is much more critical.

It's far easier to use the clutch. If you mistime your actions, gear teeth will be eroded/damaged, at least on the downshift. For the simple act of disengaging, it's less likely to cause damage.

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  • +1 I'd add that the reason that (when under load) moving the gear selector is stiff or difficult is because of the friction on the gear tooth faces. When accelerating (or decelerating on a steep grade) there is too much friction on the gears for them to smoothly disengage. Reducing the load by pulling off throttle or disengaging the clutch remove the friction from the gear teeth. – kyle_engineer Dec 16 '19 at 23:04

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