Had a large adrenalin moment today which I will say shouldn't have happened on a public road and I do not condone that type of driving.....

Coming slowly around a fairly tight radius bend in second gear and using full acceleration on a bone dry good grip tarmac surface caused a type of oversteer which I am not really too familiar with.

It felt like the LSD is spinning the rear wheels together and the torque is rotating the car, I basically steered slightly straighter (not a wild stab into it, it wasn't that sort of violent " rear end kick out" which you get in the wet) and let off the throttle but didn't violently reduce the throttle - I was amazed that the car just came back into line without any kick back the other way or anything else untoward....

I am thinking the high grip dry surface coupled with the gradual (relative) release of the throttle really saved my bacon here so the car didn't use the momentum to whip out the other way.

Could experienced high hp racers comment on the dynamics of what happened here - I obviously got the correction right but it scared the **** out of me !

How would a non LSD car react under these circumstances ? would it be safer ? would the non LSD car just spin away the excess torque (remember the car was squatting at an angle under power and didn't really have any bulit up sideways component force as I was only going 30mph when I booted it) and the car stay straight ?

WTF was the 4WD doing ? anything ? nothing ? everything ? is the 4WD visco fast enough to react and would it have been pulling the front therebye helping stabilise the rear ?

 

Sounds like your LSD was doing its job

 

Slight tire pressure adjustments may also assist you.

 

I actually experience that a fair amount at the track - a little too much torque when mid apex and the rear will start to come around. Most of the time you need to stay on the throttle and keep loading the rear wheels but AWD seems to pull you thru if you gently ease of the throttle slightly

 

Toby, what LSD do you have in your car? What is the locking rate? By your description, it doesn't sound unusual, especially in 2nd gear. When the car begins to slide, if you catch it quick enough and let up on the throttle to regain rear traction, the car will come back with steering correction, just like any other car.

 

Got to think that if the VC was in good condition it should have been distributing some of that torque forward. With the rears spinning, hence a differential in speed between the front and rear axles, the very condition that causes a working VC to transfer torque you should of had the front pulling to at least help reduce the angle it was stepping out if not almost eliminating it. Just wondering if your VC isn't a bit toasted. Mileage varies and if you don't know the history of your car there is that slim chance someone did the odd size tire thing and help speed the VC's wear. One of the reasons my car went 2WD was a toasted VC at 38K miles. Seamed a better deal than the cost of a new VC.

My car reacts the same as you discribed, not too hard to live with. I have found that both methods mentioned above work within the circumstances. Keep the power on (with moderation) to keep the rear end planted or tip out of it to reduce the spinning tires, which again with moderation. As we all know too much getting out of it can cause some snap of the rear end which may not be what you wanted at the moment.
Rick

 

Toby,

It is my understanding (and experience) of how the 4WD system works is that what you have experienced is normal.

The way the viscisious coupler works is that ONLY when there is a speed differential between the front and the rear drive, the fluid then heats up and become thicker and then begins to transfer some drive forward. There is no mechanical connection between the front and rear (like say a Subaru 4WD) and therefore there can also be some "lag" as the fluid heats up. This will of course also depend on the conditon of your VC...

Sounds like because (a) you run correct diameter tyres and (b) that you dont normally drive like that (as opposd to say me.... ) that you were driving along and using all of the stock 400HP through the rears (and hence not heating the fluid up) and as you rounded the corner and opened her up that as the K16's came on full song and the TQ overwhelmed the grip levels (I know you say it was a good grippy corner - but....) and the LSD locked up (as it is supposed to do) and began to drive both rears at the same speed.

As you had lock applied that is why it kicked sideways (and even if the VC was warm with your HP/TQ levels i doubt it would have made a difference) your fluid was cold - and hence the slide.

Quick steering inputs, an EASE (as opposed to full off) of the throttle from you and slow (relative) speed is why the car didnt snap back the other way. As TQ tapers off grip will be re - established and the car will unlock the LSD and then continue as it should.

I wouldnt be too concerned - and seriously doubt any mechanical failings with your car and am seriously surprised that you havent experienced this type of behavior before.....

A non (or less agressive) LSD car - depending on grip vs Tq levels would act differently- but is a moot point in this case - due to the VC and the 4Wd system along with the HP levels we are talking about.

Hope that helps,

Simon.

 

Even though heat does play a factor in VC function it is actually the shearing of the fluid between the VC plates that transfer the torque.

The way we used to explain it to customers was think of a stir stick in a gallon of paint. If you stir slow enough the can will not spin but as you increase the speed of the stir the can will match the stir sticks rpm. Factors in VC function are the plate slots, number of plate and viscousity of the fluid. One of the major areas heat comes into play, especially in motorsport units is when they slip enough to "hump" or actually lock solid when the plates expand enough to contact each other. The plates are purposely not deburred when machined to give a good shear edge. The wearing of these edges are one of the reasons VC's go "soft".

For those that care the DOW silicon fluid that is commonly used is the same stuff filling those happy bags you are looking at just above the staple fold.
Rick

 

Many thanks for the replies

Geoffrey, my LSD is 45/65

Eclou, the 997tt 4WD is instantaneous compared to the workings of the 993tt visco system, great explanation by Simon and Rick. Do we have a definative way of testing how in spec the visco system is ?

Simon, I have always enjoyed some "**** end out" antics but this one was slightly different. I think it was because usually I hang the tail out using the unweighting (of the rear wheels) method to get it at the point of sliding then power it through with the appropriate lock being dialled in, sometimes a "stab" and sometimes a glorious bit of proper sideways hooliganism...

This one was different because the rear wheels were already fully loaded and had a lot of grip and the diff obviously carried on giving the grip and (without asking for a p!ssing contest) my car has a lot of torque which wasn't being spun away but being deployed in the business of rotation !

For some perspective I found that the Carrera GT could powerslide quite nicely round this bend in second (at night with zero traffic) with the TC flickering away and was nothing like as fierce and intimidating as the tt....

 

It's the first time i've heard of a fluid getting thicker when warm and thinner when cold. What is the fluid in the VC?

 

Is VC oil/fluid ever changed as part of scheduled or preventative maintenance?

 

Well, your engine oil does this every day...right? Hence a 20W50. It is a "50" when hot/warm and then thins to a "20" when cooled/cold.

 

The VC Oil is the standard gear oil in the transmission, it is not separate.

Toby, what suspension do you have in your car? What you have explained seems normal to me.

 

Toby,
If you think about the "friction circle" of the rear tyres, what appears to have happened is that the combined force vector of the sideways G cornering load and forward acceleration load have exceeded the level of grip that your tyres have; the LSD appears to have done it's job properly and transferred torque to the outside wheel, whereupon both tyres have simultaneously let go at the same time. A non-LSD car would have just lit up the inside tyre.

From what I remember of the 4wd set up the response speed of the centre Torsen diff is pretty much instant but it has a torque limitation of 30% of the available drive. Given the high torque engine you are running, there will still be instances where the reduced 70% of torque transmitted to the rear tyre is still sufficient to exceed the maximum grip (i.e. combined long/lat force vector) of the tyre, maybe this was just an instance of this occuring?

The classic 911 driver's response to sudden oversteer is to let go of the steering wheel - the castor of the front end will point the front tyres in the direction the car is going nomatter what angle the rear end is at, all you need to do then is to re-grip the wheel at the appropriate time to fine tune the exit point of the bend. An afternoon's session with Don Palmer at Brunter's will confirm this if you are sure and I can confirm that this helped me to get my head around it a few years ago.....

 

This is like "using the Force"....