scoobyc

iTrader: (2)

Hi there,

Im trying to understand why DCCD has the effect of shifting torque from the front wheels to the rear.

DCCD Explained - ClubWRX Forum - Subaru Impreza WRX and STi Community and Forums

Above is a big explaination which is confusing me.


Id like to simplify an example which i hope someone can help me answer.

Lets say for arguments sake the split front to rear is 50/50. And that the front wheels are on ice which takes 10ftlb to brake traction, and the rear is on tarmac which takes 200ftlb to brake traction.

So if the engine applies a force of 100ftlb, as the front will loose traction, and as the diff is open, the force applied to the front maz 10ftlb, and to the rear max 10ftlb. The other 80 fron the engine is effectivly lost spinning the front wheels.


Now if the center diff is lockable and is locked to 10% which takes 10ftlb of torque to start slipping. please explain to me what will happen in this situation? And is the above correct?



Im not sure exactly how DCCD works. I can grasp its either locked or unlocked. How is the intermediate levels achieved? or is it a case of the force applied to the clutch has different slip levels. I.e. setting 1 the clutch will slip at 10lb, 2 20ftlb , 3 30ftlb etc etc?




I assume a locked diff is best for launches, as no power is wasted due to wheel spin, and therefore reducing the max power which can be transmitted to the wheels with traction.

Everyone talkes about dccd as if it puts more power to the rear. I.e. exiting corners more likley to loose traction at the rear, as more power is going to it. I just cant understand how this will be the case with DCCD over a normal sti which is an unlocked diff.



If neither front nore rear wheels are spinning, then surley the default power distribution of 35/65 remains making it drive exactly the same as the sti's.

Clearly im wrong, just cant quite nail down why...

help!

scoobyc

iTrader: (2)

oooor.

Is it the case that, when understeering, you can still increase the power to the rear with dccd. I.e. to get oversteer you wil lbe requiring full lock? The front may spin more but the rear will also push more/spin to even out?

The only info i can find is that set to open the power distrbution is 35/65 which is the same as a normal STI without dccd right? So it would be no more oversteery.

Im talking about classic here btw..

jho!

When you toggle the switch forward more black lines apear on your dash when you toggle back you get less

It controls black lines, it's even better than the dimmer I think.

What will they think of next????

jho!

scoobyc

iTrader: (2)

ok thats clearer then :P

So this whole DccD thing is all abuot making lines go black . awesome.

hodgy0_2

if the centre diff is open then all power/torque will be sent to the spinning wheel (assuming no LSD'd on the axles)

Adam K

The imprezas with viscous center diff have 50/50 front/rear torque.
The dccd ones have 35/65 or 39/61 or 40/60 depending on year.

I don't think any imprezas with OE dccd came without lsd rear diffs.

johnfelstead

The DCCD unit is effectively two types of differential in one, it uses a sun and planet gear arrangement to give the initial torque split drive, it also has a clutch pack LSD that clamps the front and rear drive outputs together.

With the DCCD switched off the torque transfer relies entirely on the sun/planet gears to give the torque transfer, it is effectively an open diff unit but with a gearing effect to give the 33/66 split.

When you switch the DCCD on, you bring the plated LSD into effect. As you increase the DCCD thumbwheel towards more lock, an electromagnet is energised and acts like a solenoid, pulling a ram into contact with one side of the plate assembly, the more current the electromagnet is subjected to, the harder the ram pushes against the side of the plate assembly, making the LSD clamping load tighter.

Due to the way the plates are arranged, you get two effects of this extra clamping, firstly you have a variable plated LSD, so you are changing the preload of the LSD on the fly rather than having to shim the plates manually as with a conventional plated LSD, also because it's clamping both sides of a geared sun/planet arrangement, you change the relative torque seen on the sun/planet gears, which alters the torque split accordingly,

johnfelstead

Torque split on a viscous JDM STI classic is 50/50, on a UK classic it's 45R/55F.

The best DCCD unit out there is in the MY05 widetrack blobeye, it has the advantages of Auto control using Yaw, G, Steering position and throttle position sensors, and also has a plated LSD in the rear and helical in the front, with a 65R/35F torque split.

The MY06 unit has a more front biased arrangement 59R/41F with a torque sensor in the DCCD unit which limits the amount of rear drive bias you achieve, which makes the car more understeer proned.

The MY04 relies on just a G sensor and throttle position to work out the torque split, it also uses a suretrack front LSD, which makes it also proned to understeer when using auto.

They changed the spec every year on the newage cars, with combinations of diffs and torque split, MY01 and MY02 had a 45F/55R torque split. MY03, MY04 and MY05 had a 65R/35F torque split. MY06 and MY07 had a 41F/59R torque split.

The classics all had a 66R/33F, the Type R usually had a plated rear and open front diff, the Type RA usually had a plated rear and helical front LSD.

p1doc

very informative ,i have type r classic dccd so 66r 33f how far can i shift it forward i presume to 50/50 can i make it 100r or is there a limit??
martin

johnfelstead

50/50 is achievable with the diff locked, you can get the rear wheels spinning like a RWD car if you try hard enough with the diff open.

hodgy0_2

surely you would never drive the car on tarmac with the diff locked at 50/50

you would get massive transmission windup

IggyRB320

iTrader: (9)

johnfelstead,

Seeing as you seem to know what you're talking about;

In my previous MY05 STi Type UK, it had the DCCD which I have to say got left in AUTO for the majority of the time. But I was wondering if your explanation above about the MY05 having 65R/35F split, and my current MY07 STi RB320 having 59R/41F split, being the reason!

When I had the MY05 STi, I didn't notice anything on slow full steering lock manouvers. But on the MY07, when slow reversing on full lock it's generally not smooth. There has been lots of discussion about dropping off AUTO into the green settings, to open the DIFF, to assist with this but I don't notice any difference. And surely if its in AUTO is open anyhow.

Cheers
Iggy

scoobyc

iTrader: (2)

I hear bringing the handbrake up till its on one notch and the light shows will make the diff free whatever.

I believe the way it works is even when spinning assuming the tyre doesnt degrade the 10ftlb is still being pushed back by the tarmac and so transferred through to the rear wheels. The locking the diff lets you change this. The hummers have an option where if 2 wheels are spinning, you apply the brake which effectivly forces some wheel power to the ones with grip.

This is the maths im trying to understand .

So is the classic JDM STI 50/50 open diff yes?

Ok so i can understand the different gearing on each setting. I wasnt sure that was the case. People talk about the DCCD being infinatly variabl, however if the power split is based on a planetary gearset it will be limited.

According to the article above its suggested the limited slip of the clutch cause the shift in the power. Or is it merly suggesting a change in the total powerwhich can be given once traction is lost is higher.

Ok another question. What are there different levels of lock? How is this usable other than auto in a performance car? Clearly on launches you want it locked, on the track you will want it perhaps on one lock from open. What about the other levels.

Also. as the classic doesnt have a-dccd what does everyone use for their normal driving? To have it the same as my classic now it would be on open but the power would be 65/35 instead of 50/50 yes?

Also i understand my sti jdm 98 has a lsd rear diff, but not lsd front (which i think could be of benifit).




Advice welcome

Thanks for all this so far!

evil_scoobs

What about the JDM blobeyes/ 02/03 reg cars, whats their split maximums.

Also whats best setting for 1/4 mile racing? More to the rear?

hodgy0_2

this may happen in practice on tarmac, I suppose I was comming from an off-roaders point of view, where it is often the case that one wheel loses all traction, and will spin - stopping the car, until the centre diff lock is engaged thus ensuring it only receives 50% of the available torque the rest going to the rear tyres which hopefully have traction

johnfelstead

The MY07 never goes fully open, it's always got some centre diff preload so will have a small amount of windup on tight slow movements.

johnfelstead

no, it's 66R/33F

There is a long thread on all this from a few years ago.

johnfelstead

VC equiped bugeyes are 50/50 with various combinations of plated, suretrack and open diffs

DCCD equipped are 45/55 using a plated rear and suretrack front LSD.

No idea on a 1/4 mile, whatever gets you off the line quickest after trying a few different methods.

phil_wrx

iTrader: (2)

i was told 1/4 mile setting for DCCD should be fully locked (scroll wheel forward) but knock it off before u turn

hodgy0_2

if you have pre-load on the diff is it the case that you have to be carefull replacing invidual tyres on each axle

i.e. you have to replace in pairs to keep the rolling diameter the same, otherwise they turn at different speed, thus putting strain on the diff

scoobyc

iTrader: (2)

I would imagine the diffs would have been designed with enough durability to compensate for the diameter difference in tyres due to wear? Tyres dont ware evenly anyway so i would consider that covering that base would be part of normal conditions.

Ok so some people say the JDM STI is 50/50/ others say 65/35 (ish). So if the later was true DCCD would make no difference in the rear end slideability of DCCD. if they were both 65/35, effectivly with DCCD fully open, i understand the classic sti center diff is not limited, they would behave the same?

Is there not a subaru manual somewhere with some actual facts as there is a ton of contradictory information on the internet. Ive spent a good few hours researching this now and no closer to some answers lol as every statement i read contracdicts.

Anyone driven both non-dccd classic sti and a type R in anger to compare?

rickya

iTrader: (18)

Very informative John Felstead, thanks

I also posted a video once on the development of the MY06 Hawkeye Sti & how Tomi Makinen helped development on it & advised torque split to be less rear biased than previous model.(blobeye) Probably just to make car more user friendly/safer?

johnfelstead

What i am telling you is correct, put down the internet and read what i wrote. I've driven almost every version of Impreza over the years, never in anger, just quickly.

johnfelstead

That was a bad decision in terms of handling balance, it's spot on with the MY05.

Puff The Magic Wagon!

iTrader: (2)

John is the guru - going to get my DCCD installed then get John to teach me how to use it properly (he doesn't know this yet )

rickya

iTrader: (18)

I agree, Iv tried to dial a out a bit of that extra understeer with a thicker 24mm adjustable rear ARB. Seems to have done the trick.

Jamo

iTrader: (23)

you should listen to what this man has to say, I can guarentee hes forgotton more than you know about this marque

nice to see you posting again john

gussy

iTrader: (2)

As john has also informed me in the past that the analouge green lights are just a guide the most important/infuential bit is the first one on the a-dccd between the first green light coming on switching from auto to manual

hodgy0_2

maybe for Subaru's -- but drivers of Freelanders and 4X4 Volvos its a big problem, I was talking to a tyre fitter the other day who said the 99% of Freelanders he sees have siezed or tight diffs, he can always tell coz as they turn into his yard they "skip".