One thing that I love about the impreza is that by booting it out of a low speed corner in the wet, you can get the rear out.

The thing I really don't quite understand is why? Weight is on the back (acceleration), and if there's enough power to break traction, surely that would be at the front resulting in understeer where there's more power (torque split) and less traction (weight transfer).

The only thing that I can think of is that the back end has the same "angular velocity" so is already going sideways faster to make the corner, therefore has some sideways momentum making it easier to break traction with power...

Gives me a grin every time, but got me thinking "how does it work?" this morning!

Any ideas? I'm sure I'll kick myself...

 

Que Simon.........

 

I got loads of oversteer in the snow the other month and got thinking too. I was wondering if (on a UK std turbo) it's because of the viscous coupling at the back. i.e inside back wheel starts spinning, coupling transfers some torque to outside wheel effectively making back axle like a solid axle, hence both wheels spin (aided by snow), hence no grip, hence big grin oversteer.

But then what would be happening at the front? Well, I reckon the centre viscous would try to trasnfer some torque to the front, but the open diff on the front axle just spins one of the front wheels, hence no real benefit.

When playing on some black ice a few months ago, I remember being somewhat disappointed when flooring it in a straight line seemed to mostly result in one wildly spinning front wheel. Guess there was some push from the back, but not that much.

So why is there not a viscous coupling at the front, too...!?]

 

GEJL,

Dont start this again. LOL

Have a look at.

 

Lee - wasn't intending to start this again Let's put it this way: Take a hypothetical four-wheel drive car with a viscous coupling at the back and also one in the centre, and an open diff at the front.... Would it behave like the above? I'm just wondering if this would explain the oversteer...

 

CEGL,

Could be as only one wheel at the front would be driving or spinning and the other would be trying to turn the car while both rear wheels would be driving or spinning(little grip rear)causing oversteer.

This is probably why the 22B with its more agressive rear diff has a tendency to oversteer more, although the setting on the adjustable centre diff on the 22B will have a large bearing on the situation.

After all that I'm no help anyway Sorry.

Lee

 

OOoooh... this is a biggy!

OK... some background data / info....

Tyres.. (simplistic form, so *I* can understand it!)
Tyres can produce forward / backward force (traction) and lateral / turning force (grip).
If you demand more grip from a tyre it will not be able to produce as much traction, and vice versa..

So...
Spinning tyres cannot produce as much lateral force as rolling ones.

Now..
When you spin a tyre it produces an amount of forward force.

so..
If you have your front wheels pointing right and your back wheels pointing straight (as one would expect!! ), and spin all four (mid-bend) wheels, the following will probably happen...

Rear tyres discard almost all lateral grip due to the demand caused by the spinning (the traction has all been used up), so will be producing an amount of forward force, but very little lateral force (so it is easier for the rear wheels to follow the direction they are travelling in (towards the outside of the bend)...

The fronts are also spinning, but are pointing right a bit... the forward motion produced by the tyre moves the front of the car towards the right (a bit), whilst the rear of the car is happily travelling towards the left (outside of the bend)...

hence oversteer...


now this is simplistic.. BIG TIME... but is generally true...

The fact is, diffs, tyres, power, torque, torque split, etc, etc, etc all play a big part in what happens...

There are also many many other reasons why the car would oversteer under power, but I think the above is what you were referring to?

Cheers

Simon

 

Cheers Simon, that's what I was getting at. It just got me thinking, "shouldn't I be getting terminal understeer if the front wheels are spinning?"

It's at the end of the corner when the front is almost straight that I normally "boot it" so that would explain it.

A little knowledge is a dangerous thing, especially in the hands of a muppet...

Nick.

 

It's not a Scoob but I have a Granada Scorpio for towing the Rallycar. It's a 4x4 which i know has an LSD at the rear, viscous central diff and a "free" front diff. I broke a driveshaft the other day (front o/s) and the car got me about a mile still with drive, but then it was gradually losing drive... would this be the centre diff overheating the fluid?

Owain.

 

Nope, don't want understeer thanks, but just couldn't understand why the car was doing what it was doing, especially being 4wd!

 

Hehe.....well done Simon I must try to remember all that mid turn :

 

If you want understeer?

Bridgestone RE01s on the rear, Falken 502s on the front, slightly damp surface.

Perfect combination.

By the way before anyone tells me, I know I shouldn't mix tyres like that but I got the Falkens free, they are well named the falken things. They will not be on much longer!!!