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.
When it is upshifting from a lower gear to a higher gear, under normal cirucmstances, the higher gear is alread selected for the second clutch and input shaft. As the first clutch starts to disengage, the second clutch is already starting to engage. We're talking milliseconds here so I don't see that as undo wear on the clutches. That's what clutches are for. So, depending on the aggresivness of the software, there absolutely can be a shift where torque applied to the wheels never actually drops all the way to zero.
For the sake of argument, lets say there was a malfuction and both clutches fully engaged. They would both be turning in the same direction, just 1500 rpm (give or take) different. One or both clutches would then be slipping that much and yes, wear out very quickly, but the wheels aren't going to lock up.