GREGGYG

iTrader: (2)

Thanks the King and Tony, much appreciated!

I did the car park fill lock dccd process as suggested, doing full turning circles in both directions and I beleive my dccd is operating correctly!

Thanks again,

Greg

TonyBurns

iTrader: (3)

Here is a good explanation of how it all works

http://www.driveperformance.subaru.c.../blueprint.asp

Where as Im not fully in agreement with the King, I think I understand what he is getting at, the basic fact is that you dont have "resistance" as such along the drive train, if you did then you wouldnt move but when locked the mechanical diff (with an electro magnetic clutch) will lock the diff, your resistance is then caused by the cross axle LSD's (mechanical) which compensate for the car turning, ie if your going left the inner wheel does not need to rotate as much as the outer wheel so the (either suretrac or helitical LSD on these) compensate as you have less movement in the centre lsd.
This is why ALL 4x4 with a lock facility utilise cross axle LSD's both front and rear, otherwise things would break
The vicious coupling diffs on your normal scoob dont lock so dont lock all 4 wheels to rotate together, hence they dont need front LSD's.

This system is not like say the haldrex system in VW's

Tony

arumdevil

most turbo subarus have a rear LSD

TonyBurns

iTrader: (3)

But only dccd equipped ones have a front and rear lsd

Tony

arumdevil

I know but your post was a bit misleading, implying that subarus without DCCD don't have a front OR rear LSD, which is not true. It also implies that all 4x4s with a locking centre diff have front and rear LSDs, but that is also not true.

noobyscooby

That's why Clarkson abandoned his demo STi on the A44 at Enstone last week! See Sunday Times "In Gear" section today. Only gave it 3 stars out of 5.

TonyBurns

iTrader: (3)

No it is true, you NEED front and rear LSD's if you have a LOCKING centre diff, not all 4x4's have locking centre diffs though.
The difference between a 4x4 with a locking centre diff and one without is that when you lock that diff, all 4 wheels rotate at the same rate and not independently, hence they are locked if you dont have front and rear LSD's on these then you have problems because if they all rotate at the same time because they are locked, you will break something, so the limited slip diff on the axle is there to compensate, this is why you feel the diffs "winding up" when a dccd equipped car is put in "lock" and you turn, its not the centre diff its the lsd's on the axle, if you keep going the then make a nice noise that will scare the **** out of you when they release

So the issue is that if you turn left the left (inner) wheel(s) dont need to rotate as much as the right (outer) wheels.

I hope that clears it up for you

Tony

PS, I do apologise for putting front and rear on there, should have just been front not needed on normal 4x4

arumdevil

Your statement is too general. It is NOT true that you NEED LSDs front and rear if you have a locking centre diff and nor is it true that all vehicles with a locking centre diff have LSDs front and rear. Further more it is untrue to state that a locked centre diff will cause all 4 wheels to rotate at the same speed (it WILL cause the front and rear axles to rotate at the same speed, but each will on each respective axle can still turn at different speeds).

I don't post on here enough to be familiar with yourself and how much knowledge you have, so forgive me if I explain something you already know but I feel I have to offer some explanation to back up my point.

A 4wd vehicle will generally have 3 diffs (actually, some don't have a centre diff but we'll ignore the inferior ones for now and concentrate on the ones that have 3, such as subarus, shoguns/pajeros etc). An open diff allows the wheels at either end (in the case of a front or rear diff, or on a 2wd car which has only one diff) to turn at different speeds which is necessary to stop the tyres scrubbing when turning, this is the whole reason differentials were invented in the first place. I'm sure we can all agree on that much.

And as I'm sure most of us on here know, open diffs have their down-sides. Namely that in a low traction situation all power will be sent to the side of the diff with the least traction. This is where locked diffs and limited slip diffs (LSD) come in.

A locked centre diff will ensure that even if you lose traction at one end of the vehicle, say the front, you will still get drive at the rear as the centre diff is locked and therefore still turns each axle at the same speed, regardless of weather the wheels on one axle have lost traction. This can still lead to a situation where one front and one rear wheel could spin taking all the power due to loss of traction at those wheels, even if the other two wheels have still have good traction. Although the front and rear AXLES are locked together, the wheels on those axles are still connected by an open differential which is why this loss of drive can still happen in a low traction situation.

This is why most proper 4x4s have a locking diff at the rear as well as the centre. Now the front axle is locked to the back axle and the rear wheels are locked to each-other. Now it's still possible for a front wheel to lose traction and take all the power from the other front wheel, but because the back axle is locked in you will still get drive to BOTH rear wheels (because the rear diff is locked).


Now, it is true that a locked centre diff will wind up if used on a very grippy surface, but it is NOT trued that putting LSDs at the front and rear will stop this from happening. The design of the front and rear diffs will have no effect whatsoever on the centre diff, they are completely independent. Of course it is better to have LSDs on the front and rear, and in combination with a locked centre diff this is going to maximise use of available traction in any situation, but there is no NEED to have those in the presence of a locked centre diff.

Use of a locked centre diff, regardless of the design of the other two diffs, should be restricted to low traction surfaces such as sand, snow, mud, ice, gravel etc. The centre diff should NEVER be locked on dry tarmac, indeed no LOCKING diff should be used in this situation (this is what LSDs were made for).

I hope that clarifies the reasons for my comments on your post.

D7AKE

You must have something to do with the RSOC then, I did see a Scooby at the AGM...

D7AKE

I got them from a local tyre garage, they are a make I have never heard of tbh

The king

Again. Ehm. No. Not even close.

First of all, the DCCD DOES NOT LOCK!! It actually quite far from locks. That article of yours is quite good for basics, but there is a cruical fault, which is: 100% lock = Max LIMITED (note LIMITED) slip. So 100% is the max resistance the centerdiff can create inside the center differential. This resistance im talking about, is the resistance between the gearsets, so that if the rear tires slip in example, the center diff will LIMIT the speed differance allowed between the front and rear planet-gears. Read up on differentials, and more important CLUTCH TYPE LSD too learn more.


A locked center diff does not in any way require front OR rear LSD. In fact, in a parking lot, open diffs front and rear would make the car easier to turn. Look at the Toyota Land cruiser. Some of them have TRUE centerdiff locks. They also have seperate switches for locking front and rear diffs. These diffs are normal open diffs until they are locked. You should take a drive in one of those, they dont brake, and you rarely use the front and rear locks at all.


And again. DCCD does not lock. I cant say it enough. Theres also no change in static torque split, its always 65/35 (or whatever gearset you have in there), yet with DCCD engaged, it can (and will) affect the dynamic torque split. I could write about this for weeks, but you must understand the basics.

howstuffworks.com is a good source for LSD info mate. I suggest you read it.




EDIT: Hear hear Arumdevil.

arumdevil

likewise. I did wonder if my post would fall on deaf ears but now I have some backup

ALi-B

iTrader: (1)

Ahhh, static torque split vs dynamic torque split. That's a great can of worm and something lost on many AWD "experts".

This is often evident when trying to explain the torque split of some systems, typical example is assuming Haldex equipped VAGs are 50:50 which is the maximum static split ratio, but dynamically its can be almost anything depending on conditions (yes it can send more than 50% dynamic torque to the rear ).

arumdevil

on another note:

it can handle well in snow/ice, with the right tires and frame of mind

ALi-B

iTrader: (1)

True true, but I'm still on summer tyres, (albeit with the down to 15psi to keep them flexible) and managing just fine.

That sig is aimed at more of the drifter heros thinking they can do a Petter drift round a sweeping bend then tragically understeer straight into a curb. Or the ones who find that AWD allows them to move along at a nice pace witout getting stuck, so they feel unstoppable.....but when they come to a junction they find can't stop.

The king

I dont like getting into this one, but I cant help it. From what Ive learned, Haldex = A clutch type LSD electronic centerdiff. In other words, just the same as our DCCD! (except Ill bet you the VW is a piece of **** low quality front biased understeering **** system in comparison). Yet VW claims 100% can be sent to either axle.

Soooooo, how can you send more then 50% to the rear wheels? Well, true, if front wheels are on ice and rear on tarmac. You could say all the power is sent to the rear wheels IF they dont slip.

But no matter how you twist and turn it, even with a locked centerdiff, the front wheels will still have 50% power avaliable, just not the traction to put it to the ground. So yes, 100% rear, but no. Front wheels will still be slipping, so far from all the power is sent to the rear. This is even more true if any traction control system is applying brakes on the front wheels.

But you sir is correct. Dynamic split is just all over the place, the goes for all AWD systems really.. What really makes the difference though, is that we have an initial (static) 65/35, making the scoobys drivetrain so much better (and more fun) to drive. Im no fan of the understeering preset of a 5050.

The king

You mean Scoobys can handle well. VW just cant. Its not in their nature.

Frosticles

iTrader: (1)

LOL, That was mine. I sit on the Committee of the RSOC but had a family emergency on the Sunday

arumdevil

I know it's there as a light poke at some people, but I couldn't help putting my pedants hat on to respond to it

AWD will get you going, winter tyres will stop you.

ALi-B

iTrader: (1)

What I'm trying to dispell is these contsant quotes of static split ratios. Its these rigid figues that people often quote that mislead to what really happens in the real world - when a wheel slip occurs anything can happen depending on which wheel slips and by how much. The point is that sending 10%, 20%, or 50% of power to wheel that has little or no grip doesn't really make "that" much difference - its still a wheel with little or no grip.

So yes, in all cases we have a wheel somewhere with little or no grip having power sent to it aimlessly, be it viscous or DCCD. The only difference is the speed the wheels maybe spinning at may differ...its still a wheel which has little grip and nearly all systems are sending drive to it (just the torque cannot be applied becuase there is no grip).

The reality is in the driving, and thats where it matters: having owned Imprezas for the best part of eight years, and thought I'd try something that that is supposed to be inferior, and often quoted as not being a "proper AWD" or what you say "piece of **** low quality front biased understeering **** " (why did you feel the need say this? It has totally undermined any form of point or credit you just made ).

But I digress; if its all about driving it, then the piece of **** VW actually understeered less and oversteered just as much my Impreza did ....In some cases it oversteered more as there is no turbo lag and more low rpm torque, a quick mash of the throttle in a low gear will send the back end sliding right out where my Impreza would just do nothing (or 3,2,1, turbo boost...oversteer ) maybe its because my experience is with the GenII system which has the ability to pre-engage before wheelslip occurs, but all I know is that it works just as well as my last Impreza did. Be it dry, wet or snow/ice. So I can't say which system is vastly inferior/superior as they both seem to do their objectives very well - which is usually making fast progress when pressing on. The other variables that affect the handling dynamics get in the way too much dicate which is better.

So I agree about the dynamic bit, but I disagree about the static spilt's relation to outright handling, as whilst it does have an affect, there are too many other variables that affect the overall handling dynamics, not to forget the physical conditions. IMHO.

arumdevil

I'm not sure if you're not understanding the point of an LSD or if I didn't find your post clear enough.

an OPEN diff does what you describe above, sends all power to the slipping wheel. a LOCKED diff will send equal power to both wheels/axeles (depending on what's either side of the diff, a wheel or an axle) regardless of available traction.

a LSD behaves like an OPEN diff until it slips, at which point it will effectively LOCK, or send torque to the opposite side to that which is slipping, depending on the particular design of that diff. So yes, an LSD does send torque to a spinning wheel, but the whole point is it ALSO sends torque to the other wheel instead of sending it ALL to the spinning wheel. IF the other wheel has enough traction than you will not get stuck.

My apologies if I misunderstood the gist of your post and you already knew that, others may not.

ALi-B

iTrader: (1)

Yes that kind of relates to it all. You are quite right in pointing that out, I was assuming in all cases of LSD systems. The wheel with no grip having torque sent to doesn't really make much difference so long as the wheel with the grip gets its share; Which is not always what the static split ratio dictates.

For example; Anyone had a rear driveshaft break on a viscous diff equipped Impreza (both centre and rear)? It doesn't go very far or fast.

arumdevil

but there are two points which you seem to be either cloudy about or are clouding for others in the way you are explaining things.

1. and this is a minor one really, it's not that the LSD also sends torque to the spinning wheel, it also sends torque to the wheel that is not spinning. I know it sounds pedantic but understanding that is really a crucial foundation in understanding a system incorporating LSD/s. I'm not necessarily saying that you don't understand that, but for those who don't it's helpful.

2. Relating to dynamic torque split in the example of a broken rear drive shaft - yes the LSD will transfer torque at a certain ratio (depending on what it was designed to do) but it's not sending a fixed percentage of the ENGINE torque to the other wheel, it's sending a fixed percentage of available torque to the other wheel, subject to the limitaions of the design. For example a fully locked diff will have no trouble sending half the power to the other wheel as there is effectively no diff at all, but a viscous diff isn't as srong and so if one drive shaft is broken it takes a minute amount of torque to turn that broken shaft, this will directly affect how much can be sent to the other wheel on a viscous diff.

ANyway, must go get my LPG fitted now before the cold weather returns!

The king

Written in the quote.

carpboy

iTrader: (2)

To the king, you say you have a dccd controller, is it a pro? Am thinking of either the dccdpro or neetronics version. Do you recon ones better then the other?

The king

True straight ahead. false in turns. LSDs affect handling once one wheel is going faster then the other. Thats why a rear LSD ie can make the car understeer more(and more stable).

arumdevil

ok fair point, but as this thread relates to use in snow that's moot.

The king

Im using the DCCDpro.com controller. Never tried the neetronics controller, but from what Ive heard it has more options, though Im not shure exactly what. I believe it has inputs from speed sensors, and possibly others, but again, Im yet to read up on this.

I cant say anything on the neetronics, but I can recomend the DCCDpro.com. Also, Jeff is very VERY good on customer service, and will answer any quiestions you may have.

I would dear to say the cheaper DCCDpro controller would work well enough for 99% though. On tarmac, the auto mode works really good, and you simply adjust the settings based on your conditions. in 99% of times, you find your setting on the thumbwheel, and keep it there all around the track. The G-sensors effectively reduses, or even disengages the DCCD on turn in (OEM actually never completely opens the diff!), and as you apply throttle out of the turns it starts locking again. The controller use both G sensors and TPS to decide the "lockup", much like the stock unit, but infinte adjustable.

The king

Actually, in snow the affect without slip is even more noticeable (the diff has so much more locking force compared to the traction avaliable.).

arumdevil

well as I don't actually have a DCCD in my car (50/50 viscous split) I can't really respond to that.