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Midcow3 for president pic

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I didn't intend to say that it was a new thing. The overdriven rear is how this system is able to vector torque. There are other ways to apply torque vectoring.
That wasn't really directed at you exactly. It been floating around here a long time.
 

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No worries.

There is a lot of half misrepresented or misunderstood information out there. I just hope I don't add more onto that pile.
From everything I've read I think you've got it very close to the actual spec. The only detail that still is not super clear to me is the actual (gear) ratio of the overdrive. Seems that 1.4:1 is what they're running, and that is extremely novel when compared with other "overdriven" 4WD designs, where the overdrive ratio turns the front wheels faster than the rears, and by a much smaller ratio (more like 1.05:1)
 

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Block o' Text

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Part 1, Doom and Gloom


Ford engineered in a gearing advantage for the rear of the car. The end result of which is the drive axles on the back of the car can spin faster than the fronts. This has the downside of forcing the car to be FWD most of the time.


When I say "downside"...

And "most of the time"...

Low or steady state speeds mixed with light throttle and not driving like an idiot in traffic... Yeah, FWD.


Put another way, the time when the driven wheels DO NOT MATTER.

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Part 2, The Mechanicals


The only actual differential in the car is the open front diff., mounted to the transmission the same way the Focus ST and really any other FWD transaxle is setup. The RS has a shaft bolted/welded/attached to the drive gear of the front diff/output gear of the transmission. That shaft goes through what is called a Power Takeoff Unit (PTU) which bends the rotation of the engine 90 degrees. The PTU doesn't have any capacity to decouple or adjust torque split. From the PTU there is a 3-piece driveshaft. Then at the back linking up with the driveshaft is the Rear Drive Unit (RDU). This is the thing that makes the whole setup work.

Put the car in the air, lock the RDU, put the car in gear, and the engine will be spinning the rear wheels faster than the fronts. Open the RDU and the rear wheels stop spinning and the fronts don't change speed.

The different wheel speed front-to-rear is a result of gearing either in the PTU, RDU, or both. That is why the car will be front wheel drive most of the time. Looked at the other way, the "torque split" can be WAY more aggressive and way less complex than many other AWD systems.

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Part 3, Vectoring Torques


The Rear Drive Unit (RDU) has 2 hydraulic clutch packs. 1 for each rear wheel. They can vary between 0% and 100% locked many times every second. This is how the left:right split is handled at the back of the car. And also how the front:rear split is handled.

'Torque split' or 'torque bias' as far as front:rear is a bit of a misnomer. If the front wheels have traction and the clutches in the RDU are locked it should be a 30:70 split because of the difference in gearing. If both front wheels are on ice the split becomes even more rear wheel biased. If only 1 rear wheel is on ice it can still be a 30:70 split... Situations and applications can endlessly make the numbers essentially meaningless.

Ultimately the benefit of being able to overdrive the rears is bigger than the cost of not being able to be AWD in every situation.

That beneficial magic comes with the tuning of the ECU that controls those 2 hydraulic clutches.

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Part 4, Applying What We Have Learned


The ECU monitors throttle/brake pedal position, steering position, wheel speed, gear, engine speed, and yaw rate (among other things) many times a second. This means that compared to typical All-Wheel-Drive systems (namely Haldex based systems) the RS doesn't have to wait for the detection of wheel spin to lock the RDU clutch packs. Compared to the STI or EVO, the RS is more efficient and has a wider range. It can be more pedestrian and more aggressive.

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Part 5, The Elephant In The Room


Heat will build up in the RDU clutch packs under use. This is likely the closest thing to a flaw this type of setup has. It sounds as though the car will revert to front-wheel-drive only after about 30 minutes of abuse. This is a temporary reversion. Once the RDU fluid temperatures drop, the RDU can again lockup the clutch packs and make the car all-wheel-drive.

There are 2 separate fluids used in the RDU. A gear oil, used for the center section. Then a hydraulic fluid submerging the 2 clutch packs. Using wet clutches means they have a greatly extended service life. Nissan specifies a hybrid gear oil/hydraulic fluid for the transaxle on the GTR. Their transmission fluid is both very expensive, and compromised. The clutch should not be lubricated, but the gears and bearings should be. Using different fluids like Ford appears to be doing should mean benefits for performance, reliability, and cost.

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This is my understanding of how the AWD setup on the RS is supposed to work. It may be incomplete, ineffective, or completely wrong. Let me know, I guess?




TL;DR

The Focus RS has an AWD system.
Very nicely done. Thanks.
 

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From everything I've read I think you've got it very close to the actual spec. The only detail that still is not super clear to me is the actual (gear) ratio of the overdrive. Seems that 1.4:1 is what they're running, and that is extremely novel when compared with other "overdriven" 4WD designs, where the overdrive ratio turns the front wheels faster than the rears, and by a much smaller ratio (more like 1.05:1)
Could you say where you got the 1.4/1 number? That's a 40% overspeed, and a GKN tech said that it was something like 1 tenth of that (I'll try to find the quote and the exact percentage).
 

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The 2016 Ford Focus RS Gets an Advanced Torque-Vectoring AWD System

“It’s permanently attached. The front/rear ratio mismatch is unique to the Focus RS -- to get more torque to the rear. During normal operation or straight-line driving, the overspeed isn’t really doing anything. The clutches slip to take up the difference in the ratio mismatch. But during handling events, we try to vector or send torque to one rear wheel -- the outside rear wheel in a turn. We take advantage of the ratio mismatch to get more torque to the rear axle than without a ratio mismatch,” which Ford says is about 1.8 percent.
 

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From everything I've read I think you've got it very close to the actual spec. The only detail that still is not super clear to me is the actual (gear) ratio of the overdrive. Seems that 1.4:1 is what they're running, and that is extremely novel when compared with other "overdriven" 4WD designs, where the overdrive ratio turns the front wheels faster than the rears, and by a much smaller ratio (more like 1.05:1)
4WD systems are not designed for TQ vectoring they overdrive the front to prevent binding, this is why the difference in ratios is minimal.
 

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Well remember, when you have a ratio that increase the rotation rate of the output it also reduces the torque... So if the front wheels have 300 lb ft of torque available to them and you have a 1:1.2 front wheel:rear wheel ratio the rear can't put down 300 lb ft, it will be something less. I don't know the math, but if I were to guess that a 20% rotation rate increase results in 20% torque decrease the rear could do ~240 lb ft assuming the front had 0 load.
 

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Could you say where you got the 1.4/1 number? That's a 40% overspeed, and a GKN tech said that it was something like 1 tenth of that (I'll try to find the quote and the exact percentage).
I thought I heard one of the engineers mention that ratio in one of the videos. I also heard them say that if you put the car on a lift (all wheels off the ground) then you'd get that magic 30/70 split (I'm presuming the RDU is essentially locking the clutches), but that the number really is nothing more than theory, as the software will constantly change the torque being fed to each of the rear wheels in any real-world scenario.

My feeling is that in order to get meaningful torque vectoring the overdrive is going to have to be "significant". 1 or 2% is meaningless, and my gut says 5% doesn't get the job done. I'd expect something on the order of 20% or more would seem to be necessary in order to really overcome the understeer that is inherent in FWD cars.
 

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Can we please close this thread.

The original poster ( or abbreviated OP) is a Tangerine Scream 2014 ST,ST3 owner from Calgary,AB and has 87 posts in the ST Forum.

I could be wrong, but I think he started this thread partially as a "troll bait thread" and partially because he wants an RS, but it may not be feasible to him to get a RS at this time and he was trying to justify his "not-buying remorse"

We have had plenty of very,very good threads discussing all or almost all aspects of the RS GKN AWD system. The title of this thread "AWD Issues" is somewhat misleading and premature because none of us have actually experienced any RS AWD issues, mainly because none of us have the car yet. Now maybe if Ken Block or Raj Nair and his team care to chime in, other than their big eight advertising teaser videos, we could possibly be enlightened to any RS AWD issues.

So as @Slider said so succinctly IBTL => in before the lock ===> implying I agree this thread should at a minimum be locked/closed and maybe better just removed.

LET'S CLOSE THIS THREAD!




Peace, GoodWill, Motor On RS, YMMV,

MidCow3


P.S. - Exist2Resist - I hope you get a RS in the future and that you do not encounter any AWD issues.



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Shutting this down since no actual proof was presented. We'll all have to wait and see for ourselves about this claim.


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