john banks

Only had the Scooby for 7 days 700 miles and until today I would not have known it was AWD (being a fairly granny-mode driver coming from much lesser cars).

Anyway, is my interpretation of a wet roundabout experience this am correct (BTW I am booking some skid/wet driving lessons soon):

Town roundabout (v. wet, normal geometry, OEM tyres), at whatever speed 2500rpm (UK MY00) is in 2nd gear (not very fast at all for the bend - you could go through at 40mph in the dry I suspect) about half way around the roundabout, about 0.5sec of understeer - maybe I had adjusted the throttle *EVER* so slightly (ie more) and it is sensitive at that point as the turbo is spooling up (and had just tried SUL - more responsiveness at <3000rpm & smoother). I didn't do *ANYTHING* and it sorted itself (I didn't back off -deliberately- and I didn't adjust the steering). Did I induce a little understeer with the throttle and the AWD system then removed it by putting more drive to the rear wheels or were the tyres just regaining grip after a torque "packet" in the wet?

I know I can get Prodrive setting and Toyos to stop some of the understeer, but at the moment I am obviously not finding it excessive still running in and the car's limits are MUCH higher than mine. Bumpsteer removal sounds a bit dodgy for people like me and the wife who I suspect would be more likely to get into lift off oversteer problems - at least until we have had some driver training.

Maybe a silly question, but I ask so I can understand the car's handling - can the centre and rear diffs effectively stop small amounts of understeer by redistributing the power or once you've started to understeer is that it?

RichieC

Which centre Viscous Coupling would that be then??...I wish

Richie

Diablo

Er... not sure if its viscous, but that would be the differential that sends more torque to the rear wheels of my UK scoob when the fronts loose traction if I provoke initial understeer by booting it out of wet corners, roundabouts, etc

D

Mr555

My scientific explanation, maybe nonsense, but it makes sense to me -

There are two sorts of understeer, one where the tyres are at such an angle compared to the speed of the car that they can't grip, and the front runs wide or at worst drives straight on. This can happen when firing fast into a corner, and will sort itself out when the car's speed drops to an amount that the tyres can cope with. You'll feel heavy rubbing through the wheel, and maybe even hear a sound like furniture being dragged across the floorboards if its wet, as the tyres won't squeal. In this case, it's the loss of speed created by the rubbing of the tyres on the road that sorts out your understeer. When the grip comes back, you'll feel a tug at the wheel, but the wheel itself may well feel quite heavy through the whole ordeal.

The other kind is when you induce wheelspin on a corner because the combination of tyre angle and power is more than the tyre's available grip can handle. The steering will feel lighter than plough on speed induced understeer, and the whole car will feel like it's curving slightly away from you and off the road. In this case, the Scooby's centre viscous coupling will sense that the combined speed of the front wheels is more than the back, the little vanes in the coupling itself will start to turn at different speeds in relation to each other, chopping their way through the viscous liquid which then heats and congeals, thus locking up the coupling and effectively equalising the power between front and rear. The same liquid cools quickly to free up the coupling once its trauma is over.

Sounds like you invoked the latter version, and the coupling sorted it out for you.

Try this next time you can find a large iced up carpark, free of other cars -

Drive round it in a rectangle, when you try to turn at each corner, the front wheels will spin a little, then the power will switch to the rear, sending you round the corner as you originally intended. All at very low and controllable speed.

HunterB

You need to get a grown-up Subaru - Legacy RS-B or GT-B - before you get a viscous coupling in the centre (and one at the back .... )

MorayMackenzie

HunterB,

I assume you are joking... ?

IIRC, even "humble" uk turbos have a viscious limited slip device in both the centre and rear differential units.

On that subject... don't put Silkolene Pro SRG75 into your scooby... it's simply not up to the job.

Moray

HunterB

Moray,

Happy to bow to your superior knowledge of Imprezas. However, I did read somewhere that Imps didn't have the centre limited slip diff. Always pleased to be (re)educated though

I have to say, though, that my old Impreza (one of the first MY94 turbos) didn't handle in snow like my Legacy does. Maybe things have changed in the interim, of course.

Brian

SDB

Hi jon

Welcome to the fold

It might be worth you checking out

DavidLewis

Simon,

Once over-steering is envoked, what causes the excessive understeering if you over compensate and the eventual 'fish-tailing' experienced by some mortals (ie me!!)
Is a firm press of the 'go' pedal the correct action to rectify 'fish-tailing'?

SDB

Hi David

I assume you mean the big snap pendulum effect that can happen if you oversteer big time and then over-correct?

If so this is caused by the balance and weight of the car...

It is a deep subject but basically...

You get sideways (lets say pointing right - so viewed from above the car rotates clockwise)...

The rear of the car has lots of built up energy which is making it go faster than the front...

The rear left tyre is doing lots of work to slow this down as it scrubs across the road sideways...

You then wind on oposite lock (so steering to the left) until the point where the front wheels are just travelling in a normal fasion down the road. They are basically pointing in the direction of travel...

The rears are scrubbing along and the amount of energy of in the rear of the car is slowly coming off as the tyre acts against it...

The problem is, the energy will not disperse in an even slow way, it will drop off gradually at first and then more and more rapidly as the tyre has more of an effect over it.

Suddenly the tyre has the upper hand and can agressively reduce the speed of the rear of the car and it digs in...

The front wheels are still pointing down the road so they offer no resistance at all and basically carry on at the speed they were at (there abouts). Suddenly you have a car with a rear end that has all but stopped, and a front end which is happily zipping along...

This means that the front end rotates briefly around the rears (viewed from above the car rotate anti-clockwise). This motion is normally VERY sudden.

Now comes the complex bit which I will simplify for the benefit of my fingers and your eyes!!

The left rear suspension was almost fully compressed and the weight of the rear end was being pushed towards it...

suddenly the whole thing has swapped and the weight of the rear isstarting to go in the other direction... This is helped by the springs, so the energy increases, which is then helped even more by the springs, etc, etc....

This then results in a sudden MASS of energy thrown in the oposite direction.

It is often the case that if you have got to this stage it is too late, and you are into damage limitation mode.

Cheers

Simon

john banks

Thanks for the replies. I've already studied drivingtechniques in some detail over the last few months - great site Simon

I assume the centre diff "saved" me then (it was only a little twitch really. I have been paranoid for some time about lift-off oversteer and I can see how weight transfer to the front from lifting off and then reducing the slip angle could result in a big mess.

Ideally, no one wants to lift off in any bend (let alone brake for that matter) but if you are well below the limits of the car and lifted off sharp(ishly) or braked on a wet bend could disaster still ensue or is it only on the limit? - I know I really need to practice these things in controlled conditions - lessons booked

Any thoughts on this? - I know the overall point is to drive so that you can stop in a distance you can see to be clear - realistically I would be driving along all country roads at <40mph if this was the case - surely some compromise. The point is, unexpected things do happen and you need to be able to cope with a sudden "emergency" as I know to my cost after an unfortunate tractor-Ford Focus incident earlier in the year

DavidLewis

Yep, thats the one, but sorry Simon, I dont understand, could you explain that again

Only ever happened once, my first trackday, never happened since so I must have sub-consiously learnt something (or now just drive like an old women)

Thanks for the explaination