s4awd

Motor Trend states the Center diff is 50/50 for our US Evo. Is that the "Tarmac" split on the ACD in Japanese spec Evos? Better yet does anyone know how the power is divided between Tarmac, Snow and Gravel on the acd?

Jonasan50

iTrader: (1)

I believe it would be the same as tarmac... The Eclipse GSX is split the same way.. Not a bad way to go... Much fun.....

MrAWD

First of all, Tarmac setting on the ACD equipped cars is not set to any specific ratio. It is an active setup that changes while you drive the car. In the most simplistic way, it gets less locked when you are turning (steering wheel sensor) and more locked for the straight line acceleration and braking.

Now 50/50 split would be more like the Snow setting on the ACD, but even that one gets to see an open diff sometimes (just not as much and not as long as the Tarmac option).

For the LSD with the viscous coupler, split is really never 50/50. The slip exist all the time and when more slip occurs (meaning, higher speed difference between the two sides), more locking happens at the same time. After the more locking happens, wheels decrease the difference between them and locking force goes down and let them to be more free.

So, from this said, if the diff (actually the viscous coupler) is more tight, car will be more like the Snow mode and closer to the 50/50 most of the time.

The more loose diff will stay more opened and closer to the Tarmac setting, it will corner better, but might leave some time accelerating in the straight line due to excessive tire spin.

Hope this helps!


Fedja

Fireball

iTrader: (15)

50/50 gives you the best of the AWD traction world. Too much in the front, and you get some serious push in the corners. Too much in the rear (most exotic AWD cars are 20/80 split) and too much oversteer in the corner.

MrAWD

Actually, I would prefer something like 35/65 front/rear much more than the 50/50. This would give you the best of the both worlds (cornering and the straight line acceleration) in the dry. When on the traction limited surface, just be more careful and have in mind that you are still better than those guys with only RWD or FWD!!

Fedja

s4awd

thanks for the info

Rafal

I tend to dissagree here. Usually the best setups were AWD with a rear bias (check Skyline). The reason is simple, you are using front wheels to turn so you can not apply the same amount of torque to them as to rears (it makes plenty of understeer). With a rear bias you can use power-on oversteer.
I am very afraid EVO with machanical central LSD and open front diff. will have a lot more understeer then JDM one. Subaru claims it used front SureTrac LSD to cut understeer so in USA EVO/STi character might be reversed from EVO VII/STi7 JDM.
I hope I am wrong on this one.

superz

iTrader: (7)

actually an open diff in the front will more closely resemble a rwd car. lsd with clutches for the front will give you too much understeer and will give the best straight forward traction. If you simply throw a mechanical lsd (torsen type) in the front you may have too compensate with a bigger sway bar in the rear too give better rotation. Me personally will never use a clutch type lsd in the front (cuzco), but would consider a torsen-type. A viscous center diff will provide a good balance of torque between front and rear. The reason EVO 7 had a lsd front diff and worked great was because it also had AYC to compensate for rotation making a bigger sway bar not necessary.

MrAWD

My hopes are that rear EVO diff is going to be aggressive enough to make the rear end to rotate when we need it. The lack of the front LSD is in one way a good thing (cornering wise), since outside front wheel is not gona be getting that much of the power in the corner, so it will turn better. Also, I hope that calculated that ration between the inside front wheel and two rear ones to be good enough for the rotate-able rear end!!

Hope your hopes are right and that you are wrong too!!


Fedja

iodine23

iTrader: (1)

That's a terrible generalization. The Skyline is a touring car and is rear-biased (I think 10/90) because of that. That doesn't make rear biasing the best AWD setup. It all depends on application.

Rafal

From http://www.impreza.subaru.com/micros...sti/wrxsti.jsp

Now about Suretrac from a different source
So it does not use clutches.
A torque sensing LSD is essential at front not to work under braking. And how enhancing traction at front can cause understeer? It's just opposite in my book.
My hopes are based on the whole car setup making it less understeering pig but open fron diff is definately a bad thing and adds understeer.

And if Skyline is a bad example please give me a better one.

MrAWD

The thing is that these theories are different for the straight line acceleration compared to the cornering. From the cornering perspective, outside wheel is trying (or you are trying to make it to do so) to get as much traction as it can, and speaking in the traction circle terms, to get as close and as long as possible to the 1 (one). If you succeed and get close to the maximum on that wheel, addition of any power on the same wheel will make it to go over the maximum (which is one) and you will start to understeer, since car will be no longer turning.

So, by just saying it plain like that about the open front diff and understeer, is not completely correct! Of course, since they where talking about the understeer they had to be turning, so this doesn't really work like that. For the straight line, it is a great help!

911, EVO 1-3, STi with DCCD,...


Fedja

rt turbo

iTrader: (4)

actually, i thought the gsx was split 60 front, 40 rear. my stealth is split 70 rear, 30 front to make it feel more like a rear wheel drive car.

Rafal

And open diff transferring torque to the other wheel which is unloaded and looses traction earlier helps?

STi with DCCD has 35/65.
911 Turbo has from 5% to 40% torque sent to front wheels.
I have no idea about EVOs but feel free to fill me in.

MrAWD

You are talking about the inside front wheel, which gets to be very light most of the time and on some cars you can see it up in the air too. Sending all of the front allocated power to that wheel helps cornering, because no power is going to the outside wheel, which is cornering as hard as it can and can't take any more of it!

The thing is that all of the power that goes to the inside wheel gets kind of wasted, but even that is better then send it to the wheel that is already at maximum cornering limits and has nothing else to use for the acceleration.

EVOs 1-3 had electronically controlled center diff with 30:70 split.

Fedja