Why is it impossible to change from first gear to reverse while driving, but not the other way around?

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Why is it impossible to change from first gear to reverse while driving, but not the other way around?

In: Engineering

For starters, you shouldn’t change from a forward gear to a reverse gear while moving or vice versa. This should be obvious because it causes immense stress to the the entire driveline. You should only change to a gear in the opposite direction when you are stopped.

Now, with that said, I suppose the logic that could be applied to changing from reverse to first while moving is that you’re very slowly while in reverse so the stress on the drivetrain isn’t AS incredible as it would be if you were reversing at like 30 mph. First gear is usually used to accelerate to a higher speed, so changing direction abruptly while moving fast is going to destroy very expensive things.

If you want to mention the J-turn, this is possible because the vehicle moves in reverse and turns, at which point the driver either shifts to neutral or disengages the clutch. With the engine and transmission no longer connected, everything is safe. The driver then waits for the vehicle to swing around and start rolling in what is now the forward direction using only the momentum the vehicle had when the driveline was disengaged, which is safe as far as the drivetrain is concerned. Once the vehicle is clearly moving forward, then it’s safe to engage a forward gear and proceed.

Forward gears have synchros, where reverse gear usually doesn’t. Synchros help to synchronise the speed of the output shaft of the gearbox(connected to wheels) to the speed of the input shaft(connected to clutch plate). This helps the gears mesh without grinding when shifting between gears. The reason they are often not found on reverse is that the car is assumed to be stationary when shifting to reverse and thus the output shaft would not be spinning,and the input shaft would stop spinning shortly after you press the clutch in.