03-21-2013, 02:50 PM | #89 | |
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Look I am not putting down xDrive, I own a 335i xDrive and think its great. But I also run snow tires during winter because I know handling/braking traction is 95% in the tires and not in the drivetrain. I just can't stand how BMW has marketed xDrive as being advantageous in the wet. At the very best it only has a marginal advantage. |
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03-21-2013, 02:50 PM | #90 | ||
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With no TC/ESP, he can oversteer and spin. With TC/ESP, hopefully the system will intervene and cut power before he kills himself. Note that ESP is a passive system and slows you down. With a good AWD system, some power is shifted to the front to prevent the rear wheel from losing traction. Again, AWD actively provides more traction. Definitely provides more performance headroom. Quote:
I don't remember torque vectoring being mentioned anywhere in the brochure. There is a eLSD function that simulate the function of a LSD, but that's no torque vectoring. |
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03-21-2013, 03:07 PM | #91 | |
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I also agree that if you find yourself driving on a sheet of ice where there is absolutely zero traction, then AWD and RWD will in theory be both the same. That is, you have zero control. But the reality for many people live up North is that they gonna find themselves in low traction conditions quite often, whether it's rain or snow. The advantage of xDrive simply cannot be ignored. It is fun to drift around, if I really wanna do that I will get myself a FR-S/BRZ with a proper LSD. For day to day driving, I will gladly take the xDrive over RWD. But that's because I don't live in California or Florida. |
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03-21-2013, 03:29 PM | #92 | ||
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An interesting exercise in understanding the understeer/oversteer dependency on throttle input involves driving in a very tight circle at high speed. You can actually change the radius of your circle by applying throttle more/less. This works regardless of drivetrain. Quote:
I'll look for it and post the link...
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03-21-2013, 04:10 PM | #93 | |
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This is how it is defined. With an active AWD system or any active differential, torque delivery can be varied at will usually via a computer controlled clutch. So, Haldex based VW/Audi will be considered as an active system. The new active rear diff in the F10 M5 and C7 Corvette are other examples. The Torsen based VW/Audi system or other viscous coupling based systems on the other hand are considered as passive. Because there are no means to actively control torque distribution via computer programming. Power distribution is varied when wheel slippage occurs. Of course with Audi you can also have a passive Torsen center diff coupled with an active rear diff. While you go look for mentions of torque vectoring on the f30, check out this video on xDrive as well. Both the blonde and brunette cute. http://www.bmwusa.com/Standard/Conte...ng/xDrive.aspx |
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03-21-2013, 04:24 PM | #94 | |
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On the other hand, a torque sensing differential, ie conventional limited slip differential, uses a mechanical gear set to optimize torque appropriation. There is no limitation on the actuator bandwidth other than the physics of the coupling mechanism (viscosity for example). These systems will provide optimum torque distribution in terms of maximum traction. On the other hand, one cannot enforce constraints such as: force all torque to the rear wheels, which might reduce understeer in certain instances. Side - I don't think either the M5 nor C7 have active rear differentials. Only Audi, Lancer Evo, Acura SH-AWD, Nissan Nismo stuff and BMW x5m-x6m-x6 as far as I can tell
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03-21-2013, 05:17 PM | #95 | |
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The new M5 definitely has an active rear diff. Based on the press release available, the C7 will sport an active rear diff as well. http://www.bimmerfile.com/2011/09/28...ial-in-detail/ Frankly, I don't know if you really know what you are talking about here. You don't know the fundamental difference between active and passive awd systems. Then you try to say that a Torsen based system is superior, how is that relevant to the rwd vs xdrive discussion here? I will leave you this to ponder, if a passive Torsen diff is truly superior then why does Audi use an active rear diff today (namely the Sport Differential) instead of a Torsen rear diff like they did back in the late 80s? |
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03-21-2013, 05:48 PM | #96 | |||
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Simple...rear diff eliminates understeer as it uses a feed forward control structure. You can easily anticipate torque appropriation based on inputs regardless of disturbances. The purpose of the center differential is to maximize traction regardless of performance, the only way to improve performance is to increase the rear bias via changing the nominal gearing. And Audi uses the crown center differential, not a torsen (which cannot self lock).
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03-21-2013, 07:39 PM | #97 | |||
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I often find that debating on Internet forums is a waste of time. Yet I also don't want people to come here and get misinformed or think that this forum lacks fundamental automotive knowledge. Nothing personal here.
Now, just because you make a statement, that doesn't make it true. What you are saying simply does not make sense. Quote:
Of course it's the torque vectoring that reduces the understeer, and its implementation necessitates a computer controlled active differential or using rear brakes like Porsche. In theory you can have a front active diff with torque vectoring on a FWD car. Quote:
Increase rear bias is only beneficial if there are more traction at the rear. Otherwise we will just send 100% of the power to the rear aka RWD. Quote:
It also doesn't change the fact that it is not an active center differential. So, it cannot proactive change power distribution as soon as it senses steering input. Of course when coupled with the active Sport Differential at the rear, it becomes less of an issue. |
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03-21-2013, 07:47 PM | #98 | |
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1. Please describe the system and explain why it is a feed forword control structure. List the input, output, and disturbance to the system. 2. If an active rear differential uses feed forward control structure, then does an active center differential also employ feed forward control? Why or why not? |
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03-21-2013, 07:59 PM | #99 | |||||
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Fantastic, what's your point?
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03-21-2013, 08:26 PM | #100 | |
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Here you go 1) It is a feed forward control system because it does not need to track reference signals. The system is not unstable to begin with, thus it can be appropriately tuned open loop. driver input y_1 = [SWA; throttle] sensor y_2 = [vel; yawRate] measurement y = [y_1; y_2] input u = [torque R/L] disturbance d = n/a It's actually an open-loop controller because there really is no disturbance. Mitsu is the only company I'm aware of that tries to track a reference yaw rate, so it can be done closed loop. It takes some kind of nonlinear combination of the ABS, active center diff and rear active diff. 2) The center diff uses a combination of feed forward and feedback control. The feed forward component functions similar to sports rear diff, when you mash the gas (torque goes rear) or turn the wheels (torque goes rear), torque will be appropriated as needed. On the other hand, the most worrisome disturbance it rejects is the loss of tractive force. This is not know a priori, there is no knowledge of when the road will be slick, this can only be calculated by the vehicles stability control system which employs a Kalman Filter to estimate wheel slip through ABS sensors. The center diff receives this information via CAN.
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03-21-2013, 09:55 PM | #101 | |
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I have to be honest, I went out for a Tim Hortons coffee and comeback impressed. Makes me wonder if it's the same person. Just want to point out that - There is at least one active rear differential implementation that track reference signals but the control algorithm is very tightly coupled with the entire AWD system and it's not Mitsubishi. I think we can agree that you can have both feed forward and feed forward + feedback design. If Audi Sport Differential is a simple feed forward system, to be honest I am surprised. At this day and age, it seems a bit primitive. But then again, it doesn't have active center diff to dance with. Might as well KISS. - I philosophically disagree with the Porsche implementation. Braking system IMHO should be used for reducing vehicle speed or ESP applications, not to help it go faster! Even though I do acknowledge some advantages of that design. Namely the cost and weight saving. - We certainly have a fundamental philosophical disagreement on system design. To me, a Torsen or crown gear based system is both primitive and inferior. Sure it's the feed-forward control design that you love so much. But what it really is saying is that hey all the behaviors of the system is programmed mechanically into the design and is passive in its nature. As you pointed out yourself that ideally you want less power to the front wheels during corning. Well, can the Audi system proactively shift more power to the rear as steering input is detected? NO! The GTR ATTESA E-TS is an incredible system, certainly did not archive such incredible performance using a simple feed-forward design. |
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03-21-2013, 10:20 PM | #102 | ||||
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Huh, wtf does this have to do with feed forward design? The GT-R is built from the ground up to be a race car and the control system is necessarily very advanced and highly tuned. This is not the same system as xdrive which comes on a 320ix 32k car. You pay *a lot* of money to maintain this cars drivetrain.
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03-21-2013, 11:04 PM | #104 |
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Here in "Northern Canada" I would never buy a vehicle without AWD. Add some dedicated snow tires and some driving skill and that's all you need. I still can't believe I am selling my Legacy GT and buying an F31. If the BMW is 2/3 as good as my Subarus in the snow I will be happy.
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03-22-2013, 05:30 AM | #105 | |
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Given the current state of affair, more performance can be extracted with an active differential whether it is placed at the front, center, or rear. Which is why I mentioned GT-R. It simply finds more grip than your beloved "feed forward control" passive design. Shame that earlier GTR had transmission made out of glass and even today a switch on the dash dedicated to void your warranty. A notable exception to this trend by the way is the 2014 AMG E63 4matic. Which is as KISS as it gets. It will be interesting to see how much benefit a simple AWD system brings to that beast, and how BMW reacts for that matter. I certainly hope that BMW do more than just try to set a new drift record with the M5. Oh and with the GT-R, cost of differential fluid is not that bad, it's the transmission fluid that kills you. The late model Lancer Evo, incidentally the topic focus of the article you linked to, require scheduled diff fluid change just the same, though perhaps the cost is not as outrageous. |
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03-22-2013, 06:18 AM | #106 |
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X drive is worth it on snow but sucks having higher ride height when cornering. I got a killer deal on my fully loaded 2013 335xi sedan or else id have gotten the DHP option
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03-22-2013, 08:20 AM | #107 |
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Ok, at this point I have to concede that you sir have no idea what you're talking about. Feel free to continue to read wikipedia and police the internet forums of this world with your vast knowledge. Good day!
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03-22-2013, 09:21 AM | #108 |
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We had a pretty harsh winter here in New England, and I didn't have any issue with my M3 on blizzaks. I have never got stuck!
I would never drive an xdrive, not because of the weight, but because of the steering feeling. There is a reason why M cars don't come with AWD
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03-22-2013, 09:31 AM | #109 | |
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Isn't the next M5 rumored to have AWD?
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03-22-2013, 09:37 AM | #110 | |
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Yep, I have indoor parking, and I never park it on street. M3 is easy to control mostly thanks to LSD I guess... If you are calm, and treat the throttle well you won't have any issue.
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