Originally Posted by kenwelch
It might feel as though there is "never a point where there is zero torque", but there must be a very small amount of time that both gear ratios are NOT connected to the engine. If you are beginning to disengage the clutch for one gear ratio while the next gear ratio is being engaged there is only one acceptable option...they cannot be connected to the engine at the same time.
If two gear ratios are connected at the same time (shift overlap) then the clutches must slip which causes unnecessary wear and heat. If both ratios are actually connected (engaged) at the same time then the output shaft (drive shaft) will be locked and cause the drive wheels to lock (like having an automatic trans. car in Park). If the vehicle is moving when the driveshaft locks the tires will chirp (skid) or the weakest power train link will break. Manual transmissions vehicles use a shift interlock to prevent two gear ratios from being selected at the same time. Automatic transmission vehicles have a torque converter that allows slippage.
Bottom line: You must have some time when both gears are not engaged (connected) to the engine. BMW likely designed in some “zero torque time” to prevent M-DCT owners from have early clutch failures. The wet clutch design with plenty of cooling should help also.
I suppose theoretically you could have no zero torque time (ZTT). Practically though, you will have ZTT. I doubt you could measure it without sophisticated measuring equipment and certainly could not feel it. And you bet there will be slip. Practically speaking, you always have slip. It's a matter of how much. Even the wheels slip. Maybe that's why the DCT clutch is a wet clutch.
Point here though is that practically speaking, you can't feel, and would have a hard time measuring ZTT. I believe the DCT is that good. Cheers.