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03-27-2008, 12:45 PM | #111 | |
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The jerk is not just from the torque differences at different rpm. Acutally this is only a small part of it. If you shift very fast at 8,300 rpm into the next gear, which needs 6,000 rpm at the current road speed, then there is 2,300 rpm of engien speed which must disappear. This is kinetic engergy from the rotating mass of the engine components and is converted to a surge or jerk or what ever you want to call it if the shift happens quickly. If you slow the shift down, like I think happens in D1 or D2, then that kinetic energy can dissipate as heat through internal friction of the engine slowing the rpm. |
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03-27-2008, 01:02 PM | #112 | |
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edit: Isn't the M6 only available in SMG? I am tripping out here...because if that M5 shifts like that, omfg, I may have to buy one.
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03-27-2008, 01:57 PM | #113 | |
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You have continued to exhibit a clear lack of technical knowledge and grasp here on the forum. You do offer many valuable comments and opinions but your weakness is clear. I have my weaknesses as well, but generally technical understanding and ability isn't one of them. It is a very interesting idea this whole jerk thing. Could BMW have possibly engineered the jerk back into the system when by its fundamental design there is no jerk? I just do not think it is possible. But in the future it may be. We have discussed extensively here on this forum the differences between speed (i.e. acceleration) and the perception of acceleration. It is a foregone conclusion that jerk is a more important factor in the perception of acceleration, but not actually in the measured acceleration and hence actual performance. Folks LOVE the SMG S6 smack, the jerk, it simply feels fast. But IT IS NOT AS FAST as it can be. It is not because it wastes time and bleeds speed by decelerating. The deceleration while the throttle is lifted and clutch is engaged with any MT or SMG system slows you down and produces less acceleration compared to an equivalent DCT design. We have explained why without some careful management even a DCT transmission will produce a spike/jerk. It is flywheel intertia and engine torque vs rpm. Do you even have any idea what an accelerometer trace for a shift in a MT/SMG/DCT/AT will even look like and what they mean? Let me try help as much as possible. Below are some quick sketches of acceleration vs. time. There are not position vs. time, not velocity vs. time, but acceleration vs. time. Qaulitatively the height of each curve represent how much push you feel on your back in the seat at any instant. These curves are not exact by any means, but do get all the main points across. Before clutching you are in a mode of acceleration dropping as engine torque does. When throttle is reduced and clutching the acceleration drops very sharply. If or how far you get below the x axis of zero accelertation depends on how fast you are going and how much air resistance there is. Here since you actually go to negative acceleration these shifts are likely above 50 mph or so. When you clutch out the accleration builds and you get the positive jerk as the flywheels momentum is conserved and the engine is transitioned to a lower rpm at a point it produces more torque. SMG exhibits both the dip and rise but they are very short. The AT smoothes everything over and generally lengthens the total shift time. Now with DCT you almost entirely eliminate the negative jerk via simultaneous clutching and are just left with the positive jerk (surge as BWM is calling it) from the flywheel effects and from the engine simply producing more torque at lower rpm. Now much like an automatic transmission, but accomplished by very different means the Veyron and VW DSGs very much reduce the positive spike/jerk shown in my approximated DCT graph. As well lower A modes in DCT will similarly reduce the spike shown (and very likely stretch the time out, like the automatic but more on the time scale of a very good automatic, say the IS-F). I think I am in part to blame for your misunderstanding here. I have talked much about how the BMW system will be very smooth yet still offer great performance. I was likely incorrect on their ability to have the ultimate performance with almost no jerk. My predictions here were based on both the Veyron and GTI DSG units as well. It really seems like BMW has obtained the best of both worlds. There is less jerk than a MT or SMG, however there still is jerk but it is a surge rather than containing any deceleration. I would be very willing to live with little to no jerk myself if it meant more performance, but most would not. Almost no one can separate their perception of speed with obtaining true speed, hence everyone really wants the jerk. I guess that is why the forum seems to appreciate me and enigma! Footie, please take some serious time and think this through. We are realyl trying to help you. Others: Correct my mistakes and predictions and let's keep the discussion going. P.S. Thanks for the info on IVT, I am always keen on the emerging technologies. Some reading and research needed here for sure. |
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03-27-2008, 02:00 PM | #114 | |
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This is a M6 in drivelogic mode S6 with launch control used. You got a bum ride from someone! M5 and M6 only come with SMG (SMG III, is the exact variant) everywhere in the world except in the US (maybe NA as well as US, not quite sure). Here our stubborn enthusiasts demanded a MT. M3 with DCT and LC will be quite similar! |
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03-27-2008, 02:22 PM | #115 |
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swamp,
I apologize for my remarks but when someone talks down to you on something that they have neither sampled or are 100% knowledgeable with the technology it's hard not to get a little angry and upset. The graphs are interesting but according to what I have been told by someone which should know better that the jerk/surge has been engineered into the system for not other reason than to appeal to the present following of SMG customers. There is no doubt that such an experience will give the feeling of more speed ever if the reality is not the case. P.S. I was also told that the shift time of both the new DCT and DSG gearboxes are 30ms, no more, no less. |
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03-27-2008, 02:23 PM | #116 | |
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03-27-2008, 02:26 PM | #117 | |
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Footie, tell me, where does that stored energy go if not to the rear wheels?
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03-27-2008, 02:37 PM | #118 |
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Nice graphs swamp. However, I would say that the "jerk" is not the acceleration but the rate of change of acceleration. That being the case, jerk would be identified in the graphs as the slope of the curve at any point in time. The way you have drawn the curves shows the top three (6MT, SMG and DCT) have "jerks". You can also see much greater range in 6MT and SMG as compared to the DCT. The bottom graph does not have large slopes, hence not much jerk, i.e. much smoother.
Assuming you're right with these curves, I find this a very good graphic way of conveying the concept. Great idea swamp. It's posts like this that make this forum useful. Thanks.
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03-27-2008, 02:41 PM | #119 | |
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I'll try to log the same thing for my SMG car this weekend. EDIT: Log was from this driving session http://video.google.com/videoplay?do...40764726878137
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03-27-2008, 02:46 PM | #121 |
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Race Technology DL-1. Its from one of my better laps at laguna seca in the Elise.
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03-27-2008, 02:51 PM | #122 | |
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03-27-2008, 02:56 PM | #123 |
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Based on enigma's graph I would reckon that the SMG in S6 will has a shorter drop on the graph but the rising slope between the lowest point and the highest point will be more extreme and will happen is less than the half the duration. If I am right this might explain why it's effect is more felt and extreme.
swamp, I haven't an answer to your question, but all I know is what I have been told and knowing the person and his position I reckon he should know what his talking about. And according to him it's there for no other reason than the appeal to SMG customer. I will be interesting to see a graph from all the respective modes to see what is happening during the shifts to see if there is any adjustment to the shift time. I think is here were we will finally understand what is happening. |
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03-27-2008, 02:59 PM | #124 | |
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03-27-2008, 03:04 PM | #125 | |
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03-27-2008, 03:06 PM | #126 | |
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03-27-2008, 03:12 PM | #127 |
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Cool. I have an AP-22 which I believe is made by the same company but is older and uses accelerometers only. The DL-1 also has GPS, right? I could get the same data (a vs. t) but I don't believe it will be anywhere near the time resolution you get with the DL-1. It's been a while since I've played with this toy. I'll have to break it out again.
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03-27-2008, 03:16 PM | #128 |
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One additional point to all:
In the acceleration vs. time curves, the area under the curve is proportional to the speed. And when the curve dips below zero, you're losing speed. See how the DCT stays above zero? The area under the curve is greater for the DCT than the rest over the same duration. This is the speed advantage of the DCT. And note that the little jump that the DCT curve takes adds a bit to the speed but not as much as if it were of longer duration. However, because of it's slope, it feels like you're getting a "jerk" and feels faster.
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03-27-2008, 03:18 PM | #129 | |
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Regarding the 30ms, of course this depends of mode but shouldn't really as the longer the time the less smooth it should be. I would reckon the time will vary being more to do with what you are trying to do with the gearbox, ie. change down more than one gear or decide to change down after changing up for more than two shifts, in this the gearbox software would assume you would want to shift up and thus have the higher gear already preselected. In this scenario I reckon the shift time will be greater than the 30ms being quoted. |
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03-27-2008, 03:40 PM | #130 |
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I wonder if there is an idea that the manual mode will be as good as the better SMGs out there such as the newer Ferrari F1 tranny or the Lambo e-gear.
I understand they are different the SMG is one clutch and the DCT (in theory) should be better (especially in the auto mode). it's two things to wonder about... 1) how does it behave as a autobox? Which is secondary to me. 2) how does it behave as a Manual tranny mode up and down (with throttle blips I suppose?). If the DCT is likely better than the SMG do you think mainly in the auto mode or do you think it's better in both auto and manual modes? I have driven one DSG (GTi) I have driven two SMGs (F430 and Gallardo) I was very pleased with all three trannies. I suspect I would be very happy with the DCT but I have to order without a test drive. I liked the M3 6MT, but I was not in LOVE with it. I sit far back and the 4-5 shift is a long throw. I don't think it's a deal breaker but I could go either way.. Thanks if anyone has any advice. |
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03-27-2008, 03:43 PM | #131 | |
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Being a competitor of course they think their decision is right and anyone else makign a different one is wrong. They have to say that. So by "Engineered in" they really mean "Didn't hide from driver the transfer of energy". They think this was the wrong decision, and for people looking for a status symbol they would be right. If it were going into the 550 or 335 I might even agree. Remember Audi has a different target market. They are more after the "it just works" crowd and less after the entusiast crowd.
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03-27-2008, 03:44 PM | #132 | |
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Best regards, south
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