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      03-26-2008, 09:21 PM   #89
ersin
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Numbers and physics check out ok to me. However, I'm no mechanical engineer, my degree is in Physics.

Oh, BTW, one "G" is 9.8m/s^2.
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      03-26-2008, 10:51 PM   #90
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Quote:
Originally Posted by enigma View Post
Someone feel free to check the following math.

Assuming a flywheel of ~10kg, 0.15m radius

I = 0.5 * 10 * (0.15) ^ 2 = .1125

E = 0.5 * I * w^2 = 0.05625 * w^2

At 8400 RPM: E = 0.05625 * (8400/60*2*3.14)^2 = 43.48 KJ
At 6000 RPM: E = 0.05625 * (6000/60*2*3.14)^2 = 22.18 KJ

So after shifting we have ~21KJ of energy stored by the cars flywheel (not counting the other moving engine bits) that has to go somewhere.

If the car (1650kg) is going ~44mph in 1st (20m/s) the car currently has:
1650kg * 20^2 = 660KJ of energy.

If the DCT simply disengages the previous gear and engages the next and dumps that stored energy to the rear wheels you now have 660 + 21 = 681Kj

sqrt(681Kj / 1650) = 20.32 or a sudden velocity increase of 0.32m/s or almost 1 mph. Thats the jerk, its not simulated or made up, they just don't try to hide it under the cover of "smooth" like Audi.

You could apply the energy over time, burn it in the clutches, or reduce the engine output to cover it up. None of those help you go faster.

I suspect by the time you add in the other engine components the stored energy is considerably larger. Enough to put it into the 1-2mph range.

BTW: one "G" is 9.8m/s So if they accelerate the car 0.32m/s in 0.1s. that generates a spike of ~0.3g for that brief instant over the normal acceleration.
Nice analysis. Even if the numbers nor method are perfect the point is. A couple questions/corrections:

1. How close are your flywheel mass and radius? I know we don't have actual numbers and I know you are using estimates, but what is your guess?

2. The clutch friction will cause energy to not be conserved. However, even with a lossy joining of spinning disks angular momentum will be conserved. This is the begining of the correct calculation but since you have linear and angular momentum/energy it is a bit more complicated. I don't think you can even calculate the velocity change without a reasonable estimate of the total clutch back moment of intertia.

3. Even if energy were conserved, E = .5 * m * v^2 as you stated, but didn't you leave off the 2? So you are off by sqrt(2) = 1.4 in the 20.32 number. It is 1.4 x larger.

4. Footie either does not own a calculator or does not even understand the concept of a percentage (see the recent top speed threads), don't even bother with the forumlae and physics.

ersin: Come on I expect better from a fellow physics person
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      03-26-2008, 10:59 PM   #91
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The jolt back is awesome. Just like NOS!
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      03-27-2008, 12:26 AM   #92
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Quote:
Originally Posted by rai View Post
when are the DCTs due out in the US??????
I will be driving mine sometime during the week of April 7th.
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      03-27-2008, 12:38 AM   #93
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The Veyron doesn't have the jerk because they don't want it. With 1000 HP, any slight increase in acceleration would cause it to lose traction instead of accelerate. The F1 cars also have seamless shifts because of their massive HP:weight ratios. Any slight jerk will cause unbalance.

So, if you have ENOUGH HP, you want smoooooth. If you don't have enough HP, you want all that energy transferred to the wheels during gear shift.

In theory, the best tranny is the CVT. The CVT would give you NOTICEABLE improvement over the DCT. Only problem is that the CVT has not yet been able to handle the torque.

Maybe one day, M3s will be CVT. How boring would that be? LOL. Maximum HP all the time during S6 mode and you can have an "E" mode for maximum efficiency for fuel economy to prevent the gas guzzler tax.
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      03-27-2008, 12:43 AM   #94
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Quote:
Originally Posted by swamp2 View Post
ersin: Come on I expect better from a fellow physics person
So I'm lazy. It's late. I need to get some sleep. Good night.
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      03-27-2008, 12:43 AM   #95
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I am just guessing on the flywheel dimensions. I have seen a number of them out of cars so its probably not that far off. Weights typically run 10-22lbs. Still its really hard to estimate I since the shape is far from ideal.

Clutch friction (and tire slip) will consume some of the energy.

The calculation is much more complex. You have an unknown additional mass in the engine. There is the fact that the clutch engagement needs to be treated more like a physics collision (inelastic) and conservation of momentum needs to be addressed.

4: Yes. I simply wanted to make it clear that the energy has to go somewhere. I figured even footie could understand that. If you could easily just make energy disapear then things like falling off building wouldn't be a problem.



Quote:
Originally Posted by swamp2 View Post
Nice analysis. Even if the numbers nor method are perfect the point is. A couple questions/corrections:

1. How close are your flywheel mass and radius? I know we don't have actual numbers and I know you are using estimates, but what is your guess?

2. The clutch friction will cause energy to not be conserved. However, even with a lossy joining of spinning disks angular momentum will be conserved. This is the begining of the correct calculation but since you have linear and angular momentum/energy it is a bit more complicated. I don't think you can even calculate the velocity change without a reasonable estimate of the total clutch back moment of intertia.

3. Even if energy were conserved, E = .5 * m * v^2 as you stated, but didn't you leave off the 2? So you are off by sqrt(2) = 1.4 in the 20.32 number. It is 1.4 x larger.

4. Footie either does not own a calculator or does not even understand the concept of a percentage (see the recent top speed threads), don't even bother with the forumlae and physics.

ersin: Come on I expect better from a fellow physics person
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      03-27-2008, 12:45 AM   #96
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Quote:
Originally Posted by ToothDoc View Post
So, if you have ENOUGH HP, you want smoooooth. If you don't have enough HP, you want all that energy transferred to the wheels during gear shift.
Precisly you want as much energy transfered to the rear wheels as possible WITHOUT causing more than a 4-12% tire slip (depending on tire type).
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      03-27-2008, 01:58 AM   #97
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I think we are all missing the point here. Lets stop trying to guess and hold out for a week or two. My guess is that if you want DCT to be seamless it will be with no serious impact on acceleration. The big question is whether the Auto mode would be a failure like the one in the SMG cars. After all we would all like the convenience of an auto from time to time.
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      03-27-2008, 02:21 AM   #98
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Quote:
Originally Posted by gorun View Post
I think we are all missing the point here. Lets stop trying to guess and hold out for a week or two. My guess is that if you want DCT to be seamless it will be with no serious impact on acceleration. The big question is whether the Auto mode would be a failure like the one in the SMG cars. After all we would all like the convenience of an auto from time to time.
Well with either enormous over interest on "our" part, or the seemingly ever present and severe lack of information from BMW, we are left to speculate, theorize, extrapolate, model, etc. Many of us find it fun, hence a good 20% or so of this boards posts and threads.

One point to clarify is that when you use a bit of science (be it engineering, math, physics, materials science, etc.) along with knowledge about similar cars or components, throw in some simulation and just good old common sense you really can get pretty darn good "predictions" about the future. I think being able to make such predictions is enormously rewarding. Sure we get to stroke our own egos a bit when we get it right, but that is simply part of what makes it fun.

So in short, we (especially the M-DCT "club") are excited and anxious and having some fun. Hence the answer is no, we won't just wait.

As to your big question - I think it has already been answered. My "guess", as well as that of the majority of whom I believe to be "in the know" here, has been all along that the auto mode in the more comfortable settings will be incredible and incredibly better than SMG - very much a match for a good automatic. This has been more or less confirmed in multiple locations. First in early speculations, next in the big M-DCT press release from BMW, next in my post about my visit with a dealer in Frankfurt and lastly partially confirmed again right here in this thread.
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      03-27-2008, 03:13 AM   #99
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ok, keep guessing then...
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      03-27-2008, 03:39 AM   #100
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Enigma and Swamp,

You guys are still think of DCT like you would SMG, eg. one clutch controlling all of the gears. The jerk is engineered, please go and get a bit of advice on DCT and dual clutch technology before ranting.

Quote:
Originally Posted by ToothDoc
The Veyron doesn't have the jerk because they don't want it. With 1000 HP, any slight increase in acceleration would cause it to lose traction instead of accelerate. The F1 cars also have seamless shifts because of their massive HP:weight ratios. Any slight jerk will cause unbalance.

So, if you have ENOUGH HP, you want smoooooth. If you don't have enough HP, you want all that energy transferred to the wheels during gear shift.
Correct in your comments on why you don't what jerk but to think that the Veyron would suffer from traction issues, the Veyron can put all of it's 1160hp (yes that's right) to the road even in the rain. The reason jerk is unwanted is because it will cause unbalance the chassis during cornering.

Quote:
Originally Posted by ToothDoc
In theory, the best tranny is the CVT. The CVT would give you NOTICEABLE improvement over the DCT. Only problem is that the CVT has not yet been able to handle the torque.

Maybe one day, M3s will be CVT. How boring would that be? LOL. Maximum HP all the time during S6 mode and you can have an "E" mode for maximum efficiency for fuel economy to prevent the gas guzzler tax.
ICT is the future, not CVT and BMW are a big player (among other car and gearbox manufacturers) in this technology as they are co-testing this technology with http://www.torotrak.com/, the company which invented it. Unlike CVT IVT can cope with more torque than is currently produced by any production vehicle and unlike CVT it uses special oil which increases friction, specially developed by Shell. The advantage of IVT is that it will optimise the engine's power and torque by acceleration at the optimum rev point unlike either a manual or automatic which has fixed ratios.

P.S.

And not a jerk in sight.
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      03-27-2008, 04:26 AM   #101
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In manual mode and in setting S6, jerkiness is a must.
Would be a big F..... mistake from BMW if it was smooth gearchange in S6 mode. I want to feel Im driving.

Its more important that automatic mode is smooth than the fastest setting in manual mode.
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      03-27-2008, 06:23 AM   #102
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Quote:
Originally Posted by Just_me View Post
In manual mode and in setting S6, jerkiness is a must.
Would be a big F..... mistake from BMW if it was smooth gearchange in S6 mode. I want to feel Im driving.

Its more important that automatic mode is smooth than the fastest setting in manual mode.
Too bloody right.... the fact that you will get the extra backward push (without the SMG head nod forwards!) with each change is the ONLY reason I'm just about to confirm my order for DCT..... I've never considered an auto before and if the changes ended up being totally smooth with DCT I wouldn't touch it with a barge pole...... smooth auto mode for the wife and S5/S6 for me...... win/win

I'm just praying that there is no noticable delay between pulling the paddle to the shift actually happening (especially in the fastest modes)
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      03-27-2008, 06:35 AM   #103
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I have seen weights of the Bugatti up to 4300 lbs which is a good deal and the car is 6" shorter in length and 9" shorter (height) than the M3. So likely the tranny is a good bit more robust, likely weighs a lot more than the M3s. I think part of the DCT is not wanting to add too much weight to the car and likely on the Bugatti the weight was not as big a deal as it has more than double the power to throw around.
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      03-27-2008, 06:36 AM   #104
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Quote:
Originally Posted by skierman64 View Post
I will be driving mine sometime during the week of April 7th.
please let us know wht you think. And anyone who gets one sooner that's even better.

thanks.
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      03-27-2008, 08:03 AM   #105
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Quote:
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First, a big thanks to Just Me for the breaking news on a very hot topic. Thanks as well for the initial translations and the much improved work by South!

I am very pleased and excited. If I could offer my interpretation of a part of the translation I would say folks agree that M-DCT is a very strong evolutionary step past SMG, maybe not revolutionary but that is fine becuase I think SMG is pretty darn good.



You read my mind on that one, Yoda, ha.

--------



DCT WILL be worth about 20 hp and 20 ft lb (equivalently). The larger gear ratios AND shorter shift times will do this, period. If you do not disagree that the car will be measureably faster how can you disagree that there is not a significant and equivalent hp/tq gain? You've got this one wrong.

As far as the perception of gear changes it sounds like the system offers a full range of performance, barely noticeable changes in some modes, and noticeable not not nearly as noticeable as SMG in other modes. This does not mean much as all in terms of the performance advantages. All it really means is that I was a bit wrong about the particular design direction they took and whether or not super fast could be mixed with super smooth. That being said I always agreed with enigma that there is a certain amount of engine, flywheel and drivetrain inertia and that has to go somewhere.

------



Hello, Debbie Downer. Wake up footie. Everyone loved the tranny. You can't really honestly believe that "BMW have f--ked things up BIG TIME"... Or can you? Did you hear how the one magazine tester thought the GT-R DC system made the Scuderia feel slow and anitquated? That is UNREAL praise. M-DCT will be the same.

It is smoother than SMG as you get less jerk. You need to really think about the math and curves here before you drive and before you pontificate. SMG and MT exhibit a larger jerk or derivative of acceleration becuase they both decelerate and then accelerate during a very short interval. Hence the slam/jerk. With DCT you still get the excitement and perception of the speed through some jerk but less because the shift is so fast and the entire dip in velocity and acceleration is eliminated.

Consistent with the goal of having a very tuneable car it seems BMW wins here with the tranny by offering both very smooth modes and ones with a smaller but exciting jerk.
+ I totally agree with what you said.
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      03-27-2008, 08:05 AM   #106
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Updated the third persons impressions. One still to come...


Best regards, south
Good Job with the review translation South. I think I speak for all when I say you bring great contributions to this forum.
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      03-27-2008, 09:27 AM   #107
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Quote:
Originally Posted by ToothDoc View Post
In theory, the best tranny is the CVT. The CVT would give you NOTICEABLE improvement over the DCT. Only problem is that the CVT has not yet been able to handle the torque.

Maybe one day, M3s will be CVT. How boring would that be? LOL. Maximum HP all the time during S6 mode and you can have an "E" mode for maximum efficiency for fuel economy to prevent the gas guzzler tax.
CVT =

I test drove a Toyota Highlander Hybrid that had CVT. It sounded and felt like the thing was constipated. Strain, strain, strain. I hated it! Bleh.
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      03-27-2008, 10:00 AM   #108
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      03-27-2008, 10:57 AM   #109
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Quote:
Originally Posted by footie View Post
Enigma and Swamp,

You guys are still think of DCT like you would SMG, eg. one clutch controlling all of the gears. The jerk is engineered, please go and get a bit of advice on DCT and dual clutch technology before ranting.
Footie, you are so clueless when it comes to anything technical. You are not a bad person but just clueless when it comes to anything technical.

Why don't you start a thread on a how the 6MT knob feels, is it touching your soul? Are you intimate enough with the car?
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      03-27-2008, 11:41 AM   #110
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Good Job with the review translation South. I think I speak for all when I say you bring great contributions to this forum.
Thanks, really flattering.


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