Why VDC is necessary for off-roading and low-traction situations - Club Crosstrek | Subaru XV Crosstrek Forums
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Old 03-21-2017, 10:22 AM   #1
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Lightbulb Why VDC is necessary for off-roading and low-traction situations

I know for a fact there is a small trend of Subaru drivers who have a desire to completely kill off their VDC at times completely (which despite the "VDC OFF" button and feature existing, does not actually completely turn off VDC) in order to get more-desired handling in rallycross situations. Let me clarify I don't necessarily have a problem with that, however, I would like to elaborate why VDC is necessary for improved off-road performance.

I would like it to be known that, in general, any Subaru with VDC (Vehicle Dynamics Control) is going to perform better off-road than a Subaru with open front diff and open rear diff without VDC from a torque transfer standpoint. Subaru vehicles equipped with X-MODE have a better-calibrated VDC that reacts quicker and better to traction changes expected during low-traction situations compared to vehicles that have just VDC.

The reason being is that LSD's (limited-slip differentials) cannot transfer sufficient torque. A viscous LSD in the rear has insufficient torque transferring capabilities for slow-speed off-road use, a torque-sensing LSD does not transfer torque to the wheel with most grip if the other side is in the air (much like an open diff), and of course an open diff (the most common configuration of diff) transfers the most torque to the wheel with the least grip. This paragraph and thread is specifically addressing side-to-side torque transfer.

Front-to-rear torque transfer is sufficient in Subaru's AWD systems to be considered a non-issue except for situations where the ECU/TCU intervene through excessive engine power cutoff.


Drawback of open front and rear diffs demonstrated in real-life situation,
regardless of AWD center differential and/or torque transfer system.
At the time marker of 0:14 [14 seconds], this 1st generation Subaru Forester (SF chassis) attempts a water crossing but encounters a situation where the LF (left front) and RR (right rear) wheels do not have sufficient traction.
As a result of having open front diff and rear diff and a lack of VDC, the torque of the engine is transferred to the wheels with the least grip as the vehicle spins those tires helplessly.
https://youtu.be/5MjCzHpcpfM?t=14s
YouTube video code: 5MjCzHpcpfM YouTube video title: Subaru Forester Off Road - I think I took that one a tad fast
YouTube channel code: UC_uIq1hkQuQUavV1ovSLQhw YouTube channel name: Subaru Off-Road



A controlled environment demonstrating the drawback of open diffs
At the time marker 2:35 [2 minutes 35 seconds], the individual places their hand on one of the wheels, stopping it from spinning yet the wheel on the other side continues receiving torque despite having the least grip (wheel is in air, therefore has the least traction available).
https://youtu.be/TCVbipMoWtY?t=2m35s
YouTube video code: TCVbipMoWtY YouTube video title: How an Open Differential Works!
YouTube channel code: UCCZM3brTWaHd-L_Hs_TDeEw YouTube channel name: Rick Taylor



All-in-one video demonstration
depicting both how an open diff acts during low traction situations and how a Toyota/Lexus-equivalent system of Subaru's VDC works to simulate the action of an LSD (limited-slip differential) to transfer torque from the wheels that slip to the wheels that grip, eventually bringing the vehicle to move given sufficient traction is available.

YouTube video code: LXwNllw0CLQ YouTube video title: Active Traction Control
YouTube channel code: UCDyFw9t7Tq6cUsXwPDtBOcA YouTube channel name: LexusTechNavigator



Practical off-road demonstration of Subaru VDC
transferring torque from wheels that slip to wheels with grip to eventually move the vehicle. Captions and annotations explain pretty well what's going on and how it does its job. The vehicle featured is a 3rd generation Subaru Forester XT (SH chassis).
https://youtu.be/W5g4ySOh0WE
YouTube video code: W5g4ySOh0WE YouTube video title: Testing vdc with forester 2.5XT MY09 auto.
YouTube channel code: UCUX9syGm9kBlAiMjbmgwPoA YouTube channel name: suba600



Practical off-road demonstration of Subaru VDC and Subaru X-MODE
transferring torque from the wheels that slip to the wheels with grip to eventually move the vehicle. Plenty of information provided in this demonstration.
At time marker 0:36 [36 seconds], you can see VDC working to clamp the brakes on the slipping wheels to transfer torque to wheels with grip and eventually resulting in the Forester overcoming the obstacle.
At time marker 0:53 [53 seconds], you can see VDC with X-MODE activated clamping the brakes on the slipping wheels at a much quicker reaction than with just VDC ON. This video goes on with other off-road type situations demonstrating the behavioral differences with just VDC and VDC working with X-MODE. The vehicle featured is a 4th generation Subaru Forester (SJ chassis)
https://youtu.be/cW3zwUBMGF8
YouTube video code: cW3zwUBMGF8 YouTube video title: AWD Test: 2017 Subaru Forester Diagonal and OffRoad / THE Most Complete review! / Part 6/8
YouTube channel code: UCYLv8QcoJ7NfVfTAyz6ekHA YouTube channel name: CarQuestion

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Old 03-21-2017, 02:33 PM   #2
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A very informative post. Thanks AWDfreak.
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Old 03-22-2017, 10:21 PM   #3
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Hey! I just went over this on the other board! (I'm the hamburgurgalar on the other forum)
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Old 03-23-2017, 12:28 AM   #4
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May as well post this.
And, although, the E-LSD doesn't work the same as the mechanical or viscous LSD variant, I can attest to its cornering prowess. instead of allowing you to control a slide (and lose time), this E-LSD allows you to enter, and when you have the amount of rotation needed on turn in, you simply straighten the wheel and hammer down. It's effective and pulls you straight once the crosstrek has rotated enough. No recovery needed. It just takes an adjustment to driving style. Instead of a feint to Scandiflick, it's "lift, turn in, rotate" "enough rotation? Yes. Straighten wheel and hammer down."

Essentially the vdc helps avoid countersteering. (Which some of us miss!)
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Old 04-06-2018, 07:27 PM   #5
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Sorry I am a noob but I have seen some vids where Crosstreks with X-Mode turn off their traction control when they turn their x-mode on? Which is better for situations such as snow, mud, sand, etc, traction off with x-mode on or both on?
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Old 04-10-2018, 01:15 AM   #6
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Quote:
Originally Posted by PaperClip619 View Post
Sorry I am a noob but I have seen some vids where Crosstreks with X-Mode turn off their traction control when they turn their x-mode on? Which is better for situations such as snow, mud, sand, etc, traction off with x-mode on or both on?
Turning the traction control "off" doesn't necessarily completely stop traction control, but rather reduces it to a degree that will allow sufficient momentum to become unstuck or continue driving through slick and rough conditions.

Generally, for slick off-road conditions, X-MODE ON with traction control "OFF" is ideal to minimize the chance of becoming stuck.
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