Oversteer stuff
07-03-2008, 10:22 AM
#31
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Jean,
I watched the Protomotive movie clip and the first thing that came to mind was the possibility of the outsede edge of the left rear tyre touching the kerb. Most turbo race cars run significant rear toe-in, which is good because it makes the car more stable under normal acceleration, however each tyre does have a small component force that attempts to push the wheel into the body. With high torque engines, this component force becomes more significant when one wheel is unloaded suddenly and I am guessing that it is this sideways component of the force which is causing the snap oversteer.
Food for thought?
I watched the Protomotive movie clip and the first thing that came to mind was the possibility of the outsede edge of the left rear tyre touching the kerb. Most turbo race cars run significant rear toe-in, which is good because it makes the car more stable under normal acceleration, however each tyre does have a small component force that attempts to push the wheel into the body. With high torque engines, this component force becomes more significant when one wheel is unloaded suddenly and I am guessing that it is this sideways component of the force which is causing the snap oversteer.
Food for thought?
07-03-2008, 02:49 PM
#33
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I just think it was a dumb driving error. I never usually have this problem there, it was clearly lack of focus and a mistake, but the point is that you cannot joke around when you have too much power, I was a couple of meters early on the throttle, at that point I am seeing almost 1 G lateral, there is nothing left for traction in the Sport Cups,and I have 2wd and no LSD.
See marker reference below, I am usually on thottle just a bit later, I was simply too confident IMO.
See marker reference below, I am usually on thottle just a bit later, I was simply too confident IMO.
07-03-2008, 06:24 PM
#34
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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 ?
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 ?
even though you didn't feel it.With two wheel drive you'd be in the ditch *** end first.
Jimmy
07-03-2008, 09:58 PM
#35
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Rally Drivers - the REAL heros...
OT = Anders - that is some impressive stuff - and not just from a driving perspective... There is NO way CAMS (our governing motorsport body in AU) would EVER let an event like that go on.... using parked cars and 44 Gallon drums and light poles as course markers.... Holy CRAP!!
Jean - the Pikes Peak Vid is one of the THE best motorsport films EVER....
TB - as a driving instructor in Austria (yes not AUSTRALIA) either steer faster - or drive slower....
Simon.
OT = Anders - that is some impressive stuff - and not just from a driving perspective... There is NO way CAMS (our governing motorsport body in AU) would EVER let an event like that go on.... using parked cars and 44 Gallon drums and light poles as course markers.... Holy CRAP!!
Jean - the Pikes Peak Vid is one of the THE best motorsport films EVER....
TB - as a driving instructor in Austria (yes not AUSTRALIA) either steer faster - or drive slower....
Simon.
Last edited by Full Boost; 07-04-2008 at 02:48 AM.
07-04-2008, 10:00 AM
#36
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When the 993tt was new all the press stated that between 5% and 15% of torque was constantly transmitted to the front wheels by virtue of the tyre diameter differential the front wheels rotate slightly slower than the rears maintaining the visco at the required heat level to allow this 5%-15% torque - Paul Frere further states that spinning rear wheels result in a torque transfer of approx 40%the the front which can increase to almost 100% if the rear wheels are on low grip and front high grip.
Adrain Streather in his book refutes this and reckons there is zero torque to the front wheels under normal operation and that there is a delay once the rear wheels start spinning before the fronts get any torque he maintains our cars are rear wheel drive until there is rear wheel spin...
I'm not sure who to believe but lean towards Frere and the journos at the launch who all said the same thing....
During my oversteer moment described in the first post the steering definately felt quite a lot heavier as I applied a slight corrective, could this be the front wheel drive effect loading up the steering effort ?
Any thoughts on this ?
07-04-2008, 11:39 AM
#37
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TB993tt,
The acknowledged creator of the Viscous Coupler is FFD or Ferguson Formula Developments. Based in your homeland. FFD's founder Tony Rolt and Freddie Dixon did a lot of early work with real time AWD using a VC as a center diff. For reference the 993tt has no center diff so it can not be considered a true AWD. The system in a 993tt is an on demand system. I was fortunate enough to work for FFD’s US division for 7 years.
If we look at the VC's function on it's own (not as it relates to a tt) it needs to have a differential in speed between the input and output before it will function. In a normal steady state where the drive gear ratios are the same as well as tire diameters the VC is turning at the same RPM as everything else. Per the quote above if the tire diameters in the front (hence overall ratio) is different than the rear then that would cause the VC to be transferring torque. But as referenced earlier in this post it is done by the shearing of the fluid not the heating of the fluid. And yes Mr. Frere is correct, at least in theory that if the rear tires loose 100% of the grip all torque could transfer to the front. I think though in function the 993tt would not be able to sustain that for very long.
Adrain Streather’s statement is correct in how the system functions. As for feeling some load in the steering that is probably feedback from torque being applied and that load wanting to pull the wheel to point straight forward via the castor and scrub radius. In front wheel drive terms torque steer.
Rick
The acknowledged creator of the Viscous Coupler is FFD or Ferguson Formula Developments. Based in your homeland. FFD's founder Tony Rolt and Freddie Dixon did a lot of early work with real time AWD using a VC as a center diff. For reference the 993tt has no center diff so it can not be considered a true AWD. The system in a 993tt is an on demand system. I was fortunate enough to work for FFD’s US division for 7 years.
If we look at the VC's function on it's own (not as it relates to a tt) it needs to have a differential in speed between the input and output before it will function. In a normal steady state where the drive gear ratios are the same as well as tire diameters the VC is turning at the same RPM as everything else. Per the quote above if the tire diameters in the front (hence overall ratio) is different than the rear then that would cause the VC to be transferring torque. But as referenced earlier in this post it is done by the shearing of the fluid not the heating of the fluid. And yes Mr. Frere is correct, at least in theory that if the rear tires loose 100% of the grip all torque could transfer to the front. I think though in function the 993tt would not be able to sustain that for very long.
Adrain Streather’s statement is correct in how the system functions. As for feeling some load in the steering that is probably feedback from torque being applied and that load wanting to pull the wheel to point straight forward via the castor and scrub radius. In front wheel drive terms torque steer.
Rick
07-04-2008, 12:04 PM
#38
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Can you explain exactly what this means since I thought I'd read somewhere that it was at a certain temperature threshold that the VC "stiffened up" and the torque transfer was initiated - is this not the case ?
07-04-2008, 05:45 PM
#39
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TB993tt
I have not been mentioning heat as part of a VC function to try and keep focus on the fact that a VC can start to transfer some torque in less that a full revolution. If you think about it with our cars seldom will you get prolonged wheel spin unless you are on some ice or drive very hard in the wet. Heat is created by the friction caused by the VC plates moving through the silicon fluid. This heat causes the fluid to thicken and in turn makes it harder for the plates to move through it (shear) and therefore transmits more torque.
You are correct in what you read right up to "and the torque transfer was initiated". Specifically that torque transfer starts as soon as there is a speed differential across the VC. It takes a bit of stirring the pot before heat enters the picture. By then traction may well be restorted to all 4 corners. Of course the design of a VC can vary so greatly that it's operation can encompass everything for instant solid to aways very soft.
Rick
I have not been mentioning heat as part of a VC function to try and keep focus on the fact that a VC can start to transfer some torque in less that a full revolution. If you think about it with our cars seldom will you get prolonged wheel spin unless you are on some ice or drive very hard in the wet. Heat is created by the friction caused by the VC plates moving through the silicon fluid. This heat causes the fluid to thicken and in turn makes it harder for the plates to move through it (shear) and therefore transmits more torque.
You are correct in what you read right up to "and the torque transfer was initiated". Specifically that torque transfer starts as soon as there is a speed differential across the VC. It takes a bit of stirring the pot before heat enters the picture. By then traction may well be restorted to all 4 corners. Of course the design of a VC can vary so greatly that it's operation can encompass everything for instant solid to aways very soft.
Rick
07-04-2008, 09:00 PM
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Just following the youtube links gave me a nice trip down memory lane - any self respecting UK 18 year old petrol head in the early 80's owned a MK2 Escort (just like the rally winning cars) in those days.... Mine was red like the twin headlight ones in the vid below, RS2000 Pinto engine 2 litre SOHC with Kent RL1 cam, Weber 38DGAS twin choke carb feeding a stage 2 head with Rimflow valves and Janspeed extractor manifold all good for a stonking ~110hp with 960kg (2116lbs)
Scheel seats, Pinto 2L, rusty plenum which perodically dumped water on my feet
etc etc etc ....damn....now I am going to have to go rumaging for old photos
07-04-2008, 09:09 PM
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Like most 993 TT owners, I have questioned the efffectiveness of the 4wd system at times. Is it actually working ??
I realised how brilliant it is .....when..... I had it disconnected for a week.
I entered a corner, exactly as TB993TT described. As the modified [laggy] K24's came on boost the car tried to turn itself around - into the apex of the corner.
A little bit of power to the front really does keep it straight.
Good fluid, bad fluid, worn plates....... If you are pointing straight ahead as you punch it out of an apex, your 4wd is working.
I realised how brilliant it is .....when..... I had it disconnected for a week.
I entered a corner, exactly as TB993TT described. As the modified [laggy] K24's came on boost the car tried to turn itself around - into the apex of the corner.
A little bit of power to the front really does keep it straight.
Good fluid, bad fluid, worn plates....... If you are pointing straight ahead as you punch it out of an apex, your 4wd is working.
07-04-2008, 11:08 PM
#42
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Like most 993 TT owners, I have questioned the efffectiveness of the 4wd system at times. Is it actually working ??
I realised how brilliant it is .....when..... I had it disconnected for a week.
I entered a corner, exactly as TB993TT described. As the modified [laggy] K24's came on boost the car tried to turn itself around - into the apex of the corner.
A little bit of power to the front really does keep it straight.
Good fluid, bad fluid, worn plates....... If you are pointing straight ahead as you punch it out of an apex, your 4wd is working.
I realised how brilliant it is .....when..... I had it disconnected for a week.
I entered a corner, exactly as TB993TT described. As the modified [laggy] K24's came on boost the car tried to turn itself around - into the apex of the corner.
A little bit of power to the front really does keep it straight.
Good fluid, bad fluid, worn plates....... If you are pointing straight ahead as you punch it out of an apex, your 4wd is working.
07-05-2008, 02:35 AM
#44
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If the awd wasn't working, most of the tuned cars on this thread would be crashed.
I always wondered if mine awd was coming into play, then I got into turn one at Willow too hot and jamed on the power (counter intuitive) and it pulled me though. After the session anothe driver came up to me and said, I wish I had a picture of you going through turn one with all the tires smoking. It was at that point I become a true believer.
I always wondered if mine awd was coming into play, then I got into turn one at Willow too hot and jamed on the power (counter intuitive) and it pulled me though. After the session anothe driver came up to me and said, I wish I had a picture of you going through turn one with all the tires smoking. It was at that point I become a true believer.
07-05-2008, 04:23 AM
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HI Guys, I have just come back from competing in a tarmac rally and the road was consistently wet. We were running Toyo R888's and the stages were only around 3 - 8km's long so the tyres were barely getting up to temp before the stage was finished.......I got a little sideways under really heavy braking but NEVER under acceleration.......we lit up all four wheels at the start of a couple of stages during take-off but we were able to go into corners and power out under full power every time.......AWD was working.......thank god!