Traction vs 4wd

[daveyb662]

Amateur question here...Can somebody clarify for me, is there any way of knowing when the 4wd is kicking in? Traction (psm) is indicated when the light flashes, and I believe is there to keep you on the road... but how does this differ exactly from 4wd? which I understand pushes drive to all 4 wheels when needed...

[Sunnysidebb]

daveyb662 said:

Amateur question here...Can somebody clarify for me, is there any way of knowing when the 4wd is kicking in? Traction (psm) is indicated when the light flashes, and I believe is there to keep you on the road... but how does this differ exactly from 4wd? which I understand pushes drive to all 4 wheels when needed...

Hi Davey.
The 996 4wd system is permanently engaged. There is a slight wheel speed difference between the front and rear which ensures that the centre VC is always loaded with a minimum of 5% hence giving a minimum of 5% torque to the front wheels raising to a max of 40% the faster you go.
Traction control is a computer aided system that uses the same components as ABS to monitor wheel spin by applying the brakes to the spinning wheel transferring torque to the non spinning wheel. It also uses throttle intervention to reduce engine power if the car still spins a wheel. Thats the simple explanation but thats how it works.

Edited by Sunnysidebb on Sunday 18th December 19:50

[monthefish]

Thanks Frank - useful info.

Sunnysidebb said:

The 996 4wd system is permanently engaged. There is a slight wheel speed difference between the front and rear which ensures that the centre VC is always loaded with a minimum of 5% hence giving a minimum of 5% torque to the front wheels raising to a max of 40% the faster you go.

Can you tell me more about this - i.e. is the drive to the front wheels dictated by the speed of the car, or what is happening at the wheels, and how does that 'fit in' with what the TC system is trying to do?

[willfinch36]

What about if you have PSM off? How does this affect the ratio(s) etc?

[Sunnysidebb]

monthefish said:

Thanks Frank - useful info.

Sunnysidebb said:

The 996 4wd system is permanently engaged. There is a slight wheel speed difference between the front and rear which ensures that the centre VC is always loaded with a minimum of 5% hence giving a minimum of 5% torque to the front wheels raising to a max of 40% the faster you go.

Can you tell me more about this - i.e. is the drive to the front wheels dictated by the speed of the car, or what is happening at the wheels, and how does that 'fit in' with what the TC system is trying to do?

No problem.
Keeping to basics....
The 996 4x4 system is basically permanent hard drive rear wheel drive. By which I mean there are cogs turning the rear diff. The front diff is not directly connected to the gear box. The torque transmits through a Viscous coupling VC( http://en.wikipedia.org/wiki/Viscous_coupling_unit ). The rear wheels are allowed to turn slightly faster than the front hence the input speed of the VC is faster than the output hence there is a torque movement turning the front diff. the ration never changes and is dictated by both diff ratios. As you travel faster then the amount of speed difference between the two increases which increases the torque to the front. See how a VC works.
With TC switched on it monitors the amount of wheel spin on any of the 4 wheels , if it sees one spinning it applies the brakes to that wheel alone slowing it down. if the wheel or any more wheels spin and the brakes can not slow the spinning down then the DME will intervene with the power delivery of the engine to ensure wheel spin stops. This is why when driving in snow its best to turn Off the TC ( PSM )
With TC off the wheels are allowed to spin without psm intervening with power delivery.
Note: PSM does a lot more than just TC it also monitors other parameters but that’s a different topic.lol

The 997 however has a variable centre VC and can be controlled by the DMS. So I suppose it could be used as part of the TC as well. However I haven’t looked into how the 997 works anymore than that.

It should be noted the 911 996 is not like the Cosworths , Evos, Or Subaru's which utilise a centre differential as well as a centre VC hence there is a mechanical connection to both the front and the rear diffs.


Hope that helps.
Frank

Edited by Sunnysidebb on Monday 19th December 20:45

[TB993tt]

Sunnysidebb said:

No problem.
Keeping to basics....
The 996 4x4 system is basically permanent hard drive rear wheel drive. By which I mean there are cogs turning the rear diff. The front diff is not directly connected to the gear box. The torque transmits through a Viscous coupling VC( http://en.wikipedia.org/wiki/Viscous_coupling_unit ). The rear wheels are allowed to turn slightly faster than the front hence the input speed of the VC is faster than the output hence there is a torque movement turning the front diff. the ration never changes and is dictated by both diff ratios. As you travel faster then the amount of speed difference between the two increases which increases the torque to the front. See how a VC works.

Is this correct ?

On the 993tt the difference in speed between the input and output in the VC is provided by slightly smaller diameter rear wheel/tyre combo (compared to front) so the VC fluid is kept warm by this speed differential and the 5% torque is provided by the solidity of the fluid at this temperature. As the road speed rises the speed differential does not change (does it ?) so the same 5% is there at 100mph as at 20mph
When there is wheelspin at the rear the VC fluid rapidly heats up due to the differential input/output shaft speeds and the fluid becomes less viscous (ie hardens) transmitting more torque to the front, the amount depending on how much wheelspin there is so how hot the fluid gets ?
This is how I understood the 993 system ( a Subaru VC) to work and I thought the 996tt was the same principle ?

[6C4GTS]

TB993tt said:

Is this correct ?

On the 993tt the difference in speed between the input and output in the VC is provided by slightly smaller diameter rear wheel/tyre combo (compared to front) so the VC fluid is kept warm by this speed differential and the 5% torque is provided by the solidity of the fluid at this temperature. As the road speed rises the speed differential does not change (does it ?) so the same 5% is there at 100mph as at 20mph
When there is wheelspin at the rear the VC fluid rapidly heats up due to the differential input/output shaft speeds and the fluid becomes less viscous (ie hardens) transmitting more torque to the front, the amount depending on how much wheelspin there is so how hot the fluid gets ?
This is how I understood the 993 system ( a Subaru VC) to work and I thought the 996tt was the same principle ?

As discussed previously the 993 and 996 systems are technically very different.
See here

[Sunnysidebb]

TB993tt said:

Sunnysidebb said:

No problem.
Keeping to basics....
The 996 4x4 system is basically permanent hard drive rear wheel drive. By which I mean there are cogs turning the rear diff. The front diff is not directly connected to the gear box. The torque transmits through a Viscous coupling VC( http://en.wikipedia.org/wiki/Viscous_coupling_unit ). The rear wheels are allowed to turn slightly faster than the front hence the input speed of the VC is faster than the output hence there is a torque movement turning the front diff. the ration never changes and is dictated by both diff ratios. As you travel faster then the amount of speed difference between the two increases which increases the torque to the front. See how a VC works.

Is this correct ?

On the 993tt the difference in speed between the input and output in the VC is provided by slightly smaller diameter rear wheel/tyre combo (compared to front) so the VC fluid is kept warm by this speed differential and the 5% torque is provided by the solidity of the fluid at this temperature. As the road speed rises the speed differential does not change (does it ?) so the same 5% is there at 100mph as at 20mph
When there is wheelspin at the rear the VC fluid rapidly heats up due to the differential input/output shaft speeds and the fluid becomes less viscous (ie hardens) transmitting more torque to the front, the amount depending on how much wheelspin there is so how hot the fluid gets ?
This is how I understood the 993 system ( a Subaru VC) to work and I thought the 996tt was the same principle ?

Hi Toby,

What I have written has to be taken into context with the hyper link on how a VC works.
But you don’t have to have wheel spin at the rear for torque to be sent to the front. Its why I linked the wiki about it because in its simplicity its quite complexed. You are right when you say the more torque is transmitted to the front if the rears spin, but the intention of Porsche was to give the 911 better characteristics at speed as well. The 5% I mentioned is minimum torque transfer when driving the car not 5% speed difference sorry if it came across that way . The max is 40% IIRC.
This is what the Porsche Service Information Tecknic has to say about it.....its not much.lol

Edited by Sunnysidebb on Monday 19th December 21:42

Edited by Sunnysidebb on Monday 19th December 21:43

[scarebus]

How come I have never seen the front wheels of a 4x4 911 spin up on any youtube snow vids??? Does it actually send any useful torque to the front wheels?

[TB993tt]

6C4GTS said:

As discussed previously the 993 and 996 systems are technically very different.
See here

Ah yes, I forgot that thread, I got it wrong for the 996tt then and once again now - must try harder