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  #61  
Old 09-10-2010, 10:31 AM
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Re: No front ELSD for QD II?

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Originally Posted by vonk View Post
Of course the engineers would say it's better.

Wikipedia has a nice write-up for those who want to get technical. But let me simplify, using their images.







Input torque is applied to the ring gear (blue), which turns the entire carrier (blue), providing torque to both side gears (red and yellow), which in turn drive the left and right wheels. If the resistance at both wheels is equal, the planet gear (green) does not rotate, and both wheels turn at the same rate.







If the left side gear (red) is held still (X) by brake application, the planet gear (green) rotates about the left side gear, in turn doubling the speed of the right shaft (yellow). This means that the right shaft now runs at doubled its original gear ratio. Given the same input torque, we know that when you double the gear ratio of any drive train, the output torque is cut in half.

this is nice....but the wiki is not always correct and this is not necessarily how the wk2's system works. It could be....but we dont know really. With the advanced computer systems...all of that can be altered. Thats pretty much a general diff shown.
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  #62  
Old 09-10-2010, 10:33 AM
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Re: No front ELSD for QD II?

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Originally Posted by LJS8510 View Post
Why did they keep the rear ELSD?
Im gonna guess because all WK2's are rear biased under normal conditions....meaning most of the hp/tq is applied to the rear axle
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  #63  
Old 09-10-2010, 11:00 AM
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Re: No front ELSD for QD II?

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Originally Posted by unibrain View Post
For the math inclined,

(From http://en.wikipedia.org/wiki/Torque#..._versus_torque )

The question is does this matter in the use case you have described? What other factors are in play such as varying the engine power/rpm?
The properties of a drive train are defined by non-dimensional ratios. Therefore, given sufficient structural strength, a drive trainís performance is independent of the power it transmits.
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  #64  
Old 09-10-2010, 11:40 AM
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Re: No front ELSD for QD II?

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Originally Posted by vonk View Post
The properties of a drive train are defined by non-dimensional ratios. Therefore, given sufficient structural strength, a drive trainís performance is independent of the power it transmits.
That is something that you'll need to explain to me. The words taken one at a time make sense but when strung together I get lost. We can take this offline of others aren't interested.
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  #65  
Old 09-10-2010, 11:44 AM
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Re: No front ELSD for QD II?

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Originally Posted by Scottina06 View Post
this is nice....but the wiki is not always correct and this is not necessarily how the wk2's system works. It could be....but we dont know really. With the advanced computer systems...all of that can be altered. Thats pretty much a general diff shown.
Exactly my point. They havenít told us anything about what will be in QD2.

Youíre right about wiki. However most of the time they get the principle right. Iím not implying that the story describes the Wk2 system in any way.

The last post also does not attempt to describe the WK2 system. It only describes a general open diff to explain how locking one shaft with brake application does not transfer 100% of that axle's torque across the diff to the loaded wheel
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Old 09-10-2010, 11:56 AM
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Re: No front ELSD for QD II?

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Originally Posted by unibrain View Post
That is something that you'll need to explain to me. The words taken one at a time make sense but when strung together I get lost. We can take this offline of others aren't interested.
I should have said, "The properties of a drive train are defined by gear ratios that are non-dimensional".
Does that help any?
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  #67  
Old 09-10-2010, 08:09 PM
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Re: No front ELSD for QD II?

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Originally Posted by LJS8510 View Post
Why did they keep the rear ELSD?
Exactly. If they can do miracles with engine management/brake controls/fairy dust up front, why go to the expense of installing an ELSD in the rear? Why not use the same fairy dust in the rear?

My assumptions could very well be wrong, but it just doesn't seem logical to me. If one ELSD in the rear is good, wouldn't 2 ELSDs (one front, one rear) be better? Conversely, if fairy dust in the front is wonderful, wouldn't fairy dust in the rear be doubly-wonderful?

I'd love to run a trail with a WK2 to see if I'm wrong, maybe Jeep will bring one out to Grand Slam West 2011.
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Old 09-10-2010, 08:18 PM
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Re: No front ELSD for QD II?

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Originally Posted by RedRocker View Post
Exactly. If they can do miracles with engine management/break controls/fairy dust up front, why go to the expense of installing an ELSD in the rear? Why not use the same fairy dust in the rear?

My assumptions could very well be wrong, but it just doesn't seem logical to me. If one ELSD in the rear is good, wouldn't 2 ELSDs (one front, one rear) be better? Conversely, if fairy dust in the front is wonderful, wouldn't fairy dust in the rear be doubly-wonderful?

I'd love to run a trail with a WK2 to see if I'm wrong, maybe Jeep will bring one out to Grand Slam West 2011.
Interesting logic. I recognize thatI have no mechanical skills, however, sometimes 0+1=0 and sometimes 0+1=4 which is better that 2 Jeep engineers said that the new system is better than the old one ... how that can be proven?
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Old 09-10-2010, 09:20 PM
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Re: No front ELSD for QD II?

ok...please explain to us how the elsd is better than what the engineers explained.....


maybe they needed to loose some weight in the front? maybe the elsd for the front was not available for the new axle in the front and it was for the rear? Maybe the system didnt work as well for the rear because of the transfer of power through the driveshafts?
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  #70  
Old 09-10-2010, 11:06 PM
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Re: No front ELSD for QD II?

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Originally Posted by Scottina06 View Post
ok...please explain to us how the elsd is better than what the engineers explained.....


maybe they needed to loose some weight in the front? maybe the elsd for the front was not available for the new axle in the front and it was for the rear?
This may very well be true, but that's an explanation for not using 2 ELSDs, it would not explain why 1 ELSD + fairy dust is better than 2 ELSDs.

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Originally Posted by Scottina06 View Post
Maybe the system didnt work as well for the rear because of the transfer of power through the driveshafts?
Maybe. It just doesn't seem logical to me. Seems more like marketspeak than reality. Hey, I hope I'm wrong, but I remain skeptical.
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Old 09-11-2010, 12:05 AM
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Re: No front ELSD for QD II?

First, the engineers are possibly not allowed to say much (if anything) negative about design choices like this. Second, it's quite possible that the new setup without ELSD in the front is better than the previous configuration with an ELSD in the front. But most likely the new configuration with an ELSD in front would likely be even better - but not for every scenario. Not all scenarios are low-traction situations. Third, you only get half the torque to the wheel with traction in the front, but... so what? You just have to press a little harder on the gas pedal. There's plenty of torque available from the hemi (which is required to even have QDII in the first place). Fourth, "why did they put an ELSD in the back then?" I estimate that overall they determined at least the rear needed an ELSD to reach their goals for how "good" it needs to be in low-traction scenarios, which I believe is a level above the previous generation's best.

They likely are under corporate and governmental pressure to reach certain fuel economy levels, and fuel economy was mentioned in the engineer chat on this topic. They might have drawn the line as far as total vehicle weight is concerned as well, I would guess an ELSD is significantly heavier than what's in there.

Just because overall this system design is not the best thing it could possibly be, does not mean it's not the best GC yet in terms of off-road and low-traction performance, and does not mean it "sucks". It's probably the best off-road and low-traction performing SUV you can possibly get in it's price range, bowing only to the Wrangler - which I'd argue is just an SV, not an SUV
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Old 09-11-2010, 06:31 AM
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Re: No front ELSD for QD II?

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Originally Posted by Kev- View Post
First, the engineers are possibly not allowed to say much (if anything) negative about design choices like this. Second, it's quite possible that the new setup without ELSD in the front is better than the previous configuration with an ELSD in the front. But most likely the new configuration with an ELSD in front would likely be even better - but not for every scenario. Not all scenarios are low-traction situations. Third, you only get half the torque to the wheel with traction in the front, but... so what? You just have to press a little harder on the gas pedal. There's plenty of torque available from the hemi (which is required to even have QDII in the first place). Fourth, "why did they put an ELSD in the back then?" I estimate that overall they determined at least the rear needed an ELSD to reach their goals for how "good" it needs to be in low-traction scenarios, which I believe is a level above the previous generation's best.

They likely are under corporate and governmental pressure to reach certain fuel economy levels, and fuel economy was mentioned in the engineer chat on this topic. They might have drawn the line as far as total vehicle weight is concerned as well, I would guess an ELSD is significantly heavier than what's in there.

Just because overall this system design is not the best thing it could possibly be, does not mean it's not the best GC yet in terms of off-road and low-traction performance, and does not mean it "sucks". It's probably the best off-road and low-traction performing SUV you can possibly get in it's price range, bowing only to the Wrangler - which I'd argue is just an SV, not an SUV
Well said!

If you want to crawl rocks, would you buy the most expensive way of doing it? Most Overland owners probably just want to get up their driveway in deep snow. Even the rear ELSD might be a concession for the people who have to have the top model with all the bells and whistles, whether they need them or not (me ). My guess is that cost (including warranty) was the real motivation for dropping the front ELSD. Will the rear be next for 2012? They seem to have the brake manipulation down well enough. The average user won’t cry: “I need an ELSD!”
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