996GT2 diff ramps? What are the Cup Ramps

[fioran0]

Steve Rance said:

Hi John

The diff fitted to modern 911 GT cars is very well designed and works very well but requires a certain style to get the most out of it. It locks under braking and accelleration. in both of these situations it is fantastic and really helps to settle the car. Where it is horrible is under a trail throttle, ie; the no man's land between the two. I suspect that this is where you are experiencing problems? The car will feel a lot better if you tag the brakes until you've done all of your turning. This will key the nose of the car nicely into the tarmac, stop your understeer and the diff will keep everything nicely settled while you do it. Just be mindful that your braking imputs need to be very smooth. If you start changing the ramping you could end up with a horribly unstable car under braking. The stability of the GT 911's under braking despite of it's unique chasis layout is rarely mentioned by journos but in reality the working of the diff is absolutely crucial to the car's performance as it provides a beautifully stable braking and accellerating platform. This says a lot about the quality of the engineering of the 911 and - since very few actaully mention it - a lot about some of the jounalists that drive them..

+1

[jfp]

fioran0 said:

jfp said:

I'm convinced a steeper power ramp that resists working I.e locking the diff

Can you clarify what you are meaning here?
The steeper the ramp angle, the less lock up you get. so 20% lock is something like 75 deg ramp angle.
The greater the preload, the less it takes for the diff to lock up.

It reads as though you are misunderstanding how the clutch plate type works. It doesn't work by resisting the urge to lock. You weaken preload to make it more gradual to full lock and increase ramp angle to make the amount of total lock lower.

Edited by fioran0 on Thursday 13th September 11:28

The steeper the ramp, or from 30 degrees to say 50 degrees, means its harder for the ramps to work and hence you get less lock for a given torque. Its literally like psuhing something up a hill. Pre-load resists ramps, I tried 50lb with the cup plates and it made no odds. I also tried zero and that was worse still - nothing to react to the ramps.. I do agree that with a very steep ramp, the diff may never lock.

[jfp]

Steve Rance said:

jfp said:

The current ramps cause full lock up too early in the torque delivery, hence the car pushes on with very modest throttle inputs. Thanks, Jon

Hi John

The diff fitted to modern 911 GT cars is very well designed and works very well but requires a certain style to get the most out of it. It locks under braking and accelleration. in both of these situations it is fantastic and really helps to settle the car. Where it is horrible is under a trail throttle, ie; the no man's land between the two. I suspect that this is where you are experiencing problems? The car will feel a lot better if you tag the brakes until you've done all of your turning. This will key the nose of the car nicely into the tarmac, stop your understeer and the diff will keep everything nicely settled while you do it. Just be mindful that your braking imputs need to be very smooth. If you start changing the ramping you could end up with a horribly unstable car under braking. The stability of the GT 911's under braking despite of it's unique chasis layout is rarely mentioned by journos but in reality the working of the diff is absolutely crucial to the car's performance as it provides a beautifully stable braking and accellerating platform. This says a lot about the quality of the engineering of the 911 and - since very few actaully mention it - a lot about some of the jounalists that drive them..

Steve, agree with what you are saying about 911's. I currently have a 997 GT3, 1980 930 in the garage as well, and they are all broadly similar in how they drive, how, when and what you do with the throttle is more important than say the R35 GTR I also drive. The 930 does'nt have an LSD, the GT3 does, they both drive very similarly to be honest. My (mapped) GT2 is boosting from 1,900 rpm in top gear, and the problem I have is under throttle pick up / apply on the road. The diff is so overpowering it pushes the car on at nowehere near limit handling. The diff I feel, has been dicked about with by the previous owner, and we are suffering from a problem that will be so clear when we find it I will feel a total twit. As for Journos driving, I agree. Having done many years racing my self at a competitive level I understand the difference between cocking about down the pub and winning club / national races, it's a different world.... Top Gear proved that when they pissed everyone off at the 24hr a few years ago..

[DanH]

jfp said:

Does anyone know where I can get steeper than standard diff ramps for my 996GT2? we have measured the power ramp at 30 degrees, gut feel is a 45 degree ramp would be better. Does anyone know what Cup cars use. Cheers, Jon.

If you want a more aggregative diff buy a Guards Transmission one from the US?

[fioran0]

jfp said:

The steeper the ramp, or from 30 degrees to say 50 degrees, means its harder for the ramps to work and hence you get less lock for a given torque. Its literally like psuhing something up a hill. Pre-load resists ramps, I tried 50lb with the cup plates and it made no odds. I also tried zero and that was worse still - nothing to react to the ramps.. I do agree that with a very steep ramp, the diff may never lock.

your understanding of what is going on sounds completely inside out.
preload pushes the plates inside the diff together creating a static locking force at the axle. this acts as a permanent resistance to differential forces from the wheels without the diff yet doing anything mechanically at all.

the greater the preload, the greater the differential force the car needs to generate before the preload is overcome, in english, the more the wheels are locked together when you are just driving around normally.
with zero preload the diff would act like an open diff all the time until a wheel began to slip and the mechanicals inside tried to lock the wheels together.
with huge preload the diff would act like it was just welded shut. the car would never be able to overcome the initial preload and create any differential force. as a result the mechanicals inside the diff would never try to do anything but your wheels would never be able to have any difference.

in your case with 80lbs of preload, you need one wheel to generate a force 80lbs greater than the other wheel before your rear axle begins to overcome preload and the wheels can turn with different forces (unlock).
this is why high preload causes low speed understeer and very acutely, slow turning understeer on the street. the rears cannot rotate differently without a significant initial amount of differential force occurring. preload acts as a yaw damper, the effect of which increases as the preload increases.

the ramps simply add to the force already being created by the preload when you need it.
as soon as the locking action of the plates created by preload is overcome, the ramps start to assist by pressing the plates harder and increasing total force available to resist this differential force.
as such preload doesnt resist the ramps at all. they work together to give you increasing amounts of axle lock when required.

the benefit of preload is that it allows the diff to transfer more torque quicker (ie lock faster) along with increasing the total resistance to differential forces capable of being generated. adding preload always raises the maximum resistance that the diff can generate. for street cars the preload has to be kept low enough that there can be some rotational difference generated by the wheels for slow speed cornering safety but this reduces the speed at which the diff can react to larger differences along with reducing its overall capability.

preload is not the force the diff needs to overcome in order to begin trying to lock and it neither resists nor reacts to the ramps. its a fixed value and the starting point for the diffs ability to resist breakaway. the diff is locked and the ramps dont begin to operate until preload is reached and initial breakaway occurs. at this point the diff begins to generate force by increasing the preload pressure on the discs to assist in resisting the differential forces from the wheels on the basis of the specs created by ramp angle.

with ramps, the lower the ramp angle, the greater the internal forces that are applied to the clutch plates for any given input torque.

Edited by fioran0 on Friday 14th September 10:00

[fioran0]

DanH said:

jfp said:

Does anyone know where I can get steeper than standard diff ramps for my 996GT2? we have measured the power ramp at 30 degrees, gut feel is a 45 degree ramp would be better. Does anyone know what Cup cars use. Cheers, Jon.

If you want a more aggregative diff buy a Guards Transmission one from the US?

the more aggressive guard 50/80 diff uses 45deg and 30 deg for the ramp angles.
If the OP measured his ramp angle correctly he has accel lockup that's equivelant to the decel lockup used on guards most aggressive diff

Edited by fioran0 on Friday 14th September 10:53

[Steve Rance]

Hi John

I did a lot of set up work on diffs in GT3's so i may be able to help. Can you describe exactly what the car is doing through all phases of the corner?

Edited by Steve Rance on Friday 14th September 06:52

[NJH]

Pictures speak a thousand words and all that:
http://forums.rennlist.com/rennforums/racing-and-d...

The picture showing the ramps in particular.

I believe preload is adjusted by installing different grades of belleville washer, which itself is just a washer with a slight conical shape that creates a small spring effect.

Is there much if any difference in how Porsche plate differentials work going back through the past 30 years?

[jfp]

fioran0 said:

jfp said:

The steeper the ramp, or from 30 degrees to say 50 degrees, means its harder for the ramps to work and hence you get less lock for a given torque. Its literally like psuhing something up a hill. Pre-load resists ramps, I tried 50lb with the cup plates and it made no odds. I also tried zero and that was worse still - nothing to react to the ramps.. I do agree that with a very steep ramp, the diff may never lock.

your understanding of what is going on sounds completely inside out.
preload pushes the plates inside the diff together creating a static locking force at the axle. this acts as a permanent resistance to differential forces from the wheels without the diff yet doing anything mechanically at all.

the greater the preload, the greater the differential force the car needs to generate before the preload is overcome, in english, the more the wheels are locked together when you are just driving around normally.
with zero preload the diff would act like an open diff all the time until a wheel began to slip and the mechanicals inside tried to lock the wheels together.
with huge preload the diff would act like it was just welded shut. the car would never be able to overcome the initial preload and create any differential force. as a result the mechanicals inside the diff would never try to do anything but your wheels would never be able to have any difference.

in your case with 80lbs of preload, you need one wheel to generate a force 80lbs greater than the other wheel before your rear axle begins to overcome preload and the wheels can turn with different forces (unlock).
this is why high preload causes low speed understeer and very acutely, slow turning understeer on the street. the rears cannot rotate differently without a significant initial amount of differential force occurring. preload acts as a yaw damper, the effect of which increases as the preload increases.

the ramps simply add to the force already being created by the preload when you need it.
as soon as the locking action of the plates created by preload is overcome, the ramps start to assist by pressing the plates harder and increasing total force available to resist this differential force.
as such preload doesnt resist the ramps at all. they work together to give you increasing amounts of axle lock when required.

the benefit of preload is that it allows the diff to transfer more torque quicker (ie lock faster) along with increasing the total resistance to differential forces capable of being generated. adding preload always raises the maximum resistance that the diff can generate. for street cars the preload has to be kept low enough that there can be some rotational difference generated by the wheels for slow speed cornering safety but this reduces the speed at which the diff can react to larger differences along with reducing its overall capability.

preload is not the force the diff needs to overcome in order to begin trying to lock and it neither resists nor reacts to the ramps. its a fixed value and the starting point for the diffs ability to resist breakaway. the diff is locked and the ramps dont begin to operate until preload is reached and initial breakaway occurs. at this point the diff begins to generate force by increasing the preload pressure on the discs to assist in resisting the differential forces from the wheels on the basis of the specs created by ramp angle.

with ramps, the lower the ramp angle, the greater the internal forces that are applied to the clutch plates for any given input torque.

Edited by fioran0 on Friday 14th September 10:00

I think we are basically saying the same thing.... Preload of 80lb wont stop a car turning on lock at all, whislt it sounds like a big number the clutch packs will slip easily during normal driving. Its engine torque applied that creates the lock via the ramps, and as you say the lower the ramp angle, the greater the clamp load applied to the clutch packs - that creates the lock up. I need an increased angle power ramp to either delay or stop full lock happening at such modest torque.

I have swapped the ramps as they were originally fitted by the previous owner - will let you know how I get on..