Forester SJ (2017) VDC, Traction Control and Limited Slip Diffs

scot_c

scot_c

Forum Member
Joined
Jul 29, 2020
Messages
23
Location
Scottsdale Arizona
Car Year
2017
Car Model
Forester
Transmission
CVT
I'm new to Subarus as off-road vehicles, my previous one was a '96 Toyota 4WD 4Runner, not sure if that's the model name in Australia.

I bought a used 2017 Forester 2.5i base model (no X-mode) and recently had a situation that has me thinking about possible mods. I almost high-centered the car but still had contact on all 4 wheels. I tried to get off and the 3 tires with the most traction stopped altogether while the one with the least traction (right front) spun. Had to get pulled off. I didn't think to turn off traction control. Duh. I'd run into a similar situation in a 2WD Nissan truck, where each tire in turn spun until the traction control shut down power to both. Turning off TC allowed me to get moving again.

So here are my questions: Are Vehicle Dynamic Control and Traction Control the same system on a 2017? When I push the TC button and turn it off, the multifunction display says "Vehicle Dynamic Control turned off by operator" or some such. Not Traction Control turned off. So, is it all or nothing and TC off means no VDC functions, just two open differentials with the center clutch unit varying torque between front and rear?

There's an excellent video on youtube explaining all the current Subaru AWD systems
In it he says that the WRX Sti has VDC and a Torsen liimited slip diff in the rear. I don't know what the VDC in a Sti is like, I'm sure it's tweaked compared to a Forester, but obviously a LSD can work with VDC in some fashion.
The problem with Torsen LSDs that I've read about is that if one wheel is in the air, the LSD transfers all torque to the spinning wheel instead of splitting it between the two. There are supposed to be ways to modulate the brakes to get power to the wheel with traction but I've heard they don't always work.
Here is the second question: has anyone run a Torsen type LSD in a SJ (2014-2018) Forester or Outback (I'm assuming the VDCs work the same)?
What I'm wondering is if the car has a Torsen LSD and the VDC is engaged, if one rear wheel is spinning freely, i.e. in the air, will the VDC apply braking to it and allow the LSD to work?
Also, would having a LSD interfere with the VDC under normal driving, i.e. try to override the VDC if one wheel loses partial traction?
From my experience, VDC has its limits and problems under certain circumstances. Turning off Traction Control (and perhaps all VDC) is only a partial solution as it leaves one with open diffs, not a good thing in a diagonal. Having a Torsen LSD wouldn't help in that situation, with one wheel in the air. But if the VDC would work with the LSD, it might be effective.

Any experience or ideas?

A plate/clutch LSD is out of my financial reach right now and I've had experience with lunchbox lockers on the street-not good in wet/icy conditions or applying power in tight turns, so a TorqueMaster or similar is out of the question as this is my daily driver and spends most of its time on the road.

Thanks,
Scot Carpenter
 
My understanding, in simple terms because I'm a bit simple :-) is VDC is "intelligent" skid / slide control via ABS & engine control whereas TC prevents individual wheels spinning out of control. From what I've read, when VDC is off TC is still on. My Mitsubishi ute operates in a similar manner.
 
Kevin said:
TC prevents individual wheels spinning out of control
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Kevin said:
via ABS & engine control
Click to expand...
👍

(In modern vehicles, traction-control systems utilize the same wheel-speed sensors employed by the antilock braking system. These sensors measure differences in rotational speed to determine if the wheels that are receiving power have lost traction. When the traction-control system determines that one wheel is spinning more quickly than the others, it automatically "pumps" the brake to that wheel to reduce its speed and lessen wheel slip. In most cases, individual wheel braking is enough to control wheel slip. However, some traction-control systems also reduce engine power to the slipping wheels. - from https://auto.howstuffworks.com/28000-traction-control-explained.htm)

Whereas, VDC does pretty much the same thing:

"Since most Subaru models are equipped with symmetrical full-time AWD, the VDC system relies on this first to maximize traction and restore vehicle stability at the first signs of trouble. If this doesn’t produce the desired results, VDC will then take corrective action by adjusting torque-split front to rear (CVT models only), applying the brakes to individual wheels, altering engine timing and reducing fuel flow to the engine"
From: https://www.subaru.ca/WebPage.aspx?WebSiteID=282&WebPageID=20179

X-Mode basically is an extension of what they mention happens with the CVT model above (not that that is really relevant to this conversation..)
 
Why you need anything else there with vdc allready? Just locker in rear will make huge differrence, other stuff not so much. Its better to find better angle better line wirh same awd system that is on car allready then try to force subaru be what its not and never will be.
They capable enough cars with what they are rly. If its not enough then its wrong line on angle.
 
scalman said:
Why you need anything else there with vdc allready? Just locker in rear will make huge differrence, other stuff not so much. Its better to find better angle better line wirh same awd system that is on car allready then try to force subaru be what its not and never will be.
They capable enough cars with what they are rly. If its not enough then its wrong line on angle.
Click to expand...
You're exactly right about the wrong angle! I took the wrong turn at a dirt road intersection and was backing up when I went past the other road and the rear dropped down into a ditch on the other side. Just bad driving on my part but the car was high-centered (the middle of the underside on the road surface). All 4 wheels were still on the road but 3 of them stopped turning, no power to them, and the one with least traction (right front) started spinning. It was the only one getting power. I should have tried turning Traction Control off but didn't think of it.
That's what got me thinking that VDC had its limits and a LSD with traction control off might help in similar situations.
 
scot_c said:
and the one with least traction (right front) started spinning
Click to expand...
Perhaps that was the one with the most traction, and VDC was working as designed, but it just didn't have enough traction for any result.
Unless I've got it wrong that the car was pretty much balancing on its belly.

Either that, or VDC wasn't doing its job at all.

(Edit: Durn miscreant apostrophe..)
 
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Ben Up North said:
Perhaps that was the one with the most traction, and VDC was working as designed, but it just didn't have enough traction for any result.
Unless I've got it wrong that the car was pretty much balancing on it's belly.

Either that, or VDC wasn't doing its job at all.
Click to expand...
It's possible that was the one with the most traction; I hadn't thought of that. Hard to say since I didn't turn off traction control. The car was hung up on its belly and had to be winched forward but all the tires were in contact with the ground. Luckily I have skid plates.
I had a similar experience with a 2WD truck with traction control. A rear wheel started spinning, TC applied braking to it, the other tire started spinning, TC applied braking to it and the first wheel started spinning again, etc. etc. until TC wasn't putting power to either wheel. Turned off TC and was able to get going despite wheel spinning. I assumed something like that happened this time, perhaps not.
Electronic VDC is new to me, last 4WD vehicle was a '96 Toyota 4Runner with an electric locker rear diff, so I'm still trying to figure out how it works in different off road conditions. It's great for driving on dirt/slick roads.
 
You wont make any more miracles from this car. Vdc will tey do its job but it will fail if it will be too hard situation too steep angle wheels stuck too much.
 
scalman said:
You wont make any more miracles from this car. Vdc will tey do its job but it will fail if it will be too hard situation too steep angle wheels stuck too much.
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Yeah, I didn't get it to replace my '96 Toyota 4Runner 4WD, but to get through moderate off-road conditions rather than crawl through rocks at 2 miles per hour (3.2kph). I just don't like the idea of having to use TC, i.e. the braking system, to maintain traction; the 2.5 NA needs all the help it can get up hills and through rough stuff. But adding any sort of LSD is a big investment so I'll reserve judgement until I've learned more about what the Subaru systems can do on their own.
Thanks for the comments/opinions.
 
@scot_c,
Scott: I’m interested to know if you went ahead and installed an LSD? I’m considering doing the same thing…
 
@Kevin,
Thank you, Kevin: I figured it might be a long shot… I’m a complete duffer at this forum stuff!
 
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