Thread: DSG or Manual
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      05-10-2007, 11:59 PM   #116
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Drives: E92 M3
Join Date: Sep 2006
Location: San Diego, CA USA

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Wrong physics

Originally Posted by chitown08 View Post
I think some people are looking at the time savings wrong. Esquire, using your source, lets assume .03 sec shift time for DSG. And lets also assume .30 seconds for a manual shift.

0.30 - 0.03 = 0.27 shift time savings

Here's the problem, I think swamp is looking at this like every shift in say a 0-100mph run, will shave 0.27 sec off the end time. This is not the case if I am not mistaken. If a car must shift 3 times, lets say, a DSG will not defeat a manual transmission by 3 X 0.27sec (0.81sec).

The only way this would be true is if the car physically stopped during the time it takes to shift. It does not. It coasts while you are shifting. So the difference is that a MT car is coasting/decelerating for those 0.27 sec while the DSG has already completed the shift and is again accelerating for those 0.27 sec. The net result is much smaller than 0.27 sec. You don't gain 0.27 sec per shift.

I posted this earlier in this thread. I guess you would call it physics...

If everyone already knew this, I apologize.
When looking at time to obtain a particular speed you do really get (theoretically, with all else equal and assuming a zero shift time with DCT) time improvements even greater than the sum of the shift times of the MT!

Here is the correct physics based reasoning. Draw a curve that represents speed vs. time. At any given short interval of a second or so the curve is contstant slope meaning rougly contstant acceleration. Makes "seat of the pants sense" right? Acceleration tapers off slowly but around any given second or so the "pull" is about constant. If the MT car has no power delivered to the wheels it will only have a constant velocity in vacuum with no rolling resistance. Because of the latter two effects when shifting a MT car it actually slightly decelerates and lowers it's speed. For the sake of argument though we can just assume it's speed is constant during the shift time. Meanwhile the DSG car enjoys an almost constant acceleration along its v vs. t curve (probably actually the average of the acceleration before and after the shift but again this is a refinement of the basic idea). Now draw a flat line representing a given speed, the time gain made by DCT to reach a given speed is indeed the time gain made for the shift! No matter what the slope of the line the times add up.

What I think you meant to say is that the distance gained by one vehicle is not equal to the speed of the vehicle at a shift multiplied by the shift time difference. This is true becuase one is more or less rolling at a constant speed but the other is accelerating. Here you need the more complicated distance = 1/2 acceleration x time x time formula.

Should be clear in the sketch below!

Real world evidence. A3 DSG faster than MT bt .2 sec 0-60. GTI DSG faster than MT by .1 sec. Also look at the chart I posted here long ago. Sure these are not the same car but look VERY equally matched in lower speed acceleration. The DSG car just keeps eating away at the MT until some high end power at the end of the graph seems to let the MT catch up quite a bit at the end.

Sure you won't see exactly .27 seconds x number of shifts in all cases. There is the whole effectiveness of the DCT launch to be concerned about as well as gearing differences between DCT/DSG and MT models. However, as a rough but accurate first approximation:

The benefit in time to speed is the number of shifts x shift time advantage.
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