jasonius

iTrader: (1)

I've asked this before and never really got an answer, but how come I can get the rear end of my 05 wrx to step out under hard acceleration when it has a fixed 50/50 centre diff..?

It's not like I can drift it, but the rear seems to light up a bit before I get traction then I'm gone, if a little sideways..

I would have thought with 50/50 the unloaded front would have spun up due to the weight transfer to the rear..?

Normski

Is this a standard viscous diff you're talking about?
Because if so, I would say that as you are turning and applying power at the same time, the rear wheels will slip first. This is because a tyre only has so much grip, the cornering force added to the acceleration force has pushed the tyre over the limit and so it slips. The rear of the car is lighter so has less weight pressing down on the tyre giving it less potential grip than the front.

With a viscous center diff, because the rear axle is spinning faster than the front, it tries to equalise this by transferring more torque to the front axle until they are equal again. So the torque split isn't always 50/50. The diff tries to maintain 50/50.

jasonius

iTrader: (1)

Ah, yep std viscous diff..

Makes sense and after doing more research I found out about viscous diffs being 'reactive', which is what you're explaining in your second paragraph..

It only occurs when I give it some from rolling/standing start 1st/2nd gear stuff, not necessarily whilst turning, which is why I thought the weight would be over the rear..?

Don't get me wrong, it's good fun and not a handful in a way a powerful (none TC'd) RWD would be..

Gear Head

iTrader: (2)

I always thought that the wrx had around a 60/40 split, not 50:50?

The rookie

iTrader: (1)

Always thought wrong I'm afraid, the only cars with none 50/50 are the Sti's.

Simon

Normski

To be more accurate, only models fitted with DCCD have a non 50/50 split. I.e. STi type R, STi type Ra, certain years of newage STi.
Classic STi's have viscous centre diffs.

The rookie

iTrader: (1)

I agree - I was summarising, as you agree only STi's have non 50/50, I didn't say all Sti's...as the OP has a WRX it wasn't relevant to go into detail I didn't think.

Simon

jasonius

iTrader: (1)

Having just read up on the various types of diffs on wikipedia, what would be the effect of having the front wheels on say ice and rear good traction/tarmac, whilst the car is up against something immovable (eg a snow bank)..?

50:50 viscous as in OP..

Normski

I would say the front wheels will spin.
The difference in grip between the front and rear axle and the fact that the car can't move forward would over power the centre diff, and as the torque has to go somewhere, it's able to turn the front wheels.
When I say over power the centre diff, I mean the diff would try to send torque the the rear but wouldn't be able to, as it's not a fixed mechanical connection.

The viscous diff has to allow some difference in axle speeds for cornering so it doesn't stiffen up inside unless there is a big difference.

I can remember a couple of times when in my brother's old Evo6, we got stuck. Once on a slight incline in snow, and the other on a steep incline on wet muddy grass. On both occasions one of the front wheels spun and we were going nowhere (fat low profile tyres are useless on these surfaces). A bit of man power to get us rolling and it was fine.
It's very different in my car, mine has DCCD so I can lock the centre diff meaning torque goes to all four wheels mechanically. I have never got stuck in snow.