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  #11  
Unread 25th February 2018, 09:44 PM
MiddleAgeSubie MiddleAgeSubie is online now
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I have to say that I still do not understand.

Unless one has modified the center differential to lock fully and stay that way, as opposed to constantly "lock"/"unlock," it stands to reason that the torque will simply keep going to the front wheels.

Am I missing something? Or does the rear diff lock discussion assume that the center already has been locked?
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  #12  
Unread 26th February 2018, 12:44 AM
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I would think so e.g. with DCCD
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  #13  
Unread 26th February 2018, 02:39 AM
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I'm not so sure. I have a stock 4kg centre diff with a clutch LSD rear diff & I've seen the centre keep sending power to both ends with the rear locked up.

Don't forget, a diff locker isn't some magical device that suddenly drains all the energy in a system. It's a very simple device that shares energy equally between both wheels on an axle. That's all it does. It simply applies equal torque to both wheels.

Time will tell. Eventually we'll get this made & I'll try it with my stock centre diff, or if it's been upgraded by then, someone else can
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  #14  
Unread 26th February 2018, 03:12 AM
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I am just speaking fm experience.

If the fronts are unloaded on a steep loose hill, the VTD with clutches does not do enough to prevent too much torque fm going to the fronts. I have never failed a climb due to traction but the car has struggled mightily on a few occasions. A rear locker would not have helped in those situations. A center lock would have. In its absence, momentum is the only friend.
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  #15  
Unread 26th February 2018, 05:11 AM
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Stop me if I'm wrong but..

With Forester center diff (at least on the SG with vcd) it doesn't ever put all power to the front or rear, but splits pretty evenly regardless of traction.
Ergo, with a locked diff you are still going to get an approximately 50/50 split front and rear regardless of how much spin there is in the front.

Without a locked or limited slip rear diff, as soon as the diagonally opposite wheels lose traction you are stuffed, isn't that right? or would this also occur if both wheels on one side lose traction, even with a DCCD?

Maybe I should just shut up
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  #16  
Unread 26th February 2018, 05:34 AM
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couple lockers still don't make crawler from forester but it might help in some situations as i see it. My understanding to lock rear diff first you need to lock center so you getting 50/50 locked then you could lock rear and still you would only get that 50% to rear not more. Thats where newer systems works better as they split that as you need.
DCCD in WRX STI give you maximum split 50/50 and then goes to rear more if you turn that knob.but there must be reason why subaru's DCCD was never used on more offroady models as forester or outback. maybe its just don't work on harsh off road conditions or its not better then good old VDC and x mode. And DCCD must works with LSD's to do its job best.
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Unread 26th February 2018, 07:57 AM
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Quote:
Originally Posted by MiddleAgeSubie View Post
If the fronts are unloaded on a steep loose hill, the VTD with clutches does not do enough to prevent too much torque fm going to the fronts. I have never failed a climb due to traction but the car has struggled mightily on a few occasions. A rear locker would not have helped in those situations. A center lock would have. In its absence, momentum is the only friend.
You might find a rear locker or clutch LSD will help in low grip situations to keep your momentum, so that the front doesn't steal as much of the torque.

This is the difference between reactive systems like VDC and proactive systems like (decent) LSDs & lockers

Quote:
Originally Posted by Ben Up North View Post
Ergo, with a locked diff you are still going to get an approximately 50/50 split front and rear regardless of how much spin there is in the front.
When talking about fr/rr torque split, need to mention whether it's auto or manual & which auto, they all work differently. The VTD auto can lock up its centre when the hold button is pushed in 1st or 2nd. The MPT auto can also lock its centre but only with the centre diff lock mod. The manual viscous centre diff should still send torque to both ends if working properly.

Quote:
Without a locked or limited slip rear diff, as soon as the diagonally opposite wheels lose traction you are stuffed, isn't that right? or would this also occur if both wheels on one side lose traction, even with a DCCD?
Yes that's right. If you have diagonally lifted wheels & both diffs are open, doesn't matter what kind of centre diff you have. Locked or open, you're going nowhere.

Quote:
Originally Posted by scalman View Post
there must be reason why subaru's DCCD was never used on more offroady models as forester or outback. maybe its just don't work on harsh off road conditions or its not better then good old VDC and x mode. And DCCD must works with LSD's to do its job best.
DCCD works great offroad, not as well as a locking centre diff but close.
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  #18  
Unread 26th February 2018, 08:12 AM
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Most the time when people say 50/50 torque split they mean the shafts on either side of the diff are forced to turn at the same speed. The term '50/50 torque split' would actually mean both diff outputs get the same amount of torque. When one wheel is in the air it requires close to 0 Nm of torque to spin it helplessly. This means the other shaft gets close to 0 Nm of torque, leaving the car powerless, this is an open diff.

A rear locker with stock 4Kg centre diff will be an improvement, but only a limited improvement. Lifting diagonal wheels on close to level ground or mild inclines is where you will notice it most. When you lift diagonal wheels the front two wheels both lose torque (assuming open diff) and the 4Kg centre can send enough extra torque to the rears to get the locked rear axles turning. However once the hill becomes two steep the 4Kg centre will not transfer enough torque to the rear axles to push a car up. You really need centre and rear locked to get the full benefit on steep inclines.
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  #19  
Unread 26th February 2018, 08:19 AM
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Quote:
Originally Posted by Red XS View Post
However once the hill becomes two steep the 4Kg centre will not transfer enough torque to the rear axles to push a car up. You really need centre and rear locked to get the full benefit on steep inclines.
Here's a video of me on a steep gravelly hill on a diagonal rut. Front right wheel is lifted & rear left is unweighted. Front diff is open, stock 4kg centre diff, clutch LSD rear. The wheel with grip on the rear is still getting enough torque to keep turning, which keeps the car moving forward until the front wheel comes back down

https://youtu.be/odDwsvCsqRI
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Last edited by NachaLuva; 26th February 2018 at 07:25 PM.
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  #20  
Unread 26th February 2018, 09:17 AM
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^ that link seems to take you to a google acct and wants a sign in
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