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      12-02-2012, 07:09 AM   #47
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gthal's Avatar

Drives: 2018 340i xDrive
Join Date: Jul 2007
Location: Canada

iTrader: (2)

Originally Posted by NISFAN View Post
Not exactly true on the DCT mode of operation. The DCT is like two gearboxes in one housing. When in 1st gear, second gear is pre-selected but disengaged by it's own clutch. Changing from first to second is a sequence of fading one clutch in as the other fades out. Not quite instantaneous as there is a period where both clutches are slipping, but pretty quick.

Typically, if you are accelerating up through the gears, there is logic that selects the next higher gear on the second gear set, however if you change down (out of the pre selected logic) it physically has to deselect then re-select another on the same gear set. This takes time. So if you select a gear the ECU thinks you are going to select it is quick, if you don't it needs to complete a slower change. None of this matters much to the user, as you can't physically slow down fast enough to catch the gearbox out.

The ultimate gearbox is that used in Formula 1, it is so fast it actually has two gears engaged for a split second at the same time (same gearshaft, not clutch de-coupled). This is known as seemless shift, the power is not even interrupted on up changes. But whilst on F1, not even an F1 car that pulls 5-6G deceleration needs to skip gears, so not quite understanding why a road car would need to unless the driver was a complete anus.
Agreed... what I was trying to articulate is that the DCT shifts what seems to be instantaneously almost all of the time... it may not actually skip a gear but the perception is that it does as it is so fast.
2018 340i | Mineral Gray | Current DD | C8 mid-engine Corvette to come next year
2016 M2 | Long Beach Blue | 6MT
2015 M4 | Austin Yellow | DCT
2014 Corvette Stingray | Z51 | Torch Red | 7MT
2012 MB C63AMG | 2011 E92 M3 | 2010 E92 M3 | 1996 E36 M3 (track car)

Last edited by gthal; 12-02-2012 at 08:02 AM.